Archive for autonomous platforms

Autonomous measurement of N-loss in the Eastern Tropical North Pacific ODZ: An Invitation for Collaboration

Posted by mmaheigan 
· Thursday, January 10th, 2019 

By Mark A. Altabet (SMAST/U. Mass. Dartmouth), Craig McNeil, and Eric D’Asaro (both at APL / U. Washington)

Oxygen deficient zones (ODZs) constitute a small fraction of total oceanic volume yet play an important role in regulating global ocean carbon and nitrogen cycles. They are critical for regulating the ocean’s nitrogen budget, as loss of biologically available nitrogen to N2 gas (N-loss) within ODZs is estimated to be 30 to 50% of the global total. However, temporal and spatial variability in N-loss rates have been undersampled by ship-based process studies leaving substantial uncertainty in overall rates. While local and short-term regulation of N-loss by O2 and organic matter availability is well documented, little is known about the larger scale temporal/spatial variability in N-loss that may result from physical forcings such as remote ventilation, seasonal variability in vertical exchange with the near-surface layer, and mesoscale eddies. Understanding the impact of larger scale physical forcings on N-loss as mediated through O2 and organic flux is needed to fully understand the causes and consequences of any future ODZ expansion. To achieve this, we need sustained observations by a distributed array capable of detecting synoptic variability.

To address these issues, a new NSF-funded project will carry out a multiyear, autonomous float-based observational program to answer the following questions:

  • How does biogenic N2 production in ODZs vary over weekly to annual time scales and space scales of 10s to 1000s km?
  • What are the major scales of variability and their associated oceanographic phenomena and how do they relate to control by organic matter flux and O2 concentration?
  • How does this variability influence regionally integrated N-loss?

Figure 1. The ETNP ODZ roughly defined by O2 <1.5 μmol/kg (orange, World Ocean Atlas 2013). Our new NSF-funded project will sample across these patterns of spatial and temporal variability for 2 years with 10 subsurface ODZ floats (red/blue) each measuring profiles of T, S, O2 (50 nmol/kg LOD) and N2 (0.1 μmol/kg precision), and the in situ rate of N2 change. Four Argo floats with O2 sensors and BioOptical floats provided by collaborators will supplement this array. Bright bar symbols are the planned deployment positions; dimmed bar symbols suggest possible displacements after 2 years. Ship-based measurements (yellow stars) along the deployment cruise track (magenta) will be used for float sensor calibration and identification of ETNP source water properties. The 2-year track of our prototype GasFloat is also shown (black line).

This project will exploit our ability to make in situ, ultra-high precision measurement of N2 concentration (~0.1 umol/kg) and use commercially available O2 sensors to measure O2 in the 10s of nM range. Our study area is the Eastern Tropical North Pacific (ETNP), the largest ODZ and the region of our successful pilot deployments (Figure 1). Over a multi-year period, our study will determine in situ nM-level O2 and biogenic N2 on float profiles distributed throughout the ETNP and encompassing geographic gradients in O2 and surface productivity. For the first time, our study will also determine in situ N-loss rates from changes in N2 concentration during one- to two-week Lagrangian float deployments drifting along a constant density surface (Figure 2). A pilot two-year float (‘GasFloat, Figure 1) deployment in the ETNP has documented our ability to do so. Critically, our float-based approach more closely matches the frequency and distribution of observations to the expected variability in biogenic N2 production, as compared to prior work. This study will also dramatically increase the data density in this region by acquiring >500 profiles/drifts for N2 and >1000 profiles for nM O2.

Figure 2. (a) Schematic of float system to be deployed (b) Example of float mission including 2-week isopycnal drift.

We anticipate float deployment in summer 2020 via a UNOLS vessel. Investigators interested in collaborative participation through contribution of autonomous instrumentation and/or making shipboard measurements are encouraged to contact the lead PI Mark Altabet at maltabet@umassd.edu.  Similarly, students interested in graduate research opportunities through this project should contact the lead PI.

