Archive for ocean observatories – Page 4

Shipboard LiDAR: A powerful tool for measuring the distribution and composition of particles in the ocean

Posted by mmaheigan 
· Tuesday, October 23rd, 2018 

Despite major advances in ocean observing capabilities, characterizing the vertical distribution of materials in the ocean with high spatial resolution remains challenging. Light Detection and Ranging (LiDAR), a technique that relies on measurement of the “time-of-flight” of a backscattered laser pulse to determine the range to a scattering object, could potentially fill this critical gap in our sampling capabilities by providing remote estimates of the vertical distribution of optical properties and suspended particles in the ocean.

A recent article in Remote Sensing of Environment details the development of a portable shipboard LiDAR and its capabilities for extending high-frequency measurements of scattering particles into the vertical dimension. The authors deployed the experimental system (shown in Figure 1a) during research cruises off the coast of Virginia and during a passenger ferry crossing of the Gulf of Maine (associated with the Gulf of Maine North Atlantic Time Series program-GNATS). Remote measurements of LiDAR signal attenuation corresponded well with simultaneous in situ measurements of water column optical properties and proxies for the concentration of suspended particles. Interestingly, the researchers also observed that the extent to which the return signal was depolarized (also known as the LiDAR depolarization ratio) may provide information regarding the composition of particles within the scattering volume. This is evidenced by the strong relationship between the depolarization ratio and the backscattering ratio, an indicator of the bulk composition (mineral vs. organic) of the particles within a scattering medium (Figure 1b).

Figure 1. a) LiDAR system deployed to look through a chock at the bow of the M/V Nova Star. b) Relationship between the LiDAR linear depolarization ratio (ρ) and coincident measurements of the particulate backscattering ratio (bbp/bp). The black line represents a least-squares exponential fit to the data.

As LiDAR technology becomes increasingly rugged, compact, and inexpensive, the regular deployment of oceanographic LiDAR on a variety of sampling platforms will become an increasingly practical method for characterizing the vertical and horizontal distribution of particles in the ocean. This has the potential to greatly improve our ability to investigate the role of particles in physical and biogeochemical oceanographic processes, especially when sampling constraints limit observations to the surface ocean.

 

Authors:
Brian L. Collister (Old Dominion University)
Richard C. Zimmerman (Old Dominion University)
Charles I. Sukenik (Old Dominion University
Victoria J. Hill (Old Dominion University)
William M. Balch (Bigelow Laboratory for Ocean Sciences)

Shelf-wide pCO2 increase across the South Atlantic Bight

Posted by mmaheigan 
· Thursday, August 2nd, 2018 

Relative to their surface area, coastal regions represent some of the largest carbon fluxes in the global ocean, driven by numerous physical, chemical and biological processes. Coastal systems also experience human impacts that affect carbon cycling, which has large socioeconomic implications. The highly dynamic nature of these systems necessitates observing approaches and numerical methods that can both capture high-frequency variability and delineate long-term trends.

Figure 1: The South Atlantic Bight (SAB) was divided into four sections using isobaths: the coastal zone (0 to 15 m), the inner shelf (15 to 30 m), the middle shelf (30 to 60 m), and the outer shelf (60 m and beyond). The X’s indicate the locations of the Gray’s Reef mooring (southern X) and the Edisto mooring (northern X).

In two recent studies using mooring- and ship-based ocean CO2 system data, authors observed that pCO2 is increasing from the coastal zone to the outer shelf of the South Atlantic Bight at rates greater than the global average oceanic and atmospheric increase (~1.8 µatm y-1). In recent publications in Continental Shelf Research and JGR-Oceans, the authors analyzed pCO2 data from 46 cruises (1991-2016) using a novel linear regression technique to remove the seasonal signal, revealing an increase in pCO2 of 3.0-3.7 µatm y-1 on the outer and inner shelf, respectively. Using a Generalized Additive Mixed Model (GAMM) approach for trend analysis, authors observed that the rates of increase were slightly higher than the deseasonalization technique, yielding pCO2 increases of 3.3 to 4.5 µatm y-1 on the outer and inner shelf, respectively. The reported pCO2 increases result in potential pH decreases of -0.003 to -0.004 units y-1.

Figure 2: The time series of fCO2 in the four regions of the SAB (cruise observations) and from the Gray’s Reef mooring on the inner shelf indicate an increase across the shelf. These data are the observed values, however, the trend lines for each time series are calculated using deseasonalized values using the reference year method.

Analysis of the pCO2 time-series from the Gray’s Reef mooring (using a NOAA Moored Autonomous pCO2 system from July 2006 -July 2015) yielded a rate of increase (3.5 ± 0.9 µatm y-1) that was comparable to the cruise data on the inner shelf (3.7 ± 2.2 and 4.5 ± 0.6 µatm y-1, linear and GAMM methods, respectively). Validation data collected at the mooring suggest that underway data from cruises and the moored data are comparable. Neither thermal processes nor atmospheric dissolution (the primary driver of oceanic acidification) can explain the observed pCO2 increase and concurrent pH decrease across the shelf. Unlike the middle and outer shelves, where an increase in SST could account for up to 1.1 µatm y-1 of the observed pCO2 trend, there is no thermal influence in the coastal zone and inner shelf. While 1.8 µatm y-1 could be attributed to the global average atmospheric increase, the remainder is likely due to transport from coastal marshes and in situ biological processes.  As the authors have shown, the increasing coastal and oceanic trend in pCO2 can lead to a decrease in pH, especially if there is no increase in buffering capacity.  More acidic waters can have a long term affect on coastal ecosystem services and biota.

Also see Eos Editor’s Vox on this research by Peter Brewer https://eos.org/editors-vox/coastal-ocean-warming-adds-to-co2-burden

Authors:

Multidecadal fCO2 Increase Along the United States Southeast Coastal Margin (JGR-Oceans)
Janet J. Reimer (University of Delaware)
Hongjie Wang (Texas A &M University – Corpus Christi)
Rodrigo Vargas (University of Delaware)
Wei-Jun Cai (University of Delaware)

And

Time series pCO2 at a coastal mooring: Internal consistency, seasonal cycles, and interannual variability (Continental Shelf Research)
Janet J. Reimer (University of Delaware)
Wei-Jun Cai (University of Delaware; University of Georgia)
Liang Xue (University of Delaware; First Institute of Oceanography, China)
Rodrigo Vargas (University of Delaware)
Scott Noakes (University of Georgia)
Xinping Hu (Texas A &M University – Corpus Christi)
Sergio R. Signorini (Science Applications International Corporation)
Jeremy T. Mathis (NOAA Arctic Research Program)
Richard A. Feely (NOAA Pacific Marine Environmental Laboratory)
Adrienne J. Sutton (NOAA Pacific Marine Environmental Laboratory; University of Washington)
Christopher Sabine (University of Hawaii Manoa)
Sylvia Musielewicz (NOAA Pacific Marine Environmental Laboratory; University of Washington)
Baoshan Chen (University of Delaware; University of Georgia)
Rik Wanninkhof (NOAA Atlantic Oceanographic and Meteorological Laboratory)

Marine Snowfall at the Equator

Posted by mmaheigan 
· Thursday, July 19th, 2018 

The continual flow of organic particles such as dead organisms and fecal material towards the deep sea is called “marine snow,” and it plays an important role in the ocean carbon cycle and climate-related processes. This snowfall is most intense where high primary production can be observed near the surface. This is the case along the equator in the Pacific and Atlantic Oceans. However, it is not well known how particles are distributed at depth and which processes influence this distribution. A recent study published in Nature Geoscience involved the use of high-resolution particle density data using the Underwater Vision Profiler (UVP) from the equatorial Atlantic and Pacific Oceans down to a depth of 5,000 meters, revealing that several previously accepted ideas on the downward flux of particles into the deep sea should be revisited.

