Inshore Side Research Articles

Surface velocity time series derived from satellite altimetry data in a section across the Kuroshio southwest of Kyushu

A time series of surface geostrophic velocity is developed using satellite altimetry data during 1992–2010 for a track across the Kuroshio southeast of Kyushu, Japan. The temporal mean geostrophic velocity is estimated by combining the along-track sea level anomaly and shipboard ADCP data. This approximately 6-km resolution dataset is successful in representing the Kuroshio cross-current structure and temporal variation of the Kuroshio current-axis position during 2000–2010. The authors use this dataset to examine the winter Kuroshio path destabilization phenomenon. Its seasonal features are characterized as follows: the velocity shear on the inshore side of the Kuroshio becomes stronger and the Kuroshio path state becomes unstable from the summer to winter. This evidence is consistent with the hypothetical mechanism governing the destabilization phenomenon discussed in a previous study. Furthermore, the interannual amplitude modulation of the seasonality is examined in relation to interannual variations in the winter northerly wind over the northern Okinawa Trough and the Pacific Decadal Oscillation (PDO) index. The destabilization phenomenon appears 15 times in the period 2000–2010. Ten cases are related to local wind effects, and 7 of these are also connected with the PDO index. This is probably because the winter northerly wind over the northern Okinawa Trough is regulated by the PDO signal in interannual time-scales. Only 4 cases are related to the PDO index, but their driving mechanism remains uncertain.

The California Current System in relation to the Northeast Pacific Ocean circulation

The California Current System is described in its regional setting using two modern datasets. Argo provides a broadscale view of the entire eastern North Pacific Ocean for the period 2004–2010, and the High Resolution XBT Network includes transects from Honolulu to San Francisco (1991–2010) and to Los Angeles (2008–2010). Together these datasets describe a California Current of 500–800 km width extending along the coast from 43°N to 23°N. The mean southward transport of the California Current is about 5 Sv off Central and Southern California, with about 2.5 Sv of northward flow on its inshore side. Interannual variations are 50% or more of the mean transports. The salinity minimum in the core of the California Current is supplied by the North Pacific Current and by freshwater from the northern continental shelf and modified by alongshore geostrophic and across-shore Ekman advection as well as eddy fluxes and air–sea exchange. The heat and freshwater content of the California Current vary in response to the fluctuating strength of the alongshore geostrophic flow. On its offshore side, the California Current is influenced by North Pacific Intermediate Waters at its deepest levels and by Eastern Subtropical Mode Waters on shallower density surfaces. In total, the sources of the California Current, its alongshore advection, and its strong interactions with the inshore upwelling region and the offshore gyre interior combine to make this a rich and diverse ecosystem. The present work reviews previous contributions to the regional oceanography, and uses the new datasets to paint a spatially and temporally more comprehensive description than was possible previously.

Seasonality of the Kuroshio Path Destabilization Phenomenon in the Okinawa Trough: A Numerical Study of Its Mechanism

Abstract Previous observations have indicated that the Kuroshio’s path in the northern Okinawa Trough of the East China Sea is destabilized and accompanied by meanders with periods of 1–3 months during the winter–spring period. The present study investigates the mechanism responsible for this recurrent seasonally fixed phenomenon. A hypothetical mechanism is constructed based on both a simple wind-driven Ekman-pumping model, acting within the Kuroshio, and a bifurcation model of the Kuroshio path states in the northern Okinawa Trough, established in the previous study. A high-resolution ocean general circulation model is used to examine the hypothetical mechanism. The numerical model reveals the following mechanism: the wintertime northerly wind prevailing over the Okinawa Trough blows against the Kuroshio, generating Ekman divergence, and hence upwelling within the inshore side of the Kuroshio from the sum of the earth’s rotation and the geostrophic current shear. A necessary condition for this upwelling is probably given by the exponential velocity structure of the surface Kuroshio on the inshore side of the current. This kind of wintertime upwelling acts to make the mean Kuroshio path separate from the continental slope in the northern Okinawa Trough, so that baroclinic instability destabilizes the Kuroshio path, as shown by the bifurcation model of Kuroshio path states.

