Archive for western boundary currents

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: The role of western boundary current regions in the global carbon cycle

Posted by mmaheigan 
· Friday, November 10th, 2017 

Alison R. Gray1, Jaime Palter2

1. University of Washington
2. University of Rhode Island

Estimates of contemporary global air-sea carbon dioxide (CO2) flux (Takahashi et al. 2009; Landschützer et al. 2014) suggest that subtropical western boundary currents (WBCs) and their zonal extensions are key regions of oceanic carbon uptake (Figure 1a). These narrow, intensified currents, which transport water poleward along the western edge of every ocean basin before separating from the continental shelf and turning eastward, are associated with maxima in mean velocity, eddy kinetic energy, air-sea heat flux, and nutrient transport, as well as deep mixed layers on their equatorward flanks. The prominence of these regions in the global air-sea flux of CO2 leads to a basic question: What is the role of the WBCs and their zonal extensions in the global carbon cycle? The answer, which involves the physics and biogeochemistry of the ocean-atmosphere system across a wide range of temporal and spatial scales, reveals the complex nature of these systems and their importance within the climate system, while also highlighting a number of the gaps in our current understanding.

Figure 1: (a) Climatological mean annual sea-air CO2 flux referenced to the year 2000 adapted from Takahashi et al. 2009. Blue (red) areas are ocean sink (source) regions for atmospheric CO2. (b) Surface eddy kinetic energy calculated from the 2011-2013 daily AVISO sea surface height product. The white circle in a and b indicates the location of the Kuroshio Extension Observatory mooring. WBC systems are labeled as follows: Kuroshio Extension (KE), Gulf Stream (GS), Agulhas Return Current (ARC), East Australian Current (EAC), and Brazil-Malvinas Confluence (BMC). Courtesy A. Fassbender and S. Bishop.

 

Air-sea carbon fluxes are primarily controlled by the difference between the partial pressure of CO2 in the atmosphere, which has been steadily increasing since the 1870s due to the burning of fossil fuels, and the partial pressure of CO2 in seawater (pCO2), which can be influenced by many factors (Sarmiento and Gruber 2006). The contemporary oceanic carbon uptake in the subtropical WBCs and their extensions comes about from both thermally and biologically driven decreases in pCO2. The first of these is tied to the role that WBCs play in the large-scale circulation of the ocean. These currents supply the poleward return flow for the large-scale wind-driven circulation that, through Sverdrup dynamics, generates upper ocean equatorward flow across the vast subtropical gyres (Vallis 2006). As a result, the vigorous (on the order of 1 m s-1) flows in the WBCs rapidly transport warm water from the tropics to mid-latitudes. At mid-latitudes in winter, the atmospheric storm track advects frigid air across the continents and eastward over these warm currents, leading to the largest air-sea heat fluxes in the global ocean. This physical phenomenon has a direct impact on air-sea carbon flux through the solubility effect, whereby ocean cooling reduces pCO2 and thus drives the system towards more uptake by the ocean. The second mechanism for lowering surface ocean pCO2 and producing a flux into the ocean is biological production of organic matter in the euphotic zone, which also occurs at high rates in and around WBCs and their extension regions. This biological productivity is supported by deep convective mixing on the equatorward fringes of the WBCs, which brings nutrients up from the seasonal thermocline, as well as the cross-WBC transport of nutrients. Together these two factors ¾ solubility effects and primary productivity ¾ act to decrease pCO2 in WBCs and their extensions, creating hotspots of ocean carbon uptake.

