Description: A new shipboard current profiler, a 75-kHz ocean surveyor, was operationally used during two research cruises in the tropical Atlantic and the subpolar North Atlantic, respectively. Here, a report is presented on the first experience with this instrument in two very different current regimes, in the Tropics with large vertical shears, and in the subpolar regime with mainly barotropic flow. The ocean surveyor continuously measured currents in the upper ocean from near the surface to about 500–700-m depth. The measurement range showed a dependence on the regional and temporal variations of scattering particles and on the intensity of swell and wind waves. Statistical comparisons are performed with on-station lowered acoustic Doppler current profiler (LADCP) profiles and underway measurements by classic shipboard acoustic Doppler current profiler (ADCP) measurements. Accuracy estimates for hourly averaged ocean surveyor currents result in errors of about 1 cm s–1 for on-station data and of 2–4 cm s–1 for underway measurements, depending on the regional abundance of scatterers and on the weather conditions encountered.

Description: The differences in the water mass distributions and transports in the Arabian Sea between the summer monsoon of August 1993 and the winter monsoon of January 1998 are investigated, based on two hydrographic sections along approximately 8°N. At the western end the sections were closed by a northward leg towards the African continent at about 55°E. In the central basin along 8°N the monsoon anomalies of the temperature and density below the surface-mixed layer were dominated by annual Rossby waves propagating westward across the Arabian Sea. In the northwestern part of the basin the annual Rossby waves have much smaller impact, and the density anomalies observed there were mostly associated with the Socotra Gyre. Salinity and oxygen differences along the section reflect local processes such as the spreading of water masses originating in the Bay of Bengal, northward transport of Indian Central Water, or slightly stronger southward spreading of Red Sea Water in August than in January. The anomalous wind conditions of 1997/98 influenced only the upper 50–100 m with warmer surface waters in January 1998, and Bay of Bengal Water covered the surface layer of the section in the eastern Arabian Sea. Estimates of the overturning circulation of the Arabian Sea were carried out despite the fact that many uncertainties are involved. For both cruises a vertical overturning cell of about 4–6 Sv was determined, with inflow below 2500 m and outflow between about 300 and 2500 m. In the upper 300–450 m a seasonally reversing shallow meridional overturning cell appears to exist in which the Ekman transport is balanced by a geostrophic transport. The heat flux across 8°N is dominated by the Ekman transport, yielding about –0.6 PW for August 1993, and 0.24 PW for January 1998. These values are comparable to climatological and model derived heat flux estimates. Freshwater fluxes across 8°N also were computed, yielding northward freshwater fluxes of 0.07 Sv in January 1998 and 0.43 Sv in August 1993. From climatological salinities the stronger freshwater flux in August was found to be caused by the seasonal change of salinity storage in the Arabian Sea north of 8°N. The near-surface circulation follows complex pathways, with generally cyclonic-circulation in January 1998 affected at the eastern side by the Laccadive High, and anticyclonic circulation in August 1993.

Description: Ocean deep velocity profiles were obtained by lowering a self-contained 153.6-kHz acoustic Doppler current profiler (ADCP) attached to a CTD-rosette sampler. The data were sampled during two Meteor cruises in the western tropical Atlantic. The ADCP depth was determined by integration of the vertical velocity measurements, and the maximum depth of the cast was in good agreement with the CTD depth. Vertical shears were calculated for individual ADCP velocity profiles of 140-300-m range to eliminate the unknown horizontal motion of the instrument package. Subsequent raw shear profiles were then averaged with respect to depth to obtain a mean shear profile and its statistics. Typically, the shear standard deviations were about 10(-3) s-1 when using up and down traces simultaneously. The shear profiles were then vertically integrated to get relative velocity profiles. Different methods were tested to transform the relative velocities into absolute velocity profiles, and the results were compared with Pegasus dropsonde measurements. The best results were obtained by integrating the raw velocities and relative velocities over the duration of the cast and correcting for the ship drift determined from the Global Positioning System. Below 1000-m depth a reduction of the measurement range was observed, which results either from a lack of scatterers or instrumental problems at higher pressures.

Description: 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.

