Abstract
Both the scientific and operational communities are increasingly interested in subseasonal to seasonal variations of weather and climate. The Semi-Annual Oscillation (SAO) has been studied extensively at the surface as well as in the middle atmosphere (upper stratosphere and the lower mesosphere). However, the SAO in the upper troposphere and lower stratosphere (UTLS) has been less discussed. Here we find evident SAO of temperature in the UTLS (250–175 hPa) from the subtropics to mid-latitudes (22.5–42.5º) using high-quality satellite measurements, reanalysis data, and model simulations. We show the mechanism of its formation by an energy budget analysis. The temperature in the Northern Hemisphere (NH) UTLS shows the first peak in February according to the dynamical heating and shows the second peak in July due to the dynamical heating and moist processes. Similar to the NH, the winter time maximum of temperature in the SH is related to dynamical heating and the summer time maximum is related to both moist and dynamical heating in the UTLS. Model simulations indicate that the SAO in the UTLS is partly modified by the SAO of sea surface temperatures (SSTs).
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