The annual cycle of precipitation over the southern part of Mexico and Central America exhibits a bimodal distribution with maxima during June and September and a relative minimum during July and August, known as the Mid Summer Drought (MSD). The MSD is not associated with the meridional migration of the Inter Tropical Convergence Zone (ITCZ) and its double crossing over Central America but rather, with fluctuations in the intensity and location of the eastern Pacific ITCZ. These fluctuations are related to the strength of the trade winds. During the transition from intense to weak (weak to intense) convective activity, the trade winds over the Caribbean strengthen (weaken). Such acceleration in the trade winds is part of the dynamic response of the low level atmosphere to the magnitude of the thermal forcing. The intensification of the trade winds during July and August, plus the orographic forcing of the mountains over Central America result in maximum in precipitation along the Caribbean coast, and minimum precipitation along the Pacific coast of Central America. In addition to changes in the wind speed, divergent (convergent) low-level anomalies over the "warm pool", off the west coast of southern Mexico and Central America, occur when the MSD establishes (disappears).
Maximum deep convective activity over the northern equatorial eastern Pacific and the onset of the summer rainy season over the region is reached when the sea surface temperature is above 29(C (around May). After the this, the SSTs over the eastern Pacific warm pool decrease between 28( and 29(C due to diminished downwelling solar radiation and stronger easterly winds (July and August). Such SST changes, around 28(C result in large fluctuations in deep convective activity and consequently, in less precipitation during this part of summer. Decreased deep convection allows increased downwelling solar radiation and a slightly higher SSTs, which reach a second maximum around 28.5(C by the end of August. This increase in SST results once again in stronger low-level convergence and enhanced deep convection and consequently, in a second maximum in precipitation.
The MSD signal can also be detected in other variables such as minimum and maximum continental surface temperature and even in tropical cyclone activity over the eastern Pacific, showing that air-sea-land interactions are part of the dynamics of the MSD. Similar fluctuations in precipitation over the Caribbean and the Gulf of Mexico are discussed