Western Boundary Current System Research Articles

An Intercomparison of Global Reanalysis Products for Southern Africa’s Major Oceanographic Features

A regional assessment of three global ocean reanalysis products is presented for southern Africa’s major oceanographic features. The reanalyses include Mercator Ocean’s Global Reanalysis (GLORYS), the Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) Bluelink Reanalysis (BRAN) and the Fleet Numerical Meteorology and Oceanography Center’s (FNMOC) global Hybrid Coordinate Ocean Model (HYCOM) reanalysis. The aim is to provide modelers with sufficient information for selecting the appropriate product for use as boundary conditions to force their regional ocean models, as well as to provide marine industries, relevant government agencies and academics with insight into the optimal reanalysis product for their purposes. The reanalyses are compared to both assimilated and independent observational datasets spanning various regions within the southern African marine environment. While all reanalysis products reproduce the eastern and western boundary current systems surrounding southern Africa, limitations exist. BRAN outperforms the other reanalyses in its representation of the Mixed Layer Depth, contributing to its good representation of coastal SSTs in the Benguela upwelling system, whereas GLORYS and HYCOM’s misrepresented MLD result in significant warm biases in this region. The Angola-Benguela Frontal Zone and it’s variability is best reproduced by BRAN and HYCOM. The Agulhas Current system’s major components are well reproduced by both GLORYS and BRAN. HYCOM, however, simulates considerably more early retroflections than are observed which have resulted in its mean eastward location. While all the reanalyses overestimate the occurrence of Agulhas meanders, GLORYS and BRAN resolve the associated variability best. Agulhas Current transport is best resolved by GLORYS, unlike BRAN and HYCOM which largely overestimate the magnitude of its south-westward flow, linked to their misrepresentation of the Current’s vertical structure. The bay-scale and nearshore evaluations highlighted issues pertaining to the resolution of the reanalyses and their use at such a small scale. The reanalyses are limited by their resolution, as well as by their misrepresentation of submesoscale processes or lack thereof, prompting the need for the development of regional downscaled models in and around the southern African oceans based on the global ocean reanalysis products.

Plankton community response to fronts: winners and losers

AbstractFronts are particularly productive regions of the ocean, and biodiversity hotspots for many marine species. Here we use an ocean-ecosystem model to investigate the effect of fronts on plankton ecology. We focus on energetic fronts in Western Boundary Current systems that efficiently inject nutrients into the euphotic layer and which are physical boundaries between productive and oligotrophic provinces. We found that the fronts form an environment distinct from both provinces, favorable to some plankton groups (diatoms, dinoflagellates and large carnivorous zooplankton) and less favorable to others (pico-phytoplankton, coccolithophores and small grazers), and with an overall larger diversity. In agreement with previous understanding, we find that bottom-up abiotic processes (nutrient enrichment) explain the prevalence of groups with fast growing rates (the “winners”). Importantly, our results also show that biotic interactions within the ecosystem may play a larger role than previously thought. We show that the winners can have a negative impact on other plankton species (the “losers”) through two indirect competitive processes: community shading (modification of the light environment by the plankton community leading to light-limitation of some plankton groups) and shared predation (where an increase in one functional group leads to increased grazing by a shared predator on another functional group).

Effect of the Coastline Geometry on the Boundary Currents Intruding through the Gap

The problem of a geophysical western boundary current negotiating a gap in its supporting boundary is considered. For traditional straight, parallel gaps, such systems are known to exhibit two dominant states, gap penetrating and leaping, with the transitional dynamics between states displaying hysteresis. However, for more complex geometries, such as angled or offset gap configurations, the question of multiple states and hysteresis is unresolved. In such cases, the inertia of the western boundary current is oriented into the gap, hence the assumption that increased inertia promotes gap penetrating loop current states. Here we address the problem numerically in an idealized setting. It is found that despite the inertia of the current being directed into the gap, for large western boundary current transport values, leaping states will be present. That is, we show here that the presence of multiple states with hysteresis for gap-leaping western boundary current systems is robust to both angled and offset gap geometries.

