Impact of tropical lower stratospheric cooling on deep convectiveactivity: (I) Recent trends in tropical circulation

Large changes in tropical circulation, in particular those related to the summer monsoon and cooling of the sea surface in the equatorial eastern Pacific, were noted from the mid to late 1990s. The cause of such recent decadal variations in the tropics was studied by making use of a meteorological reanalysis dataset. Cooling of the equatorial southeastern Pacific Ocean occurred in association with enhanced cross-equatorial southerlies, which resulted from a strengthening and poleward shift of the rising branch of the boreal summer Hadley circulation connected to the stratospheric Brewer‒Dobson circulation. From boreal summer to winter, the anomalous convective activity centre 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 factor causing the recent decadal change in the tropical troposphere and the ocean is a poleward shift of the rising branch of the summertime Hadley cell, which can result from a strengthening of extreme deep convection penetrating into the tropical tropopause layer, in particular over the continents of Africa and Asia, and adjacent oceans. We conjecture that this effect 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.