Building ocean biogeochemistry observing capacity, one float at a time: An update on the Biogeochemical-Argo Program

Posted by mmaheigan 
· Thursday, July 5th, 2018 

By Ken Johnson (MBARI)

The Biogeochemical-Argo (BGC-Argo) Program is an international effort to develop a global network of biogeochemical sensors on Argo profiling floats that has emerged from over a decade of community discussion and planning. While there is no formal funding for this global program, it is being implemented via a series of international research projects that harness the unique capabilities provided by BGC profiling floats. The U.S. Ocean Carbon and Biogeochemistry (OCB) Program maintains and supports a U.S. BGC-Argo subcommittee as a focal point for U.S. community input on the implementation of the global biogeochemical float array and associated science program development.

Figure 1. Steve Riser deploying a SOCCOM float from the R/V Palmer

About BGC-Argo Floats
BGC-Argo floats can carry a suite of chemical and bio-optical sensors (Figure 1 – Float Schematic).  They have enough energy to make about 250 to 300 vertical profiles from 2000 m to the surface.  At a cycle time of 10 days, that corresponds to a lifetime near 7 years.  The long endurance allows the floats to resolve seasonal to interannual variations in carbon and nutrient cycling throughout the water column.  These time scales are difficult to study from ships and ocean interior processes are hard to resolve from satellites.  BGC profiling floats extend the capabilities of these traditional observing systems in significant ways.

Figure 2. Images of Navis and APEX floats used in the SOCCOM program. These floats carry oxygen, nitrate, pH, and bio-optical (chlorophyll fluorescence and backscatter) sensors.

All of the data from profiling floats operating as part of the Argo program must be available in real-time with no restrictions on access.  The Argo Global Data Assembly centers in France and the USA both provide complete listings of all BGC profiles (argo_bio-profile_index.txt) and access to the data.  Extensive documentation of the data processing protocols is available from the Argo Data Management Team.  Individual research programs, such as SOCCOM (see below), may also provide direct data access to the observed data along with value added products such as best estimates of pCO2 derived from pH sensor data.

Regional Deployments
In 2018, it is projected that 127 profiling floats with biogeochemical sensors are will be deployed, including ~40 floats by U.S. projects. Most of the U.S. deployments (30+) will be carried out by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project (Figure 2 – Float Deployment). These floats will carry oxygen, nitrate, pH, chlorophyll fluorescence, and backscatter sensors. As part of the NOAA Tropical Pacific Observing System (TPOS) program, Steve Riser’s group (Univ. Washington) will deploy 3 BGC-Argo floats per year in the equatorial Pacific over the next 4 years. These floats will be equipped with oxygen, pH, bio-optical sensors and Passive Acoustic Listener (PAL) sensors, which provide wind speed estimates at 15-minute intervals while the floats are parked at 1000 m.  Wind speed is derived from the noise spectrum of breaking waves. Steve Emerson (Univ. Washington), with NSF support, is also deploying floats equipped with oxygen, nitrate and pH sensors in the equatorial Pacific. With funding from NSF, Andrea Fassbender (MBARI) will deploy two floats at Ocean Station Papa in the northeast Pacific in collaboration with the EXPORTS program. These floats will also carry oxygen, nitrate, pH, and bio-optical sensors.

Nearly 90 BGC floats will be deployed in 2018 by other nations in multiple ocean basins.  Much of this effort will focus on the North Pacific and North Atlantic.  The sensor load on these floats is somewhat variable. Some will be deployed with only oxygen sensors or bio-optical sensors for chlorophyll fluorescence and particle abundance. Others will carry the full suite of six sensors (oxygen, nitrate, pH, chlorophyll fluorescence, backscatter, and irradiance) that are outlined in the BGC-Argo Implementation Plan (BGC-Argo, 2016). These floats will contribute to the existing array of 305 biogeochemical floats (Figure 3 BGC Argo Map).

Community Activities
In response to the tremendous interest in the scientific community in the capabilities of profiling floats, OCB is sponsoring a Biogeochemical Float Workshop at the University of Washington in Seattle from July 9-13, 2018 to begin the process of transferring this expertise to the broader oceanographic community, bringing together potential users of this technology to discuss biogeochemical profiling float technology, sensors, and data management and begin the process of the intelligent design of future scientific experiments. The workshop will provide participating scientists direct access to the facilities of the Float Laboratory operated by Riser. This workshop builds on a previous OCB workshop Observing Biogeochemical Cycles at Global Scales with Profiling Floats and Gliders (Johnson et al., 2009). BGC-Argo will also have a prominent presence at the 6th Argo Science Workshop (October 22-24, 2018, Tokyo, Japan) and OceanObs19 (September 16-20, 2019, Honolulu, HI).