The impact of air-sea CO2 fluxes on the global climate system hinges on the fate of carbon after it enters the surface ocean, and in this regard, the WBC regions also play a crucial role.  The significant heat loss to the atmosphere and strong winds that combine to create substantial solubility-driven carbon uptake also lead to some of the deepest wintertime mixed layers in the global ocean on the equatorward side of the WBCs. These thick mixed layers are subsequently capped by lighter waters during springtime restratification and then subducted into the ocean interior to form the subtropical mode waters (STMWs) found in each basin. The carbon contained in these mode waters is thus isolated from the atmosphere. How, when, and where STMWs subsequently re-enter the ocean mixed layer governs the influence of WBC carbon uptake on atmospheric carbon concentrations and helps determine the future evolution of the ocean carbon sink (Gruber et al. 2002). Alternatively, carbon taken up in the surface ocean can enter the interior ocean via sinking organic matter that is then remineralized. Depending on the depths to which biological particles sink, this carbon can be trapped below the surface for potentially much longer timescales.

In addition to the role that STMWs play in the transfer of carbon from the surface ocean to the thermocline, the process of mode water formation has several other effects on the oceanic carbon cycle. The deepening of the mixed layer entrains subsurface waters that are higher in dissolved inorganic carbon and nutrients. The resulting increase in mixed layer carbon acts to reduce the pCO2 gradient across the air-sea interface, dampening ocean carbon uptake. The increase in nutrients, however, can also stimulate phytoplankton growth, leading to more biologically driven CO2 uptake. The balance between these processes, which can vary enormously in space and time, will regulate the total carbon uptake and storage in STMWs.  If inorganic carbon and nutrients are entrained into the mixed layer at the same ratio as is removed in sinking organic particles, then primary production will not have a net impact on oceanic CO2 uptake. However, the physical processes that govern both mixed layer dynamics and the upper ocean circulation, as well as the biological processes that determine productivity and export, are frequently decoupled in both space and time. Accordingly, variability in biologically driven carbon uptake can exist on timescales of seasons to decades or longer.

All of the processes described thus far are believed to have been operating since well before anthropogenic perturbations to the atmospheric CO2 concentration. Reconstructions of atmospheric CO2 from ice cores suggest that, averaged over decades, Holocene concentrations were near steady state (Ciais et al. 2013). Therefore, the carbon taken up by the ocean in WBC regions was balanced by outgassing elsewhere, in the climatological mean. The carbon that moves through the climate system through this pre-industrial carbon cycle, referred to as natural carbon, is often conceptually separated from the anthropogenic carbon added to the atmosphere by fossil fuel burning, of which approximately 30% has been absorbed by the ocean (Le Quéré et al. 2016). Modeling and observational studies point to WBCs as being important regions for uptake of anthropogenic carbon, and the STMWs formed on the equatorward flanks of the WBCs account for a significant portion of the anthropogenic carbon storage (Sabine et al. 2004; Iudicone et al., 2016). Climate model-based projections indicate that WBCs and their extensions will continue to be important sinks of carbon as the climate warms (McKinley et al. 2017). However, from observations it is quite difficult to disentangle the natural carbon cycle from the anthropogenic perturbation, and thus natural variability may significantly affect the rates of carbon uptake and storage that we observe.

Together, solubility and biological effects as well as anthropogenic carbon uptake act in concert to create the hotspots of ocean carbon uptake in the WBCs that we observe today. As we become progressively better able to observe and model the climate system, with longer observational records and greater resolution in both space and time, the more variability we find in these highly dynamic regions of the global ocean. WBCs are notable for significant energy at the mesoscale level, i.e., motions at the scales of approximately one month and 100 km at the WBC latitudes, as evidenced by the mean variability in the altimetric sea surface height (Figure 1b). Intense eddying occurs here because WBCs are regions of strong fronts and thus steeply tilted isopycnals. The significant potential energy inherent in this condition leads to the growth of baroclinic instabilities and the substantial mesoscale variability observed in the WBCs.

