Author Archive for mmaheigan – Page 27

Feedbacks mitigate the impacts of atmospheric nitrogen deposition in the western North Atlantic

Posted by mmaheigan 
· Thursday, April 12th, 2018 

How do phytoplankton respond to atmospheric nitrogen deposition in the western North Atlantic, an area downwind of large agricultural and industrial centers? The biogeochemical impacts of this ‘fertilization’ remain unclear, as direct oceanic observations of atmospheric deposition are limited and models often cannot resolve the important processes.

In a recent study, St-Laurent et al. (2017) simulated the biogeochemical impacts of nitrogen deposition on surface waters of the western North Atlantic by combining year-specific deposition rates from the Community Multiscale Air Quality (CMAQ) model and a realistic 3-D biogeochemical model of the waters off the US east coast. Westerly winds from the continent and large fluxes of heat and moisture over the Gulf Stream produce a ‘hotspot’ of wet nitrogen deposition along the path of the current. This nitrogen input increases the local surface primary productivity by up to 30% during the summer. However, the study also identified important processes that mitigate the impact of atmospheric nitrogen deposition in other seasons and regions. Deposition weakens vertical nitrogen gradients in the upper 20 m and thus decreases the upward transport of nitrogen to the surface layer (a negative feedback). Increases in surface phytoplankton concentrations also negatively impact light availability below the surface through shelf-shading.

Atmospheric nitrogen deposition along the US east coast. (Left) Wet deposition of oxidized nitrogen over the Gulf Stream as simulated by the Community Multiscale Air Quality model (average 2004-2008). (Right) Increase in summer surface primary productivity in response to the deposition (average 2004-2008).

These results indicate that atmospheric nitrogen deposition has important impacts on the surface biogeochemistry of the western North Atlantic but that the response is not simply proportional to the deposition. Additional research is necessary to clarify the role played by atmospheric deposition in this region in past and future centuries. While inputs of atmospheric nitrogen associated with power plants and industries have decreased since the passage of the Clean Air Act, recent studies have revealed increasing atmospheric concentrations of reduced nitrogen. Continued coordination between modeling and observing efforts (both on land and over the ocean) are needed to improve our understanding of the impacts of deposition on the biological pump in this region of the Atlantic ocean.

 

Authors:
Pierre St-Laurent (VIMS, College of William and Mary)
Marjorie A.M. Friedrichs (VIMS, College of William and Mary)
Raymond G. Najjar (Pennsylvania State University)
Doug Martins (FLIR Systems Inc.)
Maria Herrmann (Pennsylvania State University)
Sonya K. Miller (Pennsylvania State University)
John Wilkin (Rutgers University)

Volcanic carbon dioxide drove ancient global warming event

Posted by mmaheigan 
· Thursday, March 29th, 2018 

A study recently published in Nature suggests that an extreme global warming event 56 million years ago known as the Palaeocene-Eocene Thermal Maximum (PETM) was driven by massive CO2 emissions from volcanoes during the formation of the North Atlantic Ocean. Using a combination of new geochemical measurements and novel global climate modelling, the study revealed that atmospheric CO2 more than doubled in less than 25,000 years during the PETM.

The PETM lasted ~150,000 years and is the most rapid and extreme natural global warming event of the last 66 million years. During the PETM, global temperatures increased by at least 5°C, comparable to temperatures projected in the next century and beyond. While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere, the source and total amount of carbon, as well as the underlying mechanism have thus far remained elusive. The PETM roughly coincided with the formation of massive flood basalts resulting from of a series of eruptions that occurred as Greenland and North America started separating from Europe, thereby creating the North Atlantic Ocean. What was missing is evidence linking the volcanic activity to the carbon release and warming that marks the PETM.

To identify the source of carbon, the authors measured changes in the balance of isotopes of the element boron in ancient sediment-bound marine fossils called foraminifera to generate a new record of ocean pH throughout the PETM. Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, which provide information about the carbon source. When forced with these ocean pH and carbon isotope data, a numerical global climate model implicates large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM.

