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Cyclonic Gyre in the Tropical South Atlantic

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Cyclonic Gyre in the Tropical South Atlantic Deep-Sell Rtsearch. Vol. 38 . Suppl I, pp S32}-S343. 1991. Ol91'--Ol49fql noo + (] nn 1991 Pergamon Pres~ pic Pnnted In Great Britam. © Cyclonic gyre in the tropical South Atlantic ARNOLD L. GORDON* and KATHRYN T. BOSLEY· (Received 1 March 1989; in revised form 23 January 1990; accepted 12 February 1990) Abstract-A cyclonic gyre within the eastern tropical South Atlantic is resolved by an extensive oceanographic station array obtained in 1983 and 19&4. The gyre is centered near nos and SOE with a sea surface relief relative to 1500 decibars (db) of8 dyn em. The 500 db surface relative to 1500 db reveals a much diminished cyclonic circulation, shifted slightly to the south. The weak baroclinic expression of the cyclonic gyre is confined for the most part to the upper 300 db, with a surface characteristic speed of only 3 cm S- I . A transport of 5 x 1c1' m~ S-I across a line from the gyre center to the African continental margin, including the Angola Current. may best depict the gyre 0-300 db transport (relative to 1500 db). Ship drift data for the region do not show the presence of the cyclonic gyre because the wind-induced Ekman layer masks the gyre. The thermocline of the cyclonic gyre is significantly saltier and lower in oxygen than the suspected source water: the main thermocline of the South Atlantic subtropical gyre. A strong front near 18°S, the Angola-Benguela Front. separates the cyclonic gyre regime from that of the subtropical gyre. Using the regional freshwater balance. gyre thermocline residence time is determined to be between 4.4 and 8.5 years. implying an oxygen utilization of 0.3--0.5 mil-I y- I. Below the thermocline there is some evidence for southward flow of North Atlantic Deep Water along the eastern boundary. It is inferred that this flow is fed by eastward spreading of North Atlantic Deep Water along the equatorial belt. INTRODUCTION BASIN-SCALE maps of the sea surface dynamic height relative to an assumed zero flow reference level at depth (DEFANT, 1961, his Fig. 271; REID et al., 1977; TSUCHIYA, 1985) reveal eastward geostrophic flow between the equator and lOoS in the South Atlantic Ocean. This current extends from 200W to the African continent. The zonal currents across the Greenwich Meridan simulated by a multi-level wind-driven numerical model exhibit eastward flow below a shallow westward-flowing surface layer for the same region (PHILANDER and PACANOWSKI, 1986, see their Fig. 17). The large-scale dynamic topography maps mentioned above do not resolve the sea level slope along the eastern boundary, and hence do not determine whether the flow reaching the African coast turns northward into the Gulf of Guinea or to the south. Current profiler measurements taken over the continental shelf and upper slope from Port Gentil (lOS) to Pointe Noire (5 0 S; WACONGNE, 1988) show evidence of a weak, poleward-flowing undercurrent, somewhat stronger during the summer months. A connection between the eastward-flowing South Equatorial Countercurrent and the southward-flowing coastal undercurrent can be inferred from the hydrographic data of the May 1973 cruise of the R.V. Capricorne (WACONGNE, 1988, her Fig. 31a). Detailed maps of dynamic height * Lamont-Doherty Geological Observatory of Columbia University. Palisades. NY ]0964, U.S.A. S323 S324 A. L. GORDON and K. T. BOSLEY constructed for the Gulf of Guinea indicate southward flow along the coast of Angola from 10° to 200S (MERLE, 1978). Surface temperature maps (MAZEIKA, 1968; HASTENRATH and LAMB, 1977) show southward displacement of warm water to at least 15°S along the African coast, supporting the view that the eastward flow turns southward upon reaching the African Coast. A cyclonic gyre in the eastern tropical South Atlantic was first discussed in detail by MOROSHKIN et al. (1970) using an April to June 1968 oceanographic station array. The dynamic topography maps resulting from that survey show cyclonic motion to 300 m depth, centered near 14°S and 4°E, relative to 1000 decibars (db). Geostrophic velocities within the cyclonic gyre do not exceed 3 em S-I. They make reference to, but do not elaborate upon, an opposing, shallow, wind-driven surface layer. The western boundary of the cyclonic gyre is marked by the broad sweep of surface flow towards the northwest stretching across the South Atlantic, as seen on the large-scale dynamic topography maps. The northern and eastern limbs of the cyclonic gyre have been discussed separately by various authors. The eastward-flowing northern limb is referred to as the South Equatorial Countercurrent (REID, 1964; MOLINARI et al., 1981; MOLINARI, 1982). Molinari, using hydrographic