Figure 1. The Underwater Vision Profiler (UVP) during a trial in the Kiel Fjord. The UVP provided crucial data for the new study. Photo: Rainer Kiko, GEOMAR

 

It is typically assumed that the largest particle density can be found close to the surface and that density attenuates continuously with depth. However, high-resolution particle data show that density increases again in the 300-600-meter depth range. The authors attribute this observation to the daily migratory behavior of organisms such as zooplankton that retreat to these depths during the day, contributing to the particle load via defecation and mortality.

Another surprising result is the observation of many small particles below 1,000 meters depth that contribute a large fraction of the bathypelagic particle flux. This observation counters the general assumption, especially in many biogeochemical models, that particle flux at depth comprises fast sinking particles such as fecal pellets. Diminished remineralization rates of small particles or increased disaggregation of larger particles may contribute to the elevated small particle fluxes at this depth.

Figure 2. Zonal current velocity and Particulate Organic Carbon (POC) content across the equatorial Atlantic at 23˚W as observed in November 2012. From left to right: Zonal current velocity, POC content in small particle fraction and POC content in large particle fraction (adapted from Kiko et al. 2017).

 

This study highlights the importance of coupled biological and physical processes in understanding and quantifying the biological carbon pump. Further work on this important topic can now also be submitted to the new Frontiers in Marine Science research topic “Zooplankton and Nekton: Gatekeepers of the Biological Pump” (https://www.frontiersin.org/research-topics/8114/zooplankton-and-nekton-gatekeepers-of-the-biological-pump; Co-editors R. Kiko, M. Iversen, A. Maas, H. Hauss and D. Bianchi). The research topic welcomes a broad range of contributions, from individual-based process studies, to local and global field observations, to modeling approaches to better characterize the role of zooplankton and nekton for the biological pump.

 

Authors:
R. Kiko (GEOMAR)
A. Biastoch (GEOMAR)
P. Brandt (GEOMAR, University of Kiel)
S. Cravatte (LEGOS, University of Toulouse)
H. Hauss (GEOMAR)
R. Hummels (GEOMAR)
I. Kriest (GEOMAR)
F. Marin (LEGOS, University of Toulouse)
A. M. P. McDonnell (University of Alaska Fairbanks)
A. Oschlies (GEOMAR)
M. Picheral (Laboratoire d’Océanographie de Villefranche-sur-Mer, Observatoire Océanologique)
F. U. Schwarzkopf (GEOMAR)
A. M. Thurnherr (Lamont-Doherty Earth Observatory,)
L. Stemmann (Sorbonne Universités, Observatoire Océanologique)

Hotspots of biological production: Submesoscale changes in respiration and production

Posted by mmaheigan 
· Thursday, April 26th, 2018 

The biological pump is complex and variable. To better understand it, scientists have often focused on variations in biological parameters such as fluorescence and community structure, and have less often observed variations in rates of production. Production rates can be estimated using oxygen as a tracer, since photosynthesis produces oxygen and respiration consumes it. In a recent article in Deep Sea Research Part I, the authors presented high-resolution maps of oxygen in the upper 140 m of the ocean in the subtropical and tropical Atlantic, produced from a towed undulating instrument. This provides a synoptic, high-resolution view of oxygen anomalies in the surface ocean. These data reveal remarkable hotspots of biological production and respiration co-located with areas of elevated fluorescence. These hotspots are often several kilometers wide (horizontal) and ~10 m long (vertical). They are preferentially associated with edges of eddies, but not all edges sampled contained hotspots. Although this study captures only two-dimensional glimpses of these hotspots, precluding formal calculations of production rates, likely estimates of source water suggest that many of these hotspots may actually be areas of enhanced respiration rather than enhanced photosynthesis. The paper describes a conceptual model of nutrients, new production, respiration, fluorescence, and oxygen during the formation and decline of these hotspots. These data raise intriguing questions–if the hotspots do indeed have substantially different rates of production and respiration than surrounding waters, then they could lead to significant changes in estimates of production in the upper ocean. Additionally, understanding the mechanisms that produce these hotspots could be critical for predicting the effects of climate change on the magnitude of the biological pump.

(a) Oxygen concentrations and (b) fluorescence at ~1 km resolution over 300 km from 15.13°N, 57.47°W to 12.30°N, 56.42° W, as measured by sensors attached to the (c) Video Plankton Recorder II. Note that no contouring was used for this plot – every pixel represents an actual data point. Figure modified from Stanley et al., 2017. VPR image photograph by Phil Alatalo.

Authors:
Rachel H. R. Stanley (Wellesley College)
Dennis J. McGillicuddy Jr. (WHOI)
Zoe O. Sandwith (WHOI)
Haley Pleskow (Wellesley College)

WBC Series: Observing air-sea interaction in the western boundary currents and their extension regions: Considerations for OceanObs 2019

Posted by mmaheigan 
· Friday, November 10th, 2017 

Dongxiao Zhang1,2, Meghan F. Cronin2, Xiaopei Lin3, Ryuichiro Inoue4, Andrea J. Fassbender5, Stuart P. Bishop6, Adrienne Sutton2

1. University of Washington
2. NOAA Pacific Marine Environmental Laboratory
3. Ocean University of China, China
4. Japan Agency for Marine-Earth Science and Technology, Japan
5. Monterey Bay Aquarium Research Institute
6. North Carolina State University

 

Western boundary currents (WBCs) and their extensions (WBCE) are characterized by intense air-sea heat, momentum, buoyancy, and carbon dioxide (CO2) fluxes (Figure 1a). These large ocean-atmosphere exchanges contribute to the global balance of physical and biogeochemical ocean properties. Excess heat absorbed by the ocean in the tropics is transported poleward, mainly by WBCs (Trenberth and Solomon 1994; Fillenbaum et al. 1997; Zhang et al. 2001; Johns et al. 2011), and then released back to the atmosphere along the WBCs and their extensions in subtropical mid-latitudes at the subtropical and subarctic ocean boundaries (Figure 1a). For this reason, WBC regions are referred to as climatic “hot spots” (Nakamura et al. 2015). Likewise, WBC regions are ocean carbon hot spots, areas of large CO2 uptake that counterbalance the large CO2 outgassing in the tropics (Figure 1b). For OceanObs’09, Cronin et al. (2010) made strong recommendations to include multidisciplinary observations in WBC regional observing systems. The present need for such a system is more urgent than ever. While many open questions remain regarding the role of eddies in ventilation and mode water formation events and their interaction with the biological pump, new technologies are making multidisciplinary observations at these scales more feasible than ever. Use of these new tools during process studies could help to address many of the remaining open questions in WBC regions. OceanObs’19 is two years away, making it timely to review the present observing system for WBC regions and begin strategizing the necessary improvements for the next decade.

Figure 1. Annual mean air-sea net surface heat flux into the ocean from objectively analyzed flux (OAFlux; Yu and Weller 2007) and sea-to-air surface CO2 flux from Takahashi et al. (2009). White contours are the mean dynamic sea level (Rio and Hernandez 2004); star is the NOAA/PMEL KEO location. The WBC regions in the boxes are the Kuroshio-Oyashio Extension (KOE), Gulf Stream (GS), Brazil and Malvinas Currents (BMC), East Australian Current (EAC), and Agulhas Return Current (ARC). Figure adapted from Cronin et al. (2010).

 

The following sections briefly describe requirements of the ocean observing system in WBC regions, and how the current or planned observing system is meeting these requirements, such as international partnerships and collaborations. We also briefly discuss some underway process studies and raise some questions that still need to be addressed, by using the Kuroshio Extension observing system as an example that is applicable to other WBC regions. The goal of this article is to motivate discussion for developing future WBC regional observing systems in preparation for OceanObs’19.

