Implication of tropical lower stratospheric cooling in recent trends in tropical circulation and deep convective activity
2019
Abstract. Large changes in tropical circulation from the mid-to-late 1990s to the
present, in particular changes related to the summer monsoon and cooling of
the sea surface in the equatorial eastern Pacific, are noted. The cause of
such recent decadal variations in the tropics was studied using a
meteorological reanalysis dataset. Cooling of the equatorial southeastern
Pacific Ocean occurred in association with enhanced cross-equatorial
southerlies that were associated with a strengthening of the deep ascending branch
of the boreal summer Hadley circulation over the continental sector connected
to stratospheric circulation. From boreal summer to winter, the anomalous
convective activity center moves southward following the seasonal march to
the equatorial Indian Ocean–Maritime Continent region, which strengthens the
surface easterlies over the equatorial central Pacific. Accordingly, ocean
surface cooling extends over the equatorial central Pacific. We suggest that
the fundamental cause of the recent decadal change in the tropical
troposphere and the ocean is a poleward shift of convective activity that
resulted from a strengthening of extreme deep convection penetrating into the
tropical tropopause layer, particularly over the African and Asian continents
and adjacent oceans. We conjecture that the increase in extreme deep
convection is produced by a combination of land surface warming due to
increased CO 2 and a reduction of static stability in the tropical
tropopause layer due to tropical stratospheric cooling.
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