Major Western Boundary Currents Research Articles

Lagrangian characterization of the southwestern Atlantic from a dense surface drifter deployment

The Southwestern Atlantic (SWA) is characterized by its large Eddy Kinetic Energy as the result of the confluence of two major western boundary currents, the northward flowing Malvinas Current (MC) and the southward flowing Brazil Current. The SWA study was addressed in the literature based on altimetry data, in situ measurements, regional models and ocean reanalysis. The present study constitutes the first effort to sample a portion of the SWA, with a dense drifter array (N = 62) deployment. The drifters, drogued at 15 m depths, were deployed across the MC and the Argentine Continental Shelf along two zonal transects located at 47°S and 47.25°S, between the 8th and the September 9, 2021. Drifters were set to deliver their position every 10 and 60 min, providing accurate Lagrangian trajectories that provide information on a large range of space and time scales of the surface currents. Three regions are clearly identified based on the analysis of the speed of the drifters, of their trajectories and of the spectral density of their velocities: the continental shelf, the slope and the open ocean. The comparison of the trajectories of the drifters with satellite altimetry images shows that, in general, drifters follow mesoscale features that are detectable in satellite altimetry maps. The analysis of the drifter trajectories also allowed us the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data. Comparison with cloud-free, high-resolution color images, shows that drifter trajectories organized by the mesoscale flow might also locally follow sub-mesoscale features. In frontal regions it was found that drifter velocities double satellite altimetry geostrophic velocities, which suggests that the dynamics at those regions is largely dominated by ageostrophic components. The ageostrophic Ekman component might explain the direction of the drifters when strong winds from a given direction prevail for several days and the drifters are not in a region with large sea surface height (SSH) gradients. The joint analysis of drifters’ trajectory and SSH clearly depicts that mesoscale features on the open ocean region control the cross-shelf exchanges between the MC and open ocean regions as well as the strength and width of the MC. Finally, the spatial density distribution of the drifters during the first hours after deployment and within a small eddy also allowed us to characterize the flow in terms of its divergence, vorticity and strain, indicating that the MC is geostrophic and has a jet-like behavior while the eddy is largely ageostrophic and has a dominant vorticity component over strain. We conclude observing that the analysis of a dense array of drifters provides valuable information of the flow that cannot be attained solely based on satellite data.

Satellite signature of the instantaneous wind response to mesoscale oceanic thermal structures

AbstractThe thermal air–sea interaction mechanism that modulates the atmospheric mixing due to sea‐surface temperature (SST) variability is studied with long‐term consistent satellite records. Statistical analyses of daily and instantaneous wind and SST data are performed over the major western boundary currents (WBCs). This wind–SST coupling, which is mediated by atmospheric mixing, is found to be very relevant on daily, and even shorter, time scales. Co‐located and simultaneous SST and surface wind fields (from Advanced Very High Resolution Radiometer and Advanced Scatterometer data) reveal that the atmosphere responds instantaneously to the presence of SST structures with a larger coupling coefficient with respect to daily and monthly time‐averaged fields. The coupling strength varies seasonally over WBCs in the Northern Hemisphere, with wintertime coupling being the lowest. Reanalysis data show that this behaviour is related to the seasonality of the air–sea temperature difference over the region of interest. Over the Northern Hemisphere WBCs, dry and cold continental air masses drive very unstable conditions, associated with very weak thermal air–sea coupling.

A Multi-Pixel Split-Window Approach to Sea Surface Temperature Retrieval from Thermal Imagers with Relatively High Radiometric Noise: Preliminary Studies

