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

Abstract

Abstract. High-resolution hydrographic observations of temperature and salinity are used to analyze the formation and distribution of isothermal depth (ZT), mixed depth (ZM) and barrier layer thickness (BLT) in a section of the southwestern Atlantic (0°30´ N–14°00´ S; 31°24´–41°48´ W), adjacent to the northeastern Brazilian coast. Analyzed data consists of 279 CTD casts acquired during two cruises under the Brazilian REVIZEE Program. One occurred in late austral winter (August–October 1995) and another in austral summer (January–April 1997). Oceanic observations are compared to numerical modeling results obtained from the French Mercator-Coriolis Program. Results indicate that the intrusion of subtropical Salinity Maximum Waters (SMW) is the major process contributing to the seasonal barrier layer formation. These waters are brought by the South Equatorial Current (SEC), from the subtropical region, into the western tropical Atlantic boundary. During late austral winter southeastern trade winds are more intense and ITCZ precipitations induce lower surface salinity values near the equator. During this period a 5–90 m thick BLT (median = 15 m) is observed and BLT > 30 m is restricted to latitudes higher than 8° S, where the intrusion of salty waters between 8°–12.3° S creates shallow mixed layers over deep (ZT ≥ 90 m) isothermal layers. During austral summer, shallow isothermal and mixed layers prevail, when northeasterly winds are predominant and evaporation overcomes precipitation, causing saltier waters at the surface/subsurface layers. During that period observed BLT varies from 5 to 70 m and presents thicker median value of 35 m, when comparing to the winter. Furthermore, BLT ≥ 30 m is observed not only in the southernmost part of the study area, as verified during late winter, but in the latitude range 2°–14° S, where near-surface salty waters are transported westward by the SEC flow. These results indicate that the inclusion of salinity dynamics and its variability are necessary for studying mixed and barrier layer behaviors in the tropical Atlantic, where ocean-atmosphere coupling is known to be stronger.