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

Ching Yee Chang
University of Maryland


There are two parts of this talk: 1. CCSM3/CAM3 bias structures in the seasonal climate of tropical Atlantic sector, and some possible causes for the biases. 2. Observational study on the relationship between the spring meridional pattern and summer equatorial pattern of tropical Atlantic SST anomaly, and their relationship to the summer African rainfall.

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.

A diagnostic modeling study of the origin of the westerly wind bias of CAM3 by using a steady-state linearized atmospheric model followed. 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 observational study explores the relationship between the two predominate modes of Tropical Atlantic interannual variability. The spring meridional mode and summer equatorial mode are shown to be related, with the spring meridional mode leading into equatorial mode in a corresponding phase in summer. For example, if the meridional mode is expressed as cold-north/warm-south in spring, then the summer equatorial mode has warm phase in summer; and vice-versa. This modal linkage is not orchestrated by ENSO or NAO variability. It is proposed that the connection between these two phenomena is a consequence of the anomalous diabatic (latent) heating from the ITCZ rainfall due to its latitudinal displacement.

The relationship between these modes and the variability of July-August African rainfall along the Coast of Guinea is also investigated. The modal relationship enhances the prospects for prediction of boreal summer rainfall over the Coast of Guinea: tropical Atlantic SST alone can explain 40% of the summer-time West African rainfall variance with one season in advance.