Multiscale multidisciplinary air-sea interaction in WBC regions

Following the principles of the Framework for Ocean Observing (Lindstrom et al. 2012), the first step in developing an “observing system that is fit for purpose” is to define the observational requirements of the system, keeping in mind that these can only be fulfilled by an integrated system that spans multiple time and space scales.

A key characteristic of WBC regions is that their intense air-sea fluxes are associated with strong fronts and energized mesoscale and submesoscale eddies. Strong air-sea heat fluxes effectively project the SST front into the atmosphere, potentially affecting storm tracks and mid-latitude weather (Minobe et al. 2008; Small et al. 2008; Kwon et al. 2010). Carbon uptake in the WBCs is largely controlled by physical processes associated with wintertime heat loss that decreases both SST and surface water pCO2 and increases the ocean’s thermodynamic drive to absorb atmospheric CO2. Furthermore, winter heat loss in WBC regions leads to subduction and the formation of mode waters (Qiu et al. 2006; Cronin et al. 2013; Oka et al. 2015) that transport the absorbed CO2 to the ocean interior and act as an important anthropogenic CO2 sink pathway (Sabine et al. 2004). However, the biological pump also plays an important role in determining the magnitude of natural carbon uptake in the transition region between subtropical and subarctic waters (Takahashi et al. 2009; Fassbender et al. 2017; Wakita et al., 2016). Primary production and ecosystem structure and function are strongly regulated by the energetic fronts and (sub)mesoscale eddies associated with WBCs that supply nutrients to the euphotic zone via upwelling (McGillicuddy 2016; Mahadevan 2016; and the references therein) and cross-frontal exchange between high-nutrient, cold subarctic waters and low-nutrient, warm subtropical waters in the upper ocean (Ayers and Lozier 2012; Nagano et al. 2016; Nagai and Clayton 2017). Questions remain about the importance of these biological processes in local anthropogenic carbon uptake relative to other large-scale chemical and physical processes, such as changes in the seawater buffer capacity and mode and intermediate water formation rates. Within the ocean and atmosphere physics communities, it is becoming clear that to properly represent the energy balance of the climate system in a WBC region, it is necessary to resolve the multi-scale ocean and atmosphere interactions, from large-scale to frontal scale to mesoscale, and potentially even submesoscale. As discussed at the recent joint US CLIVAR and Ocean Carbon and Biogeochemistry (OCB) Ocean Carbon Hot Spots workshop, this is less clear for the biogeochemical system. While biological production is clearly sensitive to fronts and eddies – and recent data from the Kuroshio Extension Observatory (KEO) sediment trap show an accumulation peak that can be traced back to a cold-core eddy (M. Honda, pers. comm. 2017) – physical controls of solubility and buffer capacity may be more important for the carbon cycle. Participants of the Ocean Carbon Hot Spots workshop discussed the challenge of quantifying CO2 uptake and understanding air-sea exchange processes in WBC regions in the face of an incomplete observing system and lack of coverage across the fronts and eddies in these systems. Process studies will enable us to examine the complex relationships between air-sea fluxes, eddy activity, mode water formation, physical and biogeochemical properties of mode waters, and primary production. An improved understanding of these physical-chemical-biological interactions can reveal important information about the underlying mechanisms driving low-frequency decadal variations and gyre-scale circulation in WBC regions. These multi-scale, coupled interactions must be considered in the development of a more comprehensive “fit for purpose” WBC regional observing system.

Observing systems of WBC regions

Satellites represent a critical component of all observing systems, delivering global coverage. Because WBC regional fronts are often associated with clouds and rain that can disrupt satellite remote sensing, some remotely sensed fields could have systematic biases in frontal regions. Thus, the WBC regional observing system plan must include efforts to avoid biases and aliasing from improperly resolved fronts and eddies.

Strong currents, winter storms, and warm season typhoons and hurricanes can also make WBC regions challenging for in situ observations. Of all the WBC regions, the Kuroshio Extension currently has the most complete observing system, so we focus on this system as a potential roadmap for other systems.

Kuroshio Extension Observatory: NOAA surface mooring and JAMSTEC sediment trap

One of the most important observing system components in the Kuroshio Extension is NOAA’s long-term climate reference station, KEO. KEO is strategically located in the Kuroshio Extension recirculation gyre at 32.3°N, 144.5°E (star in Figure 1) on the warm side of the Kuroshio front, an ideal region for monitoring the air-sea interactions that result in mode water formation through winter and spring. Additionally, the site is frequently visited by typhoons during summer and early fall, providing case studies of air-sea interactions between warm water and strong storms. Since 2004, NOAA/PMEL’s Ocean Climate Stations group has maintained a surface mooring at KEO. The NOAA surface mooring measures the meteorological, biogeochemical, and physical ocean variables for estimating air-sea exchanges of heat, moisture, momentum, and carbon dioxide; ocean acidification; and upper ocean variability associated with air-sea interaction. Data are freely available in real time (Figure 2) and are available on GTS (Global Telecommunication System) for improving numerical weather prediction and ocean-atmosphere reanalysis.

Since 2014, Honda et al. (JAMSTEC) have maintained a sediment trap mooring at KEO (Honda pers. comm. 2017). Prior to this, the sediment trap had been deployed at the Japanese S1 mooring, located southeast of KEO at 30°N, 145°E (Honda et al. 2017). The deep sediment trap at 5000 m (800 m above sea floor) positioned next to the KEO surface mooring provides crucial information about the processes affecting nutrient supply that supports ocean productivity and biological carbon export in this subtropical oligotrophic region.

Figure 2. An example of KEO surface observations from real-time data display and delivery web pages.

 

Japanese-funded process studies JKEO, Hot-Spot, and INBOX

Over the past 13 years, there have been a number of process studies in the Kuroshio Extension region. Most notably, the US-funded Kuroshio Extension System Study (KESS) focused on the dynamics of the mesoscale meanders on the Kuroshio Extension and their interaction with the recirculation gyres north and south of the jet (Jayne et al. 2009; Donohue et al. 2008). This was followed by a study of the effect of the Kuroshio Extension front on the air-sea fluxes and interactions, through deployment of a JAMSTEC-KEO (JKEO) surface mooring north of the Kuroshio Extension front paired with the NOAA KEO mooring south of the front (Konda et al. 2010). Then from 2010 to 2015, the extremely successful Japanese process study, “Multi-Scale Air-Sea Interaction under the East-Asian Monsoon: A ‘Hot Spot’ in the Climate System” (Hot-Spot), led to a large field experiment in the region that included a surface flux mooring K-TRITON buoy deployed closer to the center of the Kuroshio Extension jet, which captured the unusual mesoscale exchanges of water mass properties across the Kuroshio Extension front (Nagano et al. 2016). Another Hot-Spot study with three research vessels, each occupying a half-degree latitude between 35°N-37°N along 143°E and transiting back and forth across the Kuroshio Extension SST front (Kawai et al. 2015), showed unprecedented details of dramatic surface latent and sensible heat flux changes and the response of the deep atmospheric boundary layer across the SST front.

While the Japanese Hot-Spot experiment focused on the physical air-sea interactions, the Japanese Western North Pacific Integrated Physical-Biogeochemical Ocean Observation Experiment (INBOX) (Inoue et al. 2016a) focused on biophysical interactions. In 2011, centered around the S1 mooring in a 150-km box, 18 Argo floats equipped with dissolved oxygen sensors were deployed. In addition, a four-month Seaglider survey was conducted between S1 and KEO in 2014. Results showed a strong association between dissolved oxygen patchiness and mesoscale and submesoscale eddies. With proper coordination through CLIVAR, leveraging international research funding could expand these Japanese-funded experiments.

