3 - The annual oscillation of the tropospheric temperature in the northern hemisphere

Summary

Mean monthly temperature data at 850, 700, 500, 300, 200 and 100 mb from about 120 radiosonde stations over the northern hemisphere are subjected to harmonic analysis to study the annual oscillation of tropospheric temperature. Two main features stand out in this analysis. Firstly, over the tropics at individual stations double maxima occur–one at the lowest level, and the other at about 300 mb. Secondly, the amplitude of the annual oscillation is large over the tropics and subtropics with the maximum value appearing over the Asiatic monsoon region. There is a phase lag varying from 30 to 45 days between the two tropospheric maxima–the upper-tropospheric one occurring later and towards the end of July. The lower-tropospheric maximum is linked with the sensible heating. The physical forcing mechanisms of the upper-tropospheric maximum are discussed. The contribution of latent heating to the high amplitude of the upper-tropospheric maximum over the monsoon region is emphasized. A schematic model for the tropospheric heating and its link with the dynamics of the Asiatic summer monsoon is suggested.

Introduction

A study of Smagorinsky (1953) showed that atmospheric motions are affected not only by the zonally-asymmetric distribution of heating and cooling, but also by the major orographic influences. The large-scale circulation associated with the summer monsoon is described in the classical literature as the interaction of the annual cycle of solar radiation and the differential effective heat capacities of land masses and oceans. The most striking feature of the Asian summer monsoon is its annual periodicity.