 

North Atlantic microfossil-derived isotope records from extinct planktonic foraminiferal species M. subbotinae relative to the onset of the PETM carbon isotope excursion (CIE). The negative trend in carbon isotope composition (A) during the carbon emission phase is accompanied by decreasing pH (decreasing δ11B, panel B) and increasing temperature (decreasing δ18O, panel C). Panels D and E zoom in on the PETM CIE, showing microfossil δ13C (D) and δ11B-based pH (E) reconstructions. Also included in E are data from Penman et al. (2014) on their original age model, with recalculated (lab-based) pH values.

 

These new results suggest that the PETM was associated with a total input of >12,000 petagrams of carbon from a predominantly volcanic source. This is a vast amount of carbon—30 times larger than all of the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves. In the following Earth System Model simulations, it resulted in the concentration of atmospheric CO2 increasing from ~850 parts per million to >2000 ppm. The Earth’s mantle contains more than enough carbon to explain this dramatic rise, and it would have been released as magma poured from volcanic rifts at the Earth’s surface.

How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change. Earth’s warming at the PETM was about what we would expect given the CO2 emitted and what we know about the sensitivity of the climate system based on Intergovernmental Panel on Climate Change (IPCC) reports. However, the rate of carbon addition during the PETM was about twenty times slower than today’s human-made carbon emissions.

In the model outputs, carbon cycle feedbacks such as methane release from gas hydrates—once the favoured explanation of the PETM—did not play a major role in driving the event. Additionally, one unexpected result was that enhanced organic matter burial was important in ultimately drawing down the released carbon out of the atmosphere and ocean and thereby accelerating the recovery of the Earth system.

 

Authors:
Marcus Gutjahr (National Oceanography Centre Southamption, GEOMAR)
Andy Ridgwell (Bristol University, University of California Riverside)
Philip F. Sexton (The Open University, UK)
Eleni Anagnostou (National Oceanography Centre Southamption)
Paul N. Pearson (Cardiff University)
Heiko Pälike (University of Bremen)
Richard D. Norris (Scripps Institution of Oceanography)
Ellen Thomas (Yale University, Wesleyan University)
Gavin L. Foster (National Oceanography Centre Southamption)

 

Early career support for Ocean Optics XXIV

Posted by mmaheigan 
· Thursday, March 29th, 2018 

OCB is providing early career support for the Ocean Optics XXIV Conference,  October 7-12, 2018 in Dubrovnik, Croatia.
Are you an early career scientist looking for a perfect place to present your latest discoveries (and you need some travel support to do so)? Look no more – Ocean Optics XXIV conference registration is open, and OCB is offering travel support to members of their community to participate. See registration page for details.

 

Widespread nutrient co-limitation discovered in the South Atlantic

Posted by mmaheigan 
· Thursday, March 15th, 2018 

Unicellular photosynthetic microbes—phytoplankton—are responsible for virtually all oceanic primary production, which fuels marine food webs and plays a fundamental role in the global carbon cycle. Experiments to date have suggested that the growth of phytoplankton across much of the ocean is limited by either nitrogen or iron. But simultaneously low concentrations of these and other nutrients have been measured over large areas of the open ocean, raising the question: Are phytoplankton communities only limited by a single nutrient?

Authors of a study recently published in Nature tested this by conducting nutrient addition experiments on a GEOTRACES cruise in the nutrient-deficient South Atlantic gyre. Seawater samples were amended with nitrogen, iron, and cobalt both individually and in various combinations. Concurrent nitrogen and iron addition stimulated increased phytoplankton growth, yielding a ~40-fold increase in chlorophyll a. Supplementary addition of cobalt or cobalt-containing vitamin B12 further enhanced phytoplankton growth in several experiments.