Spatial distribution of turbulent diapycnal mixing along the Mindanao current inferred from rapid-sampling Argo floats

The Mindanao Current (MC) bridges the North Pacific low-latitude western boundary current system region and the Indonesian Seas by supplying the North Pacific waters to the Indonesian Throughflow. Although the previous study speculated that the diapycnal mixing along the MC might be strong on the basis of the water mass analysis of the gridded climatologic dataset, the real spatial distribution of diapycnal mixing along the MC has remained to be clarified. We tackle this question here by applying a finescale parameterization to temperature and salinity profiles obtained using two rapid-sampling profiling Argo floats that drifted along the MC. The western boundary (WB) region close to the Mindanao Islands and the Sangihe Strait are the two mixing hotspots along the MC, with energy dissipation rate ε and diapycnal diffusivity Kρ enhanced up to ~ 10–6 W kg−1 and ~ 10–3 m2 s−1, respectively. Except for the above two mixing hotspots, the turbulent mixing along the MC is mostly weak, with ε and Kρ to be 10–11–10–9 W kg−1 and 10–6–10–5 m2 s−1, respectively. Strong mixing in the Sangihe Strait can be basically attributed to the existence of internal tides, whereas strong mixing in the WB region suggests the existence of internal lee waves. We also find that water mass transformation along the MC mainly occurs in the Sangihe Strait where the water masses are subjected to strong turbulent mixing during a long residence time.

Full Vorticity Budget of the Arabian Sea from a 0.1° Ocean Model: Sverdrup Dynamics, Rossby Waves, and Nonlinear Eddy Effects

Abstract The Arabian Sea, influenced by the Indian monsoon, has many unique features, including its basin-scale seasonally reversing surface circulation and the Great Whirl, a seasonal anticyclonic system appearing during the southwest monsoon close to the western boundary. To establish a comprehensive dynamical picture of the Arabian Sea, we utilize numerical model output and design a full vorticity budget that includes a fully decomposed nonlinear term. The ocean general circulation model has 0.1° resolution and is mesoscale eddy-resolving in the region. In the western boundary current system, we highlight the role of nonlinear eddies in the life cycle of the Great Whirl. The nonlinear eddy term is of leading-order importance in this feature’s vorticity balance. Specifically, it contributes to the Great Whirl’s persistence in boreal fall after the weakening of the southwesterly winds. In the open ocean, Sverdrup dynamics and annual Rossby waves are found to dominate the vorticity balance; the latter is considered as a key factor in the formation of the Great Whirl and the seasonal reversal of the western boundary current. In addition, we discuss different forms of vertically integrated vorticity equations in the model and argue that the bottom pressure torque term can be interpreted analogously as friction in the western boundary and vortex stretching in the open ocean.

Slower Long‐Term Coastal Warming Drives Dampened Trends in Coastal Marine Heatwave Exposure

AbstractLong‐term temperature changes drive coastal Marine Heatwave (MHW) trends globally. Here, we provide a more comprehensive global analysis of cross‐shore gradients of MHW and Sea Surface Temperatures (SST) changes using an ensemble of three satellite SST products during recent decades. Our analysis reveals depressed onshore SST trends in more than 2/3 of coastal pixels, including both eastern and western boundary current systems. These were well correlated with depressed trends of MHW exposure and severity, ranging from a −2 to −10 decrease in MHW days per decade and from a −2.5°C to −15°C.days per decade decrease in cumulative intensity. Results were consistent across all satellite products, indicating that these cross‐shore gradients are a robust feature of observations. ERA reanalysis data show that neither air‐sea heat fluxes nor wind driven upwelling were found to be consistent drivers. Global ocean circulation models (OFAM3 and ACCESS‐OM2) have limited ability to simulate the depressed onshore trends. A heat budget analysis performed in the Chilean coast region, where models agree with observations, showed that the gradient of temperature change was controlled by an onshore increase of longwave radiative cooling, despite an increase in upwelling. This highlights the complexity of small‐scale coastal ocean‐atmosphere feedbacks, which coarser resolution climate models do not resolve. Here, we show that global coastal regions may act as thermal refugia for marine ecosystems from aspects of climate change and pulsative (MHW) changes. Contrary to the literature, our results suggest that driving mechanisms are region dependant, stressing the necessity to improve climate models resolution.