In the following decade(s), a set of satellite missions carrying thermal infrared (TIR) imagers with a relatively high noise equivalent differential temperature (NEdT) are expected, e.g., the high resolution TIR imagers flying on the future Thermal infraRed Imaging Satellite for High-resolution Natural resource Assessment (TRISHNA), Land Surface Temperature Monitoring (LSTM) and NASA-JPL/ASI Surface Biology and Geology Thermal (SBG) missions or the secondary payload on board the ESA Earth Explorer 10 Harmony. The instruments on board these missions are expected to be characterized by an NEdT of ⪆0.1 K. In order to reduce the impact of radiometric noise on the retrieved sea surface temperature (SST), this study investigates the possibility of applying a multi-pixel atmospheric correction based on the hypotheses that (i) the spatial variability scales of radiatively active atmospheric variables are, on average, larger than those of the SST and (ii) the effect of atmosphere is accounted for via the split window (SW) difference. Based on 32 Sentinel 3 SLSTR case studies selected in oceanic regions where SST features are mainly driven by meso to sub-mesoscale turbulence (e.g., corresponding to major western boundary currents), this study documents that the local spatial variability of the SW difference term on the scale of ≃3 × 3 km2 is comparable with the noise associated with the SW difference. Similarly, the power spectra of the SW term are shown to have, for small scales, the behavior of white noise spectra. On this basis, we suggest to average the SW term and to use it for the atmospheric correction procedure to reduce the impact of radiometric noise. In principle, this methodology can be applied on proper scales that can be dynamically defined for each pixel. The applicability of our findings to high-resolution TIR missions is discussed and an example of an application to ECOSTRESS data is reported.

Ocean Mesoscale and Frontal-Scale Ocean–Atmosphere Interactions and Influence on Large-Scale Climate: A Review

Abstract Two decades of high-resolution satellite observations and climate modeling studies have indicated strong ocean–atmosphere coupled feedback mediated by ocean mesoscale processes, including semipermanent and meandrous SST fronts, mesoscale eddies, and filaments. The air–sea exchanges in latent heat, sensible heat, momentum, and carbon dioxide associated with this so-called mesoscale air–sea interaction are robust near the major western boundary currents, Southern Ocean fronts, and equatorial and coastal upwelling zones, but they are also ubiquitous over the global oceans wherever ocean mesoscale processes are active. Current theories, informed by rapidly advancing observational and modeling capabilities, have established the importance of mesoscale and frontal-scale air–sea interaction processes for understanding large-scale ocean circulation, biogeochemistry, and weather and climate variability. However, numerous challenges remain to accurately diagnose, observe, and simulate mesoscale air–sea interaction to quantify its impacts on large-scale processes. This article provides a comprehensive review of key aspects pertinent to mesoscale air–sea interaction, synthesizes current understanding with remaining gaps and uncertainties, and provides recommendations on theoretical, observational, and modeling strategies for future air–sea interaction research. Significance Statement Recent high-resolution satellite observations and climate models have shown a significant impact of coupled ocean–atmosphere interactions mediated by small-scale (mesoscale) ocean processes, including ocean eddies and fronts, on Earth’s climate. Ocean mesoscale-induced spatial temperature and current variability modulate the air–sea exchanges in heat, momentum, and mass (e.g., gases such as water vapor and carbon dioxide), altering coupled boundary layer processes. Studies suggest that skillful simulations and predictions of ocean circulation, biogeochemistry, and weather events and climate variability depend on accurate representation of the eddy-mediated air–sea interaction. However, numerous challenges remain in accurately diagnosing, observing, and simulating mesoscale air–sea interaction to quantify its large-scale impacts. This article synthesizes the latest understanding of mesoscale air–sea interaction, identifies remaining gaps and uncertainties, and provides recommendations on strategies for future ocean–weather–climate research.

Hidden Upwelling Systems Associated With Major Western Boundary Currents

AbstractWestern boundary currents (WBCs) play an essential role in regulating global climate. In contrast to their widely examined horizontal motions, less attention has been paid to vertical motions associated with WBCs. Here, we examine vertical motions associated with the major WBCs by analyzing vertical velocity from five ocean synthesis products and one eddy‐resolving ocean simulation. These data reveal robust and intense subsurface upwelling systems, which are primarily along isopycnal surfaces, in five major subtropical WBC systems. These upwelling systems are part of basin‐scale overturning circulations and are likely driven by meridional pressure gradients along the western boundary. Globally, the WBC upwelling contributes significantly to the vertical transport of water mass and ocean properties and is an essential yet overlooked branch of the global ocean circulation. In addition, the WBC upwelling intersects the oceanic euphotic and mixed layers, and thus likely plays an important role in ocean biological and chemical processes by transporting nutrients, carbon and other tracers vertically inside the ocean. This study calls for more research into the dynamics of the WBC upwelling and their role in the ocean and climate systems.

