Yves Gouriou

Mean Circulation of the Upper Layers of the Western Equatorial Pacific Ocean

E and on O.soN at l42°E. No variation in its transport (15.0 x 10 6 m 3 s·l) is found between those longitudes. We fmd indication of the Equatorial Undercurrent at 137°E-0.75°N in the geostrophic field. The northern and southern Subsurface Countercurrents are clearly identified by extrema of eastward velocity at l65°E around 3°N and 3°S (250 dbar). No evidence of a southern Subsurface Countercurrent is found at l42°E. At 137°E the northern Subsurface Countercurrent is not characterized by a local extrema of eastward velocity: the North Equatorial Countercurrent seems to extend from the surface to 400 dbar with a south ward shi ft of its core. Analysis of the seasonal variability at l6s o E indicates that the E~atorial Undercurrent transport increases by a factor 2 between January (10.7 X 10 6 m 3 s-l) and JuIy (21.5 x 10 m 3 s·l) and the Equatorial Interrnediate Current transport is halved (6.3 x 10 6 m 3 s·l in January, 3.5 x 10 6 m 3 s·l in JuIy). In contrast, the transport of the Subsurface Countercurrents does not vary substantially between those two months. The meridional distributions of salinity and potential vorticity show that the axes of the main eastward currents are associated with strong meridional property gradients, not with property extrema. The eastward currents thus represent a barrier to the riorthward extension of the high salinity Tropical Water. Relatively weak meridional gradients of salinity and potential vorticity are observed in the westward directed South Equatorial Current and Equatorial Interrnediate Current.

On the circulation in the upper layer of the western equatorial Atlantic

Hydrographic observations of pressure, temperature, salinity, dissolved oxygen, and shipboard acoustic Doppler current profiler measurements are used to study the upper layer circulation in the western equatorial Atlantic Ocean, limited to the region bounded by the 10°S and 14°N latitudes between the longitudes 30°W and 52°W. Data were obtained during four World Ocean Circulation Experiment cruises, carried out in January-March 1993, January-March 1994, September-October 1995, and April-May 1996. In the upper layer, the continuity of the northwestward flowing North Brazil Current along the American continent toward the Caribbean Sea is confirmed in boreal spring. Furthermore, part of the North Brazil Current also continues northwestward in the subthermocline layer during short periods in boreal spring, contrary to previous estimates. The North Equatorial Countercurrent (NECC) is present in boreal spring west of 40°W, fed with water of Northern Hemisphere origin only. The southeastward flowing current observed at 3°N–44°W is fed by the North Brazil Current retroflection and by a cyclonic recirculation of the southern edge of the North Equatorial Current. The upper layer of this current, at 3°N–44°W, feeds the Equatorial Undercurrent (EUC) and the NECC, when its subthermocline layer feeds the EUC and the North Equatorial Undercurrent. Near-surface eastward flow is present above the EUC during all cruises, yielding to a strong increase of the eastward warm water transport.

Hydrographic sections across the Atlantic at 7°30N and 4°30S

Abstract Transatlantic hydrographic sections along 7°30N and 4°30S, and shorter meridional ones along 35°W and 4°W in the intervening latitudinal range, provide a basin-wide description of the Atlantic water masses at their crossing of the equator. The water masses belonging to either the cold or warm segment of the global thermohaline cell enter the equatorial region mostly in the form of western boundary currents. The ways they leave it are more varied. The Ekman drift and a geostrophic western boundary current cause the export of near-surface water to the North Atlantic. A part of the southern Salinity Maximum Water, regarded as the shallowest warm water component, is thought to follow this route after experiencing strong property modification in the equatorial upwelling. The underlying South Atlantic Central Water divides into two northward paths, a direct one along the south American continental slope, hardly observed in the data because of an intense variability in the western half of the 7°30N line, and a longer one through the eastern basin, taken by water of the equatorial thermostad. There is no trace of such as eastern northward route for the Antarctic Intermediate Water, which is apparently forced northward from the equatorial region through the highly variable circulation of the western basin. The deep western boundary currents carrying southward the upper and middle components of the North Atlantic Deep Water experience a first partial shift to the eastern boundary on crossing the equator. At deeper levels, a part of the lower North Atlantic Deep Water also bifurcates eastward at the equator, but loses its identity through vertical mixing with the Antarctic Bottom Water in the equatorial fracture zones. The newly formed homogeneous bottom water proceeds eastward in the Guinea Basin, with further indication of an overflow into the Angola Basin. Beside the North Atlantic Deep Water, the deep layer of the equatorial region contains a lower-oxygen component, most clearly present between the middle and lower cores of the North Atlantic Deep Water. Previous results on this water are substantiated, namely, an arrival from the southeast, and northwestward crossing of the equator offshore from the deep western boundary current of northern water. A further northward progression of the southern water requires that the equatorial branching of the southward deep boundary current be only intermittent. A comparison of the temperatures along 7°30N in 1993 with those obtained at 8°N during the International Geophysical Year, 36 years before, reveals a net warming of the intermediate and upper deep waters, and cooling of the bottom water. This result is similar to that obtained at 24°N by other authors, yet there are signs of a southward propagation of a deep cold anomaly in the western basin, which had reached 24°N in 1992, but not yet 7°30N in 1993.

