Michael W. Douglas

The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed.

Moisture is transported in South America westward from the tropical Atlantic Ocean to the Amazon basin, and then southward toward the extratropics. A regional intensification of this circulation to the east of the Andes Mountains is called the South American low-level jet (SALLJ), with the strongest winds found over eastern Bolivia. SALLJ is present all year and channels moisture to the La Plata basin, which is analogous to the better-known Amazon basin in terms of its biological and habitat diversity, and far exceeds the latter in its economic importance to southern and central South America in terms of hydroelectricity and food production. The relatively small SALLJ spatial scale (compared with the density of the available sounding network) has a limited understanding of and modeling capability for any variations in the SALLJ intensity and structure as well as its possible relationship to downstream rainfall. The SALLJ Experiment (SALLJEX), aimed at describing many aspects of SALLJ, was carried out between 15 November 2002 and 15 February 2003 in Bolivia, Paraguay, central and northern Argentina, western Brazil, and Peru. Scientists, collaborators, students, National Meteorological Service personnel, and local volunteers from South American countries and the United States participated in SALLJEX activities in an unprecedented way, because SALLJEX was the most extensive meteorological field activity to date in subtropical South America, and was the first World Climate Research Program/Climate Variability and Prediction Program international campaign in South America. This paper describes the motivation for the field activity in the region, the special SALLJEX observations, and SALLJEX modeling and outreach activities. We also describe some preliminary scientific conclusions and discuss some of the remaining questions

Abstract Aim To explain the relationship between topography, prevailing winds and precipitation in order to identify regions with contrasting precipitation regimes and then compare floristic similarity among regions in the context of climate change. Location Eastern slope of the tropical Andes, South America. Methods We used information sources in the public domain to identify the relationship between geology, topography, prevailing wind patterns and precipitation. Areas with contrasting precipitation regimes were identified and compared for their floristic similarity. Results We identify spatially separate super‐humid, humid and relatively dry regions on the eastern slope of the Andes and show how they are formed by the interaction of prevailing winds, diurnally varying atmospheric circulations and the local topography of the Andes. One key aspect related to the formation of these climatically distinct regions is the South American low‐level jet (SALLJ), a relatively steady wind gyre that flows pole‐ward along the eastern slopes of the Andes and is part of the gyre associated with the Atlantic trade winds that cross the Amazon Basin. The strongest winds of the SALLJ occur near the ‘elbow of the Andes’ at 18° S. Super‐humid regions with mean annual precipitation greater than 3500 mm, are associated with a ‘favourable’ combination of topography, wind‐flow orientation and local air circulation that favours ascent at certain hours of the day. Much drier regions, with mean annual precipitation less than 1500 mm, are associated with ‘unfavourable’ topographic orientation with respect to the mean winds and areas of reduced cloudiness produced by local breezes that moderate the cloudiness. We show the distribution of satellite‐estimated frequency of cloudiness and offer hypotheses to explain the occurrence of these patterns and to explain regions of anomalously low precipitation in Bolivia and northern Peru. Floristic analysis shows that overall similarity among all circumscribed regions of this study is low; however, similarity among super‐humid and humid regions is greater when compared with similarity among dry regions. Spatially separate areas with humid and super‐humid precipitation regimes show similarity gradients that are correlated with latitude (proximity) and precipitation. Main conclusions The distribution of precipitation on the eastern slope of the Andes is not simply correlated with latitude, as is often assumed, but is the result of the interplay between wind and topography. Understanding the phenomena responsible for producing the observed precipitation patterns is important for mapping and modelling biodiversity, as well as for interpreting both past and future climate scenarios and the impact of climate change on biodiversity. Super‐humid and dry regions have topographic characteristics that contribute to local climatic stability and may represent ancestral refugia for biodiversity; these regions are a conservation priority due to their unique climatic characteristics and the biodiversity associated with those characteristics.

In 2006, NASA led a field campaign to investigate the factors that control the fate of African easterly waves (AEWs) moving westward into the tropical Atlantic Ocean. Aircraft and surface-based equipment were based on Cape Verde's islands, helping to fill some of the data void between Africa and the Caribbean. Taking advantage of the international African Monsoon Multidisciplinary Analysis (AMMA) program over the continent, the NASA-AMMA (NAMMA) program used enhanced upstream data, whereas NOAA aircraft farther west in the Atlantic studied several of the storms downstream. Seven AEWs were studied during AMMA, with at least two becoming tropical cyclones. Some of the waves that did not develop while being sampled near Cape Verde likely intensified in the central Atlantic instead. NAMMA observations were able to distinguish between the large-scale wave structure and the smaller-scale vorticity maxima that often form within the waves. A special complication of the east Atlantic environment is the Saharan air layer (SAL), which frequently accompanies the AEWs and may introduce dry air and heavy aerosol loading into the convective storm systems in the AEWs. One of the main achievements of NAMMA was the acquisition of a database of remote sensing and in situ observations of the properties of the SAL, enabling dynamic models and satellite retrieval algorithms to be evaluated against high-quality real data. Ongoing research with this database will help determine how the SAL influences cloud microphysics and perhaps also tropical cyclogenesis, as well as the more general question of recognizing the properties of small-scale vorticity maxima within tropical waves that are more likely to become tropical cyclones.

The present study describes some observed surface and upper‐air features of the low‐level jet (LLJ) and southerly jet (SJ). Our results suggest the existence of this low‐level circulation to the east of the Andes that transports moisture from tropical South America toward the south during the warm/wet season of 1999. We explore the synoptic variability, diurnal variation, and alternations between LLJ and SJ episodes by using a combination of surface and high‐resolution upper‐air observations (1 to 8 soundings per day) and global reanalysis. Our results show strong synoptic fluctuations; with the LLJ more frequent than SJs. The LLJ has stronger winds in the afternoon and its core of maximum winds is located between 1600 and 2000 m above the surface. Special observational efforts, such as the pilot balloon sounding network in Bolivia (Pan American Climate Studies Sounding Network [PACS‐SONET] program), the Large‐Scale Biosphere‐Atmosphere (LBA) Experiment‐WET Atmospheric Mesoscale Campaign (AMC), and Tropical Rainfall Measuring Mission (TRMM)‐LBA in Southwest Amazonia, have provided upper‐air information with high temporal and spatial resolution to describe the structure of both the LLJ and the SJ during the January–April 1999 period.