Spatial structure of inertial-period motions in a two-layered sea, based on observations

Spatial coherence and phase relationships of inertial-period motions have been inferred from an experiment with four moorings in the northern North Sea where the density profile was almost two-layered. The inertial-period currents showed antiphase between the upper and lower layers; the coherence between both layers was high. The inertial-period temperature fluctuations were in phase throughout the thermocline

Eddies in the North Brazil Current retroflection region observed by Geosat altimetry

Mesoscale fluctuations in the western tropical Atlantic are analyzed in Geosat altimetry sea surface height (SSH) and geostrophic velocity anomalies to investigate the role of eddies in the North Brazil Current (NBC) retroflection zone. The detachment of anticyclonic eddies from the NBC retroflection is observed during November through January, when the NBC retroflection into the North Equatorial Countercurrent (NECC) weakens and finally breaks down. These eddies are traced over more than 2 months between 50° and 60°W on their way toward the Caribbean, at average speeds of 15 cm s−1. In one case an apparent merger of two anticyclonic eddies occurs, one detached from the retroflection zone and one detached from the NECC. Cyclonic eddies are also observed but are generally less persistent. Mesoscale SSH variance just west of the retroflection increases by a factor of 2 from early summer to winter, mainly because of the anticyclonic eddies. Interhemispheric water mass transfer associated with the eddy flux out of the NBC retroflection may amount to an average transport of 3 Sv

Seasonal variations in the western tropical Atlantic: Surface circulation from Geosat altimetry and WOCE model results

Sea level variations and geostrophic circulation in the western tropical Atlantic are studied in an intercomparison of Geosat altimetry and the World Ocean Circulation Experiment community model effort high‐resolution model forced with climatological windstress. Overall, the annual cycles of geostrophic current fields of both products compare very well. Special comparison areas are the western North Equatorial Countercurrent (NECC) and the North Brazil Current (NBC) region. Meridional profiles of zonal velocity anomalies show a seasonal meridional migration of the NECC core centered at 5°N and a weaker eastward maximum during fall at 9°N in both products. The Geosat and model seasonal cycles of the NECC core velocity in the region 35°–45°W are highly correlated and agree with respect to the onset of eastward current acceleration and deceleration in May and December, respectively. Geosat time series from November 1986 to June 1989 show year to year differences, in particular an anomalous early NECC acceleration phase in 1987. In the NBC region 54°–58°W, flow anomalies from both Geosat and the model have two westward maxima, in March and June, which appear to be associated with eastward anomalies further offshore

On the determination of internal-wave directional spectra from moored instruments

Presented here is a model for determining the characteristic parameters of an internalwave field using cross spectra of data obtained from vertically separated and/or horizontally separated moored instruments. The model is based on the assumption that the motion consists mainly of linear free internal waves having random phase relationships. The internalwave energy is supposed to be distributed among a finite number of modes having continuous directional distribution...

Transports and pathways of the upperlayer circulation in the western tropical Atlantic