Regional imprints of changes in the Atlantic Meridional Overturning Circulation in the eddy-rich ocean model VIKING20X

Abstract. A hierarchy of global 1/4∘ (ORCA025) and Atlantic Ocean 1/20∘ nested (VIKING20X) ocean–sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50–60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the Atlantic Meridional Overturning Circulation (AMOC), and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5∘ N. This points to a dominant role of the wind forcing. The ability of the model to represent regional observations in western boundary current (WBC) systems at 53∘ N, 26.5∘ N and 11∘ S is explored. The question is investigated of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5∘ N, it is shown that in particular the outflow of North Atlantic Deepwater at 53∘ N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable of representing realistic forcing-related and ocean-intrinsic trends.

Dissolved Organic Matter Dynamics in the Epipelagic Northwest Pacific Low‐Latitude Western Boundary Current System: Insights From Optical Analyses

AbstractHigh‐resolution horizontal and vertical distribution of dissolved organic carbon (DOC), chromophoric, and fluorescent dissolved organic matter (CDOM and FDOM) were investigated in the western boundary current system of the tropical Northwest Pacific (<200 m) in autumn 2017. A strong correlation between DOC and stratification index indicated that the vertical DOC profile was primarily regulated by physical processes. The association of high aCDOM(254) with the maximum chlorophyll (Chl a) layer infers phytoplankton‐sourced dissolved organic matter (DOM). The aCDOM(325) and humic‐like FDOM (FDOMH) showed an accumulation in the deeper layer and positive correlations with apparent oxygen utilization and Chl a concentration at the maximum chlorophyll layer, suggesting that these components are related to microbial degradation of biogenic materials. Elevated Chl a at the frontal area between the North Equatorial Current (NEC) and cold Mindanao Eddy enhanced DOM production. Input waters from the NEC showed higher DOC, but lower FDOMH, than inflow waters from the New Guinea Coastal Current/Undercurrent (NGC(U)C). A mass balance model estimated a 6‐times higher lateral DOC flux from the NEC tropical‐gyre branch (12°N–7.5°N) than that from the subtropical‐gyre branch (12°N–17°N). Based on comparison with long‐term (1994–2015) average DOC fluxes for the same season, eddy and upstream processes contributed 38%, 46% and 40% of lateral DOC fluxes for the NEC tropical‐gyre branch, NGC(U)C and export North Equatorial Counter Current, respectively. These results demonstrated that the quasi‐permanent Mindanao and Halmahera eddies greatly enhance lateral export of DOM with altered properties throughout this large conjunction area.

Seasonality of the Somali Current/Undercurrent system

The connected western boundary current system in the Arabian Sea plays an important role in the exchange of heat, mass, and freshwater and their distribution. Nonetheless, the dynamics and time scales of the processes and characteristics of this boundary current are not well understood because most prior measurements were only for a limited period or only resolved the surface circulation. Seasonality of the Somali Current (SC)/Undercurrent (SUC) system is investigated using a multi-decadal time series of ocean temperature and salinity profiles. The SC is driven principally by the seasonal monsoon winds, flowing southwestward in the winter monsoon (January–March), then reverses to northeastward in the summer monsoon (June–September). The subsurface southwestward SUC is found during the transition seasons in April–June and again in September–November. A vertical normal mode decomposition shows that the surface SC is mainly controlled by the seasonally varying first baroclinic mode and the barotropic component (a combined 86.5% of the total variance), while the SUC variations can be explained by the superimposition of the first and second baroclinic modes (a combined 75.9% of the total variance). Here we show that the occurrence of the countercurrent is not only due to the baroclinic adjustment caused by the wind forcing, but also a response to the complex salinity structure caused by the intrusion of high salinity water masses. Specifically, the presence of the salty water masses from the northern Arabian Sea during the boreal summer monsoon inhibits the southwestward flowing SUC and results in a baroclinic structure that supports a subsurface northeastward flow, aligned with the surface current. This suggests that salinity dynamics play a fundamental role in the circulation of the boundary current system of the Arabian Sea.