Gulf Stream and Kuroshio synchronization.

The mid-latitude jet stream ties together the two major western boundary currents.

Pliocene to Earliest Pleistocene (5–2.5 Ma) Reconstruction of the Kuroshio Current Extension Reveals a Dynamic Current

AbstractThe Kuroshio Current Extension (KCE) is the major western boundary current of the North Pacific Subtropical Gyre. To better understand how the KCE behaved under elevated CO2 conditions and how it came into its modern configuration, we use stable isotopic analyses from mixed‐layer planktic foraminifera from three Ocean Drilling Program sites that lie to the north, underneath of, and south of the present day KCE. Our data support previous hypotheses suggesting that the KCE played a significant role in delivering moisture to the high latitudes and contributed to extensive ice growth. Samples span from 5 to 2.5 Ma and cover three tectonic and climate events. In response to constriction of the Central American Seaway, the KCE region warmed then cooled, with a potential equatorward shift of the current and intensification of the North Pacific Subtropical Gyre. During the mid‐Piacenzian Warm Period, we define the mid‐Piacenzian 1 and 2 events, which were times when the KCE warmed and subsequently developed a strong temperature/salinity gradient between the two northernmost sites. Northern Hemisphere ice growth at ∼2.9 Ma brought the KCE into its modern‐day configuration. Paradoxically, the southernmost Hole 1209A appears coolest through the study interval. This paradox is resolved by invoking seasonal preferences of the species analyzed, a finding that highlights the need to consider seasonal and ecological preferences of plankton when interpreting geochemical records in midlatitude regions. We propose several hypotheses for drivers of KCE behavior that require further testing using higher‐resolution proxy data tied to additional regional and global records.

Brazil Current Volume Transport Variability During 2009–2015 From a Long‐Term Moored Array at 34.5°S

AbstractThe Brazil Current, the western limb of the subtropical gyre of the South Atlantic Ocean, is one of the major Western Boundary Currents of the global ocean. Here, we present the first multiyear continuous daily time series of Brazil Current absolute volume transport obtained using 6+ years of observations from a line of four pressure‐recording inverted echo sounders (PIES) deployed at 34.5°S. The array was augmented in December 2012 with two current meter‐equipped PIES and in December 2013 with a moored Acoustic Doppler Current Profiler on the upper continental slope. The Brazil Current is bounded by the sea surface and the neutral density interface separating South Atlantic Central Water and Antarctic Intermediate Water, which is on average at a reference pressure of 628 ± 46 dbar, and it is confined west of 49.5°W. The Brazil Current has a mean strength of −14.0 ± 2.8 Sv (1 Sv ≡ 106 m3 s−1; negative indicates southward flow) with a temporal standard deviation of 8.8 Sv and peak‐to‐peak range from −41.7 to +20 Sv. About 80% of the absolute transport variance is concentrated at periods shorter than 150 days with a prominent peak at 100 days. The baroclinic component accounts for 85% of the absolute transport variance, but the barotropic variance is not negligible. The baroclinic and barotropic transports are uncorrelated, demonstrating the need to measure both transport components independently. Given the energetic high frequency transport variations, statistically significant seasonal to interannual variability and trends have yet to be detected.

