The ENSO effect on the tropical Atlantic variability: A regionally coupled model study

A CGCM is used to study the relative roles played by local air-sea interaction and remote ENSO effects on the tropical Atlantic interannual variability. A regional coupling strategy allows full air-sea coupling only over the Atlantic Ocean north of 30degreesS. Two experiments were conducted, respectively with either climatological or real time boundary conditions prescribed over the uncoupled portion of the global domain. The simulations show that major anomalous SST modes in the tropical South Atlantic are mainly caused by coupled ocean-atmosphere processes within the Atlantic sector. SST anomalies in the northern tropical ocean, however, are substantially affected by the Pacific ENSO, with local coupling accounting for an enhanced effect. The El Nino effect causes warmer SST in the northern tropical Atlantic in the boreal spring season, forced by weakened northeast trade winds over the Atlantic basin starting from boreal winter in the maturing El Nino phase. The warmer SST anomalies then trigger a positive air-sea feedback within the Atlantic basin, which leads to stronger anomalous cross-equatorial atmospheric flow in the spring season.