Archive for kuroshio

Enhanced-warming Kuroshio Current experiences rapid seawater acidification and CO2 increase

Posted by mmaheigan 
· Thursday, March 30th, 2023 

In order to project the future states of the climate and the marine ecosystem it is vital to understand the long-term changes in ocean carbon chemistry driven by anthropogenic influence. A paucity of data make the rates of seawater acidification and partial pressure of CO2 (pCO2) rise on ocean margins highly uncertain.

Figure 1. Graphic summary of 9 years of data from the Kuroshio Current time-series: (a) under the influences of only atmospheric CO2 increase, (b) the combined effect of atmospheric CO2 increase, SST increase, and additional DIC supply, (c) annually averaged air-sea CO2 flux decrease, (d) Projected seawater pCO2 increase under SST rise and sustained DIC increase.

A recent study in Marine Pollution Bulletin documented the rapid increase of seawater pCO2 (3.70±0.57 matm year-1) and acidification (pH at -0.0033±0.0009 unit year-1) along Kuroshio in the East China Sea (Figure 1). These findings were based on nine years of time-series data ( 2010-2018) which are now available on the website of Japan Meteorological Agency (JMA). These trends are significantly greater than the expected rates from CO2 air-sea equilibrium and those reported from other oceanic time-series studies. Interestingly, they showed the contribution of each parameter such as sea surface temperature (SST), sea surface salinity (SSS), and normalized dissolved inorganic carbon (nDIC) and total alkalinity (nTA) to the pCO2 variability. Seawater warming caused rapid rates of pCO2 increase and acidification under sustained DIC increase. The faster pCO2 growth relative to the atmosphere resulted in the CO2 uptake through the air-sea exchange declining by ~50% (~-0.8 to -0.4 mol C m-2 y-1) over the study period.

If this trend continues and the atmospheric CO2 increases at its current rate, the rapid warming Kuroshio regions could change from a sink to a source of CO2 , and cause a loss of oceanic CO2 uptake in the near future (ca. 2030-2040). Further, other “warming hotspots” in the global ocean along western boundary currents with a continuous DIC supply may exhibit similarly accelerated acidification and pCO2 rise. This could lead to a significant reduction in ocean CO2 uptake.

 

Authors:
Shou-En Tsao (Institute of Oceanography, National Taiwan University, Taiwan)
Po-Yen Shen (Institute of Oceanography, National Taiwan University, Taiwan)
Chun-Mao Tseng* (Institute of Oceanography, National Taiwan University, Taiwan)

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: Decadal variability of the Kuroshio Extension system and its impact on subtropical mode water formation 

Posted by mmaheigan 
· Friday, November 10th, 2017 

Bo Qiu1, Eitarou Oka2, Stuart P. Bishop3, Shuiming Chen1, Andrea J. Fassbender4

1. University of Hawaii at Manoa
2. The University of Tokyo
3. North Carolina State University
4. Monterey Bay Aquarium Research Institute

 

After separating from the Japanese coast at 36°N, 141°E, the Kuroshio enters the open basin of the North Pacific, where it is renamed the Kuroshio Extension (KE). Free from the constraint of coastal boundaries, the KE has been observed to be an eastward-flowing inertial jet accompanied by large-amplitude meanders and energetic pinched-off eddies (see Qiu 2002 and Kelly et al. 2010 for comprehensive reviews). Compared to its upstream counterpart south of Japan, the Kuroshio, the KE is accompanied by a stronger southern recirculation gyre that increases the KE’s eastward volume transport to more than twice the maximum Sverdrup transport (~ 60Sv) in the subtropical North Pacific Ocean (Wijffels et al. 1998). This has two important consequences. Dynamically, the increased transport enhances the nonlinearity of the KE jet, rendering the region surrounding the KE jet to have the highest mesoscale activity level in the Pacific basin. Thermodynamically, the enhanced KE jet brings a significant amount of tropical-origin warm water to the mid-latitude ocean to be in direct contact with cold, dry air blowing off the Eurasian continent. This results in significant wintertime heat loss from the ocean to atmosphere surrounding the Kuroshio/KE paths, contributing to the formation of North Pacific subtropical mode water (STMW; see Hanawa and Talley (2001) and Oka and Qiu (2012) for comprehensive reviews).

Figure 1. Yearly paths of the Kuroshio and KE plotted every 14 days using satellite SSH data (updated based on Qiu and Chen 2005). KE was in stable state in 1993–94, 2002–05, and 2010–15, and unstable state in 1995-2001, 2006–09, and 2016, respectively.