 

Chinese KEO buoy

Motivated by recent exciting findings from ultra-high-resolution coupled model simulations showing the importance of latent and sensible heat release from warm ocean eddies in forcing the atmosphere (Ma et al. 2015) and regulating the Kuroshio Extension jet (Ma et al. 2016), the Ocean University of China has successfully deployed an air-sea flux buoy, the Chinese KEO (C-KEO), north of Kuroshio Extension axis at 39ºN, 149.25ºE in October 2017. Similar to KEO, both subsurface and surface measurements at C-KEO will be transmitted and made available to the scientific community in real time when it reaches stable state. In addition, the Ocean University of China has deployed three subsurface moorings (M1: 32.4ºN, 146.2ºE; M2: 39ºN, 150ºE; and M3: 35ºN, 147.6ºE) equipped with ADCP, CTD, a current meter, and McLane Moored Profiler (M1) to monitor the subsurface eddy structure and variability. Over the past three years, they have deployed 19 Argo floats in the Kuroshio Extension region, with a cluster of floats deployed in an anticyclone eddy, providing the detailed eddy contribution to the subduction and mode water formation (Xu et al. 2016). The Qingdao National Laboratory for Marine Science and Technology, as part of its ‘Transparent Ocean’ project, support all of these observing activities.

 

Challenges and emerging technologies

Characterized by strong winds, high seas, and fast and deep currents, WBC regions are some of the most challenging environments to observe with moored surface buoys. KEO’s long history of success demonstrates that with careful planning, dedication, and international collaborations, sustained moored buoys can be maintained in WBC regions to provide long time series of high-frequency and high-quality simultaneous measurements of subsurface and surface variables. However, due to increased risk of breaking mooring lines in strong, deep jet streams, it is recommended that these long-term reference sites be placed outside of the strongest jet and in the recirculation gyre or northern flank of the jet, like KEO, or the former J-KEO and new C-KEO moorings.

Lagrangian floats have proven to be ideal for studying small-scale fronts and eddies (Shcherbina et al. 2014; Thomas et al. 2017; Inoue et al. 2016b; Xu et al. 2016). Newly available Biogeochemical (BGC)-Argo floats (Johnson and Claustre 2016) may be especially useful for monitoring ocean carbon hot spots to gain a more complete understanding of physical and biogeochemical processes in WBC regions. However, for sustained monitoring and quantifying heat or carbon uptake in eddy-rich WBC regions, a Lagrangian float array would have difficulty to maintain position in strong jets and is susceptible to sampling biases due to the tendency of the floats to be more likely trapped in cyclonic eddies (Rainville et al. 2014; Legg and McWilliams 2002). Controlled surveys by self-propelled autonomous underwater gliders will therefore be necessary to measure moving fronts and eddies and augment moored and Lagrangian components of the observing system.

In situ air-sea flux measurements across fronts and eddies have traditionally required research vessels or voluntary observing ships restricted to limited transit tracks (Fairall et al. 2003; Smith et al. 2016; Palevsky et al. 2016). However, new unmanned surface vehicles (USV) such as Wavegliders (Thompson and Girton 2017) and Saildrones (Meinig et al. 2015; Mordy et al. 2017) are now also being used for air-sea flux measurements of this nature. The Saildrone is especially well suited for collecting observations in the challenging sea conditions of WBC regions. Powered by wind and solar energy with average speed of 3-5 knots (depending on wind, with maximum speed of 7-8 knot), the Saildrone is two times faster than other USVs and has completed a voyage at sea lasting 12 months and covering 16,000 nautical miles. To make the Saildrone capable of observing air-sea exchange processes, NOAA/PMEL, the University of Washington, and Saildrone, Inc. have collaborated to successfully install sensors with equivalent or better quality than those currently used on tropical atmosphere and ocean (TAO) buoys for air-sea flux measurements, as well as a 300 kHz acoustic doppler current profiler (ADCP) for upper ocean current measurements. The standard Saildrone sensor suite also includes: the new PMEL autonomous surface vehicle CO2 system for air-sea CO2 flux measurements; sea surface dissolved oxygen, pH, and chlorophyll sensors; and subsurface backscatter sensing capability from the ADCP, making it a truly interdisciplinary observing platform (Figure 3). Most importantly, Saildrone deployments, measurements, and platform recoveries require no ship time. For example, two Saildrones were recently launched from San Francisco, CA with missions to the eastern tropical Pacific as part of the Tropical Pacific Observing System 2020 project (TPOS2020) and to participate in the field campaign of the NASA Salinity Processes in the Upper Ocean Regional Study (SPURS-2).

Figure 3. Physical and biogeochemical variables measured by Saildrone.

 

Conceptual WBC regional observing system for Ocean Obs’19

With uninterrupted satellite measurements of winds, sea surface height, SST, sea surface salinity, precipitation, and ocean color providing a large-scale context of in situ observations, a sustained WBC regional observing system should have the following components to observe multi-scale multidisciplinary processes:

  1. Long-term moored climate reference buoys in the upper ocean equipped with air-sea flux, physical, and biogeochemical sensors and sediment traps, preferably at the opposite flanks of the WBC extension jets
  2. An array of Lagrangian floats, especially BGC-Argo floats, equipped with standard biogeochemical sensors
  3. Underwater gliders for controlled observations of the subsurface ocean and across fronts and eddies
  4. Unmanned surface vehicle sections crossing WBC regional fronts and eddies around and between reference buoys
  5. Underway shipboard measurements including launch of weather balloons when crossing fronts and eddies during float deployments and glider operations

 

Before such an observing system can be fully developed, process studies are recommended to better understand key physical and biogeochemical processes operating in WBC regions and their associated temporal and spatial scales of variability. Process studies will also inform and optimize the use of newer technologies such as self-navigating platforms together with Lagrangian and Eulerian observations in WBC regions.

 

Acknowledgements

This is a NOAA PMEL contribution number 4725 and is partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement and NA15OAR4320063, Contribution No. 2017-0118.

 

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WBC Series: Fine-scale biophysical controls on nutrient supply, phytoplankton community structure, and carbon export in western boundary current regions

Posted by mmaheigan 
· Friday, November 10th, 2017 

Sophie Clayton1, Peter Gaube1, Takeyoshi Nagai2, Melissa M. Omand3, Makio Honda4

1. University of Washington
2. Tokyo University of Marine Science and Technology, Japan
3. University of Rhode Island
4. Japan Agency for Marine-Earth Science and Technology, Japan

Western boundary current (WBC) regions are largely thought to be hotspots of productivity, biodiversity, and carbon export. The distinct biogeographical characteristics of the biomes bordering WBC fronts change abruptly from stable, subtropical waters to highly seasonal subpolar gyres. The large-scale convergence of these distinct water masses brings different ecosystems into close proximity allowing for cross-frontal exchange. Although the strong horizontal density gradient maintains environmental gradients, instabilities lead to the formation of meanders, filaments, and rings that mediate the exchange of physical, chemical, and ecological properties across the front. WBC systems also act as large-scale conduits, transporting tracers over thousands of kilometers. The combination of these local perturbations and the short advective timescale for water parcels passing through the system is likely the driver of the enhanced local productivity, biodiversity, and carbon export observed in these regions. Our understanding of biophysical interactions in the WBCs, however, is limited by the paucity of in situ observations, which concurrently resolve chemical, biological, and physical properties at fine spatial and temporal scales (1-10 km, days). Here, we review the current state of knowledge of fine-scale biophysical interactions in WBC systems, focusing on their impacts on nutrient supply, phytoplankton community structure, and carbon export. We identify knowledge gaps and discuss how advances in observational platforms, sensors, and models will help to improve our understanding of physical-biological-ecological interactions across scales in WBCs.

