Longitudinal Asymmetric Trends of Tropical Cold-Point Tropopause Temperature and Their Link to Strengthened Walker Circulation

Abstract

he zonal structure of trends in the tropical tropopause layer during 1979?2014 is investigated by using reanalysis datasets and chemistry?climate model simulations. The analysis herein reveals that the tropical cold-point tropopause temperature (CPTT) trends during 1979?2014 are zonally asymmetric; that is, over the tropical central and eastern Pacific (CEP; 20°S?20°N, 160°E?100°W), the CPTT shows an increasing trend of 0.22 K decade?1, whereas over the rest of the tropical regions (non-CEP regions) the CPTT shows a decreasing trend of ?0.08 K decade?1. Model simulations suggest that this zonal asymmetry in the tropical CPTT trends can be partly attributed to Walker circulation (WC) changes induced by zonally asymmetric changes of the sea surface temperatures (SSTs). The increasing (decreasing) SSTs over the western Pacific (CEP) result in a larger zonal gradient in sea level pressure over the tropical Pacific and intensified surface easterlies. The increased pressure gradient leads to enhanced convection over the Indo-Pacific warm pool and weakened convection over the CEP, facilitating a stronger WC. The downward branch of the intensified WC induces a dynamical warming over the CEP and the upward branch of the intensified WC induces a dynamical cooling over the non-CEP regions below 150 hPa. The significant warming in the upper troposphere and lower stratosphere (UTLS) caused by the WC descending and wave activity changes in the UTLS over the CEP shifts the cold-point tropopause height to a higher level, while the radiative effects of greenhouse gases, ozone, and water vapor changes in the UTLS make less important contributions to the trend of the tropical CPTT than SST changes.