This study focuses on the improved estimation of mesoscale surface ocean circulation obtained by merging TOPEX/Poseidon (T/P) and ERS-1 and -2 altimeter measurements between October 1992 and May 1998. Once carefully intercalibrated and homogenized, these data are merged through an advanced global objective analysis method that allows us to correct for residual long wavelength errors and uses realistic correlation scales of ocean dynamics. The high-resolution (0.25°×0.25°) merged T/P+ERS-1 and -2 sea level anomaly maps provide more homogeneous and reduced mapping errors than either individual data set and more realistic sea level and geostrophic velocity statistics than T/P data alone. Furthermore, the merged T/P+ERS-1 and -2 maps yield eddy kinetic energy (EKE) levels 30% higher than maps of T/P alone. They also permit realistic global estimates of east and north components of EKE and their seasonal variations, to study EKE sources better. A comparison of velocity statistics with World Ocean Circulation Experiment surface drifters in the North Atlantic shows very good agreement. Comparison with contemporary current meter data in various oceanic regimes also produces comparable levels of energy and similar ratios of northward and eastward energy, showing that the maps are suitable to studying anisotropy. The T/P + ERS zonal and meridional components of the mapped currents usually present comparable rms variability, even though the variability in the Atlantic is more isotropic than that in the Pacific, which exhibits strong zonal changes. The EKE map presents a very detailed description, presumably never before achieved at a global scale. Pronounced seasonal changes of the EKE are found in many regions, notably the northeastern Pacific, the northeastern and northwestern Atlantic, the tropical oceans, and the zonally extended bands centered near 20°S in the Indian and western Pacific Oceans and at 20°N in the northwestern Pacific.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JC900063

Global high-resolution mapping of ocean circulation from TOPEX/Poseidon and ERS-1 and -2

The monsoon circulation of the Indian Ocean

The South Atlantic Ocean receives waters from the North Atlantic, the Weddell Sea, and from the Circumpolar Current through the Drake Passage. The circumpolar and North Atlantic waters are of widely different characteristics (temperature, salinity, oxygen and nutrients) but of overlapping density ranges, and as they enter the South Atlantic are caught up in the circulation imposed by the winds and thermohaline processes. Interleaving of these different characteristics takes place in various ranges of depth and density, and more than a dozen vertical extrema are created in the tracer fields. These are spread by lateral flow in recognizable layers. These patterns as seen on vertical sections and on isopycnals can be taken to indicate the sense of flow in various areas and depths, and I have used them, with the density field, to add the barotropic component to the baroclinic flow defined by the density field. This gives the total geostrophic flow at all depths in a manner that appears to be consonant with the tracer patterns, and that satisfies continuity of mass. The resulting flow field has the traditional western boundary current from the Weddell Sea to the equator, beneath a poleward flow north of 50°S. In the upper waters the flow pattern shows a cyclonic gyre in the Weddell Sea, a northward flow from the Antarctic along the western boundary from 55°S to 40°S, an anticyclonic gyre between 15°S and 40°S, and an eastward flow near 10°S. Below the upper layer the axis of the anticyclonic gyre shifts southward, to 35°S at 1500 m, and at greater depths the gyre is confined to the Brazil Basin, west of the Mid-Atlantic Ridge and north of the Rio Grande Rise. Below the crest of the Mid-Atlantic Ridge in the Brazil Basin waters from the south flow northward along the Ridge, and part of them crosses the Ridge at or near the equator and returns southward in the eastern basin, joined by waters from the North Atlantic. The northward abyssal flow in the Brazil Basin is not confined to the western boundary but extends some distance across the basin, diverted only slightly at 3500 m by the small deep remnant of the overlying anticyclonic gyre. From 3000 to 4000 decibars the flow within the Weddell Sea and the Argentine Basin form a single cyclonic gyre and the flow within the Cape Basin is cyclonic below 3000 m.

On the Total Geostrophic Circulation of the South Pacific Ocean: Flow Patterns, Tracers and Transports

A numerical investigation of dynamics, thermodynamics and mixed-layer processes in the Indian Ocean

http://drs.nio.org/drs/bitstream/handle/2264/25/Chap1.pdf?sequence=7

The monsoon currents in the north Indian Ocean

The Bay of Bengal, a semienclosed tropical basin that comes under the influence of monsoonal wind and freshwater influx, is distinguished by a strongly stratified surface layer and a seasonally reversing circulation. We discuss characteristics of these features in the western Bay during the northeast monsoon, when the East India Coastal Current (EICC) flows southward, using hydrographic data collected during December 1991. Vertical profiles show uniform temperature and salinity in a homogeneous surface layer, on average, 25 m deep but shallower northward and coastward. The halocline, immediately below, is approximately 50 m thick; salinity changes by approximately 3 parts per thousand. About two thirds of the profiles show temperature inversions in this layer. Salinity below the halocline hardly changes, and stratification is predominantly due to temperature variation, The halocline is noticeably better developed and the surface homogeneous layer is thinner in a low-salinity plume that hugs the coastline along the entire east coast of India, The plume is, on average, 50 km wide, with isohalines sloping down toward the coast. Most prominent in the geostrophic velocity field is the equatorward EICC. Its transport north of about 13 degrees N, computed with 1000 dbar as the level of reference, varies between 2.6 and 7.1 x 10(6) m(3) s(-1); just south of this latitude, a northwestward flow from offshore recurves and merges with the coastal current. At the southern end of the region surveyed, the transport is 7.7 x 10(6) m(3) s(-1). Recent model studies lead us to conclude that the EICC during the northeast monsoon is driven by winds along the east coast of India and Ekman pumping in the interior bay. In the south, Ekman pumping over the southwestern bay is responsible for the northwestward flow that merges with the EICC.