Reproduction dynamics of eelgrass (Zostera marina L.) populations in Ise–Mikawa Bay, central Honshu, Japan

AbstractSuitable methods for the conservation and reconstruction of eelgrass populations are needed because of decreasing eelgrass populations. For this, it is important to understand the reproduction mechanism of eelgrass populations at a molecular level as well as at a macro level. In the present study, we analyzed the dynamic changes in eelgrass populations in Ise–Mikawa Bay by monitoring their distribution from 1993 to 2005 and by genetic analysis using microsatellite polymorphism. The distribution of eelgrass populations in the eastern part of Ise Bay showed a periodic shift with a cycle of 4 years, that is, eelgrass populations appeared on the bar trough, gradually moved to the inshore side of the habitat every year, and disappeared in the fourth year. Microsatellite analysis revealed that the eelgrass populations in Ise–Mikawa Bay have high genetic diversity. The eelgrass populations in Ise Bay and those in Mikawa Bay showed genetic differentiation and genetic connectivity. The importance of upstream parent populations, seed transportation among local populations and genetic diversity were discussed for the sustainability of eelgrass populations.

Impact of shelfbreak fronts on long-range underwater sound propagation in the continental shelf area.

The dynamic shelfbreak front commonly observed in the continental shelf area has significant impact on underwater sound propagation. Analytical study with an idealized front model has shown that combining inshore refraction from the front and offshore refraction on the sloping shelf, long‐range propagating sound on the continental shelf will be trapped by the front and form “whispering gallery” modes on the inshore side of the front. In this paper, numerical propagation simulations with three‐dimensional normal mode and parabolic approximation methods are implemented to investigate the acoustic impact of a nearly realistic front model. This front model results from the MIT Multidisciplinary Simulation, Estimation, and Assimilation System which assimilated the field measurements collected during the New Jersey Shelf Shallow Water 2006 experiment. Since the front model captures very well the temporal and spatial variability of the shelfbreak frontal system in the field, the numerical propagation simulations presented here are fairly realistic. Horizontal sound refraction due to the shelfbreak front and (sub)‐mesoscale eddies are observed, and causes part of long‐range propagating sound energy to be trapped on the shelf area.

Modeling upwelling circulation off the Oregon coast

Time‐dependent, three‐dimensional, upwelling circulation on the continental shelf off the Oregon coast is studied using a primitive equation numerical model. A limited area domain with a high‐resolution curvilinear grid is utilized. The response of the coastal ocean to forcing by observed wind stress and heat flux during the summer 2001 time period of the Coastal Ocean Advances in Shelf Transport (COAST) field experiment is investigated. Model results are compared to COAST current and hydrographic measurements. The shelf velocity and density fields are generally characterized by the presence of a southward alongshore coastal jet with an upwelling density front on the shoreward side of the jet. The large variability in shelf topography associated with Heceta Bank exerts a major influence on the shelf velocity and density fields. Over the bank the alongshore coastal jet is displaced offshore, and colder upwelled water extends farther from the coast. Northward mean flow and upward motion are found inshore of the jet. Three‐dimensional flow structures in response to variable shelf bottom topography are presented, and an analysis of time‐ and space‐dependent alongshore momentum balances is applied to clarify the associated dynamics. In general, northward pressure gradients, set up over the bank during southward upwelling winds, accelerate currents on the inshore side of the jet northward when the winds relax. Analysis of term balances in the depth‐averaged equation for potential temperature shows that during upwelling, across‐shore advection makes the major contribution to cooling over most of the region, except inshore over Heceta Bank, where alongshore advection also plays a significant role.

Time‐dependent, wind‐driven flow over a shallow midshelf submarine bank

During summer 2001, high‐resolution hydrographic, velocity, and bio‐optical surveys were conducted over Heceta Bank off central Oregon. North of the bank, upwelling over simple bottom topography exhibited a classic response with a midshelf, baroclinic coastal jet and upwelled isopycnals. The coastal upwelling jet follows the bank topography as it widens offshore before reaching the southern end of the bank, where the shelf break turns almost 90° back toward the coast. The ensuing adjustment involves the offshelf transport of coastal water and the material it contains. The jet meanders anticyclonically in the “lee” of the bank before resuming its equatorward path seaward of the shelf break. A pool of high‐chlorophyll (>15 mg m−3) near‐surface water is found in the low‐velocity “lee” region created by the bank. A low‐temperature, high‐salinity bottom water pool, supplied primarily from the south, lies over the inshore region of the bank. This bottom pool is up to 40 m thick and contains elevated levels of suspended material. By creating an east‐west perturbation in the coastal upwelling front, the flow‐topography interaction introduces an alongshore pressure gradient. When southward upwelling‐favorable winds relax, this alongshore pressure gradient can drive flow back to the north. This leads to northward flow on the inshore side of the bank, ultimately setting up the potential for recirculation around the entire bank complex on a timescale of about 10 days. Strong northward winds during summer lead to downwelling within 15 km of the coast, accompanied by significant (>0.2 m s−1) wind‐driven northward currents.

