The opposite response of the South Asian high to increasing CO2 at different heights

Abstract

Based on the multimodel ensemble of 22 models in Coupled Model Intercomparison Project Phase 6 (CMIP6), the present manuscript found that in response to increasing CO₂, the South Asian high (SAH) displays the opposite response over its southern region in the upper troposphere: an anticyclonic response at 100 hPa and a weak (insignificant) cyclonic response at 200 hPa. This opposite response is a product of tropospheric warming. In response to increasing CO₂, the troposphere warms, with the potential temperatures peaking at 150–200 hPa over the northern Indian Ocean (IO). With transportation by local vertical motion (ascendance over the northeastern IO and descendance over the northwestern IO), various changes in vertical temperature advection with height form at 100 and 200 hPa. Finally, the changes contribute to a decrease in the ω response with pressures at 100 hPa but an increase in the ω response with pressures at 200 hPa over the northwestern IO. Over the northwestern IO, ω change is inversely related to pressure. At 100 hPa, the sign of ω changes with pressure, which yields distinct vorticity forcing over the northeastern and northwestern IO. This causes an anticyclonic response, which may generate zonal vorticity advection and balance the vorticity forcing. At 200 hPa, the contribution is roughly opposite to that at 100 hPa. In addition, although diabatic heating contributes to the vertical profile of ω, it yields the same-sign vorticity response at 100 and 200 hPa.