CR 27:1-8 (2004)  -  doi:10.3354/cr027001

ABSTRACT: This study uses a novel data set of satellite-derived precipitation and reanalysis winds to describe variations in the diurnal cycle during the summer season in southern Mexico, Central America, and adjacent oceans, with the purpose of elucidating local forcing mechanisms of the mid-summer drought (MSD). It is well established that precipitation peaks in the evening over land, while just off the Pacific coast the maximum is during the day. However, little is understood about the diurnal cycle of rainfall and winds associated with the MSD, a climatological phenomenon that impacts the local population. First, the MSD was quantified with the precipitation data and found to be strongest over Guatemala and El Salvador, consistent with previous studies. Over Guatemala the strength of the diurnal cycle follows the evolution of the MSD. Here evening precipitation rates are 25% higher at the beginning and end of the summer compared to mid-summer, while daytime rates remain fairly constant throughout the season. To the south, over the Pacific, both daytime and nighttime precipitation rates are higher at the beginning and end of summer as compared to mid-summer. In June, surface winds in the vicinity of the Gulf of Tehuantepec are southerly during the evening and northerly during the day, dynamically consistent with the diurnal cycle of convection. In July-August and September, tradewind systems disturb this apparent local circulation cell bringing conditions conducive to reduced and enhanced rainfall respectively. In neighboring areas where the MSD is weak, precipitation and wind change little over land and ocean for the summer season. Thus, these results support earlier studies linking solar forcing of sea surface temperatures and changes in the large-scale circulation in the development of a strong but localized MSD.

KEY WORDS: Mid-summer drought · Rainfall · Diurnal cycle · Satellite · Mexico · Central America