The Midsummer Drought over Mexico and Central America

Abstract

The annual cycle of precipitation over the southern part of Mexico and Central America exhibits a bimodal distribution with maxima during June and September-October and a relative minimum during July and August, known as the midsummer drought (MSD). The MSD is not associated with the meridional migration of the intertropical convergence zone (ITCZ) and its double crossing over Central America but rather with fluctuations in the intensity and location of the eastern Pacific ITCZ. 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 convective forcing in the ITCZ. The intensification of the trade winds during July and August and the orographic forcing of the mountains over most of Central America result in maximum precipitation along the Caribbean coast and minimum precipitation along the Pacific coast of Central America. Changes in the divergent (convergent) low-level winds over the ''warm pool'' off the west coast of southern Mexico and Central America determine the evolution of the MSD. Maximum deep convective activity over the northern equatorial eastern Pacific, during the onset of the summer rainy season, is reached when sea surface temperatures exceed 29ЊC (around May). After this, the SSTs over the eastern Pacific warm pool decrease around 1ЊC due to diminished downwelling solar radiation and stronger easterly winds (during July and August). Such SST changes near 28ЊC result in an substantial decrease in deep convective activity, associated with the nonlinear interaction between SST and deep tropical convection. Decreased deep tropical convection allows increased downwelling solar radiation and a slight increase in SSTs, which reach a second maximum (ϳ28.5ЊC) by the end of August and early September. This increase in SST results once again in stronger low-level convergence, 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 surface temperature and even in tropical cyclone activity over the eastern Pacific.

Variability of the Caribbean Low-Level Jet and its relations to climate

“…2c). The minimum rainfall in July is the well-known phenomenon of the mid-summer drought (MSD) which is more obvious in the regions of Central America and South Mexico (e.g., Magaña et al 1999;Mapes et al 2005). Unlike the zonal wind, SLP, and rainfall, the SST in the CLLJ region shows a single peak around October (Fig.…”

Regional long-term climate change (1950-2000) in the midtropical Atlantic and its impacts on the hydrological cycle of Puerto Rico

“…All runs performed will span the 3 month period of April-June, i.e., the Puerto Rican ERS (Figure 1). This part of the year is the most convenient to conduct LCLU changes studies in Puerto Rico [Velazquez-Lozada et al, 2006], and represents the end of the dry season and the onset of the midsummer drought [Magaña et al, 1999], a critical period in the annual hydrological cycle of the island, during which the atmospheric model has performed satisfactorily [Comarazamy and González, 2008]. A spin-up time of one week is specified at the start of each 3 month simulation to allow for numerical stabilization of the atmospheric model and the numerous submodels and parameterizations available in the modeling system.…”

A simple mechanism for the climatological midsummer drought along the Pacific coast of Central America

“…These three decades were also a period during which climate observations throughout the GCR, both ground-based and space-borne, were improving and expanding. Magaña et al (1999) proposed that the MSD can be explained in terms of a local mechanism involving lagged feedbacks between sea surface temperature (SST), convection, and surface insolation. The argument is as follows: seasonal insolation causes SST in the east Pacific warm pool (EPWP) to warm in early summer, which drives convection and produces the first peak in precipitation; at the height of convective activity, surface insolation is reduced due to cloud cover, thereby cooling SST, which reduces convection and produces the midsummer precipitation minimum; finally, while convection is at a minimum, increased insolation again warms SST, which results in convection and the second peak in precipitation.…”