Hydrography and circulation in the western Bay of Bengal during the northeast monsoon

A hydrographic survey during the southwest monsoon (July–August 1989) showed that along most of the western boundary of the Bay of Bengal, in an approximately 40 km wide band, isopycnals from depths up to about 70 m surfaced due to upwelling forced by local winds, in a fashion similar to that observed along eastern boundaries. Below the upwelling band there were often signatures of downwelling, suggestive of an undercurrent. There were no indications of a large-scale remotely forced western boundary current. Geostrophic velocity in the upwelling band was in the direction of the winds. The dynamic topography outside the upwelling band had cellular structures possibly indicating the presence of shelf waves with longshore wavelength of 400–500 km. The near-surface stratification was dominated by salinity, a consequence of high freshwater input to the Bay. The upwelling led to a coastward increase in salinity, except near the northern end where the freshwater influx from the Ganges and the Brahmaputra rivers overwhelmed other processes and gave rise to a freshwater plume offshore of the upwelling band. This plume moved equatorward against local winds.

Wind-driven coastal upwelling along the western boundary of the Bay of Bengal during the southwest monsoon

During June-August 1987 thirteen hydrographic sections, each approximately perpendicular to the coast, covered the coastal region off western India The southernmost section showed occurrence of upwelling, the nearshore surface temperature being about 25°C lower than farther offshore There was a shallow (approximately 75 to 100 m deep) equatorward surface current, below which there were signatures of downwelling indicative of a poleward undercurrent hugging the continental slope T-S characteristics showed that the undercurrent carried low salinity water found in the southwestern Bay of Bengal Conditions similar to those found at this section existed up to around 15N but the intensity of upwelling, and the signatures of the surface current and the undercurrent grew weaker progressively from the south to the north, and ceased to be noticeable at about 20N The width of the surface current was about 150 km, whereas the undercurrent was about 40 km wide The transport in the equatorward surface current increased from less than 05 × 106 m3/s to about 4 × 106 m3/s from the north to the southern end of the coast The winds during the observation varied between west-northwesterly near the southern end of the coast to west-southwesterly near the northern end The longshore component of the wind stress was generally equatorward Its magnitude was maximal (05 dyn/cm2) near the southern end On the basis of these observations we propose that the circulation off the west coast of India during the southwest monsoon, though weak, is dynamically similar to the wind-driven eastern boundary currents found elsewhere in the oceans

Hydrography and circulation off the west coast of India during the southwest monsoon 1987

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.

/pdf/the-western-boundary-current-of-the-seasonal-subtropical-3amsteyjxn.pdf

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

Hydrographic data collected during December 1987 and January 1988 are used to describe the poleward coastal current along the west coast of India that flows against weak winds during the northeast monsoon. Near the southern end of the coast, at about 10°N, the current was approximately 400 km wide, 200 m deep and carried the low salinity Equatorial Surface Water. The isopycnals tilted down on approaching the coast. Near the northern end of the coast, at about 22°N, the flow was restricted mainly to the vicinity of the continental slope; the current was a narrow (100 km), 400 m deep jet with a transport of about 7 × 106 m3 s−1. Along most of the coastline, a southward moving undercurrent was inferred from the distribution of salinity, temperature and dynamic topography. To understand the driving mechanism of the current, the annual cycle of the contribution of the longshore pressure gradient and that of the winds to the near-surface momentum balance was examined using available climatologies. It is seen that the longshore pressure gradient overwhelms the winds during the northeast monsoon, whereas during the southwest monsoon the winds dominate. In the Leeuwin Current off western Australia, the only other known eastern boundary current that flows against the winds, the pressure gradient dominates the winds throughout the year. The overall structure of the northeast monsoon coastal current is consistent with that predicted in the analytic model proposed by McCreary et al. (1986, Journal of Marine Research, 44, 71–92) to explain the Leeuwin Current. In the model a poleward baroclinic pressure gradient is generated by a density gradient along the coast. The latter is known to exist along the west coast of India. However, it seems likely that the current is also influenced by a barotropic pressure gradient.

A. D. Gouveia

Papers

Hydrography and circulation in the western Bay of Bengal during the northeast monsoon

Wind-driven coastal upwelling along the western boundary of the Bay of Bengal during the southwest monsoon

Hydrography and circulation off the west coast of India during the southwest monsoon 1987

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

The coastal current off Western India during the Northeast monsoon