Eddies and dipoles around South Madagascar: formation, pathways and large-scale impact

During a hydrographic cruise in March 2001, we encountered a dipole-like vortex structure directly southwest of Madagascar. The cruise formed part of the Dutch-South African Agulhas Current Sources Experiment (ACSEX). Direct current observations with a lowered ADCP showed that the multipole involved a deep-reaching central jet, with over 20 cm/s speeds still at 2000 m depth, with two contra-rotating eddies on either side. Both eddies had a size of about 250 km. A subsequent analysis of the combined TOPEX-POSEIDON/ERS altimeter dataset revealed the regular formation of dipoles around south Madagascar, where the East Madagascar Current (EMC) separates from the shelf. Between April 1995 and June 2000, 16 such features could be identified, six of which were formed between August 1996 and May 1997 and six between May 1999 and June 2000. These periods of enhanced dipole formation coincided with the negative phases of the Indian Ocean Dipole and El Niño cycles, suggesting a connection between these climate modes and the interannual variability in the periods of dipole train formation. The temperature–salinity characteristics of the eddies from the ACSEX dipole were identical to those measured in the EMC during the same cruise. The cyclonic eddy appeared to have drawn its waters from the inshore side of the EMC, suggesting it was formed as a lee-eddy on the cyclonic flank of the separating jet. The data indicate that the eddies carry their water along. At the observed scales, one dipole per year would feed 8 Sv (1 Sv=10 6 m 3/s) of EMC-derived water into the Agulhas retroflection region. In the Mozambique Basin, the eddy pairs behaved quite irregularly, with many of them splitting and interacting with other eddies in the region. The anticyclones then propagated mostly westward, while the cyclone tracks diverged more between west and southwest and were influenced by the interaction with the Mozambique Plateau. A very regular train of dipoles started in December 1999 and continued through 2000. They were spawned roughly at 2-month intervals, stayed remarkably coherent and triggered an unusually early retroflection of the Agulhas Current in late 2000. Thus, interannual variability originating in the equatorial climate modes of the Indian Ocean seems to result in variability of the Agulhas retroflection and associated ring shedding.

Distribution of chlorophyll a and Gymnodinium catenatum associated with coastal upwelling plumes off central Portugal

The composition and the distribution patterns of phytoplankton were studied during the early stages of an upwelling event in Lisbon Bay, in September 1999. The distribution of chlorophyll a and phytoplankton assemblages was asymmetric about the upwelling centre, with the highest abundance of chlorophyll a observed at the inner (coastal) side of the plume. The diatom Cylindrotheca closterium dominated in the upwelling core and the chain forming diatom Proboscia alata dominated at the outer side of the feature within mature oceanic waters. Chain forming dinoflagellates such as Alexandrium affine and the toxin producer Gymnodinium catenatum were most numerous at the inshore side of the upwelling front. These patterns were compared with the distribution of G. catenatum observed in the same area in 1985 and 1994, under conditions of fully developed upwelling when chlorophyll a and G. catenatum maxima were observed extending southwards on the inshore side of an upwelling plume. Different hydrodynamic conditions at each side of upwelling plumes associated with pronounced Capes are evoked to explain the asymmetry on the phytoplankton patterns.

A theoretical study of topographic effects on coastal upwelling and cross-shore exchange

The effects of topographic variations on coastal upwelling and cross-shore exchange are examined with a theoretical, continuously stratified, three-dimensional coastal ocean model. The model takes into account topographic variations in both alongshore and cross-shore directions and allows analytical solutions with an Ekman surface layer that faithfully represents the physical nature of the coastal upwelling system. Theoretical solutions with any analytical form of alongshore-varying topography can be solved based on the perturbation method of Killworth [J. Phys. Oceanogr. 8 (1978) 188]. Analyses of the model solutions lead to the following conclusions: (1) The variation of upwelling fronts and currents is shown to be caused by the combined effect of topography and stratification. Topographic variation causes uneven upwelling distribution and leads to density variation, which results in a varying horizontal pressure gradient field that causes the meandering currents. The variation index is dependent upon a bilinear function of their physical parameters––the ratio of the topographic variation depth to the total depth and Burger’s number of stratification. (2) Cross-shore slope is found to play a role in maintaining the meandering structure of the alongshore currents. The anticyclonic circulations can further induce downwelling on the offshore side of the current, while the cyclonic circulations enhance upwelling and form upwelling centers on the inshore side of the current. (3) Alongshore topography does not change the total upwelled water, i.e., the total Ekman pumping is conserved. However, it increases cross-exchange of water masses by transporting inshore (offshore) water near topographic features far offshore (inshore) from the mean position of the front. The applicability and limitations of the theory are also discussed.

