Mesoscale convective complexes in the Americas

Abstract

Enhanced infrared satellite imagery and conventional surface and sounding data are used to document the existence and climatological characteristics of mesoscale convective complexes (MCCs) over midlatitude South America (south of 20°S) and in the tropical region (20°N to 20°S) between North and South America. The implications of the results, with regard to the structure and dynamics of MCCs, are discussed. It is found that MCCs occur with approximately the same frequency in mid‐latitude South America as they do in mid‐latitude North America. For the most part the characteristics of mid‐latitude South American MCCs are similar to those of MCCs in the United States. The most notable difference between North and South American MCCs is that the South American systems are, on the average, about 60% larger than MCCs in the United States. In addition to the large population of mid‐latitude South American MCCs, an even larger number of low‐latitude (tropical) systems were found. In general, both the mid‐latitude and tropical populations of MCCs are nocturnal and continental; i.e., the great majority of systems occur at night over land. Of the systems that do occur over water, a significant fraction develop into tropical storms. Large populations of MCCs occur in each of the physiographically similar mid‐latitude areas of North and South America where low‐level nocturnal jets frequently develop. Very few MCCs occur over the Amazon River Basin or over the southeastern United States, even though both of these areas exhibit large amounts of deep convective activity. All of the various MCC population centers occur in latitudinal zones of westerlies or easterlies and are concentrated immediately downwind of major mountain ranges. One of the 2 years of data that was investigated was an El Niño year. During the El Niño period the number of mid‐latitude South American systems was more than double the number in the non‐El Niño year. Moreover, several MCCs formed over the anomalously warm water that appeared along the Peruvian coast. Thus on the basis of this very small sample, there may be a direct connection between MCC activity and El Niño.