Experiments conducted throughout the southeast Atlantic GEOTRACES GA08 cruise transect (left panel) demonstrated that nitrogen and iron had to be added to significantly stimulate phytoplankton growth (right panel). Supplementary addition of cobalt (or cobalt-containing vitamin B12) stimulated significant additional growth.

In addition to co-limited sites, the study identified ‘singly’ and ‘serially’ limited sites. These limitation regimes could be predicted by the measured ambient seawater nutrient concentrations, demonstrating the potential for using nutrient datasets to make confident predictions about limitation at larger spatial scales, an approach that is being more widely used in programmes like GEOTRACES,.

Finally, a complex, state-of-the-art biogeochemical ocean model suggested a much smaller extent of nutrient co-limitation than the experiments indicated. Authors attributed this to relatively restricted microbial and nutrient diversity in the model. These findings have implications for how such models are constructed if they are to represent nutrient co-limitation in the ocean and accurately project changes in ocean productivity in the future.

 

Authors:
Thomas J. Browning (GEOMAR)
Eric P. Achterberg (GEOMAR)
Insa Rapp (GEOMAR)
Anja Engel (GEOMAR)
Erin M. Bertrand (Dalhousie University)
Alessandro Tagliabue (University of Liverpool)
Mark Moore (University of Southampton)

Seagrass carbon dynamics: Gulf of Mexico

Posted by mmaheigan 
· Thursday, March 1st, 2018 

Seagrasses have died-off in great numbers, resulting in the release of stored carbon. Seagrasses represent a substantive and relatively unconstrained North American and Caribbean Sea blue carbon sink in the tropical Western Hemisphere. Fine-scale estimates of regional seagrass carbon stocks, as well as carbon fluxes from anthropogenic disturbances and natural processes and gains in sedimentary carbon from seagrass restoration are currently lacking for the bulk of tropical Western Hemisphere seagrass systems.

To address this knowledge gap, in the subtropics and tropics, a recent study yielded estimates of organic carbon (Corg) stocks, losses, and restoration gains from several seagrass beds around the Gulf of Mexico (GoM). GoM-wide seagrass natural Corg stocks were estimated to be ~37.2–37.5Tg Corg. A unique method involving quadruplicate sampling in naturally-occurring, restored, continually-historically barren, and previously-disturbed-now-barren sites provided the first available Corg loss measurements for subtropical-tropical seagrasses. GoM Corg losses were slow, occurring over multiple years, and differed between sites, depending on disturbance type. Mean restored seagrass bed Corg stocks exceeded those of natural seagrass beds, underscoring the importance of seagrass restoration as a viable carbon sequestration strategy. For restored seagrass areas, the older the restoration site, the greater the Corg stock.

Organic carbon stocks for Gulf of Mexico sediments for the top 20 cm of sediment in always barren, impacted barren, natural seagrass, and restored seagrass sites. Natural and restored seagrass beds had significantly higher organic carbon stocks than impacted barren or always barren sediments.

Seagrass restoration appears to be an important tool for climate-change mitigation. In the USA and throughout the tropics and subtropics, restoration could reduce sedimentary carbon leakage and bolster total blue carbon stores, while facilitating increased fisheries and shoreline stability. Although well-planned and executed restoration of seagrass is more difficult than mangroves or marshes, there are >1 million hectares of degraded seagrass habitats that could be restored, which would greatly increase blue carbon sinks and support diverse marine species that rely on seagrass for habitat and food.

 

Authors:
Anitra Thorhaug (Yale School of Forestry)
Helen M. Poulos (Earth Sci., Wesleyan Univ.)
Jorge López-Portillo (Inecol, Mexico)
Timothy C.W. Ku (Earth Sci., Wesleyan Univ.)
Graeme P. Berlyn (Yale School of Forestry)

Increased temperatures suggest reduced capacity for carbon

Posted by mmaheigan 
· Thursday, January 18th, 2018 

The ocean’s biological pump works to draw down atmospheric carbon dioxide (CO2) by exporting carbon from the surface ocean. This process is less efficient at higher temperatures, implying a possible climate feedback. Recent work by Cael et al. provides an explanation of why this feedback occurs and an estimate of its severity.