Structure of the North Atlantic current in stream-coordinates and the circulation in the Newfoundland basin

Accurate knowledge of the absolute North Atlantic Current (NAC) transport has been limited in the past by the difficulty of obtaining absolute velocity measurements. For this study, historical hydrography was combined with time series of round trip travel time from inverted echo sounders, bottom pressure sensor measurements, and measured velocities from deep current meters, via a “Gravest Empirical Mode” technique, to yield time series of temperature and absolute velocity sections. These sections were used to determine the mean NAC stream-coordinates structure of temperature and absolute velocity for the period from August 1993 to February 1995. This structure is narrower and stronger than the corresponding Eulerian average, with peak speeds greater than 80 cm s −1 and bottom velocities exceeding 10 cm s −1 . The calculated mean NAC transport was 131±14 (×10 6 m 3 s −1 ); temporal variations resulted in a 41×10 6 m 3 s −1 standard deviation. The stream-coordinates estimate of mean absolute transport was found to be about 10% smaller than the corresponding Eulerian mean transport of 146×10 6 m 3 s −1 as a result of partially summing the north and south transports within the Mann Eddy, a large semi-permanent anticyclonic eddy normally located just offshore of the NAC. Most transport sections across the NAC in this region include the northward transport of the inshore side of the Mann Eddy because it is difficult to discriminate between the two flows. Historical estimates place the Mann Eddy absolute transport around 50–60×10 6 m 3 s −1 ; thus, the transport crossing this transect that can be identified with the NAC is about 90×10 6 m 3 s −1 . The barotropic component of the transport (defined as the bottom velocity multiplied by the water depth) is found to contribute at least 30% of the total absolute transport, depending on the choice of baroclinic reference level. Because a time series of transport measurements was made in this study, the accuracy of the mean transport estimate is better than previous estimates, which have all been based on snapshot measurements. From a combination of these measurements with other published measurements, a scheme for the overall circulation and transport in the Newfoundland Basin is suggested.

Feeding Strategy of Japanese Sand Lance Larvae in Relation to Ciliated Protozoa in the Vicinity of a Thermohaline Front

Larval feeding and survival strategies are described on a Japanese sand lance, Ammodytes personatus Girard, collected in the vicinity of a thermohaline front in the Ise Bay, in comparison with those of other predominant larvae, Hexagrammos spp. and Sebastiscus marmoratus (Cuvier). First-feeding A. personatus larvae (3.1–3.9 mm NL) fed primarily on tintinnid ciliates, subsequently switching to copepod nauplii (4.0–7.9 mm NL larvae) and post-naupliar copepods (8.0–11.3 mm NL larvae). First-feeding Hexagrammos spp. larvae (6.5–6.9 mm NL) fed primarily on post-naupliar copepods, and first-feeding S. marmoratus larvae (<4.0 mm NL), mostly on copepod nauplii. The different food preferences of these species at first-feeding were related to their different mouth widths (0.15–0.19, 0.52–0.56 and 0.32–0.40 mm, respectively) and/or body size (3.1–3.9, 6.5–6.9 and 3.2–3.9 mm NL, respectively). Ciliate-feeding by first-feeding A. personatus larvae was strongly related to the convergence of the larvae and their prey near the thermohaline front, densities of both being greater on the inshore side of the frontal zone. In conclusion, the aggregation of ciliates near the thermohaline front may have improved feeding conditions and survival of first-feeding A. personatus larvae.

