Effect of Upper Tropospheric Vertical Thermal Contrast Over the Mediterranean Region on Convection over the Western Tibetan Plateau during ENSO Years

Abstract

ABSTRACT Using the National Centers for Environmental Prediction–Department of Energy Atmospheric Model Intercomparison Project 2 daily reanalysis datasets, the present study reveals that an upper tropospheric vertical thermal contrast (VTC) exists between pressure levels at 100 and 250 hPa over the Mediterranean region (MR) during the midsummer monsoon. An increase and decrease in the tropospheric temperature (TT) at the 100 and 250 hPa levels respectively, are found to be centred over the MR during El Niño years. The former is characterized by an anticyclonic circulation in the wind shear and the latter by a cyclonic circulation. The opposite pattern is exhibited during La Niña years. It is shown that the MR VTC remotely affects convection over the western Tibetan Plateau (WTP) by inducing favourable temperature, circulation patterns, and ascending (descending) winds on the upper vertical levels of the WTP during El Niño (La Niña) years. This effect has been further substantiated by a careful causality analysis with a new causal inference technique, which identifies a dominant causality from the MR VTC to the WTP longwave fluxes through the top of the atmosphere.