Deep Learning of Sea Surface Temperature Patterns to Identify Ocean Extremes

We performed an out-of-distribution (OOD) analysis of ∼12,000,000 semi-independent 128 × 128 pixel2 sea surface temperature (SST) regions, which we define as cutouts, from all nighttime granules in the MODIS R2019 Level-2 public dataset to discover the most complex or extreme phenomena at the ocean’s surface. Our algorithm (ULMO) is a probabilistic autoencoder (PAE), which combines two deep learning modules: (1) an autoencoder, trained on ∼150,000 random cutouts from 2010, to represent any input cutout with a 512-dimensional latent vector akin to a (non-linear) Empirical Orthogonal Function (EOF) analysis; and (2) a normalizing flow, which maps the autoencoder’s latent space distribution onto an isotropic Gaussian manifold. From the latter, we calculated a log-likelihood (LL) value for each cutout and defined outlier cutouts to be those in the lowest 0.1% of the distribution. These exhibit large gradients and patterns characteristic of a highly dynamic ocean surface, and many are located within larger complexes whose unique dynamics warrant future analysis. Without guidance, ULMO consistently locates the outliers where the major western boundary currents separate from the continental margin. Prompted by these results, we began the process of exploring the fundamental patterns learned by ULMO thereby identifying several compelling examples. Future work may find that algorithms such as ULMO hold significant potential/promise to learn and derive other, not-yet-identified behaviors in the ocean from the many archives of satellite-derived SST fields. We see no impediment to applying them to other large remote-sensing datasets for ocean science (e.g., SSH and ocean color).

Carbon pools and fluxes in the China Seas and adjacent oceans

The China Seas include the South China Sea, East China Sea, Yellow Sea, and Bohai Sea. Located off the Northwestern Pacific margin, covering 4700000 km2 from tropical to northern temperate zones, and including a variety of continental margins/basins and depths, the China Seas provide typical cases for carbon budget studies. The South China Sea being a deep basin and part of the Western Pacific Warm Pool is characterized by oceanic features; the East China Sea with a wide continental shelf, enormous terrestrial discharges and open margins to the West Pacific, is featured by strong cross-shelf materials transport; the Yellow Sea is featured by the confluence of cold and warm waters; and the Bohai Sea is a shallow semi-closed gulf with strong impacts of human activities. Three large rivers, the Yangtze River, Yellow River, and Pearl River, flow into the East China Sea, the Bohai Sea, and the South China Sea, respectively. The Kuroshio Current at the outer margin of the Chinese continental shelf is one of the two major western boundary currents of the world oceans and its strength and position directly affect the regional climate of China. These characteristics make the China Seas a typical case of marginal seas to study carbon storage and fluxes. This paper systematically analyzes the literature data on the carbon pools and fluxes of the Bohai Sea, Yellow Sea, East China Sea, and South China Sea, including different interfaces (land-sea, sea-air, sediment-water, and marginal sea-open ocean) and different ecosystems (mangroves, wetland, seagrass beds, macroalgae mariculture, coral reefs, euphotic zones, and water column). Among the four seas, the Bohai Sea and South China Sea are acting as CO2 sources, releasing about 0.22 and 13.86– 33.60 Tg C yr−1 into the atmosphere, respectively, whereas the Yellow Sea and East China Sea are acting as carbon sinks, absorbing about 1.15 and 6.92 –23.30 Tg C yr−1 of atmospheric CO2, respectively. Overall, if only the CO2 exchange at the sea-air interface is considered, the Chinese marginal seas appear to be a source of atmospheric CO2, with a net release of 6.01 –9.33 Tg C yr−1 , mainly from the inputs of rivers and adjacent oceans. The riverine dissolved inorganic carbon(DIC) input into the Bohai Sea and Yellow Sea, East China Sea, and South China Sea are 5.04, 14.60, and 40.14 Tg C yr−1 , respectively. The DIC input from adjacent oceans is as high as 144.81 Tg C yr−1 , significantly exceeding the carbon released from the seas to the atmosphere. In terms of output, the depositional fluxes of organic carbon in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 2.00, 3.60, 7.40, and 5.92 Tg C yr−1 , respectively. The fluxes of organic carbon from the East China Sea and South China Sea to the adjacent oceans are 15.25–36.70 and 43.93 Tg C yr−1 , respectively. The annual carbon storage of mangroves, wetlands, and seagrass in Chinese coastal waters is 0.36 –1.75 Tg C yr−1 , with a dissolved organic carbon (DOC) output from seagrass beds of up to 0.59 Tg C yr−1 . Removable organic carbon flux by Chinese macroalgae mariculture account for 0.68 Tg C yr−1 and the associated POC depositional and DOC releasing fluxes are 0.14 and 0.82 Tg C yr−1 , respectively. Thus, in total, the annual output of organic carbon, which is mainly DOC, in the China Seas is 81.72 –104.56 Tg C yr−1 . The DOC efflux from the East China Sea to the adjacent oceans is 15.00 –35.00 Tg C yr−1 . The DOC efflux from the South China Sea is 31.39 Tg C yr−1 . Although the marginal China Seas seem to be a source of atmospheric CO2 based on the CO2 flux at the sea-air interface, the combined effects of the riverine input in the area, oceanic input, depositional export, and microbial carbon pump (DOC conversion and output) indicate that the China Seas represent an important carbon storage area.

