Publication Date



Open access

Embargo Period


Degree Type


Degree Name

Doctor of Philosophy (PHD)


Meteorology and Physical Oceanography (Marine)

Date of Defense


First Committee Member

Brian Mapes

Second Committee Member

Amy Clement

Third Committee Member

Ben Kirtman

Fourth Committee Member

Dave Enfield


Warm season precipitation in the subtropical Americas is characterized by an abrupt drying in July and August known as the midsummer drought (MSD). The ultimate cause of the MSD is investigated in the context of large-scale atmospheric dynamics using observational data as well as a series of dedicated global modeling experiments. Observational analysis indicates that the climatological time signature as well as interannual variability of the MSD occurs in concert with the westward displacement of the North Atlantic subtropical high (NASH) and sign change of the mean barotropic zonal wind near 20-30ºN. Decomposition of a fully-closed zonal momentum budget reveals stationary eddy momentum flux divergence (SEMFD) on the western sector of the Asian monsoon Tibetan high as the primary mechanism governing the occurrence of zonal mean easterlies at subtropical latitudes in July. A stronger Asian monsoon in midsummer is significantly correlated with stronger subtropical easterlies and consequently, with a stronger MSD in the western Atlantic (WATL). These results form the observational basis for a series of experiments using the Community Atmosphere Model version 4 (CAM4). The effects of a progressively enhanced Asian monsoon on the barotropic zonal wind are examined in CAM4 with an emphasis on downstream summer climate impacts in the greater WATL. The strength of the monsoon is controlled by making the South Asian land surface darker (via lower soil albedo), leading to forcings of 5, 10, and 20 Wm-2 in net radiation. This leads to an enhanced Rossby wave source region over the Balkan Peninsula at 45?N, northwest of the upper level Tibetan high. Equatorward propagation of Rossby waves causes SEMFD to the south of this source region which 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 NASH on its equatorward flank. The western intensification of the NASH causes drying in the WATL and neighboring land masses primarily due to near surface wind divergence in the anticyclone. Coupled air-sea interactions exhibit a positive enhancement of these monsoon-driven impacts, particularly off the Pacific coast of Mesoamerica, where increased easterly trades on the southwest corner of the NASH causes evaporative cooling of sea surface temperatures. These modeling results confirm the mechanistic hypothesis deduced from observations and are demonstrated to be insensitive to how heating over South Asia is introduced in the model. Related impacts of the mechanisms found here include steering of tropical cyclones on the western edge of the NASH, and interpretations for paleoclimate signatures.


subtropical high; midsummer drought; mean zonal wind; Asian monsoon; summer climate; subtropics