Description: Fifteen profiling floats were injected into the deep boundary current off Labrador. They were ballasted to drift in the core depth of Labrador Sea Water (LSW) at 1500-m depth and were deployed in two groups during March and July/August 1997. Initially, for about three months, the floats were drifting within the boundary current, and the flow vectors were used to determine the mean horizontal structure of the Deep Labrador Current, which was found to be about 100 km wide with an average core speed of 18 cm s−1. North of Flemish Cap the boundary current encounters complicated topography around “Orphan Knoll,” and there the LSW outflow splits up into different routes. One obvious LSW path is eastward through the Charlie Gibbs Fracture Zone and another route is a narrow recirculation toward the central Labrador Sea. A surprising result was that none of the floats were able to follow the boundary current southward to the Grand Banks area and exit into the subtropics. Trajectories and temperature profiles of the eastward drifting floats indicate the importance of the North Atlantic Current for dispersing the floats, even at the level of LSW.

Description: The deep circulation and related transports of the southern Labrador Sea are determined from direct current observations from ship surveys and a moored current-meter array. The measurements covered a time span from summer 1997 to 1999 and show a well-defined deep boundary current extending approximately out to the 3300-m depth contour and weak reverse currents farther offshore. The flow has a strong barotropic component, and significant baroclinic flow is only found in the shallow Labrador Current at the shelf break and associated with a deep core of Denmark Strait Overflow Water. The total deep-water transport below σΘ = 27.74 kg m−3 was 26 ± 5 Sv (Sv ≡ 106 m3 s−1) comprising Labrador Sea Water (LSW), Gibbs Fracture Zone Water (GFZW), and Denmark Strait Overflow Water (DSOW). Intraseasonal variability of the flow and transport was high, ranging from 15 to 35 Sv, and the annual means differed by 17%. A seasonal cycle is confined to the shallow Labrador Current; in its deeper part, where the mean flow is still strong, no obvious seasonality could be detected. The transport of the interior anticyclonic recirculation was estimated from lowered acoustic Doppler current profiler stations and geostrophy, yielding about 9 Sv. Thus, the net deep-water outflow from the Labrador Sea was about 17 Sv. The baroclinic transport of GFZW and DSOW referenced to the depth of the isopycnal σΘ = 27.80 kg m−3 is only about one-third of the total transport in these layers. Longer-term variations of the total transports are not represented well by the baroclinic contribution.

Description: The current system east of the Grand Banks was intensely observed by World Ocean Circulation Experiment (WOCE) array ACM-6 during 1993–95 with eight moorings, reaching about 500 km out from the shelf edge and covering the water column from about 400-m depth to the bottom. More recently, a reduced array by the Institut für Meerskunde (IfM) at Kiel, Germany, of four moorings was deployed during 1999–2001, focusing on the deep-water flow near the western continental slope. Both sets of moored time series, each about 22 months long, are combined here for a mean current boundary section, and both arrays are analyzed for the variability of currents and transports. A mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper-layer extrapolation and isopycnal subdivision of the mean transports into deep-water classes. The offshore part of the combined section is dominated by the deep-reaching North Atlantic Current (NAC) with currents still at 10 cm s−1 near the bottom and a total northward transport of about 140 Sv (Sv ≡ 106 m3 s−1), with the details depending on the method of surface extrapolation used. The mean flow along the western boundary was southward with the section-mean North Atlantic Deep Water outflow determined to be 12 Sv below the σθ = 27.74 kg m−3 isopycnal. However, east of the deep western boundary current (DWBC), the deep NAC carries a transport of 51 Sv northward below σθ = 27.74 kg m−3, resulting in a large net northward flow in the western part of the basin. From watermass signatures it is concluded that the deep NAC is not a direct recirculation of DWBC water masses. Transport time series for the DWBC variability are derived for both arrays. The variance is concentrated in the period range from 2 weeks to 2 months, but there are also variations at interannual and longer periods, with much of the DWBC variability being related to fluctuations and meandering of the NAC. A significant annual cycle is not recognizable in the combined current and transport time series of both arrays. The moored array results are compared with other evidence on deep outflow and recirculation, including recent models of different types and complexity.