Mesoscale features can impact the oceanic carbon cycle in a number of different ways. Mesoscale motions strongly affect mixed layer depths, with implications for mode water formation, subduction, and ventilation; the net effect is to increase the stratification of the upper ocean. Many studies have shown that mesoscale eddies, in releasing the potential energy associated with tilted isopycnals, pump heat out of the deep ocean and towards the surface (Gregory 2000; Gnanadesikan et al. 2005; Palter et al. 2014). Given that natural carbon increases with depth in the ocean due to the remineralization of sinking organic matter, this eddy-driven vertical exchange is expected to reduce the strength of the biological pump. On the other hand, anthropogenic carbon concentrations are highest at the ocean’s surface and decrease downward, so that the global-average effect of mesoscale eddies would be expected to decrease the ocean uptake of anthropogenic CO2, relative to an ocean without such eddies.  To infer the current generation of climate models’ ability to represent the net effect of eddies on carbon through common parameterizations, we take cues from an analysis of ocean heat uptake in models of varying resolution (Griffies et al. 2015): Parameterizations were successful in their qualitative representation of the upward transport of heat, albeit at a reduced efficiency relative to the simulations that resolved a rich spectrum of mesoscale eddies. This conclusion suggests that, in the model analyzed, parameterized eddies might underestimate the upward pumping of natural carbon from the ocean interior and overestimate the unbalanced downward transport of anthropogenic carbon by the time-mean circulation.

As a result of the significant horizontal and vertical shear associated with the high levels of mesoscale activity found in the WBCs, these regions are also hotspots of variability at the submesoscales, or motions at scales of approximately 10 km and one day. Such motions, which can be generated through a number of different mechanisms, can be associated with substantial instantaneous vertical velocities and are thought to be critical in the restratification of the mixed layer. Submesoscale motions can lead to strong physical export of particles (Omand et al. 2014), and their effects are only partially parameterized in current generation of climate models.  While a number of ambitious field campaigns have been conducted in recent years in order to better understand these types of motions (e.g., Shcherbina et al. 2015), there are still many open questions regarding the impact of small-scale instabilities on the uptake and storage of carbon through both biophysical coupling and effects on mode water subduction and ventilation.

Diverse physical and biological processes, at a range of temporal and spatial scales, clearly contribute to making the WBCs hotspots of oceanic carbon uptake. While the large-scale mean state of the system is relatively well-understood, a number of challenges remain that currently limit our understanding of the role that WBCs and their extensions play in the global carbon cycle. The spatial and temporal variability in the natural carbon cycle of these systems is not well-characterized, due to the difficulties inherent in both observing and modeling these extremely turbulent regions. Given that the WBC uptake of total carbon (the sum of the natural and anthropogenic components) is determined by the balance of large fluxes both into and out of the ocean, variability in the processes that govern these fluxes can have important effects on the ocean carbon cycle. Although our knowledge of mesoscale and submesoscale physics has increased immensely over the past few decades, how these motions impact biogeochemical cycling at small scales and how these effects feed back on the larger-scale carbon uptake and storage are still open questions. In addition, the Southern Hemisphere WBCs are generally much less studied than their counterparts in the North Atlantic and North Pacific basins, despite the fact that these systems connect to and interact with the Southern Ocean where model estimates indicate approximately 50% of the ocean uptake of anthropogenic carbon has occurred (Frölicher et al. 2015). Indeed, one of the biggest bottlenecks in both quantifying and understanding the role of WBCs in the global carbon cycle remains the chronic scarcity of observations in the Southern Hemisphere.

As we work toward addressing these issues, it remains critically important to develop a mechanistic understanding of the myriad processes that are involved in carbon uptake in the WBCs and their extensions. In this way, we will be able to translate this knowledge into better predictions of future changes in the role of WBCs in the global carbon cycle.

 

References

Ciais, P., and Coauthors, 2013: Carbon and Other Biogeochemical Cycles. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, Eds., Cambridge University Press, Cambridge, United Kingdom, and New York, USA, doi:10.1017/CBO9781107415324.015.

Frölicher, T. L., J. L. Sarmiento, D. J. Paynter, J. P. Dunne, J. P. Krasting, and M. Winton, 2015: Dominance of the Southern Ocean in anthropogenic carbon and heat uptake in CMIP5 models. J. Climate, 28, 862–886, doi:10.1175/JCLI-D-14-00117.1

Gnanadesikan, A., R. D. Slater, P. S. Swathi, and G. K. Vallis, 2005: The energetics of ocean heat transport, J. Climate, 18, 2604–2616, doi:10.1175/jcli3436.1.

