Differences between the Spring and Autumn Circulation of the Northern Hemisphere

Abstract

The phase of the annual cycle in the mean temperature of the Northern Hemisphere troposphere lags the conventional meteorological seasons (centered on April and October) by about two weeds. Hence, the conventional spring season is colder and generally more winter-like than the autumn season. When the transition seasons are redefined so as to bring them more closely into line with the annual cycle in tropospheric mean temperature, a distinctive pattern of differences emerges, which is dominated by a zonally symmetric component. These differences are investigated, making use of three different climatological datasets. Spring is characterized by higher pressure over the polar regions, and lower pressures in the vicinity of 40°N at all levels in the troposphere and lower stratosphere. The amplitude of this difference pattern increases with height. Consistent with the pressure pattern, the belt of strongest zonally averaged westerlies is located farther equatorward, by 5?10° latitude, during spring than during autumn. These differences are observed during virtually every year. It is hypothesized that the difference in the position of the jet stream is responsible for some of the marked climatic contrasts between the two seasons observed over parts of the hemisphere. Differences between the climatological mean stationary wave patterns in the two seasons are documented and discussed. Differences in the phase tilts of the waves in the horizontal plane are consistent with the hypothesis that the, orographic forcing shifts equatorward during spring and poleward during autumn with the strongest zonally averaged zonal winds. The subtropical jetstreams are almost twice as strong during spring as during autumn, and outgoing longwave radiation data indicate that the tropical rain belt shifts northward from spring to autumn.