Effects of the Mean Walker Circulations on the Zonal Variability of Tropical CISK Waves

Abstract

An idealized zonally asymmetric mean state in the Tropics is used to investigate the effects of atmospheric mean Walker circulations and sea surface temperatures on the zonal variability of tropical waves with positive- only wave?CISK heating. First, the effects of the zonally symmetric vertical and meridional shears of the mean zonal flow are investigated with different vertical profiles of convective heating and temperature lapse rate. With the observed vertical profile of temperature lapse rate in the Tropics, the effect of the vertical shear of the mean zonal flow on the fast waves (period about 2 weeks) associated with a maximum heating in the upper troposphere is found to be opposite to that on the slow waves (period about 4 weeks) with a maximum heating in the middle troposphere, while the vertical shear effects with an idealized constant lapse rate are quite similar. The meridional shear of the mean zonal flow acts only to reduce the effects of the vertical shear of the mean zonal flow. Second, with the existence of atmospheric mean Walker circulations, tropical waves increase and decrease alternately when propagating around the globe. It is found that the influence of the mean Walker circulations is mainly from the effects of the vertical shear of the mean zonal flow, and the tendencies of the zonal variations for the fast waves and the slow waves are almost opposite. Third, the amplitudes of zonal variations of the tropical waves as a consequence by the atmospheric mean Walker circulations are comparable to those arising from the zonally asymmetric SSTs in the Tropics. It is found that the combined effect of the mean state makes the fast waves propagate over the colder regions without decreasing, while the slow waves strongly anchor over the warmer regions, implying other processes might be important in sustaining the slow waves over the colder regions where convection is weaker.