The mean warm water transfer toward the equator along the western boundary of the South Atlantic is investigated, based on a number of ship surveys carried out during 1990–96 with CTD water mass observations and current profiling by shipboard and lowered (with the CTD/rosette) acoustic Doppler current profiler and with Pegasus current profiler. The bulk of the northward warm water flow follows the coast in the North Brazil Undercurrent (NBUC) from latitudes south of 10°S, carrying 23 Sv (Sv ≡ 106 m3 s−1) above 1000 m. Out of this, 16 Sv are waters warmer than 7°C that form the source waters of the Florida Current. Zonal inflow from the east by the South Equatorial Current enters the western boundary system dominantly north of 5°S, adding transport northwest of Cape San Roque, and transforming the NBUC along its way toward the equator into a surface-intensified current, the North Brazil Current (NBC). From the combination of moored arrays and shipboard sections just north of the equator along 44°W, the mean NBC transport was determined at 35 Sv with a small seasonal cycle amplitude of only about 3 Sv. The reason for the much larger near-equatorial northward warm water boundary current than what would be required to carry the northward heat transport are recirculations by the zonal current system and the existence of the shallow South Atlantic tropical–subtropical cell (STC). The STC connects the subduction zones of the eastern subtropics of both hemispheres via equatorward boundary undercurrents with the Equatorial Undercurrent (EUC), and the return flow is through upwelling and poleward Ekman transport. The persistent existence of a set of eastward thermocline and intermediate countercurrents on both sides of the equator was confirmed that recurred throughout the observations and carry ventilated waters from the boundary regime into the tropical interior. A strong westward current underneath the EUC, the Equatorial Intermediate Current, returns low-oxygen water westward. Consistent evidence for the existence of a seasonal variation in the warm water flow south of the equator could not be established, whereas significant seasonal variability of the boundary regime occurs north of the equator: northwestward alongshore throughflow of about 10 Sv of waters with properties from the Southern Hemisphere was found along the Guiana boundary in boreal spring when the North Equatorial Countercurrent is absent or even flowing westward, whereas during June–January the upper NBC is known to connect with the eastward North Equatorial Countercurrent through a retroflection zone that seasonally migrates up and down the coast and spawns eddies. The equatorial zone thus acts as a buffer and transformation zone for cross-equatorial exchanges, but knowledge of the detailed pathways in the interior including the involved diapycnal exchanges is still a problem

The shallow overturning circulation of the Indian Ocean

The Indian Ocean differs from the other two large oceans in not possessing an annual-mean equatorial upwelling regime. While the subtropical cells (STCs) of the Atlantic and Pacific Oceans connect subtropical subduction regimes with tropical upwelling via equatorward thermocline flows and coastal undercurrents, much of the upwelling in the Indian Ocean occurs in the coastal regimes of the northern hemisphere. Consequently, the counterpart of the STCs of the other oceans has to be a cross-equatorial cell connecting the southern subtropical subduction zone via the Somali Current with the upwelling areas off Somalia and Oman. The southward return flow is by interior Ekman transports. This annual-mean picture is accomplished by a dominance of the summer monsoon, during which only northern upwelling occurs, over the winter monsoon. Pathways of the thermocline flows related to the shallow overturning circulations are investigated here and estimates of subduction and upwelling are presented. From the observed mean northward flow of thermocline waters within the Somali Current and the interior southward cross-equatorial return flow the magnitude of the cross-equatorial cell is estimated at 6 Sv, with part of the thermocline waters being supplied by the Indonesian Throughflow. From observations we estimate that the northern upwelling occurs dominantly through the offshore outflows of the Somali Current by the Southern Gyre and Great Whirl and to a lesser degree off Oman. However, we also present model results suggesting a much lower role of Somali upwelling and a significant contribution from open-ocean upwelling in cyclonic domes around India and Sri Lanka. An interesting aspect of the Indian Ocean cross-equatorial cell is the mechanism by which the Ekman transport crosses the equator. Typically, Ekman transports during the summer (winter) monsoon are southward (northward) on both sides of the equator, while mean meridional winds on the equator are in the respective opposite direction. Earlier model evidence had suggested that this type of forcing should lead to an equatorial roll with northward surface flow and southward subsurface flow during the summer monsoon and reverse orientation during the winter monsoon. Observational evidence is presented here, based on shipboard ADCP sections, moored stations and surface drifters, confirming the existence of the equatorial roll. It is strongly developed in the western Indian Ocean during the SW monsoon where the wind conditions for the roll are best met. While in the central Indian Ocean and during the winter monsoon the roll appears to be a more transient phenomenon, superimposed by equatorial-wave currents. The evidence further suggests that the roll is mostly confined to the surface-mixed layer and is, therefore, of little consequence for the meridional heat transport

The flow field off southwest India at 8°N during the southwest monsoon of August 1993