Figure 3. May 2018 map of the location of BGC-Argo floats that have reported in the previous month and sensor types on these floats. From jcommops.org.

BGC-Argo Publications
Several resources now highlight the capabilities of profiling floats to accomplish scientific observing goals. A web-based bibliography of biogeochemical float papers hosted on the Biogeochemical-Argo website currently includes >100 publications and continues to grow. A special issue of Journal of Geophysical Research: Oceans focused on the SOCCOM program is in progress with 11 papers now available and a dozen more forthcoming. These papers include summaries of the technical capabilities of floats and the biogeochemical sensors, comparisons of float bio-optical data with satellite remote sensing observations, seasonal and interannual assessments of air-sea oxygen flux, under-ice biogeochemistry, carbon export, comparisons of pCO2 estimated from floats with pH vs. time-series data, and net community production. The connection of float observations with numerical models is a special focus of the program and this is highlighted in several papers, including a description of the Biogeochemical Southern Ocean State Estimate (SOSE), which is a data assimilating BGC model. Results from Observing System Simulation Experiments (OSSEs) used to assess the number of floats needed for large-scale observations are also reported. The BGC-Argo steering committee is developing a community white paper for the OceanObs19 conference in September 2019. BGC-Argo also develops and distributes a community newsletter.

For more information, visit the BGC-Argo website or reach out to the U.S. BGC-Argo Subcommittee.

 

 

 

 

Updates and Plans for the First EXPORTS Field Campaign

Posted by mmaheigan 
· Thursday, February 1st, 2018 

Contacts: David Siegel (UCSB; EXPORTS Science Lead) & Ivona Cetinić (NASA GSFC/USRA; EXPORTS Project Scientist)

 

EXPORTS in a Nutshell

Ocean ecosystems constitute a significant fraction of the world’s primary production, fixing CO2 and creating oxygen while playing critical roles in sequestering CO2 from the atmosphere. An improved understanding of the cycling and fate of oceanic organic carbon will not only allow for better prediction of how these processes may change in the future, but it will help underpin the societal value of these ocean ecosystem services. The EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign aims to provide answers to these questions.

The goal of EXPORTS is to develop a predictive understanding of the export and fate of global ocean net primary production (NPP) and its implications for the Earth’s carbon cycle in present and future climates (oceanexports.org). To develop this quantitative understanding, EXPORTS will measure and model the export pathways that remove fixed organic carbon from the upper ocean and drive the attenuation of these vertical fluxes within the ocean interior. EXPORTS datasets will be used to develop and test numerical predictive and satellite-data diagnostic models of NPP fates and their carbon cycle impacts. EXPORTS builds on decades of NASA-funded research on developing and validating satellite data-driven models of regional to global NPP and hence, EXPORTS will contribute to NASA’s upcoming Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) mission.

 

A Brief History of EXPORTS

The NASA EXPORTS field campaign is the result of an initial open competition in 2012 by the NASA Ocean Biology and Biogeochemistry (OBB) Program to identify scoping workshops for future field campaigns. This was followed by many years of committee-based planning, community vetting of science and implementation plans, and final peer review.  The NASA EXPORTS Science and Implementation Plans were made publicly available by the NASA OBB program. In February 2016, the National Science Foundation held the Biology of the Biological Pump (BoBP) workshop aimed in part to leverage NASA’s planned investment in the EXPORTS field program. In August 2016, NASA announced it would support data mining and observational system simulation experiment (OSSE) projects to help with planning the NASA EXPORTS field campaign and five projects were funded under this pre-EXPORTS call.

In early 2017, NASA released a call for proposals for the EXPORTS field program and the competition for inclusion on the NASA EXPORTS Science Team and its leadership. The call also included the implementation approach for the EXPORTS field program, with two major cruises to collect in situ data, followed by a synthesis and analysis phase to be competed in the future. At the same time, NSF released a Dear Colleague Letter (DCL) stating they would consider proposals that leveraged the NASA investment with objectives that supported the BoBP plan. From the NASA competition, 11 projects were selected for support (Table 1). Three NSF proposals have been recommended for support (at the time of this writing, the awarding of these grants is not yet official), bringing the count to a total of 41 PIs and co-PIs that are supported by NASA and NSF on EXPORTS/BoBP. This level of investment likely makes EXPORTS the largest coordinated U.S.-funded biogeochemical field program since the Joint Global Ocean Flux Study (JGOFS) nearly 2 decades ago. Table 1 lists the funded projects, PI, and co-PIs, project titles, and links to two page descriptions for each project.

