The Dependence of the Low-Level Equatorial Easterly Jet on Hadley and Walker Circulations

Abstract

How the time-mean Hadley and Walker circulations affect the formation of a low-level equatorial easterly jet is investigated. Experiments are conducted for equinoctial conditions using a general circulation model, the Community Climate Model (CCM1), that includes a Kuo convective scheme and a lower boundary that is specified to be water at a fixed sea surface temperature (SST). Several zonally symmetric SST forcings are used to determine how various Hadley circulations affect the tropical zonal wind field. A zonal wavenumber one equatorial SST anomaly superimposed on a zonally symmetric SST distribution forces a wind field that includes both Hadley and Walker circulations. The Hadley circulation experiments produce equatorial easterlies and low-level jets on the poleward sides of the intertropical convergence zone (ITCZ) 10° to 15° from the equator. In an experiment with a single, dominant off-equatorial ITCZ in the Northern Hemisphere, the Southern Hemisphere jet moves to within 7.5° of the equator; yet none of the Hadley circulation cases produce a low-level easterly jet on the equator because they lack a mechanism to vertically confine the flow. The experiment that includes a zonally overturning cell on the equator produces a low-level equatorial easterly jet in the cold tongue region that is similar to the observed jet over the central to eastern Pacific. That case shows that east of the equatorial warm pool the Walker circulation and its induced Kelvin wave response provide the necessary upper-level westerly flow and subsidence to vertically confine the low-level easterlies into a jet. Spring and fall climatological runs of the CCMI with land surfaces, seasonally varying SSTs and insolation, and a moist convective adjustment scheme support the hypothesis that the Walker circulation provides the vertical confinement necessary to form a low-level equatorial easterly jet in the region east of the equatorial convective center, regardless of the Hadley circulation in that region. The eddy vertical-flux convergence of moisture in the Kuo convective scheme produces a dry tongue in the Walker circulation simulation below the low-level equatorial easterly jet. The CCM1 climatologies show that the dynamics of the jet do not depend on this feature. Betts, Albrecht, and Kloesel have observed a similar feature just above the boundary layer in the central to eastern Pacific and, without referring to the low-level jet, they have hypothesized a mechanism in which convection forms this dry layer. Analysis of the simulations performed here suggests that the model's parameterized convective physics utilize the same mechanism to form the dry tongue in the vicinity of the low-level equatorial easterly jet; however, since the mechanism of Betts, Albrecht, and Kloesel has not yet been confirmed through observational studies, the relationship between the observed and modeled dry tongue remains speculative.