The flow field off the southwest coast of India at 8N was investigated during RV Sonne cruise 89 in August 1993 by direct velocity observations from shipboard- and lowered-ADCP and geostrophic computations from CTD stations. The upper ocean between 75E and 76°52′E near the South Indian shelf was governed by a northward flow with a subsurface velocity maximum of 25 cm s−1 at about 100 m depth. This flow, organized as a poleward undercurrent hugging the continental slope, is typical for the southwest monsoon season. The northward transport in August 1993 was 4.7 Sv (1 Sverdrup = 106 m3 s−1) for the upper 300 m from the shipboard ADCP. Earlier geostrophic observations showed southward surface flow above the poleward undercurrent, but in August 1993 the northward flow reached to the surface and in the geostrophy calculations, i.e. without the southward Ekman flow near the surface, there was even no clear subsurface core. The T-S characteristics show that Bay of Bengal Water (BBW) was carried with this flow, and low wind conditions seemed to be connected to the flow of BBW from the southern tip of Sri Lanka toward the southwest coast of India. Further offshore, two meridional current bands were identified in the upper 300 m of the ocean. West of the coastal undercurrent a band of southward flow existed with velocities up to 35 cm s−1 above and to the east of the Chagos-Laccadive Ridge, from 72°10′E to 75E. The associated geostrophic transport in the upper 500 m was 5.2 Sv. As the T-S relation was different from that of the northward flow, this current band was not a local recirculation of the poleward undercurrent. Further west, the flow was weak, but intensified toward the central Arabian Sea, between 66E and 69°20′E, where another southward current band was found with velocities of up to 20 cm s−1 and a total geostrophic transport for the upper 300 m of −7.2 Sv

The warm water inflow into the western tropical Atlantic boundary regime, spring 1994

During March 1994 a survey of the western boundary of the tropical Atlantic, between 10 degrees N and 10 degrees S, was carried out by conductivity-temperature-depth and current profiling using shipboard and lowered acoustic Doppler current profilers. In the near-surface layer, above sigma. = 24.5, the inflow into the boundary regime came dominantly from low latitudes; out of the 14 Sv that crossed the equator in the upper part of the North Brazil Current (NBC), only 2 Sv originated from south of 5 degrees S, while 12 Sv came in from the east at 1 degrees-5 degrees S with the South Equatorial Current (SEC). After crossing the equator near 44 degrees W, only a minor fraction of the near-surface NBC retroflected eastward, while a net through flow of about 12 Sv above sigma. = 24.5 continued northwestward along the boundary, By contrast, in the isopycnal range sigma. = 24.5-26.8 encompassing the Equatorial Undercurrent (EUC), the source waters of the equatorial circulation were dominantly of higher-latitude South Atlantic origin. While only 3 Sv of eastern equatorial water entered the region through the SEC at 3 degrees-5 degrees S, there was an inflow of 10 Sv of South Atlantic water in the North Brazil Undercurrent (NBUC) along the South American coast that originated south of 10 degrees S, The transport of 14 Sv arriving at the equator along the boundary in the undercurrent layer was almost entirely retroflected into the EUC with only marginal northern water additions along its path to 35 degrees W. The off-equatorial undercurrents in the upper thermocline, the South and North Equatorial Undercurrents carried only small transports across 35 degrees W, of 5 Sv and 3 Sv, respectively, dominantly supplied out of SEC recirculation rather than out of the boundary current. Still deeper, three zonal undercurrents were observed: the westward-flowing Equatorial Intermediate Current (EIC) in the depth range 200-900 m below the EUC, and two off-equatorial eastward undercurrents, the Northern and Southern Intermediate Countercurrents (NICC, SICC) at 400-1000 m and 1 degrees-3 degrees latitude. In the lower part of the NBUC there was an Antarctic Intermediate Water (AAIW) inflow along the coast of 6 Sv, and there was a clear connection at the AAIW level to the SICC by low salinities and high oxygens and a weaker suggestion also that some supply of the NICC might be through AAIW out of the deep NBUC

Cruise report SONNE cruise SO128 ARABWOCE Male - Muskat 8.-29. January 1998

Untersuchung der Monsunzirkulation im Arabischen Meer Investigation of the monsoon circulation of the Arabian Se