Relationship between tropical heating and global circulation: Interannual variability

Abstract

This study examines the relationship between tropical heating and global circulation in the troposphere through singular value decomposition (SVD) analysis and numerical simulations based on a global barotropic spectral model. SVD analysis was applied to the 200‐mbar stream function field and the outgoing longwave radiation (OLR) to extract the most recurrent coupled mode. The stream function vector of the first mode indicates an out‐of‐phase relationship between the stream function in the northern and southern hemispheres. Tight gradients in the pattern along the equator suggest a large fluctuation of zonal‐mean zonal wind associated with this seesaw. Its eddy component appears to modulate the stationary eddies. The major structure of the corresponding OLR vector is a dipole near the equator with one center at 120°E and another one at 160°W. The first mode exhibits a strong interannual variability and is strongly correlated with the occurrence of El Niño and La Niña. To understand the relationship between the stream function and OLR, a series of numerical experiments using a barotropic model was carried out, specifying Rossby wave source associated with differing idealized divergence patterns in the tropics. The steady state responses in all experiments exhibit a zonally symmetric structure resembling the stream function vector of the first mode. The pattern was mainly forced by the zonal‐mean component of Rossby wave source that is contributed mostly by zonal‐mean divergent winds under the constraint of zero global‐mean divergence. The same zonal‐mean forcing is also responsible for the forced eddy stream function perturbations that exhibit similar structures in all experiments.