On the crossing of the equator by intermediate water masses in the western Atlantic ocean: Identification and pathways of Antarctic Intermediate Water and Upper Circumpolar Water (original) (raw)

1999, Journal of Geophysical Research

The flow of intermediate water masses across the equator in the Atlantic Ocean is of fundamental interest in the context of the giobai meridional circulation C{2II abbOldl•t•U Wl[11 [11• • o i l• vv Antarctic Intermediate Water (AAIW) and the Upper Circumpolar Water (UCPW) at between 500-and 1200-m depths in the western equatorial Atlantic (5øS -7ø30'N). These have been deduced from hydrological and geochemical tracer (nutrients and chlorofluorocarbons) data sets from CITHER 1 (• L'Atalante, January-March 1993), ETAMBOT 1 (• Le Noroit, September-October 1995), and ETAMBOT 2 (• Edwin Link, April-May 1996) cruises. Both the AAIW and UCPW enter, on the isopycnals 60-27.25 (676 ß 36 dbar) and 60 = 27.40 (919 ß 35 dbar), respectively, the equatorial belt as narrow, northwestward flows around the northeast tip of Brazil near 5øS. During transit within this zone the core properties of UCPW erode more than those of AAIW. Flow patterns of both the water masses show westward spreading and eastward recirculations on either side of the equator. Temporal v•iations in spreading and recirculation occur at both levels, but they are more pronounced at the AAIW level, in agreement with earlier observations in the upper layers. At the northern bound•y of the equatorial belt (7ø30'N) the AAIW flows along the western boundary while the UCPW, instead, recirculates into the interior of the ocean. complex exchanges between the intermediate waters and the overlying warm waters, on one hand, and the underlying cold waters, on the other hand, occur mainly in the western equatorial Atlantic. However, notwithstanding the importance of the intermediate water to the thermohaline circulation between the North and South Atlantic Oceans, this layer, except for a few recent studies [Suga and Talley, 1995; Bub and Brown, 1996], has received little attention. The northward spreading AAIW can be traced by its salinity minimum accompanied by a high-oxygen anomaly [Wast, 1935]. The oxygen maximum disappears as it meets the strong oxygen minimum characteristic of the equatorial zone [Reid, 1989]. Just beneath the salinity minimum in the lower part of the intermediate layer there is a potential temperature minimum that has been identified by Reid [1989] as the signature of the Upper Circumpolar Water (UCPW). Thus the AAIW and UCPW constitute the deepest layer of the relatively warm Atlantic waters. The equatorial circulation within this layer, lying approximately between 500 and 1200 m, differs from that of the overlying South Atlantic Central Water layer [Stramma and Schott, 1996]. The trajectories of the floats at 800 m depth in the study of Richardson and Schmitz [1993] also reveal the complex pathways of the water masses in the western tropical Atlantic. Nevertheless, in the equatorial Atlantic, the AAIW and UCPW are often treated as a single layer [Stramma and Schott, 1996], and even now the circulation of UCPW has not been differentiated from that of the AAIW. For example, Tsuchyia et al. [1994] consider the temperature minimum and the associated silicate maximum north of 21øS as the lower boundary of the AAIW rather than the northward extension of the UCPW silicate maximum. 20,911 20,912 OUDOT ET AL.' INTERMEDIATE WATER MASSES IN WESTERN ATLANTIC The large amount of data, in particular, on the geochemical (nutrients and chlorofluorocarbons) tracers, collected during the trans-Atlantic CITHER 1 cruise on World Ocean Circulation Experiment Hydrographic Programme (WHP) lines A6 and A7 and the ETAMBOT cruises in the western equatorial Atlantic, gave us an opportunity to study and differentiate the circulation patterns of AAIW and UCPW in the equatorial Atlantic, especially on its western boundary. The two trans-Atlantic Conductivity-temperature-depth-oxygen (CTDO2) tracer sections with closely spaced stations along 7ø30'N and 4ø30'S (WHP lines A6 and A7) and the two meridional sections along 3ø50'W and 35øW between these two latitudes were occupied in January-March 1993 (CITHER 1 cruise; Figure la)[Andrig et al., 1998; Arhan et al., 1998; Oudot et al., 1998]. The western equatorial Atlantic basin stations of the CITHER 1 cruise were reoccupied 0 ß ß ß ß 1000 ß ß ß ß 0