The Impact of the Asian Summer Monsoon Circulation on the Tropopause

Previous studies have identified two important features of summertime thermodynamics: 1) a significant correlation between the low-level distribution of equivalent potential temperature theta(e) and the potential temperature theta of the extratropical tropopause and 2) a northwestward shift of the maximum tropopause theta relative to the maximum low-level theta(e). Here, the authors hypothesize these two features occur because of the Asian monsoon circulation. The hypothesis is examined using a set of idealized prescribed sea surface temperature (SST) aquaplanet simulations. Simulations with a zonally symmetric background climate exhibit a weak moisture-tropopause correlation. A significant correlation and northwestward shift occurs when a zonal wave-1 SST perturbation is introduced in the Northern Hemisphere subtropics. The equivalent zonal-mean subtropical warming does not produce a significant correlation. A mechanism is proposed to explain the moisture-tropopause connection that involves the circulation response to zonally asymmetric surface heating and its impact on the tropopause defined by the 2-potential-vorticity-unit (PVU; 1 PVU = 10(-6) K kg(-1) m(2) s(-1)) surface. While the circulation response to diabatic heating is well known, here the focus is on the implications for the tropopause. Consistent with previous research, surface heating increases the low-level theta(e) and produces low-level convergence and a cyclonic circulation. The low-level convergence is coupled with upper-level divergence via convection and produces an upper-level anticyclonic circulation consistent with Sverdrup balance. The anticyclonic vorticity lowers the PV northwest of the surface heating via Rossby wave dynamics. The decreased PV leads to a northwestward shift of the 2-PVU surface on fixed pressure levels. The theta value to the northwest of the surface heating is higher, and consequently the maximum tropopause theta increases.