High North American Monsoon Lowermost Stratospheric Water Vapor: Signatures of Convective Injection from MLS and Other Satellite Measurements

Abstract:

The summer North American monsoon anticyclone (NAMA) and the Asian monsoon anticyclone (AMA) enclose most of the highest 100 hPa and 83 hPa water vapor (H2O) mixing ratios in the 11-year Aura Microwave Limb Sounder record, both in terms of mean values and in high outliers. The highest NAMA H2O outliers in the central continental United States are almost certainly the result of direct, local injection of ice into the lower stratosphere (LS) by convection that overshoots the tropopause. However, the relative importance of these direct injections in supplying the bulk of the NAMA LS humidity anomaly compared to that of transport from convection on the tropical side of the anticyclone has not been conclusively determined. Large-scale circulation models have reproduced the high NAMA LS water vapor with tropical convection followed by advection and ascent, however the handling of convection in such models is generally a source of uncertainty. NAMA H2O at 100--83 hPa is close to saturation at the low tropopause temperatures on the tropical side of the anticylone, and cold anomalies above this convection have been shown to act as cold traps, leading to an anticorrelation between NAMA LS humidity and tropical convective intensity. In this work we use MLS and other satellite-based observations to investigate the importance of direct convective injection in supplying the high H2O anomalies within the NAMA LS anticyclone.