 

Although the ocean is known to be a turbulent medium, variations in both the level of mesoscale eddy activity and the formation rate of STMW in the KE region are by no means random on interannual and longer timescales. One important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits clearly defined decadal modulations between a stable and an unstable dynamical state (e.g., Qiu & Chen 2005, 2010; Taguchi et al. 2007; Qiu et al. 2007; Cebollas et al. 2009; Sugimoto and Hanawa 2009; Sasaki et al. 2013; Pierini 2014; Bishop et al. 2015). As shown in Figure 1, the KE paths were relatively stable in 1993–95, 2002–05, and 2010–15. In contrast, spatially convoluted paths prevailed during 1996–2001 and 2006–09. When the KE jet is in a stable dynamical state, satellite altimeter data further reveal that its eastward transport and latitudinal position tend to increase and migrate northward, its southern recirculation gyre tends to strengthen, and the regional eddy kinetic energy level tends to decrease. The reverse is true when the KE jet switches to an unstable dynamical state. In fact, the time-varying dynamical state of the KE system can be well represented by the KE index, defined by the average of the variance-normalized time series of the southern recirculation gyre intensity, the KE jet intensity, its latitudinal position, and the negative of its path length (Qiu et al. 2014). Figure 2a shows the KE index time series in the satellite altimetry period of 1993–present; here, a positive KE index indicates a stable dynamical state and a negative KE index, an unstable dynamical state. From Figure 2a, it is easy to discern the dominance of the decadal oscillations between the two dynamical states of the KE system.

Figure 2. (a) Time series of the KE index from 1993‑present; available at http://www.soest.hawaii.edu/oceanography/bo/KE_index.asc. (b) Year-mean SSH maps when the KE is in stable (2004 and 2011) versus unstable (1997 and 2008) states. (c) SSH anomalies along the zonal band of 32°-34°N from satellite altimetry measurements. (d) Time series of the PDO index from 1989-present; available at http://jisao.washington.edu/pdo/PDO.latest.

 

Transitions between the KE’s two dynamical states are caused by the basin-scale wind stress curl forcing in the eastern North Pacific related to the Pacific Decadal Oscillation (PDO). Specifically, when the central North Pacific wind stress curl anomalies are positive during the positive PDO phase (see Figure 2d), enhanced Ekman flux divergence generates negative local sea surface height (SSH) anomalies in 170°–150°W along the southern recirculation gyre latitude of 32°–34°N. As these wind-induced negative SSH anomalies propagate westward as baroclinic Rossby waves into the KE region after a delay of 3–4 years (Figure 2c), they weaken the zonal KE jet, leading to an unstable (i.e., negative index) state of the KE system with a reduced recirculation gyre and an active eddy kinetic energy field (Figure 2b). Negative anomalous wind stress curl forcing during the negative PDO phase, on the other hand, generates positive SSH anomalies through the Ekman flux convergence in the eastern North Pacific. After propagating into the KE region in the west, these anomalies stabilize the KE system by increasing the KE transport and by shifting its position northward, leading to a positive index state.

The dynamical state of the KE system exerts a tremendous influence upon the STMW that forms largely along the paths of the Kuroshio/KE jet and inside of its southern recirculation gyre (e.g., Suga et al. 2004; Qiu et al. 2006; Oka 2009). Figure 3a shows the monthly time series of temperature profile, constructed by averaging available Argo and XBT/CTD/XCTD data inside the KE southern recirculation gyre (see Qiu and Chen 2006 for details on the constructing method). The black line in the plot denotes the base of the mixed layer, defined as where the water temperature drops by 0.5°C from the sea surface temperature. Based on the temperature profiles, Figure 3b shows the monthly time series of potential vorticity. STMW in Figure 3b is characterized by water columns with potential vorticity of less than 2.0 x 10-10 m-1s-1 beneath the mixed layer. From Figure 3, it is clear that both the late winter mixed layer depth and the low-potential vorticity STMW layer underwent significant decadal changes over the past 25 years. Specifically, deep mixed layer and pronounced low-potential vorticity STMW were detected in 1993–95, 2001–05, and 2010–15, and these years corresponded roughly to the periods when the KE index was in the positive phase (cf. Figure 2a).