Mechanisms of nutrient supply

Nutrient supply to the euphotic zone occurs over a range of scales in WBC systems. The Gulf Stream and the Kuroshio have been shown to act as large-scale subsurface nutrient streams, supporting large lateral transports of nutrients within the upper thermocline (Pelegrí and Csanady 1991; Pelegrí et al. 1996; Guo et al. 2012; Guo et al. 2013). The WBCs are effective in transporting nutrients in part because of their strong volume transports, but also because they support anomalously high subsurface nutrient concentrations compared to adjacent waters along the same isopycnals (Pelegrí and Csanady 1991; Nagai and Clayton 2017; Komatsu and Hiroe pers. comm.). It is likely that the Gulf Stream and Kuroshio nutrient streams originate near the southern boundary of the subtropical gyres (Nagai et al. 2015a). Recent studies have suggested that nutrients in the Gulf Stream originate even farther south in the Southern Ocean (Williams et al. 2006; Sarmiento et al. 2004). These subsurface nutrients can then be supplied to the surface through a range of vertical supply mechanisms, fueling productivity in the WBC regions.

We currently lack a mechanistic understanding of how elevated nutrient levels in these “nutrient streams” are maintained, since mesoscale stirring should act to homogenize them. While it is well understood that the deepening of the mixed layer toward subpolar regions (along nutrient stream pathways) can drive a large-scale induction of nutrients to the surface layer (Williams et al., 2006), the detailed mechanisms driving the vertical supply of these nutrients to the surface layer at synoptic time and space scales remain unclear. Recent studies focusing on the oceanic (sub)mesoscale (spatial scales of 1-100 km) are starting to reveal mechanisms driving intermittent vertical exchange of nutrients and organisms in and out of the euphotic zone.

Recent surveys that resolved micro-scale mixing processes in the Kuroshio Extension and the Gulf Stream have reported elevated turbulence in the thermocline, likely a result of near-inertial internal waves (Nagai et al. 2009, 2012, 2015b; Kaneko et al. 2012, Inoue et al. 2010). In the Tokara Strait, upstream of the Kuroshio Extension, where the geostrophic flow passes shallow topography, pronounced turbulent mixing oriented along coherent banded layers below the thermocline was observed and linked to high-vertical wavenumber near-inertial internal waves (Nagai et al. 2017; Tsutsumi et al. 2017). Within the Kuroshio Extension, measurements made by autonomous microstructure floats have revealed vigorous microscale temperature dissipation within and below the Kuroshio thermocline over at least 300 km following the main stream, which was attributed to active double-diffusive convection (Nagai et al. 2015c). Within the surface mixed layer, recent studies have shown that downfront winds over the Kuroshio Extension generate strong turbulent mixing (D’Asaro et al. 2011; Nagai et al. 2012). The influence of fine-scale vertical mixing on nutrient supply was observed during a high-spatial resolution biogeochemical survey across the Kuroshio Extension front, revealing fine-scale “tongues” of elevated nitrate arranged along isopycnals (Figure 1, Clayton et al. 2014). Subsequent modeling work has shown that these nutrient tongues are ubiquitous features along the southern flank of the Kuroshio Extension front, formed by submesoscale surface mixed layer fronts (Nagai and Clayton 2017).

Microscale turbulence, double-diffusive convection, and submesoscale stirring are all processes associated with meso- and submesoscale fronts. The results from the studies mentioned above support the hypothesis that WBCs are an efficient conduit for transporting nutrients, not only over large scales but also more locally on fine scales, as isopycnal transporters, lateral stirrers, and diapycnal suppliers. It is the sum of these transport processes that ultimately fuels the elevated primary production observed in these regions.

Figure 1. Vertical sections of nitrate (μM) observed across the Kuroshio Extension in October 2009. The panels are organized such that they line up with respect to the density structure of the Kuroshio Extension Front. Cyan contour lines show the mixed layer depth (taken from Nagai and Clayton 2017).

Phytoplankton biomass, community structure, and dynamics

WBCs separate regions with markedly different biogeochemical and ecological characteristics. Subpolar gyres are productive, highly seasonal, tend to support ecosystems with higher phytoplankton biomass, and can be dominated by large phytoplankton and zooplankton taxa. Conversely, subtropical gyres are mostly oligotrophic, support lower photoautotrophic biomass, and are not characterized by a strong seasonal cycle. In turn, these subtropical regions tend to support ecosystems that comprise smaller cells and a tightly coupled microbial loop. As boundaries to these diverse regions, WBCs are the main conduit linking the equatorial and polar oceans and their resident plankton communities. Within the frontal zones, mesoscale dynamics act to stir water masses together and can transport ecosystems across the WBC into regions of markedly different physical and biological characteristics. Furthermore, mesoscale eddies can modulate vertical fluxes via the displacement of ispycnals during eddy intensification or eddy-induced Ekman pumping, or generating submesoscale patches of vertical exchange. At these smaller scales, vigorous vertical circulations ¾ with magnitudes reaching 100 m/day ¾ can fertilize the euphotic zone or transport phytoplankton out of the surface layer.

Numerous studies have hypothesized that the combination of large-scale transport, mesoscale stirring and transport, and submesoscale nutrient input leads to both high biodiversity and high population densities. Using remote sensing data, D’Ovidio et al. (2010) showed that mesoscale stirring in the Brazil-Malvinas Confluence Zone brings together communities from very different source regions, driving locally enhanced biodiversity. In a numerical model, in which physical and biological processes can be explicitly separated and quantified, Clayton et al. (2013) showed that high modeled biodiversity in the WBCs was due to a combination of transport and local nutrient enhancements. And finally, in situ taxonomic surveys crossing the Brazil-Malvinas Confluence (Cermeno et al. 2008) and the Kuroshio Extension (Honjo and Okada 1974; Clayton et al, 2017) showed both enhanced biomass and biodiversity associated with the WBC fronts. Beyond these local enhancements, WBCs might play a larger role in setting regional biogeography. Sugie and Suzuki (2017) found a mixture of temperate and subpolar diatom species in the Kuroshio Extension, suggesting that the boundary current might play a key role in setting downstream diatom diversity.

However tantalizing these results are, they remain relatively inconclusive, in part because of their relatively small temporal and spatial scales. Extending existing approaches for assessing phytoplankton community structure, leveraging emerging ‘omics and continuous sampling techniques, larger regions might be surveyed at high taxonomic and spatial resolution. Combining genomic and transcriptomic observations would provide measures of both organism abundance and activity (Hunt et al. 2013), as well as the potential to better define the relative roles of growth and loss processes. With genetically resolved data and appropriate survey strategies, it will be possible to conclusively determine the presence of these biodiversity hotspots. A better characterization and deeper understanding of these regions will provide insight into the long-term and large-scale biodiversity, stability, and function of the global planktonic ecosystem.

Organic carbon export via physical and biological processes

Export, the removal of fixed carbon from the surface ocean, is driven by gravitational particle sinking, active transport, and (sub)mesoscale processes such as eddy-driven subduction. While evidence suggests that WBCs are likely hot spots of biological (Siegel et al. 2014; Honda et al. 2017a) and physical (Omand et al. 2015) export fluxes out of the euphotic zone, only a small handful of studies have explored this. Recent results from sediment trap studies at the Kuroshio Extension Observatory (KEO) mooring, located just south of the Kuroshio Extension, suggest that there is a link between the passage of mesoscale eddies and carbon export (Honda et al. 2017b). They observed that high export events at 5000 m lagged behind the passage of negative (cyclonic) sea surface height anomalies (SSHA) at the mooring by one to two months (Figure 2). In other regions, underway measurements (Stanley et al. 2010) and optical sensors on autonomous platforms (Briggs et al. 2011; Estapa et al. 2013; Estapa et al. 2015; Bishop et al. 2016) have revealed large episodicity in export proxies over timescales of hours to days and spatial scales of 1-10 km.

