A mid-summer drought (MSD) is experienced in southern Mexico and Central America, as well as in the adjacent Caribbean, Gulf of Mexico and eastern Pacific seas. The aim of this paper is to describe the regional characteristics of the drought and to propose some possible forcing mechanisms. Satellite and in-situ data is used to form a composite of the evolution of a typical MSD, which highlights the coincidence of low level easterly flow of dry tropical western Atlantic air towards the tropical

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... astern Pacific with the drought. The MSD is also coincident with heavy precipitation over the Sierra Madre Occidental (part of the North American Monsoon). Reanalysis data is used to show that the divergence of the anomalous easterly flow during the MSD is the main factor governing the variations in precipitation. There is no evidence that the local sea surface temperature (SST) variations are driving changes in the convection. A linear baroclinic model is used to show that the seasonal progression of the Pacific Inter-Tropical Convergence Zone (ITCZ), which moves northward following warm SST during the early summer, and of the Atlantic (Bermuda) sub-tropical high, which moves westward, are the most important remote factors that contribute towards the low level easterly flow and divergence during the MSD. It is also found that the circulation associated with the MSD precipitation deficit helps to maintain the deficit by inducing low level anticyclonic flow and easterlies across Central America. Surface heating over land also plays a role: a large thermal low over northern USA in early summer is accompanied by enhanced subsidence over the North Atlantic. This thermal low is seen to reduce considerably in mid-summer, allowing the high pressure anomalies in the Atlantic and Pacific oceans to extend into the Gulf of Mexico. These anomalies are maintained until the withdrawal of the Bermuda sub-tropical high in late summer (coincident with a surge in Atlantic tropical cyclones) which induces anomalous surface cyclonic flow with westerlies fluxing moisture from the Pacific ITCZ towards Central America