The zonal currents and transports at 35°W in the tropical Atlantic

The total of 13 existing cross-equatorial shipboard current profiling sections taken during the WOCE period between 1990 and 2002 along 35°W are used to determine the mean meridional structure of the zonal top-to-bottom circulation between the Brazilian coast, near 5°S, and 5°N and to estimate mean transports of the individual identified shallow, intermediate and deep current branches. One of the results is that, on the equator, a mean westward Equatorial Intermediate Current below the Equatorial Undercurrent exists.

Why Were Sea Surface Temperatures so Different in the Eastern Equatorial Atlantic in June 2005 and 2006

Abstract A comparison of June 2005 and June 2006 sea surface temperatures in the eastern equatorial Atlantic exhibits large variability in the properties of the equatorial cold tongue, with far colder temperatures in 2005 than in 2006. This difference is found to result mainly from a time shift in the development of the cold tongue between the two years. Easterlies were observed to be stronger in the western tropical Atlantic in April–May 2005 than in April–May 2006, and these winds favorably preconditioned oceanic subsurface conditions in the eastern Atlantic. However, it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid-May 2005 over a broad region extending from 20°W to the African coast and from 6°S to the equator. This particular event underscores the ability of local intraseasonal wind stress variability in the Gulf of Guinea to initiate the cold tongue seaso...

Isopycnal and diapycnal circulation of the upper equatorial Atlantic Ocean in 1983–1984

Isopycnal circulation and diapycnal processes in the equatorial Atlantic Ocean are investigated using eight cruises from November 1982 to July 1984. An eastward decrease of the transport of the equatorial undercurrent is observed averaging 18.2 106 m3 s−1 at 35°W to 10.2 106 m3 s−1 at 4°W. There is also a meridional convergence at the undercurrent level and a larger meridional divergence in the surface layer of 15 106 m3 s−1 between 35°W and 4°W. In the eastern equatorial Atlantic, the undercurrent core density as well as the salinity above the undercurrent experience a large seasonal cycle. During the season where the eastern Atlantic thermocline is closest to the surface and surface waters are coldest, the current peaks in denser waters and subsurface salinity maximum are weaker. An analysis of the salinity on isopycnal surfaces indicates that there is significant diapycnal mixing in the upper thermocline. In the eastern Atlantic, the upper thermocline vertical heat diffusivity coefficient scaled over 1° of latitude varies between 3 cm2 s−1 during the upwelling season and nearly 0 early in the year. This seasonal mixing is an important element of undercurrent dynamics. The turbulent heat flux at the base of the surface layer averages 50 W m−2 between 1.5°N and 1.5°S. The surface layer heat budget implies an average oceanic gain of 60 W m−2 between 1.5°N and 1.5°S which could result from exchanges with the atmosphere. The heat flux and fresh water seasonal cycles needed to close the budgets are not realistic, however. The eastern equatorial Atlantic thermocline seasonal upwelling is associated with a large inflow into the surface layer. Over the two years, this flow averaging 11–12 × 106 m3 s−1 originates from the upper thermocline, and diapycnal transports near the core of the undercurrent or below are found to be small. Below the core of the undercurrent, we also find that mixing is not intense: for instance, at σθ = 26.5, in the upper part of the thermostad, vertical heat diffusivity is only 0.6 cm2 s−1 (assuming that mixing takes place over 1° of latitude). The uncertainties on these budgets are however large, and assumptions on the dynamics that were used could not be checked.