Gregory, J. M., 2000: Vertical heat transports in the ocean and their effect on time-dependent climate change. Climate Dyn., 16, 501-515, doi:10.1007/s003820000059.

Griffies, S. M. and Coauthors, 2015: Impacts on ocean heat from transient mesoscale eddies in a hierarchy of climate models. J. Climate, 28, 952–977, doi:10.1175/JCLI-D-14-00353.1.

Gruber, N., C. D. Keeling, and N. R. Bates, 2002: Interannual variability in the North Atlantic ocean carbon sink. Science, 298, 2374–2378, doi:10.1126/science.1077077.

Iudicone, D., and Coauthors, 2016: The formation of the ocean’s anthropogenic carbon reservoir. Scientific Reports, 6, 35473. http://doi.org/10.1038/srep35473

Landschützer, P., N. Gruber, D. C. E. Bakker, and U. Schuster, 2014: Recent variability of the global ocean carbon sink. Global Biogeochemical Cycles, 28, 927–949. http://doi.org/10.1002/2014GB004853

Le Quéré, C., and Coauthors, 2016: Global Carbon Budget 2016. Earth Syst. Sci. Data., 8, 605-649. DOI:10.5194/essd-8-605-2016.

McKinley, G. A., A. R. Fay, N. S. Lovenduski, and D. J. Pilcher, 2017: Natural variability and anthropogenic trends in the ocean carbon sink. Annu. Rev. Mar. Sci., 9, 125–50. http://doi.org/10.1146/annurev-marine-010816-060529

Omand, M. M., E. A. D’Asaro, C. M. Lee, M. J. Perry, N. Briggs, I. Cetini, and A. Mahadevan, 2015: Eddy-driven subduction exports particulate organic carbon from the spring bloom. Science, 348, 222–225. http://doi.org/10.1126/science.1260062

Palter, J. B., S. M. Griffies, B. L. Samuels, E. D. Galbraith, A. Gnanadesikan, and A. Klocker, 2014: The deep ocean buoyancy budget and its temporal variability. J. Climate, 27, 551–573, doi:10.1175/JCLI-D-13-00016.1.

Sabine, C. L., and Coauthors, 2004: The oceanic sink for anthropogenic CO2. Science, 305, 367–371. http://doi.org/10.1126/science.1097403

Sarmiento, J. L., and N. Gruber, 2006: Ocean Biogeochemical Dynamics. Princeton University Press, New Jersey, USA, ISBN:9781400849079.

Shcherbina, A. Y., and Coauthors, 2015: The LatMix summer campaign: Submesoscale stirring in the upper ocean. Bulletin of the American Meteorological Society, 96, 1257–1279. http://doi.org/10.1175/BAMS-D-14-00015.1

Takahashi, T., and Coauthors, 2009: Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the global oceans. Deep-Sea Res. Part II: Top. Stud. Oceanogr., 56, 554–577, doi:10.1016/j.dsr2.2008.12.009.

Vallis, G. K., 2006: Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation.  Cambridge University Press, Cambridge, UK, ISBN: 9780521849692.

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: Western boundary currents as conduits for the ejection of anthropogenic carbon from the thermocline

Posted by mmaheigan 
· Friday, November 10th, 2017 

Keith B. Rodgers1, Ping Zhai1, Daniele Iudicone2, Olivier Aumont3, Brendan Carter4, Andrea J. Fassbender5, Stephen M. Griffies6, Yves Plancherel7, Laure Resplandy8, Richard D. Slater1, Katsuya Toyama9

1. Princeton University
2. Stazione Anton Dohrn, Italy
3. Sorbonne Universités, LOCEAN/IPSL, France
4. University of Washington
5. Monterey Bay Aquarium Research Institute
6. NOAA Geophysical Fluid Dynamics Laboratory
7. University of Oxford, UK
8. Princeton University
9. Japan Meteorological Agency, Japan