Large Variability of Dissolved Inorganic Radiocarbon in the Kuroshio Extension of the Northwest North Pacific

ABSTRACTRadiocarbon (14C) in dissolved inorganic carbon (DIC) was measured for water samples collected from six deep stations in the Kuroshio Extension (KE) region in the northwestern North Pacific in April–May 2015. Vertical profiles of Δ14C-DIC indicate that bomb-produced 14C was present from the surface to ~1500 m water depth. Large variations in Δ14C-DIC values (300‰) were observed at 500 m water depth among the stations and the differences were likely controlled by transport and mixing dynamics of different water masses in the region. The major Pacific western boundary currents, such as Kuroshio and Oyashio and regional mesoscale eddies, could play important roles affecting the observed Δ14C-DIC variability. The depth profiles of both Δ14C-DIC and DIC concentrations can be predicted by the solution mixing model and can be used as conservative tracers of water mass movement and water parcel homogenization in the ocean.

Late Quaternary water temperature variations of the Northwest Pacific based on the lipid paleothermometers TEXH86, UK´37 and LDI

The Kuroshio Current (KC) and Oyashio Current (OC) are the two major western boundary currents of the Pacific Ocean and their interplay exerts a major control on the climate evolution of the northwestern Pacific region as well as East Asia. Although millennial scale variations in the strength and flow pattern of the KC are well documented, only little is known on the long-term evolution of this ocean current and its role in affecting regional and global climate over geological time scales. Here, we present surface and thermocline temperature records covering the last two glacial-interglacial cycles of IODP (Integrated Ocean Drilling Program) Site C0011, SE of Japan, using the lipid paleothermometers TEXH86 (tetraether index of tetraethers consisting of 86 carbon atoms), UK´37 (unsaturated ketone index) and LDI (long-chain diol index). Lower average water temperatures (20.1–20.7°C in TEXH86, 21.6–22.0°C in UK´37, and 20.7–21.9°C in LDI) during marine isotope stages (MIS) 2 and 6 are considered to indicate a reduction in warm water mass export from the Western Pacific Warm Pool (WPWP) to northern mid-latitudes via the KC due to decreased subtropical gyre circulation in the North Pacific. A synchronous southward displacement of the KC/OC interfrontal zone resulted in an overall stronger influence of colder and more polar waters at Site C0011. MIS 1, 3 and 5 are characterized by generally higher water temperatures (21.7–22.1°C in TEXH86, 23.2–24.3°C in UK´37, and 23.1–24.3°C in LDI), likely reflecting an increased northward transport of subtropical waters to the study site. Higher Holocene than Eemian water temperatures are attributed to a stronger KC and the formation of its short meander south of Japan, whereas a less strong KC during the Eemian likely favored the formation of the large meander path. Better correlations between the different lipid paleothermometers during cold MIS are considered to indicate more similar production seasons and habitat depths of the biological proxy sources with the latter factor likely driven by thermocline shoaling.

