Equatorial Mean Zonal Wind in a Global Nonhydrostatic Aquaplanet Experiment

Abstract

Timeaveraged zonalmean equatorial winds in an aquaplanet experiment were investigated using 30day simulation data from a global 7km mesh nonhydrostatic experiment with explicit moist physics. The simulated mean winds included equatorial easterly (westerly) in the lower (upper) troposphere and Hadley cells with a single upward branch on the equator. The mean zonal wind in the equatorial region, where moist convection was abundant, was investigated in detail. Each term of the momentum equation for the mean zonal wind was diagnosed. The advection terms were the major components that maintained the simulated mean wind.The advection terms were divided into those associated with mean (zonal and temporal) and deviation winds. In the equatorial region (1°N-1°S), the advection due to deviation winds led to westerly acceleration, which was balanced by the vertical transport of lowlevel easterly through mean upward motion. To identify the primary source of the westerly acceleration, deviation winds were decomposed into several classes by zonal scales, and momentum flux divergence was calculated for each class. The role of diabatic heating was also examined in a similar way. It was found that the momentum transport associated with planetaryscale disturbances was the primary source in the upper troposphere, of which moist Kelvin wave modes accounted for a significant part. In the middle to lower troposphere, synopticscale and mesoscale components were responsible for the westerly acceleration. It was also found that the westerly acceleration in the upper (middle to lower) troposphere by deviation winds mainly occurred on cloudfree (convective) grid points.