Transports and Pathways of the Upper-Layer Circulation in the Western Tropical Atlantic

Abstract

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 10S, carrying 23 Sv (Sv 10 6 m 3 s 1) above 1000 m. Out of this, 16 Sv are waters warmer than 7C 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 5S, 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 44W, 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.

Pathways and variability at intermediate depths in the tropical Atlantic

“…Vertical and horizontal structure Schmid et al [2001] found, using an equatorial LADCP section, that the zonal flow at the equator between 500 ¤ and more than 1000 ¤ . This range is quite similar to the range of 400-600 ¤ published elsewhere [Ponte et al, 1990;Böning and Schott, 1993;Schott et al, 1998;Molinari et al, 1999;Gouriou et al, 1999;Gouriou et al, 2001;Send et al, 2002]. Some of the earlier observations show that quite small vertical extents of the jets can be found at depths that include the AAIW layer (e.g.…”

“…Stramma and Schott [1999] (in 1990 to 1996) show the North Brazil Undercurrent as a current with the velocity maximum between 100 and 400 ¤ , and a vertical extent from the surface to at least 1000 ¤ [Schott et al, 1998]. At several latitudes portions of the water in the western boundary current feed into the zonal currents, primarily the SICC and NICC (Figure 1, top).…”

“…Transport estimates from several studies indicate that the strength of the western boundary current depends on the latitude [Fu, 1981;Holfort, 1994;Schott et al, 1998;Schmid et al, 2000]. Such changes can be partially attributed to zonal transports that feed into the boundary current or are fed by it.…”

“…For example, seasonal reversals of the zonal flow at intermediate depth have often been observed close to the equator [e.g. Schott et al, 1998;Boebel et al, 1999a;Gouriou et al, 1999;Molinari et al, 1999;Richardson and Fratantoni, 1999;Schmid et al, 2001]. It has been proposed that planetary waves cause the variability.…”

Caracterização hidrográfica da plataforma continental do maranhão a partir de dados oceanográficos medidos, remotos e modelados

“…Estas medidas in situ foram obtidas durante os cruzeiros oceanográficos do Programa REVIZEE/SCORE-NO, e serão complementadas neste estudo com informações obtidas por satélites e por resultados de simulação obtidos através do experimento numérico com um modelo oceânico de alta resolução espacial (1/6 • ). Trata-se de umaárea de grande dinâmica, resultado da ação combinada do aporte deáguas continentais, dos ventos alíseos, e da influência da Corrente Norte do Brasil (CNB) e sua componente em subsuperfície a Subcorrente Norte do Brasil (SCNB), que transporta grande volume d'água para noroeste, paralelamenteà costa (Johns et al, 1998;Schott et al, 1998;Silva, 2006).…”