Satellite‐derived sea surface temperature fronts on the continental shelf off the northeast U.S. coast

The distribution and variability of sea surface temperature (SST) fronts, over the shelf and slope along the east coast of North America from Cape Hatteras to Nova Scotia, are studied using a 12 year time series (1985–1996) of advanced very high resolution radiometer (AVHRR) images. After the masking of cloud‐contaminated pixels, an edge‐detection algorithm identifies surface temperature fronts in each image. Maps of the seasonal probability of detecting a front indicate substantial spatial and temporal variability in the occurrence of SST fronts. Over the continental shelf south of New England during spring through autumn, surface fronts are rare and observed only in scattered locations. North of Nantucket Shoals, both tidal mixing fronts and fronts associated with the Eastern Maine Coastal Current occur during the summer. A major finding of this study is the observation of fronts in winter over the inner and middle shelf from Cape Hatteras to the Bay of Pundy. These fronts, peaking during January‐March, are characterized by cold SST on their inshore (shallow) side and appear to result from the influence of surface cooling on shallow nearshore waters. The shelfbreak front is found to vary strongly with season, being detected most frequently during spring and autumn. South of Hudson Canyon, it essentially disappears during the summer, while from Hudson Canyon to Northeast Channel, it weakens during summer but nevertheless remains detectable in SST.

Secondary circulation associated with a shelfbreak front

Evidence for secondary circulation associated with a shelfbreak front is obtained from a high‐resolution, cross‐shelf section of hydrographic, optical and velocity fields. Convergence in the bottom boundary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a mid‐water region of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the offshore side of the front in the upper water column is inferred from a 20‐m downward bend of the subsurface phytoplankton layer. These observations are in agreement with recent model predictions for secondary circulation near an idealized shelfbreak front. Convergence in measured cross‐shelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9±2 m day−1 of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10–16 days.

On the branching of the Kuroshio west of Kyushu

Trajectories of satellite‐tracked surface drifters released in the continental margin area west of Kyushu indicate a separation of the inshore side of the Kuroshio that appears to form the beginning of the Tsushima Current. The trajectories also indicate an anticyclonic sense of circulation in the area between the separated branch and the eastward directed Kuroshio main stream, consistent with long‐term geomagnetic electrokinetograph measurements of surface currents, which show a broad southward flow along the west Kyushu coast. A theory is put forth that ties together these observed flow features. In the theory, the incidence is considered of an inertial current upon a step rise in topography (characterized by a step depth of H0 to simulate the west Kyushu coastal bathymetry) in an f plane, two‐layered, inviscid ocean with a quiescent lower layer. The inertial current is of a constant potential vorticity f/H, where H (&gt;H0) is the undisturbed upper layer depth and is bounded inshore by a free streamline along which the upper layer depth is a constant hc (&lt;H). The steady state downstream configuration of a separation branch and a deflected main stream along the step is connected to the upstream approach current through the consideration of the constancy of the Bernoulli function and the conservation of potential vorticity and mass. In the immediate neighborhood of the incidence, a complicated flow region is anticipated in which recirculation is featured and an anticyclonic gyre is expected further onto the step. The volume‐integrated balance of along‐step momentum of the constant potential‐vorticity flow allows the determination of the angle the branch current makes with the step as a function of the angle of incidence and two depth ratios, hc/H and H0/H. The bifurcation quotient, defined as the ratio of the branch current transport to that of the approach current, is found to be also such a function, except it is independent of the incidence angle. These functions yield results that are qualitatively in agreement with the observed bifurcation of the Kuroshio west of Kyushu that gives rise to the Tsushima Current.

Observations of the countercurrent on the inshore side of the Kuroshio northeast of Taiwan

Intensive current measurements in the area northeast of Taiwan indicate subsurface, southwestward flow existed between the inshore edge of the Kuroshio and the East China Sea continental slope. At 70 km away from Taiwan, this countercurrent has a mean speed about 30 cm s−1 at mid-depth. Closer to Taiwan, the flow turns along with the topography, and subjects to sidewall and bottom friction. Both the magnitude and the vertical shear of this countercurrent are comparable with that inferred from hydrographic survey. The wind field features short-period (a few days) fluctuations associated with the cold front passages, however, this is not reflected on the current records. It appears that the countercurrent is fairly steady. Together with similar reversing flow found at places much further to the north, the overall pattern seems to be a general quasi-steady feature along most part of the shelf edge of the East China Sea.

Optimum multiparameter analysis of the water mass structure in the western North Atlantic Ocean