Pioneering developments of marine renewable energy in Australia

The history of ocean renewable energy developments in Australia is reviewed. A layperson’s description of the physical operating principle is given for the main classes of technology that have been tested in Australian waters. The Australian marine domain possesses among the world’s most energetic wave-energy resources, driven by powerful mid-latitude westerly winds. The northern coast of Western Australia has tidal ranges significant on a global scale, and some geographical features around the continent have local tidal resonances. The East Australian Current, one of the world’s major western boundary currents, runs along the eastern Australian seaboard, offering potential for ocean-current energy. Sea-water temperatures in the tropical north-east of Australia may permit ocean thermal energy conversion. While this abundance of resources makes Australia an ideal location for technology development, the population is highly concentrated in a few large cities, and transmission infrastructure has developed over a century to supply cities from traditional power plants. Several wave-power developments have resulted in demonstration of deployments in Australian waters, three of which have been grid connected. Trials of tidal devices have also occurred, while other classes of ocean renewable energy have not yet been trialled. The prospects for marine renewable energy in Australia are discussed including non-traditional applications such as coastal protection and energy export.

A new mechanism for ocean–atmosphere coupling in midlatitudes

AbstractThe role of moist convection in ‘transferring’ upward surface ocean conditions throughout the troposphere is studied using reanalysis data for the extratropical Northern and Southern Hemispheres in winter. It is found that conditions for the development of a convective air column from the sea surface to the tropopause are met frequently over all major western boundary currents and their extension in the oceanic interior (sometimes for as much as 50% of the time). These frequent occurrences are shown to be jointly controlled by oceanic advection of warm waters and, on the atmospheric side, the downward displacement of the tropopause associated with synoptic weather systems.Based on these results, it is proposed that the oceans can influence the atmosphere directly through convection in midlatitudes, as is commonly thought to occur in the Tropics. Analysis of the Richardson numberRifound at low levels suggests that moist symmetric instability (0< Ri≤ 1) is a key process involved in linking surface ocean temperatures to atmospheric lapse rates, in addition to standard upright convection. These lowRiprocesses are not currently parametrized in climate models, which raises the possibility that the extratropical oceanic influence on climate might be underestimated in the current generation of models. Copyright © 2011 Royal Meteorological Society

Coastal and shelf circulation in the vicinity of Camamu Bay (14°S), Eastern Brazilian Shelf

The Camamu Bay (CMB) is located on the narrowest shelf along the South American coastline and close to the formation of two major Western Boundary Currents (WBC), the Brazil/North Brazil Current (BC/NBC). These WBC flow close to the shelf break/slope region and are expected to interact with the shelf currents due to the narrowness of the shelf. The shelf circulation is investigated in terms of current variability based on an original data set covering the 2002–2003 austral summer and the 2003 austral autumn. The Results show that the currents at the shelf are mainly wind driven, experiencing a complete reversal between seasons due to a similar change in the wind field. Currents at the inner-shelf have a polarized nature, with the alongshore velocity mostly driven by forcings at the sub-inertial frequency band and the cross-shore velocity mainly supra-inertially forced, with the tidal currents playing an important role at this direction. The contribution of the forcing mechanisms at the mid-shelf changes between seasons. During the summer, forcings in the two frequency bands are important to drive the currents with a similar contribution of the tidal currents. On the other hand, during the autumn season, the alongshore velocity is mostly driven by sub-inertial forcings and tidally driven currents still remain important in both directions. Moreover, during the autumn when the stratification is weaker, the response of the shelf currents to the wind forcing presents a barotropic signature. The meso-scale processes related to the WBC flowing at the shelf/slope region also affect the circulation within the shelf, which contribute to cause significant current reversals during the autumn season. Currents at the shelf-estuary connection are clearly supra-inertially forced with the tidal currents playing a key role in the generation of the along-channel velocities. The sub-inertial forcings at this location act mainly to drive the weak ebb currents which were highly correlated with both local and remote wind forcing during the summer season.

