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Annual Cycle and Variability of the North Brazil Current

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Annual Cycle and Variability of the North Brazil Current JANUARY 1998 JOHNS ET AL. 103 Annual Cycle and Variability of the North Brazil Current W. E. J OHNS AND T. N . L EE Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida R. C. BEARDSLEY,J.CANDELA, AND R. LIMEBURNER Woods Hole Oceanographic Institution, Woods Hole, Massachusetts B. CASTRO Instituto Oceanogra®co Universidade SaÄo Paulo, SaÄo Paulo, Brazil (Manuscript received 18 October 1996, in ®nal form 5 June 1997) ABSTRACT Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 48N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv [ 106 m3 s21) in July±August to a minimum of 13 Sv in April±May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by ¯ow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3±5 Sv and appears to be fairly constant throughout the year, based on the available current meter records and shipboard ADCP surveys. The NBC transport cycle is in good agreement with linear wind-driven models and appears to be in near-equilibrium with remote wind stress curl forcing across the tropical Atlantic for much of the year. However, the mean transport of the NBC is 15 Sv larger than can be explained by wind forcing alone, indicating a strong thermohaline component. Mesoscale variability in the region is dominated by ¯uctuations with periods near 25±40 days and 60±90 days. The 25±40-day ¯uctuations are strongly surface trapped and are most energetic in early summer during the acceleration phase of the NBC. The lower-frequency ¯uctuations have a deeper reaching baroclinic structure, are present year-round, and are associated with the propagation of large anticyclonic eddies north- westward along the coast. It is hypothesized that these features may serve as a catalyst for the eddy shedding process seen in the NBC retro¯ection in earlier observations. 1. Introduction conduit for cross-equatorial transport of South Atlantic The North Brazil Current is a major low latitude west- upper-ocean waters as part of the Atlantic meridional ern boundary current in the Atlantic that transports up- overturning cell. This northward thermohaline return per-ocean waters northward across the equator. Its coun- ¯ow originates from a combination of intermediate wa- terparts in the other ocean basins are the Somali Current ters ¯owing though Drake Passage from the Paci®c in the Indian Ocean (DuÈing and Schott 1978; Swallow Ocean (Rintoul 1991) and warmer thermocline waters et al. 1991) and the New Guinea Coastal Current in the ¯owing westward around South Africa from the Indian Paci®c (Lukas et al. 1991). However, due to differences Ocean (Gordon 1986; Gordon et al. 1992). On the mean in basin geometry and forcing, each of these current these two sources, plus northward ¯ow of Antarctic Bot- systems is rather distinct in character. In the Atlantic tom Water, must combine to balance an export of ap- 6 3 21 the NBC plays a dual role, ®rst, in closing the wind- proximately 15 Sv (Sv [ 10 m s ) of North Atlantic driven equatorial gyre circulation and feeding a system Deep Water from the Atlantic (Roemmich and Wunsch of zonal countercurrents and, second, in providing a 1983; Schmitz and McCartney 1993). The formation region of the NBC is generally agreed to be near 108S where waters ¯owing westward in the South Equatorial Current (SEC) ®rst begin to concen- Corresponding author address: Dr. William E. Johns, RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL trate into a northward boundary current (Fig. 1a). Near 33149. 58S the northward transport of the NBC determined by E-mail: [email protected] geostrophic calculations and current pro®ling is ap- q 1998 American Meteorological Society Unauthenticated | Downloaded 09/25/21 07:05 PM UTC 104 Unauthenticated | Downloaded 09/25/21 07:05 PM UTC JANUARY 1998 JOHNS ET AL. 105 proximately 15±20 Sv (Stramma 1991; Stramma et al. variation is determined by remote wind stress (i.e., 1995; Schott et al. 1995). Most of this transport is con- Sverdrup forcing) in the interior, versus local wind tained in the thermocline, with only 3±5 Sv occurring forcing in the coastal region. For the tropical Atlantic in the surface layer; hence Stramma et al. (1995) refer between 08±58N, linear Sverdrup dynamics predicts a to the northward boundary current here as the North maximum northward boundary current transport dur- Brazil Undercurrent (NBUC). Between 58S and the ing boreal summer and fall, when