The Cabo Frio Upwelling System, Brazil

Abstract

Upwelling processes are most common and most intense on the eastern side of the Atlantic and Pacific oceans. The Cabo Frio upwelling system (23°S, 42°W) is an anomaly in that it is on the west side of the Atlantic Ocean on the coast of the state of Rio de Janeiro, Brazil (Fig. 7.1). The name “capo frigidio” (cold cape) first appeared on marine charts of Portuguese navigators at the beginning of the 15th century. A change in coastal direction from north-south to east-west at Cabo Frio and the proximity of the 100-m isobath lead to a topography which promotes upwelling of deep South Atlantic central water (SACW). However, the principal mechanism of upwelling is north-northeast wind. The process of SACW upflow follows two stages (Moreira da Silva 1973). The first stage begins in mid-August/September (late winter) and depends on the position of the main flow of the Brazil Current in relation to the continental slope and on variations in frequency and intensity of NE winds. During strong upwelling, SACW advances beyond the continental slope and invades the shelf to depths of about 50–80 m where is remains until April (mid-fall). The second stage of upwelling, mainly from September to March, is influenced by changes in intensity and direction of local winds, which cause alternating warming and cooling of near-surface waters. Two or 3 days of northeasterly winds (10 m s−1) bring SACW to the surface and cold front passages (8- to 10-day cycles), which cause the wind to shift to the west, reverse the process (downwelling). Upwelled surface water forms a narrow belt (<1 km) around Cabo Frio with the plume flowing west-southwest (50 cm s−1), owing to NE winds and the Coriolis effect. However, upwelling is not limited to the cold surface water zone, but south of Cabo São Tomé the penetration of the deep SACW over the shelf causes stratification, with thermocline differences of up to 10 °C within a few meters of depth during much of the year.