Equatorial Energy Accumulation and Emanation Regions: Impacts of a Zonally Varying Basic State

Abstract

Previous studies have suggested that the regions of mean anomalous perturbation kinetic energy which exist in the vicinity of the equatorial upper-tropospheric westerlies are the result of the propagation of extratropical synoptic and low frequency waves through the equatorial ?westerly duet? where a subsequent wave energy convergence occurs. The proposition that these perturbed equatorial regions may arise from remote equatorial energy sources is investigated. It is shown that three criteria must be met. The first two, the existence of wave energy sources along the equator and a mechanism to transport that energy longitudinally, are accounted for relatively easily with existing theory of divergent, trapped equatorial modes. The third criterion, the requirement of a mechanism for an accumulation of transient energy in the equatorial stretch flow (i.e., nonzero ?x), is not immediately obvious and requires exploration to develop new concepts. Using simple WKBJ arguments it is shown that within a realistic parameter range, a combination of longitudinal stretch in the basic state along the equator and the characteristics of the equatorial trapped waves satisfy the third criterion. The equatorial waves must possess a divergent structure which insists on equatorial trapping. It is shown that purely barotropic modes, which cannot be equatorially trapped, do not represent the real atmospheric structure at low latitudes. Regions of negative longitudinal stretch along the equator (i.e., westerlies decreasing, or easterlies increasing, towards the cast) are shown to be wave energy accumulation regions. Regions with positive stretch, on the other hand, are wave energy depletion regions. A free-surface barotropic model with fully nonlinear basic states, containing both stretch and shear, confirm the results of the simpler model, i.e., regardless of the position of the energy source within the tropical atmosphere the wave energy accumulates in the same region; namely, on the eastern side of the westerly maximum. Thus, the third criterion is accounted for by the longitudinal trapping of the equatorial Rossby waves in specific regions as they move through the longitudinal stretch flow. It is argued that the existence of energy depletion and accumulation regions provides for the existence of an equatorial teleconnection system allowing for remote regions of the tropics to be connected by the equatorial transient modes. A number of alternate hypotheses such as the instability of the longitudinal varying basic flow are tested, although, in this case, the basic state is found to be very stable, indicating a robustness of the wave energy accumulation hypothesis. The numerical results indicate that the equatorial wave accumulation regions are also emanation regions of equatorial transient influence to higher latitudes. That is, wherever the equatorial regions are excited, a wave train to higher latitudes will originate from the same longitude belt along the equator, producing a phase locked response at higher latitudes given the same mean basic equatorial flow. It is argued that the concept of tropical emanation regions provides a significant modification to the tropical-extratropical wave train teleconnection theory. It is shown that thew new theories of atmospheric teleconnections (i.e., the equatorial and the modified tropical-extratropical theories) may allow interpretation of some rather difficult questions that have been posed in both modeling and observational studies.