A long-standing question regarding the ocean carbon cycle is whether western boundary currents (WBCs) and their extension regions provide an important pathway for anthropogenic carbon (Cant) uptake, thereby contributing to the known importance of these regions in the climate system. Successive versions of the Lamont Doherty Earth Observatory air-sea carbon dioxide (CO2) flux climatologies (Takahashi et al. 2002, 2009) indicate that, at the very least, there is a broad correspondence between the maxima in CO2 fluxes (uptake) and surface heat fluxes (release of heat to the atmosphere over the western subtropical gyres and WBCs). Motivated to understand the mechanistic controls on the ocean carbon cycle in these regions, a number of modeling and observationally based studies have drawn on multiple platforms to constrain fluxes both at the surface and in the interior of this region (Fassbender et al. 2017, and references therein; Nakano et al. 2011).

In particular, Nakano et al. (2015) and Iudicone et al. (2016) have emphasized the value of invoking a density-based framework for understanding the relationship between heat and carbon fluxes in WBCs and their extension regions building on the earlier studies of Iudicone et al. (2008, 2011). Within this framework, WBCs are best understood in the context of the shallow subtropical cell overturning structures (McCreary and Lu 1994; Lu and McCreary 1995) that connect the equatorial upwelling regions with their poleward subtropical water mass formation regions. The role of the subtropical cells in the climate system is to export excess heat absorbed by the coupled system in the equatorial regions to the subtropics, where heat is released along WBCs to the atmosphere. The heat released in WBC regions results in the densification of surface waters and the filling of the subtropical mode water reservoirs. In summary, this overall heat exchange process manifests itself in the ocean as a poleward divergence of warm surface waters in the upper branch of the subtropical cells and a subsurface convergence of cooler thermocline waters. The modeling studies of Nakano et al. (2011, 2015) and Iudicone et al. (2016) argue that the upper branches of the subtropical cells that feed WBCs accumulate Cant over broad scales and that this accumulation is a first-order process in setting the Cant inventory of the subtropical and subpolar mode waters reservoirs. In particular, Nakano et al. (2011) demonstrated that the earlier study of Rodgers et al. (2008), which had argued instead for convection in mode water formation regions to determine Cant uptake, was not supported by the large-scale Lagrangian diagnostics. Although Iudicone et al. (2016) was able to identify an integrated net transfer of Cant to higher density class waters associated with WBCs, given their global focus, they did not consider the specific pathways over which such transfers can occur.

Before proceeding to a mechanistic evaluation of the pathways and mechanisms regulating the transmission of Cant to higher density water masses in WBCs in a global model, it is instructive to first consider a simple budget of Cant over the Southern Hemisphere given by a global ocean carbon cycle model. The model considered is a global non-eddying (nominally 1°) configuration of the Geophysical Fluid Dynamics Laboratory’s (GFDL’s) MOM5-BLING model (Griffies 2012; Galbraith et al. 2011), forced at the surface with CORE-II normal-year forcing (Large and Yeager 2009). The model was spun up for 1000 years, and from 1860-1995 two runs were performed with identical climatological circulation states. For one of the runs, the surface boundary condition for CO2 gas exchange followed observational reconstructions, and for the second run, pre-industrial CO2 was maintained in the atmosphere. Following the definition of Cant of Zhai et al. (2017), who used the same modeling configuration, Cant in the model is the difference between the carbon variables for these two runs. Given our focus on waters for which s0  ≤  27.1, we use potential density for our analysis, following the method presented by Iudicone et al. (2016).

 

Figure 1. For the Southern Hemisphere, the simulated density-binned inventory of anthropogenic carbon (Cant) in 1995 (solid black), the cumulative air-sea fluxes of Cant over 1861-1995 (dashed black), and the density-binned inventories of Cant from the GLODAPv1 data product of Sabine et al. (2004) (solid blue). For each case, the units are 1015 moles of Cant per Δσ=0.1 kg m-3 density interval.

