On the mechanism of the large-scale seasonally varying upwelling in the region of the tropical tropopause
Yulaeva, Elena Valentinovna
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This work investigates the physical mechanisms that determine the large-scale upwelling through the tropical tropopause, one of the principal components of stratosphere-troposphere mass exchange. The global-scale mean meridional circulation in the lower stratosphere and the upward branch of the tropospheric Hadley circulation which form the two interrelated parts of atmospheric dynamics in the region of the tropical tropopause are considered together. The comparative roles of two main mechanisms: non-local extratropical wave-induced zonal forcing and local radiative and (tropospheric) cumulus heating in controlling the mean meridional circulation in the tropical stratosphere and troposphere are investigated. The study includes objective analysis of observational data and numerical simulation of the upwelling with a specially designed two-dimensional, zonally symmetric spectral model based on the transformed Eulerian mean equations. The analysis of the NCAR/NCEP Reanalysis data reveals the seasonally varying circulation patterns in the upper troposphere and in the lower stratosphere. The observed mean meridional circulation in the lower stratosphere can be viewed as a superposition of two components: an equatorially symmetric component forced by the monsoon part of the diabatic heating, and an equatorially asymmetric component forced by the seasonally varying wave-induced zonal forcing in the stratosphere. At any given time, the mean meridional mass stream function in the upper troposphere can be considered as a combination of two elements: a circulation related to the oceanic ITCZ and a circulation related to the diabatic heating in the continental monsoons.The experiments with the model are designed to elucidate two aspects of the climatological-mean annual march. The first aspect concerns the annual cycle in the temperature of the tropical tropopause, which is considered to be a manifestation of the annual march in the upwelling in the region of the tropical tropopause, with a minimum (in upwelling) in July-August and a maximum in January-February. The modeling results prove that the seasonal variability of the upward mass flux through the tropical tropopause is controlled by eddy induced zonal forcing in the stratosphere. The second aspect concerns the marked asymmetry between the Northern Hemisphere tropospheric zonal wind fields in the transition seasons, with the westerly jetstream located at higher latitudes in October-November than during April-May. The experiments with the model suggest that the difference in the cumulus heating associated with a northward migration of the ITCZ from May to October appears to be the main mechanism responsible for this asymmetry.
- Atmospheric sciences