Abstract
This paper explores climate variability of the lower troposphere and boundary layer in the tropical Atlantic sector through a series of modeling simulations with a diagnostic primitive equation model. The focus is on the role that realistic diabatic heating and its vertical placement as well as surface temperature have in inducing/reinforcing the local monthly wind circulation, the role that thermal and momentum transients play in the Tropics, the potential for feedbacks, and the way through which other basins influence the tropical Atlantic region. NCEP–NCAR reanalysis data for the period 1958–93 are used to provide forcing and model verification.
In the first part of the paper local effects are considered. It is found that the most important terms controlling anomalous surface winds over the ocean are boundary layer temperature gradients and diabatic heating anomalies at low levels (below 780 mb). Anomalous diabatic heating at mid- and upper levels (430–690 mb) contributes to the near-surface circulation poleward of 15° over the warm hemisphere. Anomalous diabatic heating over the African continent influences zonal winds well into the ocean. It is found that the anomalies of surface latent heat flux induced by the interhemispheric distribution of anomalies provide positive feedback on both sides of the equator, in the deep Tropics and west of 20°W. It provides negative feedback off the northwest coast of Africa.
In the second part the relative importance of remote forcing is considered. It is found that anomalous heating associated with interhemispheric gradients of surface temperature in the tropical Atlantic influence winds in the northern extratropics in a wavelike pattern during boreal spring. Anomalous heating associated with equatorial anomalies of surface temperature influence winds in the southern extratropics in a wavelike pattern during boreal summer. In contrast, the influence of heating in the midlatitudes is confined to the northern subtropics. Anomalous ENSO-related diabatic heating influences near-surface winds in the tropical Atlantic, which resembles the local response to interhemispheric gradients of surface temperature. This remote influence induces changes in the intensity of the Atlantic Walker and Hadley circulations as a consequence of the direct effect of heating in the eastern tropical Pacific.
Corresponding author address: Alfredo Ruiz-Barradas, 3420 Computer and Space Sciences Bldg., Department of Meteorology, University of Maryland, College Park, MD 20742-2425. Email: alfredo@atmos.umd.edu