Any implementation of the EXPORTS field program must result in the quantification of the major export pathways that remove fixed organic carbon from the upper ocean and sequester it at depth. NASA is uniquely poised, given the global vantage point of space-based observations, to use Earth observing satellite data to meet this objective, while also understanding observational requirements for future advanced Earth Observing missions.  Quantification of major carbon export pathways requires the simultaneous measurement of 1) sinking particle fluxes (and their composition), 2) the export of organic carbon to depth via vertically migrating zooplankton, and 3) the vertical transport of dissolved and suspended particulate organic carbon to depth, where it is remineralized by different microbial communities. To develop predictive links to satellite ocean color-retrievable parameters, the quantification of export pathways must be augmented by research programs focused on, but not limited to, the elucidation of plankton community structure, rates of NPP and grazing, and optical oceanography. Complicating this further is the stochastic nature of export flux determinations that necessitates a fully four-dimensional sampling design while maintaining a long-term perspective. This reasoning led to the Agency selection of projects listed in Table 1.

The planning of the EXPORTS field campaign is well underway. The first field deployment is planned to take place in the summer of 2018 in the Northeast Pacific, while the tentative second cruise will be in the North Atlantic Ocean in the spring of 2020. NASA has formed a project office staffed of Agency and EXPORTS PIs to direct EXPORTS’ progress. The EXPORTS Science Team, which comprises the funded PIs, is participating on near-weekly teleconferences, and co-chief scientists have been selected. An initial EXPORTS kickoff meeting was held in September 2017 in the Washington, DC area. There, the PIs organized themselves into working groups focused on creating short methodological descriptions for each measurement to be made. This documentation will be critical for the metadata, the project data management, and for ensuring legacy of the program through a set of NASA Technical Memoranda. This has also proven to be an excellent way to foster cross-project collaborations. A second PI meeting is scheduled for mid-February 2018, leveraging the upcoming Ocean Sciences Meeting.

 

EXPORTS First Field Deployment

The first EXPORTS field deployment will be to the Northeast Pacific Ocean in late summer 2018. Two ships, the R/V Roger Revelle and the R/V Sally Ride, will be deployed for 27 days of coordinated sampling around Station P (50°N 145°W), while EXPORTS’ autonomous component will ensure a longer-term presence. The choice of Station P as an anchor point for the field campaign was made based on results from the data mining and OSSE projects and the availability of a long-term data set for this site, as well as the many sampling partnerships afforded by ongoing programs. Canada’s Line P long-term hydrographic/biogeochemistry program has been running since 1949, and they currently conduct three annual transect cruises from British Columbia to Station P. Other useful partnerships include NOAA Pacific Marine Environmental Laboratory’s (PMEL) air-sea interaction buoy and the NSF’s Ocean Observatories Initiative’s (OOI) global node at Station P.

 

Figure 1: Cartoon depicting many of the individual elements to be deployed during the 2018 EXPORTS sampling program in the North Pacific.

The EXPORTS 2018 field deployment will comprise four basic components (depicted in Figure 1 above). First, several autonomous vehicles will be deployed before the ship observations. An instrumented Lagrangian float will be deployed at depth and used to set the spatial center of the sampling program, while an instrumented Seaglider will be used to provide vertical and some horizontal spatial information around the Lagrangian float’s drift. In addition, and if approved by the OOI Facility Board, instrumented gliders deployed at the Station P OOI global node will be used to supplement the autonomous vehicle data streams.

Second, the R/V Roger Revelle will be the Process Ship, and will follow the Lagrangian float. The Process Ship will focus on rates (NPP, sinking particle fluxes, grazing, net community production, zooplankton respiration and fecal particle production, aggregate formation, etc.) and vertical information (microbial community structure and particle size spectra) in the water mass surrounding the float. Rate measurements will be made using water sampled with a trace metal-clean rosette system, and sinking particle fluxes from neutrally buoyant sediment traps (NBSTs) and sediment trap array. In particular, microbial community structure will be measured using a variety of techniques, including high-throughput microscopic imaging systems, meta-community genomic sequencing, isolation and experimentation on individual marine snow aggregates, and gel trap-collected sinking particles. The Process Ship will also conduct a complete optical oceanographic sampling program ensuring links to remotely sensed parameters. Drs. Deborah Steinberg (VIMS) and Jason Graff (OSU) have volunteered to be co-chief scientists for the R/V Revelle.

