Asian monsoon forcing of subtropical easterlies in the community atmosphere model: Summer climate implications for the western atlantic

The effects of a progressively enhanced Asian summer monsoon on the mean zonal wind are examined in a series of experiments using the Community Atmosphere Model version 4 (CAM4). The response of the barotropic mean zonal wind varies in a linear fashion with the forcings of 5, 10, and 20 W m22 in net radiation over South Asia. The authors increase the strength of the monsoon by making the South Asian land surface hotter (via lower soil albedo). This leads to an enhanced Rossby wave source region over the Balkan Peninsula at 458N, northwest of the upper-level Tibetan high (TH). Equatorward propagation of Rossby waves causes stationary eddy momentumflux divergence (SEMFD) to the south of this source region. This local area of SEMFD produces easterly tendencies of the barotropic part of the mean zonal wind in the subtropics. As the easterly mean flow strengthens, so do low-level easterlies across the subtropical Atlantic, leading to a westward displacement of the North Atlantic subtropical high (NASH) on its equatorward flank. The western intensification of theNASH causes drying in the west Atlantic and neighboring land masses primarily because of near-surface wind divergence in the anticyclone. These modeling results confirm the mechanisms deduced in the authors' recent observational analysis of the mean seasonal cycle's midsummer drought.