The seasonal evolution of the diurnal variation of the low-level winds around the Gulf of California. Is there a link to vegetation green-up during the wet season?

Sea-land breeze circulations are ubiquitous along the Gulf of California. Sea breezes are well developed because of the strong heating over the desertic regions along both sides of the Gulf. However, the sea surface temperature has a large seasonal range, especially over the northern Gulf. In addition, after the start of the summer rains there is a rapid foliation of the seasonal dry forest found along the eastern side of the Gulf. It was hypothesized that the rapid vegetation change and associated change in the land surface characteristics (albedo and evapotranspiration) might modify the sea-land breeze circulations. This might in turn affect the diurnal evolution of rainfall over the region. The recently ended North American Monsoon Experiment (NAME) afforded an opportunity to determine the possible seasonal variation of the sea-breeze intensity and its relationship with the onset of the rainy season. This presentation will describe the effort to measure the diurnal cycle of the winds and its seasonal variation. A network of 7 pilot balloon stations made observations twice-daily for approximately four months, with two of these making more frequent observations during special periods. Surface observations from automated surface stations were also available. The monthly mean winds from the different stations will be shown, as well as divergence estimates over different subregions of the pilot balloon array. The changes in sea-breeze intensity and the area-averaged divergence estimates will be compared with the seasonal evolution of the Gulf surface temperature and mean land surface temperature changes and the observed rainfall onset over the region.