Dry air valley in the upper troposphere over the eastern Mediterranean-western Tibetan Plateau

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Based on AIRS satellite observations and ERA-Interim reanalysis data, we investigated the seasonal evolution of the dry air valley (DAV) in the upper troposphere over regions from the eastern Mediterranean to the western Tibetan Plateau (EM-WTP). We find that monsoon-related descending motion is the main driver for the DAV, and its contribution to the DAV is 61%. The intensity of DAV increases rapidly in June, reaches its maximum in late July and early August, then weakens gradually until it disappears in early October. From May to July, zonal temperature gradient over the Mediterranean Sea increases and the southerly wind in the western South Asia High enhances, resulting in an enhancement of descending motion over the Mediterranean. Meanwhile, the northerly wind and zonal vorticity gradient over south of the Aral Sea strengthen due to the thermal-dynamical effect of the Tibetan-Iranian Plateau, consequently, descending motion over south of the Aral Sea intensifies. From August to September, the descending motion weakens quickly over the Mediterranean but remains nearly steady over south of the Aral Sea. As a result, the east-west water vapor gradient increases over the DAV, and water vapor in the DAV converges horizontally due to the transport of westerlies. Accordingly, the DAV intensity decreases. Apart from the descending motion, the dry air intrusion from the lower stratosphere associated with tropopause fold events also has an impact on the DAV intensity. However, the contribution of the tropopause fold events to the DAV is rather small, no >10%.