On the crossing of the equator by intermediate water masses in the westernAtlantic Ocean: Identification and pathways of Antarctic Intermediate Water and Upper Circumpolar Water

The flow of intermediate water masses across the equator in the Atlantic Oc ean is of fundamental interest in the context of the global meridional circ ulation cell associated with the formation of the North Atlantic Deep Water . This paper describes the flow and pathways of the Antarctic Intermediate Water (AAIW) and the Upper Circumpolar Water (UCPW) at between 500- and 120 0-m depths in the western equatorial Atlantic (5 degrees S - 7 degrees 30'N ). These have been deduced from hydrological and geochemical tracer (nutrie nts and chlorofluorocarbons) data sets from CITHER 1 (RN L'Atalnte, January -March 1993), ETAMBOT 1 (RN Le Noroit, September-October 1995), and ETAMBOT 2 (RN Edwin Link, April-May 1996) cruises. Both the AAIW and UCPW enter, o n the isopycnals sigma(theta) = 27.25 (676 +/- 36 dbar) and sigma(theta) = 27.40 (919 +/- 35 dbar), respectively, the equatorial belt as narrow, north westward flows around the northeast tip of Brazil near 5 degrees S. 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 variations in spreading and recirculation occur at both levels, but they are more pro nounced at the AAIW level, in agreement with earlier observations in the up per layers. At the northern boundary of the equatorial belt (7 degrees 30'N ) the AAIW flows along the western boundary while the UCPW, instead, recirc ulates into the interior of the ocean.