In a highly simplified view, carbon export depends on the balance between two temperature-dependent processes: 1) The autotrophic production and 2) the heterotrophic respiration of organic carbon. Cael and Follows (Geophysical Research Letters 2016) recently developed a mechanistic model based on established temperature dependencies for photosynthesis and respiration to explore feedbacks between export efficiency and climate. Heterotrophic growth rates increase more so than phototrophic rates with increasing temperature, which suggests that at higher temperatures, community respiration will increase relative to production, thereby decreasing export efficiency. Although simplistic, the model captures the temperature dependence of export efficiency observations.

Figure: Schematic of the mechanism on which the Cael and Follows (2016) model is based. (a) Photosynthesis (dark grey) and respiration (light grey) respond to temperature differently, yielding (b) a decline in export efficiency at higher temperatures.

More recently, Cael, Bisson, and Follows (Limnology and Oceanography 2017) applied this model to sea surface temperature records and estimated a ~1.5% decline in globally-averaged export efficiency over the past three decades of increasing ocean temperatures as a result of this metabolic mechanism. This ~1.5% decline is equivalent to a reduced ocean sequestration of approximately 100 million fewer tons of carbon annually, comparable to the annual carbon emissions of the United Kingdom. The model provides a framework in which to consider the relationship between climate and ocean carbon export that might also elucidate large-scale (e.g., glacial-interglacial) atmospheric CO2 changes of the past.

Authors:
B. B. Cael (MIT/WHOI)
Kelsey Bisson (UCSB)
Mick Follows (MIT)

Lasers shed light on giant larvacean filtration impact on the ocean’s biological pump

Posted by mmaheigan 
· Thursday, January 4th, 2018 

To accurately assess the impacts of climate change, we need to understand how atmospheric carbon is transported from surface waters to the deep sea. Grazers and filter feeders drive the ocean’s biological pump as they remove and sequester carbon at various rates. This pump extends down into the midwater realm, the largest habitat on earth. Giant larvaceans are fascinating and enigmatic occupants of the upper 400 m of the water column, where they build complex filtering structures out of mucus that can reach diameters greater than 1 m in longest dimension (Figure 1A). Because of the fragility of these structures, direct measurements of filtration rates require us to study them in situ. We developed DeepPIV, an ROV-deployable instrument (Figure 1B) to directly measure fluid motion and filtration rates in situ (Figure 1C).

Figure 1. (A) Traditional view of a giant larvacean illuminated by white ROV lights. (B) DeepPIV instrument is seen attached to Monterey Bay Aquarium Research Institute’s (MBARI) MiniROV. (C) DeepPIV-illuminated interior view of a giant larvacean house, where particle motion in ambient seawater serves as a proxy for fluid motion. White arrows in (A) and (C) indicate larvacean head/trunk; white arrow in (B) indicates DeepPIV.

The filtration rates we measured for giant larvaceans are far greater than for any other zooplankton filter feeder. When combined with abundance data from a 22-year time series, the grazing impact of giant larvaceans indicates that within 13 days, they can filter the total volume of water within their habitable depth range (~100-300 m; based on maximum abundance and measured filtration rates). Our results reveal that the contribution of giant larvaceans to vertical carbon flux is much greater than previously thought. Small larvaceans, which are present in the water column in even larger quantities than giant larvaceans, may also have a measurable impact on carbon fluxes. New technologies such as DeepPIV are yielding more quantitative observations of midwater filter feeders, which is improving our understanding of the roles that deep-water biota play in the long-term removal of carbon from the atmosphere.

