Tropospheric Temperatures and Southern Hemisphere Circulation

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Abstract
This note documents improvements in the Southern Hemisphere midlatitude circulation associated with a revised convective scheme that was implemented in a version of the NCAR Community Climate Model. Perpetual July and January runs are performed for a standard control and an experiment with a revised convective parameterization. No other boundary conditions in the model are changed (e.g., sea-surface temperatures or sea ice, which are prescribed from observations). The most notable result is a warmer troposphere at all latitudes that is closer to the observed than to the control. Since the warming is greatest in the tropics, the equator-to-pole temperature gradient is increased in both July and January. As a consequence, the midlatitude u wind maximum is stronger at both times of the year in the lower and mid-troposphere. Sea-level pressure is also lower at high southern latitudes in the circumpolar trough surrounding Antarctica. Greatest improvements of the Southern Hemisphere (SH) circulation in the model experiment with the warmer tropical troposphere occur in southern summer, the time of year when the climate of the control simulation was most poorly simulated. Midlatitude eddy heal transport increases somewhat with the revised convective scheme, but greatest changes are seen in large increases of poleward eddy momentum transport. Results show that notable improvement in the SH mid- and high-latitude circulation in this case can be realized by changing a model parameterization whose greatest temperature effects are in the tropical troposphere.

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