 

Figure 3. Monthly time series of (a) temperature (°C) and (b) potential vorticity (10-10 m-1 s-1) averaged in the KE’s southern recirculation gyre. The thick black and white lines in (a) and (b) denote the base of the mixed layer, defined as where the temperature drops by 0.5°C from the surface value. Red pluses (at the top of each panel) indicate the individual temperature profiles used in constructing the monthly T(z, t) profiles. The potential vorticity, Q(z,t) = fα∂T(z,t)/∂z, where f is the Coriolis parameter and α the thermal expansion coefficient.

 

The close connection between the dynamical state of the KE system and the STMW formation has been detected by many recent studies based on different observational data sources and analysis approaches (Qiu and Chen 2006; Sugimoto and Hanawa 2010; Rainville et al. 2014; Bishop and Watts 2014; Oka et al. 2012; 2015; Cerovecki and Giglio 2016). Physically, this connection can be understood as follows. When the KE is in an unstable state (or a negative KE index phase), high-regional eddy variability infuses high-potential vorticity KE and subarctic-gyre water into the southern recirculation gyre, increasing the upper-ocean stratification and hindering the development of deep winter mixed layer and formation of STMW. A stable KE path with suppressed eddy variability (in the positive KE index phase), on the other hand, favors the maintenance of a weak stratification in the recirculation gyre, leading to the formation of a deep winter mixed layer and thick STMW.

Since the STMW is renewed each winter, due to combined net surface heat flux and wind stress forcing that modulate on interannual timescales, a question arising naturally is the timescale on which the dynamical state change of the KE system is able to alter the upper ocean stratification and potential vorticity inside the recirculation gyre. If the influence of the KE dynamical state acts on interannual timescales, one may expect a stronger control on the STMW variability by the wintertime atmospheric condition (e.g., Suga and Hanawa 1995; Davis et al. 2011). Intensive observations from the Kuroshio Extension System Study (KESS) program, spanning the period from April 2004 to July 2006, captured the 2004–05 transition of the KE system from a stable to an unstable state. The combined measurements by profiling Argo floats, moored current meter, current and pressure inverted echo sounder (CPIES), and the Kuroshio Extension Observatory (KEO) surface mooring revealed that the KE dynamical state change was able to change the STMW properties both significantly in amplitude and effectively in time (Qiu et al. 2007; Bishop 2013; Cronin et al. 2013; Bishop and Watts 2014). Relative to 2004, the low-potential vorticity signal in the core of STMW was diminished by one-half in 2005, and this weakening of STMW’s intensity occurred within a period of less than seven months. These significant and rapid responses of STMW to the KE dynamical state change suggests that the variability in STMW formation is more sensitive to the dynamical state of the KE than to interannual variations in overlying atmospheric conditions over the past 25 years.

The decadal variability of STMW in the KE’s southern recirculation gyre is able to affect the water property distributions in the entire western part of the North Pacific subtropical gyre (Oka et al. 2015). Measurements by Argo profiling floats during 2005–14 revealed that the volume and spatial extent of STMW decreased (increased) in 2006–09 (after 2010) during the unstable (stable) KE period in its formation region north of ~28°N, as well as in the southern, downstream regions with a time lag of 1-2 years. Such decadal subduction variability affects not only physical but also biogeochemical structures in the downstream, interior subtropical gyre. Shipboard observations at 25°N and along the 137°E repeat hydrographic section of the Japan Meteorological Agency exhibited that, after 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon. Changes in dissolved inorganic carbon, pH, and aragonite saturation state were opposite their long-term trends.

KE State and the Ocean Carbon Cycle

Western boundary current (WBC) regions display the largest magnitude air-to-sea carbon dioxide (CO2) fluxes of anywhere in the global ocean. STMW formation processes are thought to account for a majority of the anthropogenic CO2 sequestration that occurs outside of the polar, deep water formation regions (Sabine et al. 2004; Khatiwala et al. 2009). Once subducted and advected away from the formation region, mode waters often remain out of contact with the atmosphere on timescales of decades to hundreds of years, making them short-term carbon silos relative to the abyssal carbon storage reservoirs. One of the physical impacts on carbon uptake via air-sea CO2 flux is due to the temperature dependence of the solubility of pCO2 in the surface waters. Cooler surface waters during the wintertime months reduce the oceanic pCO2 and subsequently enhance the CO2 flux into the ocean. This carbon uptake corresponds with the timing of peak STMW formation.