Read the full journal article: http://advances.sciencemag.org/content/3/5/e1602374.full

Authors: (All at MBARI)
Kakani Katija
Rob E. Sherlock
Alana D. Sherman
Bruce H. Robison

Zooplankton play a key and diverse role in the ocean carbon cycle

Posted by mmaheigan 
· Thursday, December 7th, 2017 

How does the enormous diversity of zooplankton species, life cycles, size, feeding ecology, and physiology affect their role in ocean food webs and cycling of carbon?

In the 2017 issue of Annual Review of Marine Science, Steinberg and Landry review the fundamental and multifaceted roles that zooplankton play in the cycling and export of carbon in the ocean. Carbon flows through marine pelagic ecosystems are complex due to the diversity of zooplankton consumers and the many trophic levels they occupy in the food web–from single-celled herbivores to large carnivorous jellyfish. Zooplankton also contribute to carbon export processes through a variety of mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations).


Figure 1.  Pathways of cycling and export of carbon by zooplankton in the ocean.

Climate change and other stressors are already affecting zooplankton abundance, distribution, and life cycles, and are predicted to result in widespread changes in zooplankton carbon cycling in the future. These changes will affect both the larger marine food web that depends upon zooplankton for food (fish) or recycled products for growth (primary producers) and the amount of carbon exported into the deep sea–where far from contact with the atmosphere it no longer contributes to global warming.

 

Authors:

Deborah K. Steinberg, Virginia Institute of Marine Science, The College of William and Mary
Michael R. Landry, Scripps Institution of Oceanography

The sea surface microlayer in a future ocean

Posted by mmaheigan 
· Tuesday, November 28th, 2017 

The sea surface microlayer (SML) is the boundary interface between the atmosphere and ocean, spanning the uppermost ~1 mm of the ocean. Covering 70% of the Earth’s surface, the SML supports a rich diversity of life, serving as an incubator for eggs and larvae. The SML controls air-sea interactions  and serves as an important hotspot of microbially mediated biochemical activity. A recent review paper by Wurl et al. highlights the important role of the SML in climate and ecosystem function and how it might change in the future.

Figure Caption: The sea surface microlayer comprises a complex biofilm and serves as a biochemical micro-reactor with distinct microbial communities and short-term carbon accumulation.

 

The SML is directly exposed to meteorological forces such as UV radiation, precipitation, and diurnal warming. Since these forces will be impacted by climate change, the SML is also likely to experience changes in the future. For example, projected increases in primary productivity in the upper sunlit layer of the ocean may enhance the supply of surface-active organic material to the SML with accompanying feedbacks on the molecular diffusion and conduction processes that drive exchange of heat and climate-relevant gases such as CO2 between the ocean and atmosphere. Furthermore, changes in UV flux may enhance the SML’s role as a biochemical reactor in which unique microbial communities and photochemical reactions occur, including the transformation of deposited atmospheric particles like dust into bioavailable nutrients like iron to fuel phytoplankton production. Increasing levels of manmade pollutants such as pharmaceuticals and micro-plastics are accumulating in the SML. These and other pollutants have the potential to disrupt biochemical and photochemical processes in the SML, as well as the unique and diverse food webs it supports.

Moving forward, novel techniques and a holistic approach will be needed to improve our understanding of highly complex  SML dynamics. Multidisciplinary data sets that link microbial community structure and function with biogeochemistry will be needed, and eventually, the SML should be included in computer models used to forecast future changes in climate, marine ecosystems, and biogeochemistry.

 

Authors:
Oliver Wurl (Carl von Ossietzky Universität Oldenburg, Institute for Chemistry and Biology of the Marine Environment)
Werner Ekau (Leibniz Centre for Tropical Marine Research, Bremen)
William M. Landing (Florida State University, Department of Earth, Ocean, and Atmospheric Science)
Christopher J. Zappa (Lamont-Doherty Earth Observatory, Columbia University)

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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