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Characteristics of Intermediate Water Flow in the Benguela Current As Deep-Sea Research II 50 (2003) 87–118 Characteristics of intermediate water flow in the Benguela current as measured with RAFOS floats P.L. Richardsona,*, S.L. Garzolib a Department of Physical Oceanography, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, 3 Water Street, P.O. Box 721, USA b Atlantic Oceanographic and Meteorological Laboratory, NOAA, 4301 Rickenbacker Causeway, Miami, FL 33149, USA Received 28 September 2001; accepted 26 July 2002 Abstract Seven floats (not launched in rings) crossed over the mid-Atlantic Ridge in the Benguela extension with a mean westward velocity of around 2 cm=s between 22S and 35S. Two Agulhas rings crossed over the mid-Atlantic Ridge with a mean velocity of 5:7cm=s toward 2851: This implies they translated at around 3:8cm=s through the background velocity field near 750 m: The boundaries of the Benguela Current extension were clearly defined from the observations. At 750 m the Benguela extension was bounded on the south by 35S and the north by an eastward current located between 18S and 21S. Other recent float measurements suggest that this eastward current originates near the Trindade Ridge close to the western boundary and extends across most of the South Atlantic, limiting the Benguela extension from flowing north of around 20S. The westward transport of the Benguela extension was estimated to be 15 Sv by integrating the mean westward velocities from 22S to 35S and multiplying by the 500 m estimated thickness of intermediate water. Roughly 1.5 Sv of this are transported by the B3 Agulhas rings that cross the mid-Atlantic Ridge each year (as observed with altimetry). This value of the Benguela extension transport is the first one to have been obtained from long-term (two-year) observations and across the full width of the Benguela extension. r 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction equator to the northern North Atlantic where the water is cooled and transformed into deep water A quantitative knowledge of Antarctic Inter- which returns to the South Atlantic (Schmitz, mediate Water (AAIW) circulation patterns and 1996; Gordon et al., 1992; Sloyan and Rintoul, the associated heat and salt transport is important 2001). The low-salinity AAIW layer is one of the to climate studies because AAIW is a key part of most prominent features of the Atlantic and can be the Atlantic meridional overturning circulation clearly traced from the South Atlantic across the (MOC). Thermocline and intermediate water from equator and into the northern North Atlantic. the South Atlantic flows northward across the The upper limb of the Atlantic MOC originates in the southeastern Atlantic where South Atlantic *Corresponding author. Tel.: +508-289-2546; fax: +508- Current water merges with Indian Ocean leakage 457-2163. in the Benguela Current (Reid, 1989; Peterson and E-mail address: [email protected] (P.L. Richardson). Stramma, 1991; Stramma and England, 1999; de 0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0967-0645(02)00380-6 88 P.L. Richardson, S.L. Garzoli / Deep-Sea Research II 50 (2003) 87–118 Ruijter et al., 1999). The Benguela Current flows (Gordon et al., 1992). Estimates of the amount northward as the eastern part of the subtropical of intermediate water entering the Benguela gyre of the South Atlantic (Fig. 1). Accurate Current from the Agulhas range from near zero knowledge of the circulation and amounts of (Rintoul, 1991) to around 50% (Gordon et al., source water in the Benguela Current has re- 1992). Knowledge of the relative amounts and mained lacking because of the complexities of the subsequent pathways is important because Agul- ocean circulation and the lack of suitable has leakage water is warmer and saltier than South direct measurements especially in the intermediate Atlantic Current Water of Drake Passage origin water, which contributes about half of the and could lead to a greater northward heat and transport of the upper limb of the MOC salt flux. NICC NSEC EIC Eq. SICC ESEC SEUC 5S CSEC SECC 10S SSEC 15S 20S 25S 30S Benguela Current 35S AC Brazil Current South Atlantic Current 40S FC 45S Antarctic Circumpolar Current 50S 70W 60W 50W 40W 30W 20W 10W 0 10E 20E Fig. 1. Schematic circulation at the level of Antarctic Intermediate Water (500–1200 m) by Stramma and England (1999). The Agulhas Current (AC) sheds current rings near 40S 18E that translate northwestward in the Benguela Current and its extension. The outlined box (15S–40S, 20E–32W) shows the area of the Benguela Current float experiment and trajectories. Note that the main pathway of intermediate water to the North Atlantic is first northward in the Benguela Current, then westward in its extension, and finally northward along the western boundary north of 25S. Near the equator some intermediate water is entrained into zonal currents as described by Stramma and England (1999). P.L. Richardson, S.L. Garzoli / Deep-Sea Research II 50 (2003) 87–118 89 AAIW enters the subtropical gyre of the South the subtropical gyre) flows northward in the MOC Atlantic near its southwestern corner in the Brazil– along the western boundary and two-thirds flows Falkland confluence zone and also along the southward in the Brazil Current (Boebel et al., southern edge of the South Atlantic Current 1999b; see also Schmid et al., 2000). (McCartney, 1977; Tsuchiya et al., 1994; Boebel Agulhas Current leakage is dominated by et al., 1999a). Some South Atlantic Current water extremely energetic eddies. Ship and satellite data enters the Benguela Current and some merges with have revealed the presence of numerous large the Agulhas Return Current and continues east- (300–400 km diameter) rings of the Agulhas ward past Africa. Part of this eastward flow loops Current embedded in the Benguela Current. The into the subtropical gyre of the Indian Ocean and, rings form as the western part of the Agulhas via Agulhas Current leakage, feeds into the retroflection pinches off from the main current Benguela Current and the South Atlantic sub- (Lutjeharms and Van Ballegooyen, 1988). Ap- tropical gyre. Because the subtropical gyres of the proximately 5–6 rings form per year (Goni et al., South Atlantic and Indian Ocean are partially 1997; Schouten et al., 2000). Olson and Evans connected, it is problematic to use water properties (1986) found that Agulhas rings are among the to clearly identify sources of the Benguela Current most energetic eddies in the world. The Agulhas and to trace the source waters downstream. ring shedding area has the highest levels of eddy Gordon et al. (1992) estimate that approxi- kinetic energy in the southern hemisphere, with mately 65% of the Benguela Current thermocline values reaching 1000–4000 cm2=s2 (Johnson, 1989; and as much as 50% of the Intermediate Water is Patterson, 1985; Ducet et al., 2000). derived from the Indian Ocean. In their scheme, Intermediate water of Indian Ocean origin is the main supply of upper layer water flowing north quickly dissociated from the rings in the Cape across the equator is drawn from AAIW; the lower Basin west of Cape Town, and this water is thermocline and intermediate water passes to blended with water from the South Atlantic lower latitudes of the South Atlantic where Current and also from the tropical Atlantic significant amounts of intermediate water is (McDonagh et al., 1999). This rapid mixing in transferred into the thermocline which could the Cape Basin has made it difficult to determine account for the amount of South Atlantic thermo- accurately the amount of Indian Ocean intermedi- cline water passing northward through the Straits ate water that enters the Benguela Current in rings of Florida as determined by Schmitz and Richard- because source characteristics are rapidly obscured son (1991). In a recent study, Sloyan and Rintoul in the Cape Basin. (2001) concluded that the Atlantic MOC is closed Difficulties in estimating the circulation of by cold, fresh intermediate water that is modified intermediate water in the Benguela Current are to warm, salty varieties by air sea fluxes and the low mean velocities at that level compared to interior mixing in the Atlantic and southwestern the ring velocities and the weak shear in mean Indian Ocean. They suggest that the lower velocity profiles of geostrophic velocity (Garzoli thermocline and intermediate water ðTo101CÞ in and Gordon, 1996). Although there is significant the Benguela Current comes from the South shear in Agulhas rings, there is little shear in the Atlantic Current and Agulhas Current in equal region they are embedded. The small shear and amounts. Unresolved issues are the relative paucity of direct velocity measurements to provide amounts (at different depths) of Indian Ocean a reference for geostrophic velocity has limited our water and South Atlantic water that flow north- knowledge of the subsurface circulation patterns ward in the MOC as opposed to being recycled in the Benguela Current. One notable set of direct around the western side of the Atlantic subtropical current measurements comes from the BEST gyre back into the South Atlantic Current. A (Benguela Sources and Transport) program recent study using floats suggests that around a moored current meter array coupled with inverted third of the intermediate water in the western echo sounders along 30S (Garzoli et al., 1996; Benguela extension (the westward return flow of Pillsbury et al., 1994). These measurements 90 P.L. Richardson, S.L. Garzoli / Deep-Sea Research II 50 (2003) 87–118 spanned the whole Benguela Current in the Cape The Benguela Current Experiment is one of Basin from the coast of Africa out to the Walvis three components of KAPEX (Cape of Good Ridge near 3E (but they missed possible flow Hope Experiments) which is the focus of a series of associated with rings and the portion of the papers in this issue of Deep-Sea Research II.
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