Figure 2. Time series of ocean temperature in the upper ~550 m (less than 550 dbar) at station KEO between July 2014 and June 2016. The daily data shown in the figure are available on the KEO database. White contour lines show the temporal variability in the daily satellite-based sea surface height anomaly (SSHA). White open bars show the total mass flux (TMF) observed by the time series sediment trap at 5000 m (based on a figure in Honda et al. 2017b).

Another avenue of carbon export from the surface ocean results from grazing and vertical migration. Vertically migrating zooplankton feed near the surface in the dark and evade predation at depth during the day. Fronts generated by WBCs produce gradients in zooplankton communities, both in terms of grazer biomass and species compositions (e.g., Wiebe and Flierl, 1983), and influence the extent and magnitude of diel vertical migrations. Submesoscale variability in zooplankton abundance can be observed readily in echograms collected by active acoustic sensors, but submesoscale variability in zooplankton community structure and dynamics remains difficult to measure. Thus, the nature of this variability remains largely unknown.

Future research directions

Building a better understanding of how physical and biogeochemical dynamics in WBC regions interact relies on observing these systems at the appropriate scales. This is particularly challenging because of the range of scales at play in these systems and the limitation of existing in situ and remote observing platforms and techniques. As has been outlined above, the ecological and biogeochemical environment of WBCs is the result of long range transport from the flanking subtropical and subpolar gyres, as well as local modification by meso- and submesocale physical dynamics in these frontal systems.

Another challenge in disentangling the relationships between physical and biogeochemical processes in WBCs is the difficulty in measuring rates rather than standing stocks. In such dynamic systems, lags in biological responses mean that the changes in standing stocks may not be collocated with the physical process forcing them. Small-scale lateral stirring spatially and temporally decouples net community production and export while secondary circulations contribute to vertical transport. As much as possible, future process studies should include approaches that can explicitly quantify biological rates and physical transport pathways. New platforms are beginning to fill these observational gaps: BGC-Argo floats, autonomous platforms (e.g., Saildrone), high-frequency underway measurements, and continuous cytometers (including imaging cytometers) are all capable of generating high-spatial resolution datasets of biological and chemical properties over large regions. Gliders and profiling platforms (e.g., WireWalker) are making it possible to measure vertical profiles of biogeochemical properties at high frequency. Operating within a Lagrangian framework, while resolving lateral gradients of physical and biogeochemical tracers with ships or autonomous vehicles, may someday allow us to quantitatively partition the observed small-scale variability in biogeochemical tracers between that attributable to biological or physical processes.

 

 

 

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Sugie, K., and K. Suzuki, 2017: Characterization of the synoptic-scale diversity, biogeography, and size distribution of diatoms in the North Pacific. Limnol. Oceanogr., 62, 884-897, doi:10.1002/lno.10473.

Tsutsumi, E., T. Matsuno, R. C. Lien, H. Nakamura, T. Senjyu, and X. Guo, 2017: Turbulent mixing within the Kuroshio in the Tokara Strait. J. Geophys. Res. Oceans, 122, 7082-7094, doi:10.1002/2017JC013049.

Wiebe, P., and G. Flierl, 1983: Euphausiid invasion/dispersal in Gulf Stream cold-core rings. Mar. Fresh. Res., 34, 625–652, doi: 10.1071/MF9830625.

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Phytoplankton increase projected for the Ross Sea in response to climate change

Posted by mmaheigan 
· Thursday, October 26th, 2017 

How will phytoplankton respond to climate changes over the next century in the Ross Sea, the most productive coastal waters of Antarctica? Model projections of physical conditions suggest substantial environmental changes in this region, but associated impacts on Ross Sea biology, specifically phytoplankton, remain unclear.

In a recent study, Kaufman et al (2017) generated and analyzed model scenarios for the mid- and late-21st century using a combination of a biogeochemical model, hydrodynamic simulations forced by a global climate projection, and new data from autonomous gliders. These scenarios indicate increases in the production of phytoplankton in the Ross Sea and increases in the downward flux of carbon in response to environmental changes over the next century. Modeled responses of the different phytoplankton groups to shoaling mixed layer depths shift the biomass composition more towards diatoms by the mid 21st century. While diatom biomass remains relatively constant in the second half of the 21st century, the haptophyte Phaeocystis antarctica biomass increases as a result of earlier seasonal sea ice melt, allowing earlier availability of low light, in which P. antarctica thrive.

 

Modeled climate scenarios for the 21st century project phytoplankton composition changes and increases in primary production and carbon export flux, primarily as a result of shoaling mixed layer depths and earlier available low light.

The projected responses of phytoplankton composition, production, and carbon export to climate-related changes can have broad impacts on food web functioning and nutrient cycling, with wide-ranging potential effects as local deep waters are transported out from the Ross Sea continental shelf. Future changes to this ecosystem have taken on a new relevance as the Ross Sea became home this year to the world’s largest Marine Protected Area, designed to protect critical habitat for highly valued species that rely on the Ross Sea food web. Continued coordination between modeling and autonomous observing efforts is needed to provide vital data for global ocean assessments and to improve our understanding of ecosystem dynamics and climate change impacts in this sensitive and important region.

 

For other relevant work on observing phytoplankton characteristics in the Ross Sea using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

And for assimilation of bio-optical glider data in the Ross Sea please see: https://doi.org/10.5194/bg-2017-258.

 

Authors:
Daniel E. Kaufman (VIMS, College of William and Mary)
Marjorie A. M. Friedrichs (VIMS, College of William and Mary)
Walker O. Smith Jr. (VIMS, College of William and Mary)
Eileen E. Hofmann (CCPO, Old Dominion University)
Michael S. Dinniman (CCPO, Old Dominion University)
John C. P. Hemmings (Wessex Environmental Associates; now at the UK Met Office)

 

The changing ocean carbon cycle

Posted by mmaheigan 
· Thursday, July 6th, 2017 

Since preindustrial times, the ocean has removed from the atmosphere 41% of the carbon emitted by human industrial activities (Figure 1). The globally integrated rate of ocean carbon uptake is increasing in response to rising atmospheric CO2 levels and is expected to continue this trend for the foreseeable future. However, the inherent uncertainties in ocean surface and interior data associated with ocean carbon uptake processes make it difficult to predict future changes in the ocean carbon sink. In a recent paper, McKinley et al. (2017), review the mechanisms of ocean carbon uptake and its spatiotemporal variability in recent decades. Looking forward, the potential for direct detection of change in the ocean carbon sink, as distinct from interannual variability, is assessed using a climate model large ensemble, a novel approach to studying climate processes with an earth systems model, the “large ensemble.” In a large ensemble, many runs of the same model are done so as to directly distinguish natural variability from long-term trends.


This analysis illustrates that variability in CO2 flux is large enough to prevent detection of anthropogenic trends in ocean carbon uptake on at least decadal to multi-decadal timescales, depending on location. Earliest detection of trends is most attainable in regions where trends are expected to be largest, such as the Southern Ocean and parts of the North Atlantic and North Pacific. Detection will require sustained observations over many decades, underscoring the importance of traditional ship-based approaches and integration of new autonomous observing platforms as part of a global ocean carbon observing system.