Description: Sea-surface height data acquired by the TOPEX/POSEIDON satellite over the Arabian Sea from October 1992 to October 1998 are analyzed. Strong seasonal fluctuations are found between 61 and 101N, which are mainly associated with westward propagating annual Rossby waves radiated from the western side of the Indian subcontinent and that are continuously forced by the action of the wind-stress curl over the central Arabian Sea. An analysis of hydrographic data acquired during August 1993 and during January 1998 at 81N in the Arabian Sea reveals the existence of first- and second-mode annual Rossby waves. These waves, which can be traced as perturbations in the density fields, have wavelengths of 12�103 and 4.4�103km as well as phase velocities of 0.38 and 0.14 m/s, respectively. The waves are associated with a time-dependent meridional overturning cell that sloshes water northward and southward. Between 581 and 681E in the central Arabian Sea, we found a Rossby-wave induced transport in the upper 500m of about 10 Sv southward in August 1993 and northward in January 1998. Below 2000 m, there was still a northward transport of 3.2 Sv in August 1993 and a southward transport of 4.8 Sv in January 1998. A comparison of steric height differences between August 1993 and January 1998 calculated from the observed density fields as well as calculated from the reconstructed density fields using first- and second-mode annual Rossby waves agree quite well with the corresponding sea-surface height differences. Implications resulting from the reflection of annual Rossby waves, like fluctuations of the western boundary currents, are discussed.

Description: During December 1991 to April 1992 measurements with moored acoustic Doppler current profiler (ADCP) stations and shipboard surveys were carried out in the convection regime of the Gulf of Lions, northwestern Mediterranean. First significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late December. Mixed layer deepening to about 400 m, eroding the salinity maximum layer of saltier and warmer Levantine Intermediate Water and causing temporary surface-layer warming, followed during a second cooling period of late January. During a mistral cooling period from 18 to 23 February 1992, convection to 1500-m depth was observed, where the size of the convection regime was 50–100 km extent. Vertical velocities 40–640 m deep, recorded by four ADCPs of a triangular moored array of 2 km sidelength in the center of the convection regime, exceeded 5 cm s−1 and were not correlated over the separation of the moorings. Horizontal scales estimated from event duration and advection velocity were only around 500 m, in agreement with scaling arguments for convective plumes. Plume activity during nighttime cooling was larger than daytime daytime. Significant evidence for rotation of the plumes could not be found. Overall, plume energy, and the degree of mixing accomplished by them, was much lower than observed during a stronger mistral in February 1987. The mean vertical velocity over the mistral period, determined from the four ADCPs, was near zero, confirming the role of plumes as mixing agents rather than as part of a mean downdraft in a convection regime. The cyclonic rim current around the convection regime was confined to a strip of 〈20 km width with an average velocity of about 10 cm s−1, which is in agreement with near-zero vertical mean velocity in the interior based on potential vorticity conservation. A relation between variations of the larger-scale cyclonic North Mediterranean Current along the boundary and the deep convection could not be identified. An unexplained feature still is the cover of the convection regime by a shallow layer of light water that moves in rather quickly from the sides after the cooling ends.

Description: The bottom and deep circulation in the Somali Basin are investigated on the basis of hydrographic and direct velocity profiles from three shipboard surveys carried out during the southwest monsoon in 1995 and of velocity time series from the WOCE mooring array ICM7. The inflow of bottom water into the Somali Basin through the Amirante Passage drives a thermohaline circulation, which may be modulated by the monsoon wind forcing. Details of the abyssal circulation have been discussed controversially. Deep velocity records from the mooring array in the northern Somali Basin are dominated by fluctuations with periods of 30–50 days and amplitudes above Full-size image (〈1 K). Despite this strong variability annual record averages indicate the existence of a deep western boundary current (DWBC) below Full-size image (〈1 K) at the base of the continental slope south of Socotra Island as part of a cyclonic bottom circulation. The southwestward DWBC transport off Socotra Island is estimated to Full-size image (〈1 K). The bottom and deep water exchange between the Somali and Arabian Basin north of 7°N is estimated from two cross-basin geostrophic velocity sections referenced by vertically averaged LADCP currents. For the bottom water, an eastward transport into the Arabian Basin of Full-size image (〈1 K) and Full-size image (〈1 K) was determined in June and August, respectively, while for the deep-water layer above Full-size image (〈1 K) eastward transports of Full-size image (〈1 K) in June and Full-size image (〈1 K) in August were obtained.