Seasonal and Interannual Ocean-Atmosphere Variability in the Tropical Atlantic: Observed Structure and Model Representation

Abstract

Tropical Atlantic is clamped by the South America (the Amazon) in the west and Africa (the Sahel) in the east. These two regions have been undergoing significant climate/environment changes for decades. In order to use climate models to study the impacts of these changes, climate models need to be able to well simulate the seasonal climate of the tropical Atlantic sector.

The first part of this dissertation focuses on the representation of the seasonal cycle in the CCSM3 coupled atmosphere-ocean model. CCSM3 SST has a north-south dipole pattern of bias centered at the latitude of the thermal equator, resembling the observed pattern of interannual climate variability in boreal spring. Along the equator in boreal spring CCSM3 exhibits striking westerly winds at the surface, reminiscent of the pattern of climate variability in boreal summer. The westerly winds cause deepening of the eastern thermocline that keeps the east warm despite enhanced coastal upwelling. Next, a comparison is made with a simulation using historical SST to force the atmospheric model (CAM3) in order to deduce information about the origin of bias in CCSM3. The patterns of bias in CAM3 resemble that in CCSM3, indicating that the source of the bias in CCSM3 may be traced to difficulties in the atmospheric model.

The next chapter presents a modeling study of the origin of the westerly wind bias CAM3 by using a steady-state linearized atmospheric model. The results indicate that underestimation of rainfall over the eastern Amazon region can lead to the westerly bias in equatorial Atlantic surface winds. They suggest that efforts to reduce coupled model biases, especially seasonal ones, must target continental biases, even in the deep Tropics where ocean-atmosphere interaction generally rules.

The fourth chapter investigates the relationship between the two predominate modes of Tropical Atlantic interannual variability. The leading modes of Tropical Atlantic SST variability in boreal spring and summer are shown to be related, with the spring meridional mode leading into summer equatorial mode. The presence of a meridional mode with warm SST anomalies in the southern tropics in spring leads to a warm phase equatorial mode in summer, and vice-versa. This modal linkage occurs independently of climate variability in other ocean basins (e.g., ENSO). Atmospheric diabatic heating associated with a meridional shift of the Inter-Tropical Convergence Zone plays an important role in this relationship. The identification of this relationship enhances the prospects for prediction of boreal summer rainfall over the Guinea Coast of equatorial Africa.