Third, the R/V Sally Ride will be the Survey Ship making spatial patterns about the Process Ship on scales from roughly 1 km to nearly 100 km. The focus of the Survey Ship will be collecting horizontal spatial information on particle export (234Th disequilibrium), net community production (O2/Ar), organic carbon stocks, phytoplankton composition, and inherent and apparent optical properties. The Survey Ship will also deploy a suite of instrumentation to characterize the particle size spectrum from 20 nm to nearly a cm. It will also be responsible for validating the calibration of the autonomous vehicles’ bio-optical instrumentation and the development of the biogeochemical proxies. Norm Nelson (UCSB) and Mary Jane Perry (self-affiliated) have agreed to be the co-chief scientists on the R/V Sally Ride.

Last, EXPORTS needs a long-term sampling presence to tie the ship-based observations to climatically relevant time and space scales. The Lagrangian float and Seaglider will sample for ~6 months, bracketing EXPORTS’ intensive ship observations, and thus providing some long-term perspectives to the ship sampling. Partnering programs like Line P and the OOI Global Node will allow for some additional in situ sampling opportunities and broader temporal context. Further, the PMEL mooring and a profiling float project recommended for funding by NSF will extend the long-term biogeochemical observations.

The integration of the observations will generate a data set that will not only be invaluable for building new algorithms for retrievals of new and refined data products from NASA’s current fleet of Earth Observing Satellites, but also will be critical in the development of new sets of requirements for future satellite observations of our Earth system. As described in the EXPORTS Implementation Plan, the likelihood of the EXPORTS achieving its predictive goals will increase as the number and variety of observations available to develop and test novel algorithms increases. Hence, the EXPORTS program is particularly motivated to collaborate with international partners who would be interested to share their data sets to address these important issues.

 

An Amazing Opportunity for Ocean Science

EXPORTS is the first large-scale, coordinated opportunity aimed at understanding the ocean’s biological pump since the JGOFS program. Hence, the EXPORTS team is planning to create a long-term legacy for these one-of-a-kind datasets. NASA is supporting a full-time data manager to make sure that all of the information collected is easily accessible to all; as per NASA policies, all data will be freely available. Efforts are being made to ensure the intercalibration and interoperability of measurements made across different platforms, thus ensuring continuity of the datasets. EXPORTS also plans to over-collect whole water, filtered particulate, and trap-collected samples that can be used for many purposes, both now by collaborators, and in the future as analytical methodologies become more powerful.

The timing for EXPORTS could not be better. Our understanding of the biological pump and in particular, the fate of ocean NPP has rapidly advanced over the past decade. We now know that the biological pump is four-dimensional, which complicates our observational approaches, and that food web and aggregate dynamics, microbial community composition and function, individual organism physiology and behavior, and submesoscale turbulent transport are all components that need to be quantified. Further, our observing tools and capabilities have witnessed giant leaps over just the past couple of years. Novel imaging instruments can now measure particle and aggregate size distributions and identify and quantify plankton abundances. Genomic approaches enable the characterization of plankton communities and their physiology. Novel hyperspectral optical measurements of ocean reflectance as well as component inherent optical properties provide strong links to present and future satellite ocean color missions. High-resolution numerical models now enable the elucidation of submesoscale (100s m to ~10 km) processes that include food webs and biogeochemistry, while autonomous vehicles provide persistent and spatially distributed observations that complement the shipboard sampling. It seems the time for EXPORTS is now.