Seasonal-to-decadal modes of global sea level variability derived from merged altimeter data

Motivated by the first availability of more than 16 years (1992–2009) of merged altimeter data from up to four concurrent satellite missions with TOPEX/Poseidon, ERS-1 and 2, ENVISAT, Jason-1, and Geosat Follow-On, the global sea level is reexamined with special emphasis on its distinctive scales of variability. The global sea level variability is dominated by eleven statistically significant and geographically robust principal modes at seasonal-to-decadal timescales. The identified modes can be divided into four regimes, i.e., the seasonal regime with two principal components at 3 and 6 months, the annual regime with a single 12-month component of global dominance, the interannual regime with seven principal components at 1.55, 1.74, 1.94, 2.34, 3.07, 4.20, and 5.40 years, as well as the decadal regime with a 9.28-year subdecadal component. The frequency dispersion of the identified modes is found to be at ∼ 0% level for the annual regime, at ∼ 5% level for the seasonal and interannual regimes, and at ∼ 10% level for the decadal regime. In the space domain, the locations of principal modes are found to be geographically correlated. The seasonal regime is characterized by a small number of localized amplitude highs in regional seas. The annual regime is associated with major western boundary currents of the Gulf Stream and Kuroshio. For the interannual regime, mode active zones are observed in the western Pacific warm pool (WPWP), El Niño, and western Indian Ocean regions, among which the WPWP has a well-defined two-core structure, and the northern core coincides with the most energetic zone of the decadal regime. Another interesting finding is that east–west (zonal) and equatorial–extratropical (meridional) teleconnections are evident for the major signals in a rather symmetric and systematic pattern. Joint spatiotemporal analysis reveals that multi-modality at a given location and multi-regionality for a given mode are fundamental features in global sea level variability.

Investigating the Global Impacts of the Agulhas Current

The Agulhas Current is the major western boundary current of the Southern Hemisphere [Lutjeharms, 2006] and a key component of the global ocean “conveyor” circulation controlling the return flow to the Atlantic Ocean [Gordon, 1986]. As such, it is increasingly recognized as a key player in ocean thermohaline circulation, with importance for the meridional overturning circulation (MOC) of the Atlantic Ocean.Unusual dynamics pervade the motion of this warm‐water current—as it moves west around the southern tip of Africa, it is retroflected back east by the Antarctic Circumpolar Current. Not all waters are captured by this sudden diversion of course—parts of the Agulhas Current leak away into the South Atlantic Ocean (Figure 1).

Spatial and temporal variability of surface water in the Kuroshio source region, Pacific Ocean, over the past 21,000 years: evidence from planktonic foraminifera

The Kuroshio Current is the major western boundary current of the North Pacific Ocean and has had a large impact on surface water character and climate change in the northwestern Pacific region. The Kuroshio Current becomes a distinctive surface flow in the Ryukyu Arc region after diverging from the North Equatorial Current and passing through the Okinawa Trough. Therefore, the Ryukyu Arc area can be called the Kuroshio source region. We reconstructed post-21-ka time–space changes in surface water masses in the Ryukyu Arc region using 15 piston cores which were dated by planktonic δ 18O stratigraphy and AMS 14C ages. Our analysis utilized spatial and temporal changes in planktonic foraminiferal assemblages which were classified into the Kuroshio, Subtropical, Coastal, and Cold water groups on the basis of modern faunal distributions in the study region. These results indicate that the Kuroshio Current and adjacent surface water masses experienced major changes during: (1) the Last Glacial Maximum (LGM), and (2) the so-called Pulleniatina minimum event (PME) from ∼4,500 to 3,000 yr BP. The Kuroshio LGM event corresponds to severe global cooling and is marked by decreases in planktonic δ 18O values and estimated sea-surface temperature (SST) with the dominance of the Cold water group of planktonic foraminifera. Cooling within the Kuroshio source region was enhanced during the LGM event because the Kuroshio Current was forced eastward due to the formation of a land bridge between Taiwan and the southern Ryukyu Arc which prohibited its flow into the Okinawa Trough. Except for the severe reduction and disappearance of the Pulleniatina group, no clear cooling signal was identified during the PME based on δ 18O values, estimated SST values and variations in the composition of planktonic foraminiferal faunas. The PME assemblages are marked by high abundances of Neogloboquadrina dutertrei, a distinctive Kuroshio type species, along with other species assigned to the Coastal and Central water groups. Subtle ecological differences exist between Pulleniatina obliquiloculata and N. dutertrei; i.e. P. obliquiloculata exhibits lower rates of reproduction under conditions of lower primary productivity in the central Equatorial Pacific Ocean. El Niño-like conditions in the Equatorial Pacific Ocean result in lower rates of surface transport in the Kuroshio Current. In turn, this response triggers lower rates of primary productivity in central equatorial surface waters as well as in the upstream Kuroshio source region, ultimately resulting in a lower abundance of P. obliquiloculata. Thus, we interpret the PME as a possible proxy signal of El Niño-like conditions and enhancement of the El Niño Southern Oscillation climate system after the PME in the tropical and sub-tropical Pacific Ocean.

