Spatio-temporal variations in surface characteristics over the North American Monsoon region

摘要

In this paper we summarize the surface characteristics for six locations in western Mexico and southwestern USA (from a subhumid climate in Jalisco, Mexico to the Sonoran Desert climate in Arizona, USA), that lie along a meridional transect within the North American Monsoon (NAM) core region using available MODerate Resolution Imaging Radiometer (MODIS) satellite data and supplementary surface instrumental data for two of these sites in Sonora, Mexico. The climate analysis for each site is carried out for the period 2000-2008, that includes all available MODIS data. A comparison of seasonal and annual variability in surface conditions for the enhanced vegetation index (EVI), albedo and land surface temperature (LST) at each site is presented. With the help of available surface data from field observations, a more detailed analysis of Rayon and Rosario de Tesopaco sites is presented. The qualitative behavior and climate response of three types of vegetation: desert shrub, subtropical shrub, and tropical deciduous forest ecosystems are analyzed under the influence of the NAM summer wet season. The onset of the NAM warm wet season in early summer, is one of the main precursors of generalized EVI growth in all the NAM region. At all the sites, it is observed that the mean daytime LST cools several degrees as the NAM fully develops. During the warm wet season, in the case of open and sparse vegetation regions such as desert shrub and subtropical shrub, albedo values fall slightly during the NAM season, while in closed and dense tropical deciduous forest regions albedo shows a slight increase. Differences in soil reflectivity at these sites are probably responsible for this rather unexpected behavior. Additionally it is found that, desert shrub and subtropical shrub regions in northern latitudes show large LST and small EVI/albedo seasonal variability, whilst tropical deciduous forests in lower latitudes show much larger EVI/albedo and smaller LST seasonal variability. Thus MODIS data proves to be a valuable tool for assessing the dynamics of seasonal and interannual surface characteristics helpful in determining climate patterns.