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

The flow of intermediate water masses across the equator in the Atlantic Ocean is of fundamental interest in the context of the global meridional circulation 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 1200‐m depths in the western equatorial Atlantic (5°S–7°30′N). These have been deduced from hydrological and geochemical tracer (nutrients and chlorofluorocarbons) data sets from CITHER 1 (R/V L'Atalante, January‐March 1993), ETAMBOT 1 (R/V Le Noroit, September‐October 1995), and ETAMBOT 2 (R/V Edwin Link, April‐May 1996) cruises. Both the AAIW and UCPW enter, on the isopycnals σθ = 27.25 (676±36 dbar) and σθ = 27.40 (919±35 dbar), respectively, the equatorial belt as narrow, northwestward flows around the northeast tip of Brazil near 5°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 pronounced at the AAIW level, in agreement with earlier observations in the upper layers. At the northern boundary of the equatorial belt (7°30'N) the AAIW flows along the western boundary while the UCPW, instead, recirculates into the interior of the ocean.