Please see a relevant OCB outreach tool on ocean carbon uptake developed by McKinley and colleagues:
OCB teaching/outreach slide deck Temporal and Spatial Perspectives on the Fate of Anthropogenic Carbon: A Carbon Cycle Slide Deck for Broad Audiences  – also download explanatory notes

Satellite Laser Lights Up Polar Research

Posted by mmaheigan 
· Thursday, April 13th, 2017 

What controls annual cycles and interannual changes in polar phytoplankton biomass? Answers to this question are now emerging from a satellite light detection and ranging (lidar) sensor, which can observe the polar oceans throughout the extensive periods when measurements from traditional passive ocean color sensors are impossible. The new study uses active lidar measurements from the CALIOP satellite sensor to construct complete decade-long record of phytoplankton biomass in the northern and southern polar regions. Results of the study show that annual cycles in biomass are driven by rates of acceleration and deceleration in phytoplankton division, with bloom termination coinciding with maximum division rates irrespective of whether nutrients are exhausted. The study further shows that interannual differences in bloom strength can be quantitatively related to the difference between the winter minimum to summer maximum in division rates. Finally, the analysis indicated that ecological processes had a greater impact than ice cover changes on integrated polar zone phytoplankton biomass in the north, whereas ice cover changes were the dominant driver in the south polar zone. Despite being designed for atmospheric research, CALIOP has provided the first demonstration that active satellite lidar measurements can yield important new insights on plankton ecology in the climate sensitive polar regions. This proof-of-concept creates a foundation for a future ocean-optimized sensor with water-column profiling capabilities that would launch a new lidar era in satellite oceanography.

 

 

Authors:

Michael J. Behrenfeld (Oregon State Univ.)
Yongxiang Hu (NASA Langley Research Center)
Robert T. O’Malley (Oregon State Univ.)
Emmanuel S. Boss (Univ. Maine)
Chris A. Hostetler (NASA Langley Research Center)
David A. Siegel (Univ. California Santa Barbara)
Jorge Sarmiento (Princeton Univ.)
Jennifer Schulien (Oregon State Univ.)
Johnathan W. Hair (NASA Langley Research Center)
Xiaomei Lu (NASA Langley Research Center)
Sharon Rodier (NASA Langley Research Center)
Amy Jo Scarino (NASA Langley Research Center)

A framework for ENSO predictability of marine ecosystem drivers along the US West Coast

Posted by mmaheigan 
· Thursday, February 16th, 2017 

The US West Coast eastern boundary upwelling system supports one of the most productive marine ecosystems in the world and is a primary source of ecosystem services for the US (e.g., fishing, shipping, and recreation). Long-term historical observations of physical and biological variables in this region have been collected since the 1950s (e.g., the CalCOFI program and now including the coastal ocean observing systems), leading to an excellent foundation for understanding the ecosystem impacts of dominant climate fluctuations such as the El Niño-Southern Oscillation (ENSO). In the northeast Pacific, ENSO impacts a wide range of physical and biotic processes, including temperature, stratification, winds, upwelling, and primary and secondary production. The El Niño phase of ENSO, in particular, can result in extensive geographic habitat range displacements and altered catches of fishes and invertebrates, and impact vertical and lateral export fluxes of carbon and other elements (Jacox et al., this issue; Anderson et al., this issue; Ohman et al., this issue). However, despite empirical observations and increased understanding of the coupling between climate and marine ecosystems along the US West Coast, there has been no systematic attempt to use this knowledge to forecast marine ecosystem responses to individual ENSO events. ENSO forecasting has become routine in the climate community. However, little has been done to forecast the impacts of ENSO on ecosystems and their services. This becomes especially important considering the occurrence of recent strong El Niño events (such as 2015-16) and climate model projections that suggest that ENSO extremes may become more frequent (Cai et al. 2015).

The joint US CLIVAR/OCB/NOAA/PICES/ICES workshop on Forecasting ENSO impacts on marine ecosystems of the US West Coast (Di Lorenzo et al. 2017) held in La Jolla, California, in August 2016 outlined a three-step strategy to better understand and quantify the ENSO-related predictability of marine ecosystem drivers along the US West Coast (Figure 1). The first step is to use a high-resolution ocean reanalysis to determine the association between local ecosystem drivers and regional forcing patterns (RFPs). The identification of ecosystem drivers will depend on the ecosystem indicators or target species selected for prediction (Ohman et al., this issue). The second step is to objectively identify the tropical sea surface temperature (SST) patterns that optimally force the RFPs along the US West Coast region using available long-term large-scale reanalysis products. While the goal of the first two steps is to understand the dynamical basis for predictability (Figure 1, blue path), the final third step (Figure 1, orange path) aims at quantifying the predictability of the RFPs and estimating their prediction skill at seasonal timescales. This third step can be implemented using the output of multi-model ensemble forecasts such as the North America Multi-Model Ensemble (NMME) or by building efficient statistical prediction models such as Linear Inverse Models (LIMs; Newman et al. 2003).

Figure 1. Framework for understanding and predicting ENSO impacts on ecosystem drivers. Blue path shows the steps that will lead to Understanding of the ecosystem drivers and their dependence on tropical Pacific anomalies. Orange path shows the steps that will lead to quantifying the Predictability of marine ecosystem drivers along the US West Coast that are predictable from large-scale tropical teleconnection dynamics.

 

Important to the concept of ENSO predictability is the realization that the expressions of ENSO are very diverse and cannot be identified with a few indices (Capotondi et al. 2015; Capotondi et al., this issue). In fact, different expressions of sea surface temperature anomalies (SSTa) in the tropics give rise to oceanic and atmospheric teleconnections that generate different coastal impacts in the northeast Pacific. For this reason, we will refer to ENSO as the collection of tropical Pacific SSTa that lead to deterministic and predictable responses in the regional ocean and atmosphere along the US West Coast.

In the sections below, we articulate in more detail the elements of the framework for quantifying the predictability of ENSO-related impacts on coastal ecosystems along the US West Coast (Figure 1). Our focus will be on the California Current System (CCS), reflecting the regional expertise of the workshop participants. Specifically, we discuss (1) the ecosystem drivers and what is identified as such; (2) RFP definitions; and (3) the teleconnections from the tropical Pacific and their predictability.

Ecosystem drivers in the California Current System

The impacts of oceanic processes on the CCS marine ecosystem have been investigated since the 1950s when the long-term California Cooperative Oceanic Fisheries Investigations (CalCOFI) began routine seasonal sampling of coastal ocean waters. The CalCOFI program continues today and has been augmented with several other sampling programs (e.g., the coastal ocean observing network), leading to an unprecedented understanding of how climate and physical ocean processes, such as upwelling, drive ecosystem variability and change (e.g., see more recent reviews from King et al.2011; Ohman et al. 2013; Di Lorenzo et al. 2013).

The dominant physical oceanographic drivers of ecosystem variability occur on seasonal, interannual, and decadal timescales and are associated with changes in (1) SST; (2) upwelling velocity; (3) alongshore transport; (4) cross-shore transport; and (5) thermocline/nutricline depth (see Ohman et al., this issue). This set of ecosystem drivers emerged from discussions among experts at the workshop. Ecosystem responses to these drivers include multiple trophic levels, including phytoplankton, zooplankton, small pelagic fish, and top predators, and several examples have been identified for the CCS (see summary table in Ohman et al., this issue).

While much research has focused on diagnosing the mechanisms by which these physical drivers impact marine ecosystems, less is known about the dynamics controlling the predictability of these drivers. As highlighted in Ohman et al. (this issue), most of the regional oceanographic drivers (e.g., changes in local SST, upwelling, transport, thermocline depth) are connected to changes in large-scale forcings (e.g., winds, surface heat fluxes, large-scale SST and sea surface height patterns, freshwater fluxes, and remotely forced coastally trapped waves entering the CCS from the south). In fact, several studies have documented how large-scale changes in wind patterns associated with the Aleutian Low and the North Pacific Oscillation drive oceanic modes of variability such as the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation (Mantua et al. 1997; Di Lorenzo et al. 2008; Chhak et al. 2009; Ohman et al., this issue; Jacox et al., this issue; Anderson et al., this issue; Capotondi et al., this issue) that influence the CCS. However, these large-scale modes only explain a fraction of the ecosystem’s atmospheric forcing functions at the regional-scale. Thus, it is important to identify other key forcings to gain a more complete mechanistic understanding of CCS ecosystem drivers (e.g., Jacox et al. 2014; 2015).