 

Figure 1 shows the observationally derived density-binned Cant inventory from the GLODAPv1 product (Sabine et al., 2004) with the density-binned Cant inventories in 1995 and the cumulative air-sea Cant fluxes over 1860-1995 simulated by the model. The cumulative fluxes were density-binned by month and then summed separately for each individual density class over the full period, 1860-1995. The base of the directly ventilated thermocline in the model at 30°S, calculated following the method of Sallée et al. (2013), has been identified to be at s0 = 26.4 in our model configuration. Comparing the density-binned inventories of Cant from GLODAPv1 (Sabine et al. 2004) with the model state in 1995 reveals that the model captures the first-order structure of the total Cant inventories over mode and intermediate waters. This diagnostic view reveals that Cant uptake by gas exchange tends to quantitatively explain storage patterns in waters lighter than the base of the thermocline, as these lighter waters are less likely to be transferred into the interior via subduction. In contrast, in the ocean interior for waters denser than the base of the thermocline, storage tends to exceed uptake (see Iudicone et al. 2016, for a discussion and analytical approximation of these distributions). This is consistent with the idea that WBCs and their extension regions may be serving as “gateways” for the net transfer of Cant from thermocline to sub-thermocline waters.

Figure 2. Maps of fluxes of Cant across the σ0=26.4 horizon for the year 1995, derived using the water mass transformation diagnostics presented in Equations 1 and 2 of Zhai et al. (2017): (a) the buoyancy-driven component, (b) the diffusive transformation term, (c) the contribution from tracer diffusion, and (d) the total diapycnal fluxes. The units are gC m-2 yr-1 and positive values indicate a net transfer from lighter to denser water masses.

 

In order to identify the specific mechanisms whereby WBCs and their extension regions sustain exchanges between thermocline water masses and subpolar water masses across the base of the thermocline, we consider, for the year 1995, a decomposition into the three dominant drivers in Figure 2. We begin with the contribution from buoyancy exchange with the atmosphere (Figure 2a). The sign convention is such that net buoyancy loss to the atmosphere, resulting in densification of water parcels that contain Cant, results in a positive flux. Thus over the WBCs and their extension regions, the diagnostic reveals a structural and significant annual mean flux of Cant across s0 = 26.4 from subtropical to subpolar water masses (positive), with a smaller flux in the opposing sense from subpolar to subtropical water masses (negative). The diffusive transformation contribution (Figure 2b) reveals a smaller net flux of Cant from subpolar water masses into the thermocline across s0 = 26.4. Thus, this term opposes in its sign the buoyancy-driven component over the WBC and extension regions. A smaller amplitude flux derives from the tracer diffusion contribution (Figure 2c). The total diapycnal flux is shown in Figure 2d, which includes additional terms such as cabbeling – i.e. when two separate water parcels mix to form a third that sinks. Taken together, the results emphasize an interplay of mechanistic drivers over the WBC regions that sustain diapycnal fluxes across the thermocline base and the central importance of heat loss to the atmosphere among the drivers of diapycnal exchanges.

Figure 3. Overturning schematics for the Southern Ocean (three-dimensional domain Y<30°S) for (a) mass fluxes, and (b) for Cant fluxes over the year 1995 in the MOM5-BLING simulation. The coarse graining into thermocline water (TW), subantarctic mode water (SAMW), and Antarctic intermediate water (AAIW) has been accomplished using the algorithmic approach of Sallée et al. (2013). The net freshwater forcing (precipitation minus evaporation, or pme) is shown at the sea surface for the mass fluxes. The mass units are Sverdrups (109 kg s-1) and for Cant are PgC yr-1.