Dynamics of larval fish assemblages over a shallow coral reef in the Florida Keys

Few time series collections have been made of the larval ichthyofauna in waters directly above shallow coral reefs. As a result, relatively little is known regarding the composition and temporal dynamics of larval fish assemblages in shallow-reef waters, particularly those near a major western boundary current. We conducted a series of nightly net tows from a small boat over a shallow reef (Pickles Reef) along the upper Florida Keys during four new moon and three third-quarter moon periods in July (two new moons), August, and September 2000. Replicate tows were made after sunset at 0–1 m and at 4–5 m depth to measure the nightly progression in community composition, differences in depth of occurrence, and abundance and diversity with lunar phase. A total of 66 families was collected over the 3-month period, with a mean (±SE) nightly density of 23.7±2.1 larvae per 100 m3 and diversity of 24.2±0.9 taxa per tow. A total of 28.8% of the catch was composed of small, schooling fishes in the families Atherinidae, Clupeidae, and Engraulidae. Of the remaining catch, the top ten most abundant families included reef fishes as well as mangrove and oceanic taxa (in descending order): Scaridae, Blennioidei (suborder), Gobiidae, Paralichthyidae, Lutjanidae, Haemulidae, Labridae, Gerreidae (mangrove), Balistidae, and Scombridae (oceanic). These near-reef larval fish assemblages differed substantially from those collected during previous offshore collections. Taxa such as the Haemulidae were collected at a range of sizes and may remain nearshore throughout their larval period. Overall, the abundance and diversity of taxa did not differ with depth (although within-night vertical migration was evident) or with lunar phase. Temporal patterns of abundance of larval fish families clustered into distinct groups that in several cases paralleled family life-history patterns. In late July, a sharp shift in larval assemblages signaled the replacement of oceanic water with inner shelf/bay water. In general, the suite and relative abundance of taxa collected each night differed from those collected on other nights, and assemblages reflected distinct nightly events as opposed to constant or cyclical patterns. Proximity to the Florida Current likely contributes to the dynamic nature of these near-reef larval assemblages. Our results emphasize the uniqueness of near-reef larval fish assemblages and point to the need for further examination of the biophysical relationships generating event-related temporal patterns in these assemblages.

Underestimation of Latent and Sensible Heat Fluxes above the Agulhas Current in NCEP and ECMWF Analyses

The Agulhas Current is the major western boundary current of the Southern Hemisphere. South of Africa it retroflects back into the southwest Indian Ocean, transporting relatively warm water into the midlatitudes. Large sensible and latent heat transfers from the Agulhas Current and its retroflection to the atmosphere occur throughout the year, but particularly during winter. This study suggests that the NCEP and ECMWF models tend to underestimate these fluxes because they are unable to adequately represent the air‐sea fluxes over the warmest waters in the core of the current. This core is only 80‐100 km wide and it is suggested that the SST data used by these models do not have fine enough spatial resolution to properly represent the Agulhas Current and its mesoscale variability.