The Interannual Variability in the Tropical Atlantic Ocean Simulated by a Regionally Coupled Ocean-Atmosphere GCM

The interannual fluctuations in the tropical Atlantic Ocean are affected by both air-sea interactions within the Atlantic Sector and remote forcing, such as those from ENSO. To study the relative roles played by the remote ENSO effect and the air-sea coupling within the Atlantic, we have conducted a series of experiments using a coupled ocean-atmosphere general circulation model (CGCM). The atmospheric component of the regional CGCM is the COLA AGCM with a horizontal T42 resolution and 18 vertical levels. The oceanic component is the Poseidon global quasi-isopycnal ocean model with 14 vertical layers. For our purpose, the atmospheric and oceanic components are fully coupled only in the Atlantic region. The surface fluxes at the air-sea interface outside this region are prescribed for both the OGCM and the AGCM. Two experiments have been conducted using this regional CGCM (RCGCM). One prescribes climatological boundary conditions to drive the atmosphere and the ocean in the uncoupled region (referred to as CF run). The other prescribes CPC monthly SST and surface stress observations from NCEP reanalysis for the period of 1950-1998 (referred to as RF run). In CF run, we eliminate any remote ENSO effect in the Atlantic Sector and the simulated interannual variability within the Atlantic Ocean can only arise from regional air-sea interaction and forcing due to the atmospheric internal variability. In the RF case, signals from ENSO induced atmospheric teleconnection can be simulated as realistically as those from an uncoupled AGCM, while their interactions with the ocean in the Atlantic Sector can be picked up through regional coupling. Our preliminary analysis of these two experiments show that the RCGCM in general has a realistic mean state and annual cycle in the tropical Atlantic Ocean, though the mean model SST has a basin-wide cold bias and the annual cycle in the Caribbean Sea has larger errors. Moreover, the CF run's interannual variability has weaker strength and less organized than the observed one in the northern tropical Atlantic Ocean. However, its major patterns in the equatorial and the southern oceans are similar to those from the observations, suggesting that the Atlantic Ocean can still maintain an intrinsic variability. On the other hand, the RF run shows that remote forcing enhances and modulates the Atlantic SST variability. Specifically, El Niño induced perturbations in the Atlantic sector causes stronger cross-equatorial flow, weaker northeast trades, and warmer SST over northern basin of the tropical Atlantic from boreal winter to spring seasons. The ENSO induced SST anomalies can persist into boreal summer season in the northern ocean.