data obtained during four cruises from July 1978 to March 1980, made a distinction between two bands of eastward flow: (1) the South Equatorial Undercurrent located between 3° and 50 S with a subsurface maximum speed of 3 40 em s -1 and a transport ranging from 5 to 23 x Ht' m s -I (Sv)( relative to 1000 db); and (2) the South Equatorial Countercurrent located between 7° and 90 S with maximum speed of 10 em s -1 and a transport range from 3 to 7 Sv. The Angola Current (CURRIE, 1953) forms the eastern boundary of the gyre. This current carries equatorial water southward to the vicinity of Cape Frio, 15° to 18°S, where it encounters the cold, northward-flowing Benguela Current. The confluence of these two currents causes a sharp thermal front, called the Angola-Benguela Front, in the upper 50 m, along with a highly variable circulation pattern (HART and CURRIE, 1960; SHANNON, 1985; SHANNON etal., 1987). A narrow filament of the Benguela water may extend north to 12°S before returning south immediately west of the Angola Current (MOROSHKIN et al .• 1970). It is possible that the position of the front is set by the shelf width or change of the orientation of the coastline, though change in wind stress also has been suggested (SHANNON et al., 1987). Seasonal variations in the position of the Angola-Benguela Front seem to be associated with the wind field, and there is evidence of EI Nino-like episodes in the South Atlantic in which warm surface water extends further south along the Angola coast, suppressing Benguela upwelling (SHANNON et al., 1987). These events are also associated with more westward penetration of the Agulhas Retroflection (SHANNON et al., 1986). Warm events occurred in the years 1963 and 1984, the first such appearances since the early 1950s (SHANNON et al., 1986) . Anomalous warming of the eastern tropical South Atlantic during the 1984 episode is clearly shown in the June 1984 vs the June 1983 satellite-derived sea surface temperature images appearing on the cover of Eos (LEGECKIS, 1989). An XBT section obtained in May 1987 from the R.R.S. Discovery (Fig. 1) provides a detailed thermal view of a meridional slice across this gyre. The 12°e isotherm reaches its shallowest depth of 150 m near 12-13°S, with a secondary shallowing near 4°-50 S. while the lOoC isotherm attains its shallowest depth of approximately 250 m near 18°S. The double­ peaked 12°C isotherm is consisten.t with the tw~ axes of eastward current mentioned by MOLINARI (1982). The poleward shIft of the 10"C Isotherm crest, relative to that of the l20C 3] ~~37[~ w u ItIr ~:a.!> J ITO II., MO 1!1O M5 '40 116 130 1fl 120 tIS 110 105 100 9$ 110 85 10 7S 10 Vtl$ SIlO 55 O ~~~~~~~~~~~~~uILu~~~;U~~~W+Uu~~~~~~~~~+U~~~~~'" , 28 26 ~ , Z2 " , : I!O ;', 20 ~ ;"';Z~' v!.. ., ~r,;4~",-~~. , ,JE_ ~~~~§~~~~~ : \~.\ ~~~~/ ~~~Jr'''' ~f- ,.. \.....- ' , L: /' ~ ~/'''vv~''~' ~~ '-\J'\ ' 100 ,../"~\~~~ --.' . i.. '" __ /I ~ ./\ ,, ' -"", ,~" ~ \../---/\, ''''---V _ t _ '"'vV (\ ' \ - , , ,/ \~~~ ~ \../ . /' (' A /' \\-IJ\\v' ,,'\ F /' ,--" ,, - ' ," / / r- ,-.. ,/" v r--J\) \../ ............. J V L r-.. ' _ I~ / \ ~ \...1Z , ("'I n (' , '-' ,..-- \ .-. V ' 12 \ n / r-., ) -- r~, / \..-- ~ v ' \ , '. ~ I v / / '"-,,,,' ' J ../' ,\....-, -, \ 200 I 'JV "\ I - \ J '\ ~ , r i \ ' ,'A, ~ r'-...'./'~\t !'\r-,('j\Ifv ,1 J\ L.-. ' f " V , ' ; ~ ~ ~~ 12 J J\: '- ~ ' !\ i\..r----...., ~ ~ - ~ j\ I ~)." -\ \ ,>, l0 --"J'v~I\f ' _ ~ ./ /'J , 300r , 'v In'< E " \0, 1\ ) 'V {\/\ 1\ f\ I 0 \ , ( 0 Y '----..... ) " .J V '"' f V ', ('-0' \ \ Iv . r ~, n / ' t" V\i 0- t- ',. ,..-- '-', \J 10 r ,~ il) ' ~ '--' \) \ r' r "- w I a _ J ; ~' '\ IIV", I, ' _ (lr \ \; ! A~: \~ ( 'J'''JV' , ',' / ,r"'- r I ~ V \ / . \l r ' V / l 400 j ,; .. I '\ (\' ,'\} ,c .. ,_/ ' ----J ,r .J J I~ \ f \ J'\ /\ : )' ,1\ ' \)' f\ ! /rf ", '8\ // '--' ~:'\) ~ r\~ 8 -~ \~"\ :f\ v .,V '-----J \J v \;\ I \V I 'V - - j \ f', ( I ,Gf\J \_/v ", J I ' '.. ( v 500 ~ V '-'\ ~~ r' ~' AJV(\J \' . '- - '- - - - 6_ 'v-J, ~ J /'- /\/ ~\ /" i '-----6 V ' __ __""-- "'- .../ ~ __ .- :,'\ \J \. - I \ ) 6 - ' ,, " rjv '- ~ - , ANGOLA BASIN TEMPERATURE ('C) GOO Leg 3 )\1" ll,lder Itl - .- N DISCOVERY 165 B j 1500 1400 1300 1200 1100 1000 900 600 100 6QO :;00 400 300 ZOO 100 0 DISTANCE (NourlCol 1.1,1 ... ) E WI ION 0" ,!s 2° 3' 4' 5' 6° .,. 8" go 10° II" 12- 13" 14° ISO 16" 17" 18- 19' 20" 21° 22" 23' 24° 25° i6° 2":,5'$ 9"3015'W LATITUDE (Degrees) 12°454'E en Fig. 1. Temperature for the upper 700 m with surface salinity, obtained by XBT and surface sampling in May 1987 from the R.R.S Discovery. The section w extends from near Luderitz (-2JDS, -I2°E) to Liberia (JON, -WOW) (see Fig. 2). ~ S326 A. L. GORDON and K. T. BOSLEY isotherm, reflects a similar shift of the cyclonic gyre observed in the dynamic topography pattern, discussed below.
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