The Impact of the South Asia High Bimodality on the Chemical Composition of the Upper Troposphere and Lower Stratosphere

The South Asia High (SAH) is the dominant feature of the circulation in the upper troposphere and lower stratosphere (UTLS) during the boreal summer,and the upper tropospheric anticyclonic circulation extends into the lower stratosphere.The preferred locations of the center of the SAH occur in two different regions,and the center can be located over the Iranian Plateau or over the Tibetan Plateau.This bimodality has an impact on the distribution of chemical constituents in the UTLS region.We analyzed water vapor (H 2 O),carbon monoxide (CO),and ozone (O 3) data derived from the Aura Microwave Limb Sounder (MLS) and total column ozone data from the Ozone Monitoring Instrument (OMI).For the Iranian Plateau mode of the SAH,the tropospheric tracers exhib-ited a positive anomaly over the Iranian Plateau and a negative anomaly over the Tibetan Plateau,whereas the stratospheric tracer exhibited a negative and a positive anomaly over the Iranian Plateau and the Tibetan Plateau,respectively.For the Tibetan Plateau mode,however,the distribution of the anomaly was the reverse of that found for the chemical species in the UTLS region.Furthermore,the locations of the extrema within the anomaly seemed to differ across chemical species.The anomaly extrema for H 2 O occurred in the vicinity of the SAH ridgeline,whereas CO and O 3 exhibited a northward shift of 4-8 degrees.These impacts of the variation in the SAH on the chemical constitutes in the UTLS region can be attributed in part to the dynamical structure delineated by the tro-popause field and the temperature field at 100 hPa.