Oscillations of the intertropical convergence zone and the genesis of easterly waves

Abstract

We examine the eastern Pacific Ocean Intertropical Convergence Zone (ITCZ) both in its mean state and transient phases using a combined diagnostic, theoretical and numerical modeling approach. We note that the ITCZ is perpetually in a transient state with strong variability occurring on 4-8 day time scales. Transients, about half the amplitude of the mean ITCZ, propagate northwards from the near-equatorial southern hemisphere eventually increasing the convection in the vicinity of the mean ITCZ convection. It is argued that the mean ITCZ is continually inertially unstable with incursions of anticyclonic vorticity advected across the equator resulting in the creation of a divergence-convergence doublet. The low-level convergence generates convection and vortex tube stretching which generates cyclonic vorticity counteracting the northward advection of anticyclonic vorticity. During a cycle, the heating in the mid-troposphere near 10°N oscillated between 6 and 12 K/day at the inertial frequency of the latitude of the mean convection. The shallow meridional circulation, noted in the mean field in other studies, appears to be a result of the transient nature of the ITCZ. It is hypothesized that westward propagating equatorial waves result from the inertial oscillation of the ITCZ. To test that the waves are formed in situ in the eastern Pacific and not remnants of waves propagating from the Atlantic or promoted by the Central and South American orography, several numerical experiments are undertaken using a high-resolution regional model spanning the western Atlantic Ocean and the eastern Pacific. In the control case, the model is initialized at all boundaries with full high-frequency observations. In two additional experiments, these transients are filtered out, and a third experiment is run with the topography over a large part of Central and South America removed. In all experiments, westward propagating waves are formed in the region of high CEPG suggesting that the hypothesis of in situ development may be correct.