Role of Tropical Lower Stratospheric Cooling on Recent Tropical Tropospheric Change

Large changes in circulation have occurred around the end of 1990's in the troposphere and the ocean as well as in the lower stratosphere, such as an advancement of the onset of Asian summer monsoon, cooling of the equatorial eastern Pacific connected to the hiatus of the global warming, and cooling in the tropical lower stratosphere. Whether or not a causal relationship exists among these phenomena, is an interesting yet challenging question.

For this study, we made use of JRA-55 meteorological reanalysis data, cloud top height and convective overshooting frequency data obtained from different satellite observations. The result of the analysis suggests that the fundamental factor causing the recent decadal change in the tropical troposphere and ocean is a poleward shift of the rising branch of the summertime Hadley cell associated with an intensified extreme deep convection penetrating into the tropical tropopause layer (TTL) over and around continents. In particular over African and Asian sectors where extreme deep convections frequently occur. It is conjectured that such change can be produced by a reduction of static stability in the TTL resulting from the combined effects of stratospheric cooling and surface warming due to increased CO2.