Observations of SST‐Induced Wind Perturbations in the Leeuwin Current

AbstractSea surface temperature (SST) perturbations modify low‐level winds through the influence of air‐sea heat flux across the marine atmospheric boundary layer, mostly observable in the major western boundary current systems. In this study, we investigate the effects of SST on surface winds in a poleward‐flowing eastern boundary current system, the Leeuwin Current (LC), using the Advanced Very High‐Resolution Radiometer (AVHRR) SST and wind data from Cross‐Calibrated Multi‐Platform (CCMP) during 2009–2018. There exist robust linear relationships between perturbation SST and perturbation wind speed and their derivatives in the LC. The regression slopes between SST perturbations and wind perturbations appear to be higher in austral summer when the alongshore northward winds are strongly opposing the LC, however, the relationships in winter are statistically more robust when the LC and associated cross‐shore SST fronts are the strongest. Consistent with previous studies in the western boundary currents, crosswind perturbation wind speed gradient is the largest for the winds 80° (−100°) to the right (left) of the SST gradient in the LC region. The perturbation wind stress curl induced by the SST signature of the LC causes positive wind stress curl anomalies on the offshore edge of the LC, driving anomalous downwelling and would reduce surface chlorophyll a concentrations along the LC SST front as observed in satellite data, which may have implications in regional biogeochemistry.

Concentration of dissolved lead in the upper Northwestern Pacific Ocean

During a cruise in 2016, samples of clean seawater were collected, and the first high-quality dataset of total dissolvable Pb concentration ([TDPb]) was developed for the upper 750 m of the low-latitude Northwestern Pacific Ocean (NWPO). The distribution of [TDPb] was influenced by the low-latitude NWPO current system. The surface [TDPb] ranged from 22 to 60 pmol/kg, with two significant maxima occurring in the North Equatorial Current (NEC) and the Kuroshio Current (KC). The vertical [TDPb] ranged from 16 to 70 pmol/kg. The [TDPb] profile to the north of 13°N, where the NEC occupys, was characterized by a significant sub-surface maximum, and this feature extended eastward to the North Equatorial Countercurrent (NECC) and possibly, to the Mindanao Current (MC). In the area south of 5°N, which is affected by currents originating from the eastern equator and the southern hemisphere, [TDPb] was greatly reduced and its distribution was uniform vertically. The vertical Pb maximum characteristic was continuous throughout the North Pacific subtropical gyre at σθ = 25.2–26.2, which was associated with high surface Pb at 30°N–40°N in Northwestern Pacific. The lateral transport of Pb in the NEC along 130°E amounted to approximately 16.8–22.5 Gg/yr, which could be considered to be the convergence budget of the advection transport of the North Pacific. With the bifurcation of the NEC, approximately 4.5–9.0 Gg/yr of Pb was distributed into the northward-flowing KC which rejoins the subtropical circulation of the North Pacific, while another 7.8–18.0 Gg/yr of Pb was distributed into the southward-flowing MC, which joins the tropical circulation of the Pacific or enters the Indian Ocean via the Indonesian through-flow (ITF). The distribution pattern of Pb in the low-latitude western Pacific is influenced by the western boundary current system and possible changes associated with the climate change.