Atmospheric and oceanic regional forcing patterns

The dominant large-scale quantities that control the CCS ecosystem drivers are winds, heat fluxes, and remotely forced coastally trapped waves (Hickey 1979). Regional expressions or patterns of these large-scale forcings have been linked to changes in local stratification and thermocline depth (Veneziani et al. 2009a; 2009b; Combes et al. 2013), cross-shore transport associated with mesoscale eddies (Kurian et al. 2011; Todd et al. 2012; Song et al. 2012; Davis and Di Lorenzo 2015b), and along-shore transport (Davis and Di Lorenzo 2015a; Bograd et al. 2015). For this reason, we define the regional expressions of the atmospheric and remote wave forcing that are optimal in driving SST, ocean transport, upwelling, and thermocline depth as the RFPs. To clarify this concept, consider the estimation of coastal upwelling velocities. While a change in the position and strength in the Aleutian Low has been related to coastal upwelling in the northern CCS, a more targeted measure of the actual upwelling vertical velocity and nutrient fluxes that are relevant to primary productivity can only be quantified by taking into account a combination of oceanic processes that depend on multiple RFPs such as thermocline depth (e.g., remote waves), thermal stratification (e.g., heat fluxes), mesoscale eddies, and upwelling velocities (e.g., local patterns of wind stress curl and alongshore winds; see Gruber et al 2011; Jacox et al. 2015; Renault et al. 2016). In other words, if we consider the vertical coastal upwelling velocity (w) along the northern CCS, a more adequate physical description and quantification would be given from a linear combination of the different regional forcing functions w = Σn ∝n * RFPn rather than w = ∝*Aleutian Low.

The largest interannual variability in the Pacific that impacts the RFPs is ENSO, which also constitutes the largest source of seasonal (3-6 months) predictability. During El Niño and La Niña, atmospheric and oceanic teleconnections from the tropics modify large-scale and local surface wind patterns and ocean currents of the CCS and force coastally trapped waves.

ENSO teleconnections and potential seasonal predictability of the regional forcing patterns

While ENSO exerts important controls on the RFPs in the CCS, it has become evident that ENSO expressions in the tropics vary significantly from event to event, leading to different responses in the CCS (Capotondi et al., this issue). Also, as previously pointed out, the CCS is not only sensitive to strong ENSO events but more generally responds to a wide range of tropical SSTa variability that is driven by ENSO-type dynamics in the tropical and sub-tropical Pacific. For this reason, we define an “ENSO teleconnection” as any RFP response that is linked to ENSO-type variability in the tropics.

ENSO can influence the upwelling and circulation in the CCS region through both oceanic and atmospheric pathways. It is well known that equatorial Kelvin waves, an integral part of ENSO dynamics, propagate eastward along the Equator and continue both northward (and southward) along the coasts of the Americas as coastally trapped Kelvin waves after reaching the eastern ocean boundary. El Niño events are associated with downwelling Kelvin waves, leading to a deepening of the thermocline, while La Niña events produce a shoaling of the thermocline in the CCS (Simpson 1984; Lynn and Bograd 2002; Huyer et al. 2002; Bograd et al. 2009; Hermann et al. 2009; Miller et al. 2015). The offshore scale of coastal Kelvin waves decreases with latitude, and the waves decay while propagating northward along the coast due to dissipation and radiation of westward propagating Rossby waves. In addition, topography and bathymetry can modify the nature of the waves and perhaps partially impede their propagation at some location. Thus, the efficiency of coastal waves of equatorial origin in modifying the stratification in the CCS is still a matter of debate. To complicate matters, regional wind variability south of the CCS also excites coastally trapped waves, which supplement the tropical source.

In the tropics, SST anomalies associated with ENSO change tropical convection and excite mid-troposphere stationary atmospheric Rossby waves that propagate signals to the extratropics, the so-called atmospheric ENSO teleconnections (Capotondi et al., this issue). Through these atmospheric waves, warm ENSO events favor a deepening and southward shift of the Aleutian Low pressure system that is dominant during winter, as well as changes in the North Pacific Subtropical High that is dominant during spring and summer, resulting in a weakening of the alongshore winds, reduced upwelling, and warmer surface water. These changes are similar to those induced by coastal Kelvin waves of equatorial origin, making it very difficult to distinguish the relative importance of the oceanic and atmospheric pathways in the CCS. In addition, due to internal atmospheric noise, the details of the ENSO teleconnections can vary significantly from event to event and result in important differences along the California Coast (Figure 2).

Figure 2. Schematic of ENSO teleconnection associated with different flavors of tropical SSTa. (a) Atmospheric teleconnections of the canonical eastern Pacific El Niño tend to impact the winter expression of the Aleutian Low, which in turn drives an oceanic SSTa anomaly that projects onto the pattern of the Pacific Decadal Oscillation (PDO). (b) Atmospheric teleconnections of the central Pacific El Niño tend to impact the winter expression of the North Pacific High, which in turn drives an oceanic SSTa anomaly that projects onto the pattern of the North Pacific Gyre Oscillation (NPGO). The ENSO SSTa maps are obtained by regressing indices of central and eastern Pacific ENSO with SSTa. The other maps are obtained by regression of SSTa/SLPa with the PDO (a) and NPGO (b) indices.

 

El Niño events exhibit a large diversity in amplitude, duration, and spatial pattern (Capotondi et al. 2015). The amplitude and location of the maximum SST anomalies, whether in the eastern (EP) or central (CP) Pacific, can have a large impact on ENSO teleconnections (Ashok et al. 2007; Larkin and Harrison 2005). While “canonical” EP events induce changes in the Aleutian Low (Figure 2b), CP events have been associated with a strengthening of the second mode of North Pacific atmospheric variability, the North Pacific Oscillation (NPO; Figure 2a; Di Lorenzo et al. 2010; Furtado et al. 2012). In addition, it is conceivable that EP events have a larger Kelvin wave signature than CP events, resulting in different oceanic influences in the CCS.

In summary, while the ENSO influence on the CCS physical and biological environments is undeniable, several sources of uncertainty remain about the details of that influence. This uncertainty arises in the physical environment on seasonal timescales from many sources, including the diversity of ENSO events, the intrinsic unpredictable components of the atmosphere, and the intrinsic unpredictable eddy variations in the CCS. We also need to distinguish between physically forced ecosystem response versus intrinsic biological variability, which is potentially nonlinear and likely unpredictable. Skill levels need to be quantified for each step of the prediction process (i.e., ENSO, teleconnections, local oceanic response, local ecosystem response) relative to a baseline—for example the persistence of initial condition, which is also being exploited for skillful predictions of the large marine ecosystem at the seasonal timescale (Tommasi et al., this issue). The target populations should be exploitable species that are of interest to federal and state agencies that regulate certain stocks. Models are currently being developed to use ocean forecasts to advance top predator management (Hazen et al., this issue). The implementation of this framework (Figure 1) for practical uses will require a collaborative effort between physical climate scientists with expertise in predicting and understanding ENSO and biologists who have expertise in understanding ecosystem response to physical climate forcing.

Authors

Emanuele Di Lorenzo (Georgia Institute of Technology)
Arthur J. Miller (Scripps Institution of Oceanography)

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Veneziani, M., C. A. Edwards, and A. M. Moore, 2009: A central California coastal ocean modeling study: 2. Adjoint sensitivities to local and remote forcing mechanisms. J. Geophy. Res.-Oceans, 114, doi:10.1029/2008jc004775.

 

 

 

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