SUBDUCTION OF SOUTH ATLANTIC SUBTROPICAL MODE WATERS

ABSTRACT . The formation of the Subtropical Mode Waters (STMW) in the South Atlantic, part of the South Atlantic Central Water (SACW), by the subduction process, transferring mixed layer fluid into the permanent thermocline, is investigated using results of numerical simulations with the HYbrid Coordinate Ocean Model (HYCOM). Subduction rates were estimated by the kinematic method, adding the lateral induction of fluid through the sloping base of winter mixed layer with the vertical velocities at the base of winter mixed layer. Subduction rates above 100 m/year were found over the South Atlantic Subtropical Front, with maximum rates larger than 200 m/year in three distinct regions. The subduction pattern is dominated by the contribution of lateral induction, specially over the Subtropical Front, with rates significantly larger than the maximum rate of Ekman pumping. Different STMW were identified, associated with maximum layers thickness in isopycnals representative of upper and middle portion of SACW. The regions of maximum subduction rates were associated with the formation of the STMW. Keywords : mixed layer, ventilation, SACW, permanent thermocline, lateral induction. RESUMO . A formacao de Aguas Modais Subtropicais (AMS) no Atlântico Sul, que fazem parte da Agua Central do Atlântico Sul (ACAS), transferindo fluido da camada de mistura para a termoclina permantente pelo processo de subduccao, foi estudada a partir dos resultados de simulacoes numericas com um modelo oceânico de coordenadas hibridas (HYCOM – Hybrid Coordinate Ocean Model). A subduccao foi calculada pelo metodo cinematico, somando as contribuicoes da inducao lateral de fluido atraves da base da camada de mistura e as velocidades verticais na base da camada de mistura de inverno. Foram encontradas taxas de subduccao superiores a 100 m/ano ao longo da Frente Subtropical do Atlântico Sul, com tres nucleos distintos de maxima subduccao atingindo mais de 200 m/ano. A inducao lateral mostrou-se o processo dominante na subduccao, especialmente ao longo da frente, com taxas significativamente superiores ao bombeamento de Ekman. Foram identificadas diferentes AMS associadas as maximas espessuras de camadas representativas das porcoes media e superior da Agua Central do Atlântico Sul (ACAS). As regioes de maximas taxas de subduccao estao associadas a formacao das AMS. Palavras-chave: camada de mistura, ventilacao, ACAS, termoclina permanente, inducao lateral.