Seasonal changes in the mixed and barrier layers in the western Equatorial Atlantic

Abstract

A B S T R A C TClimate is closely related to the dynamics of the surface layer of the tropical Atlantic and the exchange between this latter and the atmosphere, and wearther forecasting will improve with increasing understanding of the processes that govern the relative distribution of thermodynamic properties of the water column. This paper focuses on the isolation of warm surface waters from the cold ones of the deep ocean by a salinity induced barrier layer (BL) in the western equatorial Atlantic (3 o S-7 o N; 40 o -52ºW), based on 487 CTD profiles (REVIZEE-1995(REVIZEE- -2001. The main process contributing to the seasonal BL formation is the discharge of low salinity waters from the Amazon river. During boreal late winter/spring (Mar-May; high river discharge), deeper isothermal (Z T ) and mixed layers (Z M ) prevail and the formation of a 16m-thick BL was clearly determined the formation of a salt-induced marked pycnocline within a deeper isothermal layer. However, during the boreal autumn (Oct-Dec; low river discharge), density stratification was mainly determined by temperature R E S U M OO clima está fortemente relacionado com a dinâmica da camada superficial do Atlântico tropical e com as trocas entre esta e a atmosfera, e a previsão do tempo melhorará à medida em que ganhemos um melhor entendimento dos processos que governam a distribuição relativa das propriedades termodinâmicas na coluna d'água. O presente trabalho focaliza o isolamento das águas quentes superficiais das águas frias profundas pela camada de barreira (CB) induzida pela salinidade no Atlântico Equatorial Oeste (3 o S-7 o N; 40 o -52ºW), com base em 487 perfis de CTD (REVIZEE -1995(REVIZEE - -2001. O principal processo que contribue para a formação sazonal da CB é a descarga fluvial de águas doces do rio Amazonas. Durante o final do inverno/primavera boreal (Mar-Mai; alta descarga), prevalecem camadas isotérmica (Z T ) e de mistura (Z M ) mais profundas e a formação de uma CB com 16m de espessura foi governada pelo estabelecimento de uma forte picnoclina induzida pela salinidade, no interior da camada isotérmica. Entretanto, durante o outono boreal (Out-Dez; baixa descarga), estratificações em densidade foram principalmente controladas pela distribuição de temperatura (Z M ≅ Z T ; ECB = Z M -Z T ≅ 0). Embora não tenha registrado uma CB sobre a plataforma Amazônica, uma CB máxima (40m) foi formada próxima à quebra da plataforma a 45°W.

Salinity-induced mixed and barrier layers in the southwestern tropical Atlantic Ocean off the northeast of Brazil

“…Thus, the increased thickness of the BL appears more influenced by horizontal advection of salt than the surface freshwater balance. This conclusion was also obtained by Silva et al (2005) for the northern area of Brazil as well as by Zeng et al (2009) at the southeast of Vietnam.…”

“…Since the SBE911plus CTD has two thermometers, with an accuracy of about 0.001 • C, the error in computing Z T for a T = 0.5 • C is around 0.2% for a local Z T . Moreover, previous studies in the western tropical Atlantic also used the 0.5 • C criterion for the isothermal layer and its equivalent in density change (Silva et al, 2005), whose are found to be the most adaptable due to their robustness (Tanguy et al, 2010). When density stratification is exclusively controlled by temperature, the isothermal layer depth becomes equivalent to the mixed layer depth, and BLT = 0.…”

“…In the western tropical Atlantic, these charts suggest that the subduction of subtropical salty surface waters toward the equator during winter, and their advection by the equatorial current system, might be the mechanism that forms the BL. This hypothesis is partially supported by the presence of subsurface Salinity Maximum Waters (SMW), as called by Defant (1936), along the western Atlantic boundary (Pailler et al, 1999;Stramma et al, 2005;Silva et al, 2005). These authors argue that this salty water, which is formed in the South Atlantic subtropical gyre, enters the region within the NBUC that flows northwestward along the Brazilian continental slope.…”

Distribution of the Planktonic Shrimp Lucifer (Thompson, 1829) (Decapoda, Sergestoidea) off the Amazon

“…In the present study, the Amazon Shelf, due to the influences of both Para and Amazon rivers, is an unusually energetic coastal region that is subjected to strong seasonally driven forces, such as the Amazon River discharge, North Brazil Current transport, and trade-winds. Due to its proximity to the Equator, moderate along-shelf geostrophic currents may be generated by weak cross-shelf pressure gradients in the area's northern and southern regions (Silva et al, 2005). The formation and characteristics (depth and thickness) of the barrier and mixed layer vary seasonally, and a barrier layer (BL) is formed in distinct areas by different processes.…”

Caracterização hidrográfica da plataforma continental do maranhão a partir de dados oceanográficos medidos, remotos e modelados

“…Neste sentido, Gibbs (1970), e mais recentemente Silva et al (2005a;2005b); Silva (2006), além de outros, têm demonstrado como situação extrema que a pluma d'água superficial com baixo valor de salinidade, resultante da descarga do rio Amazonas, pode atingir centenas de quilômetros da costa na porção oeste e leste do oceano Atlântico equatorial, no qual sofre deslocamento espacial e sazonal por processos advectivos (ventos e correntes). No caso da influência dos aportes continentais oriundos do Golfão Maranhense, esta pluma espalhou-se até uma distância de cerca de 85 km da costa no mês de Junho/1999,época que corresponde aos maioresíndices de precipitação na região (Fig.…”