Interannual and Intraseasonal Interactions Between Greening Process and Soil Moisture in the North American Monsoon Region in Northwestern Mexico

In the North American Monsoon System (NAMS) Tier 1 core area, summer precipitation accounts for more than fifty percent of the annual total. Early in this season precipitation events produce rapid greening of the vegetation cover. While this greening has been shown to results in major changes in land surface water and energy cycles locally, the nature of the resulting interactions with regional scale fluxes, and its intraseasonal and interannual variability, are less clear. We focus here on the greening process and soil moisture relationships at the regional scale, as key components of the hydrological cycle associated with the greening. Specifically, using MODIS LAI from 2000 to 2007, we identify spatiotemporal patterns of variability of the greening process. We also evaluate assimilation of MODIS LAI into the Variable Infiltration Capacity (VIC) land surface model in order to estimate the implications of interannual variations in vegetation greenness on surface hydrological conditions. In these experiments, we force VIC with daily observed precipitation, minimum and maximum temperatures, and wind speed at 1/16 degree spatial resolution over the NAME tier 1 core area in northwest Mexico. We use VIC soil moisture estimates both with a fixed seasonal cycle of leaf area index, and with MODIS LAI, to evaluate interannual differences in the hydrological response to changes in vegetation during the pre-monsoon and monsoon months of wet and dry years and during the specific greening events between 2000 and 2007. These experiments allow us to evaluate the role of soil moisture persistence on the intraseasonal and interannual variability of the greening process and to investigate factors that may affect the strength of the greening-soil moisture relationship.