A climatological-dynamical analysis of tropopause folds over Southwest Asia in the period of 1989–2018

Tropopause folding is believed to be connected to the development of cyclonic systems in midlatitudes as well as the larger-scale, lower-frequency processes in the lower latitudes. To investigate such connections, a statistical and dynamical analysis of tropopause folds occurred in Southwest Asia is carried out for the period of 1989–2018. Tropopause folds are identified using the Era-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts. The spatial–temporal distributions of tropopause folding show that the frequency of folding is higher in the winter hemisphere than in the summer hemisphere and greater values of the frequency are found mostly within an elongated zonal band which is situated between 20 and 40 degrees latitude. Southwest Asia also has strong positive anomalous values of tropopause folding frequency and achieves the values more than five times the average of the Northern Hemisphere in the summer, especially in the northern latitudes of this region including Afghanistan, Iran and the eastern Mediterranean. This can be attributed mainly to the formation of summer monsoon in the subtropical latitudes of the Indian Ocean. In this regard, the examination of the effects of El Niño and La Niña on the spatial–temporal distributions of tropopause folding reveals that the values of 3-month running average of tropopause folding frequency coincident with La Niña tend to be larger than those with El Nino in both the hemispheres and Southwest Asia. This result is consistent with the previous findings on the tendency for higher occurrence of anticyclonic (cyclonic) baroclinic wave life cycles during La Niña (El Niño). Dynamically, the folds occurred in the study area are associated with the formation of waves at 500 hPa level and subsequently the creation of surface cyclones. In areas of high frequency of folding events, strong meridional shear of the zonal winds and jet stream amplification are observed at 200 hPa level. The analysis of wave activity flux and its divergence for the selected month of June 2015 shows two regions of wave propagation, one with a wave-packet like signature of flux divergence/convergence centers in the upper troposphere and one over the lower troposphere. The upper-tropospheric wave packet ends with an area of flux convergence related with the receipt and decay of Rossby waves, while in the lower troposphere, a flux divergence region as a source of stationary waves associated with the Indian monsoon activity is identified.