Click for PDF of article and table

Table 1: EXPORTS Science Team

Lead PI Co-PIs Project Title
Michael Behrenfeld
(OSU) – NASA		 Emmanuel Boss (UMaine), Jason Graff (OSU), Lionel Guidi (LOV), Kim Halsey (OSU), & Lee Karp-Boss (UMaine) First Step – Linking Remotely-Detectable Optical Signals, Photic Layer Plankton Properties, and Export Flux  
Ken Buesseler
(WHOI) – NASA		 Claudia Benitez-Nelson (USC) & Laure Resplandy (Princeton) Elucidating Spatial and Temporal Variability in the Export and Attenuation of Ocean Primary Production using Thorium-234 
Craig Carlson
(UCSB) – NASA		 Dennis Hansell (RSMAS) Evaluating the Controls of Dissolved Organic Matter Accumulation, its Availability to Bacterioplankton, its Subsequent Diagenetic Alteration and Contribution to Export Flux
Meg Estapa
(Skidmore) -NASA		 Ken Buesseler
(WHOI), Colleen Durkin (MLML) & Melissa Omand (URI)		 Linking Sinking Particle Chemistry and Biology with Changes in the Magnitude and Efficiency of Carbon Export into the Deep Ocean 
Craig Lee
(UW) – NASA		 Eric D’Asaro (UW), David Nicholson (WHOI), Melissa Omand (URI), Mary Jane Perry (self-affiliated) & Andrew Thompson (CalTech) Autonomous Investigation of Export Pathways from Hours to Seasons
Adrian Marchetti (UNC) – NASA Nicolas Cassar (Duke) & Scott Gifford (UNC) Quantifying the Carbon Export Potential of the Marine Microbial Community: Coupling of Biogenic Rates and Fluxes with Genomics at the Ocean Surface
Susanne Menden-Deuer
(URI) – NASA		 Tatiana Rynearson (URI) Quantifying Plankton Predation Rates, and Effects on Primary Production, Phytoplankton Community Composition, Size Spectra and Potential for Export 
Collin Roesler (Bowdoin) – NASA Heidi Sosik (WHOI) Phytoplankton community structure, carbon stock, carbon export and carbon flux: What role do diatoms play in the North Pacific and North Atlantic Oceans? 
David Siegel
(UCSB) – NASA		 Adrian Burd (UGA), Andrew McDonnell (UAF), Norm Nelson (UCSB) & Uta Passow (UCSB) Synthesizing Optically and Carbon Export-Relevant Particle Size Distributions for the EXPORTS Field Campaign
Deborah Steinberg (VIMS) – NASA Amy Maas (BIOS) Zooplankton-Mediated Export Pathways: Quantifying Fecal Pellet Export and Active Transport by Diel and Ontogenetic Vertical Migration in the North Pacific and Atlantic Oceans 
Xiaodong Zhang (UND) – NASA Deric Gray (NRL), Lionel Guidi (LOV) & Yannick Huot (Sherbrooke) Optically Resolving Size and Composition Distributions of Particles in the Dissolved-Particulate Continuum from 20 nm to 20 mm to Improve the Estimate of Carbon Flux
Bethany Jenkins (URI) – NSF* Mark Brzezinski (UCSB) & Kristen Buck (USF) Collaborative Research: Diatoms, Food Webs and Carbon Export – Leveraging NASA EXPORTS to Test the Role of Diatom Physiology in the Biological Carbon Pump
Ben Van Mooy (WHOI) -NSF* Environmental Lipidomics of Suspended and Sinking Particles in the Upper Ocean
Andrea Fassbender (MBARI) – NSF* Constraining Upper-Ocean Carbon Export with Biogeochemical Profiling Floats

*Project recommended for funding by NSF, but not officially funded as of this publication.

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: Frontiers in western boundary current research

Posted by mmaheigan 
· Friday, November 10th, 2017 

WBC Series Guest Editors: Andrea J. Fassbender1 and Stuart P. Bishop2

1. Monterey Bay Aquarium Research Institute
2. North Carolina State University

Western boundary current (WBC) regions are often studied for their intensity of air-sea interaction and mesoscale variability, yet research addressing the implications of these characteristics for biogeochemical cycling has lagged behind. WBCs, and their extension jets, display a wide breadth of physical processes that give rise to variability ranging from submesoscale (1-10 km) to basin scale (1000 km). WBC extension jets can act as both barriers and conduits for biological and chemical exchanges between subpolar-subtropical water masses, likely serving an important role in local chemical fluxes and biological community composition. Additionally, WBC regions are known for their formation of subtropical mode waters, carrying their source water biogeochemical signatures into the ocean interior. Interactions between (sub)mesoscale processes, mode water formation, and cross frontal exchanges of chemicals and organisms remain an important and nascent area of research.