the ITCZ shifts to equator the NBC increases its transport due to predom- its northernmost position near 108N, and a reversal to inantly surface in¯ow from the SEC, leading to an an- a weak southward boundary current during boreal nual mean transport at the equator of 32 Sv in the upper spring when the ITCZ retreats to the equator (Mayer 600 m (Schott et al. 1993, hereafter S93). Seasonal vari- and Weisberg 1993). No evidence for such a reversal ability of the NBC between 108S and the equator appears has been found for the NBC, suggesting that either to be quite small; for example, S93 ®nd an annual cycle the remote wind forcing signal is overpowered by at the equator of only 63 Sv, with maximum transport northward thermohaline ¯ow carried by the NBC, or during June±August and minimum transport during De- that it is obscured by propagation delays from the cember±February. interior or local forcing effects. North of the equator water begins to leave the NBC In an effort to determine the pathways of the upper- and ¯ow into the interior, feeding a system of zonal ocean thermohaline ¯ow through the tropical Atlantic, countercurrents: the North Equatorial Countercurrent and the manner in which this ¯ow interacts with the (NECC), the Equatorial Undercurrent (EUC), and North seasonal wind-driven circulation, an international ®eld Equatorial Undercurrent (NEUC)(Metcalf and Stalcup program (WESTRAX, Brown et al. 1992) was con- 1967; Cochrane et al. 1979). At the surface the NBC ducted during 1989±91 concentrating on the western continues to 68±78N where it retro¯ects seasonally into boundary region between 08 and 108N. Moored current the NECC from approximately June±January. Shipdrifts meter observations were collected in the North Brazil and drifter trajectories show that eastward ¯ow in the Current near the equator (S93), near 48N (this study), NECC ceases in the western Atlantic from February± and near 68N (Colin et al. 1994), to investigate the struc- May and is replaced by weak westward ¯ow (Richard- ture and variability of the NBC at several locations along son and Walsh 1986); therefore, any surface water trans- the coast. Earlier current meter measurements in the ported northward in the NBC at this time is presumed NBC retro¯ection region near 88N are also available to continue northward along the coast and join the west- and those results are reported in Johns et al. (1990, 1993) ward ¯ow of the North Equatorial Current entering the and Fratantoni et al. (1995). In addition, measurements Caribbean. In the thermocline layer a portion of the were collected from moored current meters deployed on NBC ¯ow separates at or just north of the equator to the North Brazil shelf near 48N during February±May feed the EUC, while another portion continues to 38± 1990, and from shipboard ADCP surveys of the shelf 48N where it appears to feed seasonally into the NEUC. region during the WESTRAX period, as part of a sep- Some of this thermocline ¯ow may also recirculate back arate international ®eld program (AMASSEDS, Nit- into the NBC around a semipermanent anticyclonic eddy trouer et al. 1991) designed to investigate the transport or gyre (the ``Amazon'' eddy: Bruce et al. 1985) cen- of freshwater and suspended sediment from the Amazon tered near 28N. Mesoscale energy is known to be large River into the Atlantic. in the region (Wyrtki 1976; Johns et al. 1990; Didden The emphasis in this paper is on the currents and and Schott 1993) and has made the identi®cation of transports in the upper water column, extending from seasonal mean ¯ow patterns from synoptic ship surveys the surface to approximately 800 m, where the transition dif®cult. from the northward ¯owing NBC regime to the south- Models (e.g., Philander and Pacanowski 1986) in- ward ¯owing deep western boundary current (DWBC) dicate that the NBC has a large seasonal cycle north regime occurs. Mean ¯ows and statistics from the deeper of the equator, related to the seasonal migration of current measurements are also presented, but a detailed the intertropical convergence zone (ITCZ) and asso- discussion of these records is deferred until further syn- ciated changes in the wind stress curl across the in- thesis with regional measurements collected in the terior of the basin. A question left to be answered for WESTRAX time frame. the NBC, and for low-latitude western boundary cur- The organization of the paper is as follows. Data rents in general, is the extent to which their transport sources used in the analysis are described in section ← FIG.
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