Deep jets in the equatorial Atlantic Ocean

Deep velocity profiles taken in the equatorial Atlantic Ocean show equatorially trapped deep jets with similar features to those of the Indian and Pacific Oceans: a zonal velocity of the order of 10 to 20 cm s−1 and a meridional scale of 1°. In the Pacific and Indian Oceans the zonal extent of the jets is at least 15° of longitude. Owing to the lack of synoptic measurements, we have no information on the zonal scale in the Atlantic Ocean, but we present here zonal velocity profiles, made at a 16-month interval, that have identical baroclinic structure in the western (35°W) and central basin (13°W). The Atlantic jets have a vertical scale larger (400–600 m) than those observed in the Pacific Ocean (250–400 m). Our measurements confirm the opposite directions of the jets for different seasons in the Atlantic Ocean. Furthermore, for a given season, the vertical profiles of zonal velocity at 35°W–0° are astonishingly similar at a 5-year interval. As in the Pacific and Indian Oceans, the jets are embedded in a large-vertical-scale current that changes direction with time, The few profiles available in the equatorial Atlantic Ocean suggest a seasonal reversal of the jets, but neither this nor the temporal variability of the large-scale current has been adequately resolved.

Tracer distributions and deep circulation in the western tropical Atlantic during CITHER 1 and ETAMBOT cruises, 1993-1996

This paper presents CFC and nontransient tracer observations in the western equatorial Atlantic Ocean on repeated sections along 7°30′N, the 35°W meridian, and a transect crossing the Ceara Rise. Three World Ocean Circulation Experiment cruises have been carried out in this area, in February-March 1993 (CITHER 1) and September-October 1995 and April-May 1996 (ETAMBOT 1 and 2). Together with the tracer data, the direct current measurements are used to deduce the circulation pathways. The data confirm the principal circulation features of the Upper North Atlantic Deep Water within the area. The Deep Western Boundary Current flows southward along the continental slope, while, adjacent to the DWBC, there is a northward flow corresponding to the DWBC recirculation whose origin appears variable. The largest variability is observed along the 35°W meridional section, where the DWBC bifurcates eastward north of 3°S but the eastward equatorial flow does not appear to be permanent. At the Middle North Atlantic Deep Water level, CFC distributions exhibit the most important contrast between waters of northern and southern origin. The circulation at the Lower North Atlantic Deep Water level, mainly controlled by the topography, looks more permanent because the DWBC is regularly observed during the three cruises, and there is no evidence for a northwestward recirculation along the Mid-Atlantic Ridge. The transient behavior of CFC distributions, which is superimposed on local circulation effects, can be seen clearly through their temporal evolution within the DWBC. The variability of the deep circulation observed during the 1993–1996 interval rules out a dominant variability in response to semi annual forcing.

Intercomparison of the upper layer circulation of the western equatorial Atlantic Ocean: In situ and satellite data

Thanks to the agreement found between in situ measurements and TOPEX/POSEIDON data in the western tropical Atlantic Ocean, a realistic picture of the spatial and temporal variability over the 1992–1997 period is obtained. The sea level variability clearly emphasizes three ranges of variability. The intraseasonal variability is associated with propagating features north of the equator, consistent with a first baroclinic Rossby wave characteristic. The annual variability, which represents the largest part of the variability, describes the seasonal cycle of the North Equatorial Countercurrent; but, more interesting, is a clear year-to-year variability in the northernmost part of the area. The surface currents also reveal an intraseasonal tendency with a peak of energy at 62 days at 5°N. Upper layer volume transport across 38°W and between 3°N and 9°N shows a regular seasonal contrast mostly due to the 3°N–6°N region. Year-to-year variations are also clearly evidenced. The eastward transport loses about 35% of its strength in the second half of 1995 compared with 1994. This event is followed by a stronger than usual westward transport in early 1996, and in early 1997, the transport seems abnormally eastward. However, contrary to the seasonal cycle, this interannual variability occurs mainly in the 6°N–9°N band. Interhemispheric transport computed across 7°30′N, between 50°W and 35°W, is northward during the whole period 1992–1995. A seasonal cycle can be detected with maximum transport during boreal winter and minimum in spring. An interesting result is the increasing tendency of the northward transport from April 1993 until boreal fall 1995, when the maximum value for the period is reached.

A comparison of kinematic evidence for tropical cells in the Atlantic and Pacific oceans.

Kinematic evidence for the existence of Tropical Cells (TC) in the Atlantic Ocean is offered. Mean sections of meridional velocity, its horizontal divergence and vertical velocity are estimated from twelve available sections centered at about 35°W. Of the twelve sections, six were occupied in March and April, thus there is a boreal spring bias to the observations. Equatorial upwelling and offequatorial downwelling, between 3°N and 6°N, represent the southern and northern boundaries of a northern hemisphere TC. Uncertainties for the estimates of average quantities are large. However, favorable comparisons with observational representations of Pacific TCs provide support for the existence of a northern hemisphere Atlantic TC.