 

The net cycling of Cant through the ocean’s overturning structures in 1995 can be better appreciated by considering the net transfers between three coarse-grained layers over the Southern Hemisphere:  subtropical thermocline waters (TW) (s0 < 26.4), subantarctic mode water (SAMW), (26.4 < s0 < 27.1), and a deeper layer that will be referred to as Antarctic intermediate water (AAIW) (27.1<s0). Although the deeper layer also aggregates water masses denser than AAIW, our interest is in quantifying fluxes across the SAMW/AAIW interface. For mass (Figure 3a), it can be seen that the principal formation source of SAMW is from AAIW (70%), with only 30% emanating from TW. This stands in stark contrast to the formation sources for Cant in SAMW (Figure 3b), where the TW formation source dominates over the AAIW source within the overturning circulation. In fact, the net of TW-to-SAMW formation sources is of nearly the same amplitude as the net gas exchange uptake of Cant by the SAMW layer over 1995, suggesting highly efficient transfer of Cant to the ocean interior. We wish to emphasize the strong degree of amplification in the TW formation source of Cant relative to mass for SAMW. While the Revelle factor (Revelle and Suess 1957) is expected to contribute to this amplification, a detailed attribution study of the discrepancies between mass and Cant is yet to be realized.

The model results presented here underscore an important role for WBCs and their extension regions in the ejection of Cant from the thermocline into denser waters. Building on the density framework for understanding Cant pathways first developed and presented by Iudicone et al. (2011; 2016), our analyses reveal important net diapycnal transfers of Cant to the ocean interior, consistent with the uptake pathways emphasized in Nakano et al. (2011). Furthermore, these analyses substantiate direct attribution of heat fluxes in WBCs and their extension regions as first-order contributors of Cant storage in sub-thermocline waters associated with the shallow subtropical cell overturning structures.

Ejection of Cant from the thermocline in WBCs and their extension regions has implications for the climate system for two reasons. First, the Revelle factor of low-latitude and thermocline waters is known to be less than that of circumpolar waters (Sabine et al. 2004), meaning that despite higher temperatures, low-latitude waters have an enhanced capacity to absorb Cant from the atmosphere than high-latitude waters. Thus filling a large subpolar reservoir such as SAMW with a subtropical formation source may lead to more efficient storage with respect to a circumpolar formation source. Second, denser subpolar interior water masses are expected to have longer interior renewal or re-emergence timescales for their Cant than subtropical waters (Toyama et al. 2017), and the longer the delay before re-emergence, the weaker the Revelle factor impact will be on regulating future Cant uptake by the ocean. Given the potential significance of the Revelle factor in regulating carbon-climate feedbacks, it will be important to determine whether this entry pathway for Cant might change under future perturbations to the physical state of the ocean.

Viewed in light of the study of Toyama et al. (2017), the net transfer of Cant from thermocline to subpolar water masses across s0=26.4 should be associated with re-emergence of Cant from the thermocline into the ocean’s mixed layer over the Southern Ocean (Toyama et al. 2017). We think it is important to combine the Lagrangian diagnostics for re-emergence applied in that study and the water mass transformation diagnostics applied here within a consistent modeling framework. It is also important to test the sensitivity of the formation sources for the important subtropical and subpolar mode water reservoirs to resolution for eddy-permitting or eddy-resolving model configurations. Of equal importance is the development of new observational constraints on the surface and near-surface formation sources of mode waters, through the development and application of quasi-conservative tracers of water mass transformations. One promising quasi-conservative tracer for this purpose is oceanic radiocarbon, which for the Southern Hemisphere has distinct subtropical and circumpolar surface ocean signatures.

 

 

References

Fassbender, A. J., C. L. Sabine, M. F. Cronin, and A. J. Sutton, 2017: Mixed-layer carbon cycling at the Kuroshio Extension Observatory. Global Biogeochem. Cycles, 31, doi:10.1002/2016GB005547.

Galbraith, E. D., and Coauthors, 2011: The impact of climate variability on the distribution of radiocarbon in CM2Mc, a new Earth System Model. J. Climate, 24, 4230-4254, doi:10.1175/2011JCLI3919.1.

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

Posted by mmaheigan 
· Friday, November 10th, 2017 

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

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

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

Mechanisms of nutrient supply

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

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

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

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

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

Phytoplankton biomass, community structure, and dynamics

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

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

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

Organic carbon export via physical and biological processes

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

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

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

Future research directions

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

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

 

 

 

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