In addition to the physical dynamics, many questions remain regarding the role of WBC regions in the global carbon cycle. Recent work suggests that these domains exhibit physically mediated export of biogenic particles and are gateways for anthropogenic carbon injection into the ocean interior. Such recent discovery that WBC processes may be strongly linked to the biological carbon pump and anthropogenic carbon storage speaks to the challenges associated with observing these ocean realms. While much has been learned from pairing satellite remote sensing with in situ physical oceanographic observations, biogeochemical analyses have historically been limited by the lack of necessary observing tools. Thus, there remains a critical knowledge gap on the role of WBCs in the global carbon cycle and other biogeochemical cycles.

With OceanObs’19 approximately two years away, the recent Ocean Carbon Hot Spots workshop assessed community interests and perspectives, revealing that it is an opportune time to make use of novel autonomous observing platforms and biogeochemical sensors to unravel some of the mysteries surrounding the role of WBC extensions in marine biogeochemical cycling. The articles herein present some of the most pressing research questions and observing hurdles related to WBCs from the perspectives of physical, chemical, and biological oceanographers and modelers working in this arena.

Series Articles:

Fine-scale biophysical controls on nutrient supply, phytoplankton community structure, and carbon export in western boundary current regions, S. Clayton, P. Gaube, T. Nagai, M.M. Omand, M. Honda

Decadal variability of the Kuroshio Extension system and its impact on subtropical mode water formation B. Qiu, E. Oka, S.P. Bishop, S. Chen, A.J. Fassbender

Western boundary currents as conduits for the ejection of anthropogenic carbon from the thermocline K.B. Rodgers, P. Zhai, D. Iudicone, O. Aumont, B. Carter, A. J. Fassbender, S. M. Griffies, Y. Plancherel, L. Resplandy, R.D. Slater, K. Toyama

The role of western boundary current regions in the global carbon cycle A.R. Gray, J. Palter

Observing air-sea interaction in the western boundary currents and their extension regions: Considerations for OceanObs 2019 D. Zhang, M.F. Cronin, X. Lin, R. Inoue, A.J. Fassbender, S.P. Bishop, A. Sutton

 

US CLIVAR Variations Issue PDF (compiled articles)

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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An autonomous approach to monitoring coral reef health

Posted by mmaheigan 
· Thursday, July 20th, 2017 

Coral reefs are diverse, productive ecosystems that are highly vulnerable to changing ocean conditions such as acidification and warming. Coral reef metabolism—in particular the fundamental ecosystem properties of net community production (NCP; the balance of photosynthesis and respiration) and net community calcification (NCC; the balance of calcification and dissolution)—has been proposed as a proxy for reef health. NCC is of particular interest, since ocean acidification is expected to have detrimental effects on reef calcification.

Traditionally, these metabolic rates are quantified through laborious methods that involve discrete sampling, which, due to a limited number of observations, often fails to characterize natural variability on time scales of minutes to days. In a recent paper in JGR, Takeshita et al. (2016) presented the Benthic Ecosystem and Acidification Measurement System (BEAMS), a fully autonomous system that simultaneously measures NCP and NCC at 15-minute intervals over a period of weeks. BEAMS utilizes the gradient flux method to quantify benthic metabolic rates by measuring chemical (pH and O2) and velocity gradients in the turbulent benthic boundary layer.

Two BEAMS were simultaneously deployed on Palmyra Atoll located approximately one km apart over vastly different benthic communities. One site was a healthy reef with approximately 70% coral cover, and the other was a degraded reef site with only 5% coral cover that was dominated by a non-calcifying invasive corallimorph Rhodactis howesii. Over the course of two weeks, BEAMS collected over 1,000 measurements of NCP and NCC from each site, yielding significantly different ratios of NCP to NCC between the two sites. These initial results suggest that BEAMS is capable of detecting different metabolic states, as well as patterns consistent with degrading reef health.

BEAMS is an exciting new autonomous tool to monitor reef health and study drivers of reef metabolism on timescales ranging from minutes to months (and potentially years). Additionally, autonomous measurement tools increase the potential for widespread and comparable observations across reefs and reef systems. Such knowledge will greatly improve our ability to predict the fate of coral reefs in a changing ocean.

 

Authors: 
Yui Takeshita (Monterey Bay Aquarium Research Institute)

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

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