Hydrographic data of temperature, salinity, oxygen, nitrate, phosphate, and silicate at 81 stations with 435 samples on 3 sections between the Azores, the Grand Banks of Newfoundland, and the Bermuda Islands are used to determine the mixing of water masses by optimum multiparameter analysis over the depth range 100–1500 m. The method optimally utilizes all information from our hydrographic data set by solving an overdetermined set of linear mixing equations for all parameters using the method of least squares residuals. It is shown that the method gives quantitative information on the influence of the various water masses of the western North Atlantic. The Gulf Stream and the North Atlantic Current appear as broad bands transporting large amounts of Western North Atlantic Central Water at their warm flank. Western Subarctic Intermediate Water and Shelf Water supplied by the Labrador Current and containing significant amounts of Labrador Current Water are found on their inshore side. The area of the Azores front is found in the vicinity of the Comer Seamounts, where the uniform water mass distribution of the Sargasso Sea changes into a more complex structure that reflects the influence of water masses originating in the Labrador Sea. Small‐scale structures, like eddies or Gulf Stream rings, are also detectable by this analysis method. Comparison with dynamic height analysis supports the circulation pattern of the North Atlantic Current as a continuation of the Gulf Stream, and of the southeastward flowing Azores Current originating in the area of the Southeast Newfoundland Rise.

Origin of high phytoplankton concentration in deep chlorophyll maximum (DCM) in a frontal region of the Southwestern Mediterranean Sea (algerian current)

In June 1990, we investigated the Atlantic flow along the Algerian coast in the Western Mediterranean. Intensive and fine resolution sampling along 13 longitudinal transects, consisting of 4–12 stations (3–5 nautical miles apart), was carried out with physical, chemical and biological measurements. This sampling enabled us to describe quasi-synoptically the spatial distribution over a large region between 1° and 4°E. Two oligotrophic systems, where the integrated chlorophyll was lower than 30 mg m −2, were separated by an almost continuous rich band characterized by integrated chlorophyll higher than 60 mg m −2. This chlorophyll-rich band, with concentrations greater than 2 mg m −3 (up to 15.8 mg m −3), was found along the boundary of the Atlantic flow, indicating that the phytoplanktonic biomass distribution is determined by hydrodynamics. The patchiness of phytoplankton, both in chlorophyll concentration and in species abundance, along a density surface resulted from interactions between Atlantic and Mediterranean waters. Also, a first assessment of the biological response to the mesoscale hydrodynamical features can be deduced from the comparative analysis of the physical, chemical and biological parameters. Large anomalies in nutrient and chlorophyll distributions indicated strong vertical advections along the front, either following density surface (isopycnal mixing) or crossing it (diapycnal mixing). Important isopycnal mixing of nutrients were evidenced on the inshore side of the current, while diapycnal mixing was observed only on the seaward side. These two processes seem to bring nutrients into the photic zone, leading to an enhanced in situ growth. A cross-front circulation is proposed to explain (i) the vertical distribution of chemical parameters near the front and (ii) the associated phytoplankton patchiness.

On the North Atlantic Circulation

A new, speculative, and, we hope, provocative summary of the North Atlantic circulation is described, including both horizontal currents (wind‐driven) and the primarily (thermohaline) meridional flows that involve the transformation of warm to cold water at high latitudes. Our picture is based on a synthesis of a variety of independent investigations that are contained in the literature as opposed to a presentation of the results of one technique or the point of view of one author. We describe a thermohaline cell (the so‐called thermohaline conveyor belt) that is concentrated within the Atlantic and Southern oceans (rather than essentially global), with the most important upwelling sites being in the circumpolar and the equatorial current regimes. We concentrate on deep water formation and its replacement relative to intermediate‐water formation. It has been pointed out recently that the formation of 13 Sv (1 Sv = 106m³ s−1) of southward flowing North Atlantic Deep Water is compensated for in the upper ocean by northward cross‐equatorial transport. We suggest that this thermocline layer flow passes through the Straits of Florida, transits the Gulf Stream system on its inshore side, and exits through the North Atlantic Current system after recirculation and modification. There is now a clear observational basis for the structure of recirculating gyres on the southern and northern sides of the Gulf Stream. We suggest a recirculation for the North Atlantic Current as well. We also describe a C‐shaped component to the southern Gulf Stream recirculation and identify a roughly 10‐Sv circulation in the eastern North Atlantic associated with the Azores Current. Recirculations play an important role in deep boundary current regimes and in water mass formation and modification. The transport of the deep western and northern boundary currents in the North Atlantic Ocean may be boosted (roughly doubled or tripled) by counterclockwise recirculating gyres and by additions of modified bottom or intermediate water. While the North Atlantic is the most completely observed ocean, there are still significant gaps in our knowledge of its circulation.

The western boundary current of the seasonal subtropical gyre in the Bay of Bengal

Hydrographic data collected during March–April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal north of about 10° N carrying warmer waters of southern origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 × 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March‐April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the curl of wind stress, which has a well‐defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January‐May. The pattern of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress curl over the bay is the sole mechanism to force the gyre.