As mentioned above, the formation of STMW is modulated by the dynamic states of the KE, with less STMW forming during unstable states and more during stable states. To complicate matters, more enhanced levels of surface chlorophyll (Chla) have also been observed from satellite ocean color during unstable states (Lin et al. 2014), which points to the potential importance of biophysical interactions on carbon uptake. Elevated levels of Chla can further modify the pCO2 of surface waters and enhance carbon export at depth from sinking of particulate organic matter following an individual bloom. Given that submesoscale processes result from deep wintertime mixed layers and from the presence of the larger mesoscale lateral shear and strain fields (McWilliams 2016), it is expected that submesoscale processes are also important in STMW formation during unstable states of the KE. An open question in the research community is to what extent do elevated levels of mesoscale and submesoscale eddy activity modulate STMW formation and carbon uptake during unstable states of the KE? With large variations in STMW formation occurring in concert with decadal variability in the mesoscale eddy field, it is possible that submesoscale processes may impact STMW formation through restratification of the mixed layer within density classes encompassing STMW and timing of the spring bloom. These mesoscale and submesoscale processes may then also impact the uptake of CO2 in the North Pacific on interannual to decadal timescales.

 

 

References

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Cronin, M. F., N. A. Bond, J. T. Farrar, H. Ichikawa, S. R. Jayne, Y. Kawai, M. Konda, B. Qiu, L. Rainville, and H. Tomita, 2013: Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre. Deep-Sea Res. II, 85, 62-74, doi:10.1016/j.dsr2.2012.07.018.

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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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Biophysical drivers of vigorous carbon cycling near the Kuroshio Extension

Posted by mmaheigan 
· Thursday, April 27th, 2017 

The Kuroshio Current and its Extension jet in the western North Pacific Ocean form a dynamic western boundary current (WBC) region characterized by large air-sea exchanges of heat and carbon dioxide gas (CO2). The jet is known to oscillate between stable and meandering states on multi-year timescales that alter the eddy field and depth of winter mixing in the southern recirculation gyre. These dynamic state changes have been shown to imprint biogeochemical signatures onto regional mode waters that can be distributed widely throughout the North Pacific and remain out of contact with the atmosphere for decades.

Figure. ~7 years of (a) AVISO daily sea surface height (SSH) anomalies and (b) upper-ocean temperature from the NOAA Kuroshio Extension Observatory (KEO) surface mooring. Black and gray lines in b show the mixed layer depth (MLD) and 17C contour, respectively. Spring bloom periods are indicated in blue in a. The semi-regular upwelling of cold water and corresponding depression of SSH is caused by cold-core eddies that pass the KEO mooring. Winter ventilation depths increase by ~100 m after 2010 when the extension jet entered a stable phase.

To better characterize carbon cycling in this region, ~7 years of daily-averaged autonomous CO2 observations from NOAA’s Kuroshio Extension Observatory (KEO) surface mooring were used to close the mixed layer carbon budget. High rates of net community production (NCP; >100 mmol C m-2 d-1) were observed during the spring bloom period, and a mean annual NCP of 7±3 mol C m-2 yr-1 was determined. Biological processes near KEO largely balance the input of carbon that occurs annually through winter mixing; however, physical processes that deviate from climatology were not resolved in this study. Therefore, it remains unclear how transient features such as eddies influence biological carbon production and export through altered nutrient supply and active vertical transport of organic material. Further work is required to determine how biophysical interactions during mesoscale and submesoscale disturbances contribute to local carbon cycle processes and variability in regional mode water carbon inventories.

Ocean Carbon Hot Spots, an upcoming workshop focused on understanding biophysical drivers of carbon uptake in WBC regions, will be held September 25-26, 2017 at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, California. The primary objective of the workshop is to develop a community of observationalists and modelers working on the topic, and to identify critical observational needs that would improve model parameterizations. Ocean Carbon Hot Spots will be co-sponsored by US CLIVAR, US OCB, MBARI, and OMIX.

Written by Andrea J. Fassbender, Monterey Bay Aquarium Research Institute

 

Mixed-layer carbon cycling at the Kuroshio Extension Observatory (Global Biogeochemical Cycles) 

Authors:
Andrea J. Fassbender (Monterey Bay Aquarium Research Institute)
Christopher L. Sabine (NOAA Pacific Marine Environmental Laboratory)
Meghan F. Cronin (NOAA Pacific Marine Environmental Laboratory)
Adrienne J. Sutton (Joint Institute for the Study of the Atmosphere and Ocean, University of Washington)

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