Spatial patterns in planktonic cnidarian distribution in the western boundary current system of the tropical South Atlantic Ocean

Abstract In marine western boundary systems, strong currents flowing coastward spread oceanic water masses over the continental shelves. Here we propose to test the hypothesis according to which oceanic cnidarian species may dominate western boundary system regions even in coastal waters. For that purpose we use a set of data collected above the shelf, slope and around oceanic seamounts and islands in the Western Tropical South Atlantic. Samples were acquired with a plankton net with 300 μm mesh size over 34 stations during an oceanographic cruise carried out in October 2015. Results reveal a diverse cnidarian assemblage in the area, extending the known distribution of many species. In addition, the Fernando de Noronha Chain and most of the narrow continental shelf presented a typical oceanic cnidarian community, dominated by holoplanktonic siphonophores. In this western boundary system, this condition was likely driven by the strong currents, which carry the oceanic tropical water and associated planktonic fauna toward the coast. A specific area with reduced influence of oceanic currents presented typical coastal species. The pattern we observed with the dominance of oceanic plankton communities up to coastal areas may be typical in western boundary systems characterized by a narrow continental shelf.

Otolith δ18O and microstructure analyses provide further evidence of population structure in sardine Sardinops sagax around South Africa

Abstract Sardine Sardinops sagax is an ecologically and economically important Clupeid found off the entire South African coast that includes both coastal upwelling and western boundary current systems. Although the management of the sardine fisheries historically assumed a single, panmictic population, the existence of three, semi-discrete subpopulations has recently been hypothesized. We conducted otolith δ18O and microstructure analyses to investigate nursery habitat temperatures and early life growth rates, respectively, of sardine collected from three biogeographic regions around South Africa’s coast to test that hypothesis. Analyses indicated that for both summer- and winter-captured adults and summer-captured juveniles, fishes from the west coast grew significantly slower in water that was several degrees cooler than those from the south and east coasts. This suggests that mixing of sardines between regions, particularly the west and other coasts, is relatively limited and supports the hypothesis of semi-discrete subpopulations. However, the west-south differences disappeared in the results for winter-captured juveniles, suggesting that differences in early life conditions between regions may change seasonally, and/or that all or most winter-captured juveniles originated from the west coast. Further elucidating the interactions between South African sardine subpopulations and the mechanisms thereof is important for sustainable harvesting of this species.

3D characterisation of the thermohaline structure in the southwestern tropical Atlantic derived from functional data analysis of in situ profiles

The dynamic of the thermohaline structure of the upper ocean, which depends on ocean–atmosphere interactions, drives most near surface oceanic processes, including the control of gases and heat fluxes, and nutrient availability in the photic layer. The thermohaline structure of the southwestern tropical Atlantic (SWTA), a key region for diagnosing variation of the Atlantic Meridional Overturning Circulation, has prime impact on global climate. Characterising the thermohaline structure is typically based on the application of classical statistical methods on vertical profiles. Such approach has important limitations since classical methods do not explicitly contemplate the vertical nature of the profiles. Functional Data Analysis (FDA) is a new alternative to solve such drawbacks. Here, we apply an FDA approach to characterise the 3D canonical thermohaline structure of the SWTA in austral spring and fall. Our results reveal a clear spatial pattern with the presence of three areas with significantly different thermohaline structure. Area 1, mostly located along the continental slope, reflects the western boundary current system, with low static stability and high frequency of occurrence of barrier layer (BL). Conversely, Area 2, located along the Fernando de Noronha chain, presents strong static stability with a well-marked thermocline. This area, under the influence of the eastern Atlantic, is characterised by a low BL frequency, which is seasonally modulated by the latitudinal oscillation of the Intertropical Convergence Zone, controlling the regime of precipitation. In turn, Area 3 behaves as a transition zone between A1 and A2 with the presence of the water core of maximum salinity in subsurface, and therefore presence of strong-moderate BL. Beyond this study, FDA approach emerges as a powerful way to describe, characterise, classify and compare ocean patterns and processes. It can be applied to in situ data but could also be used to deeply and comprehensively explore ocean model output.

A Water Mass Classification Approach to Tracking Variability in the East Australian Current

The East Australian Current (EAC) is a southward flowing western boundary current that transports relatively warm and nutrient-depleted subtropical water along Australia's east coast. The EAC is a highly variable system that is formed by temporally-varying mixtures of water in the Coral Sea that do not form a linear density gradient or conform to a set range of temperature and salinity values. It can therefore be difficult to track EAC dynamics across both space and time using traditional analytical approaches. In order to more accurately quantify variability and trends in penetration of the EAC we develop a novel machine-learning classification approach to quantify variability in coastal EAC dynamics along a latitudinal gradient within the EAC extension zone in southeastern Australia. Applying our method to data from a 22-year free running regional hydrodynamic model revealed significant decadal-scale changes to EAC dynamics in the region. The annual period (generally in the austral summer) when the EAC is the dominant water mass in the region increased by approximately 2 months over the model time series. The encroachment of the EAC's traditional period of summer dominance into winter may have significant ecological implications through the acceleration of poleward range extensions by vagrant tropical species, facilitation of community phase shifts from temperate to tropical assemblages, and a phenological shift in the timing of major phytoplankton blooms. These results highlight the need to further understand the rapid changes occurring within western boundary current systems, and illustrates how classification approaches may assist in uncovering patterns in these highly variable systems.

Investigating the Diversity of Marine Bacteriophage in Contrasting Water Masses Associated with the East Australian Current (EAC) System.

Virus- and bacteriophage-induced mortality can have a significant impact on marine productivity and alter the flux of nutrients in marine microbial food-webs. Viral mediated horizontal gene transfer can also influence host fitness and community composition. However, there are very few studies of marine viral diversity in the Southern Hemisphere, which hampers our ability to fully understand the complex interplay of biotic and abiotic factors that shape microbial communities. We carried out the first genetic study of bacteriophage communities within a dynamic western boundary current (WBC) system, the east Australian current (EAC). Virus DNA sequences were extracted from 63 assembled metagenomes and six metaviromes obtained from various depths at 24 different locations. More than 1700 bacteriophage genomic fragments (>9 kbps) were recovered from the assembled sequences. Bacteriophage diversity displayed distinct depth and regional patterns. There were clear differences in the bacteriophage populations associated with the EAC and Tasman Sea euphotic zones, at both the taxonomic and functional level. In contrast, bathypelagic phages were similar across the two oceanic regions. These data provide the first characterisation of viral diversity across a dynamic western boundary current, which is an emerging model for studying the response of microbial communities to climate change.

Eddy‐Driven Cross‐Shelf Transport in the East Australian Current Separation Zone

AbstractIn western boundary current systems, sharp velocity gradients between the poleward flowing jet and coastal waters generally act to inhibit cross‐shelf exchange. Downstream of jet separation, dynamic mesoscale eddies dominate the flow. In the East Australian Current System, counter‐rotating eddy dipoles are often present which, in the appropriate configuration, have potential to drive cross‐shelf transport. However, this eddy dipole mode is poorly understood in the framework of cross‐shelf exchange and the effect of these structures on shelf waters is uncertain. Using 25 years of satellite altimetry, as well as in situ sampling of a typical dipole event, we investigate the characteristics of eddy‐driven cross‐shelf exchange. We show that the maximum onshore velocity is driven by an eddy dipole structure and occurs in a defined latitudinal band between 33°S and 34°S more than 50% of the time. We sample a typical eddy dipole and find a strong onshore jet, 37 km wide, with velocities up to 1.78 m s and a transport of at least 16 Sv. Hydrographic data from an autonomous underwater glider show that this jet manifests on the shelf as a subsurface intrusion of warm salty water extending from offshore up onto the midshelf. In the light of climatic changes in western boundary current transport and the increase in their eddy kinetic energy, understanding eddy‐driven cross‐shelf exchange is important to predict future changes to the shelf water mass.

Structure and dynamics of undercurrents in the western boundary current of the Bay of Bengal

The structure and variability of undercurrents in the East India Coastal Current (EICC), which is the western boundary current system in the Bay of Bengal (BoB), and the mechanisms of their formation are examined in this study. We used current data collected by Acoustic Doppler current profilers (ADCP) moored off Cuddalore (~ 12oN), Kakinada (~ 16.5oN), Visakhapatnam (~ 17.7oN), and Gopalpur (~ 19.4oN) and simulations for the period 2013–2014 from a high-resolution model configured for the BoB. The undercurrents were observed at all these locations, mainly during summer (June–August) and winter (October–December). Undercurrents were seen at relatively shallow depths (75 m), and their occurrences were more frequent off Cuddalore, whereas they were deep (100–150 m) and less frequent in the northern part of the east coast (off Visakhapatnam and Gopalpur). Numerical simulations showed that the interaction of the westward propagating anticyclonic eddies with the equatorward EICC weakened the strong surface flow and reversed the weak subsurface flow in the northern part of the western BoB. This interaction resulted in the formation of the poleward undercurrent here. Once these mesoscale eddies dissipated due to the interaction with the continental slope, the poleward undercurrents vanished and equatorward flow in the subsurface reappeared. The observed undercurrents near the shelf break region (75–200 m) in the southern part of the coast (off Cuddalore) were associated with small subsurface eddies (diameter of about 20–30 km), which developed due to large zonal gradient in the alongshore component of EICC. Subsurface anticyclonic circulations of larger spatial extent (diameter > 200 km) were responsible for the observed undercurrents in the deeper levels (deeper than 250 m) off Cuddalore. We further show that intraseasonal variability of undercurrents near the shelf break off Cuddalore was directly linked to intraseasonal variability in the strength of surface EICC itself. Results from this study suggest that the undercurrents observed below the EICC were not continuous poleward flow, but they were part of distinct anticyclonic eddies.

On the Role of Turbulent Mixing Produced by Vertical Shear Between the Brazil Current and the Intermediate Western Boundary Current

AbstractAn intensification of the vertical shear is observed below the surface mixed layer at 21°S due to the mutually opposing flows of the Brazil Current and the Intermediate Western Boundary Current. The propensity to develop turbulence and mixing due to vertical shear over intense stabilizing density gradients is an important characteristic of such environments. For the first time, microscale measurements were made in the Brazil Current‐Intermediate Western Boundary Current system, providing direct quantitative values of the turbulent fluctuations. Peaks of relative strong dissipation rates of turbulent kinetic energy (O(10 ) W/kg) were observed close to the base of the surface mixed layer. On the other hand, prominent peaks of turbulent kinetic energy dissipation rates of up to 2 orders of magnitude higher than the background were observed at deeper levels, where stratification begins to lose intensity. Analyzing such peaks, caused by intense vertical shear or weak stratification—and sometimes both—, allows a characterization of the local mixing processes and the role played by vertical exchanges of biogeochemical properties. Based on the estimated nitrate gradient and the vertical diffusivity, we show that turbulent mixing driven by vertical shear plays an important role in the supply of nitrate to the upper layer.

Empirical Evidence Linking the Pacific Decadal Precession to Kuroshio Extension Variability

AbstractRecent research has revealed a new mode of quasi‐decadal climate variability—the Pacific Decadal Precession (PDP)—that has teleconnected impacts upon the state of the ocean and atmosphere across the North Pacific. Previous research has revealed similar decadal scale variability in the location, intensity, and stability of the North Pacific's western boundary current system and in particular the Kuroshio Extension (KE) current, suggesting potential links between the large‐scale (~1,000 km and larger) ocean/atmosphere variations associated with the PDP and mesoscale (~100 km) variations in the KE. Empirical analyses using self‐consistent historical state estimates of the ocean and atmosphere over the North Pacific—derived from the 20th Century Reanalysis Version 2c (20CRv2c) and Simple Ocean Data Assimilation – sparse input version 3 (SODAsi.3) data sets for the period 1851–2013—confirm these links exist. Further, these empirical analyses suggest that the PDP both forces and responds to KE variability on decadal time scales. Our synthesis of these empirical results with previously published empirical, theoretical, and numerical results argues for the presence of a self‐sustaining decadal oscillation of the ocean/atmosphere system across the extratropical North Pacific in which large‐scale ocean‐atmosphere coupling positively reinforces the atmospheric circulations that generate KE variability while mesoscale ocean‐atmosphere coupling allows the subsequent KE variability to reverse these atmospheric circulations.