Influence of interannual variation in snow mass in modulating the onset and intensity of the North American monsoon circulation.

This study addresses the hypothesis that interannual variations in the North American Monsoon System (NAMS) are modulated by the effect of snowpack from the previous spring on land surface hydrologic processes. This hypothesis is identical to that proposed over 100 years ago to explain land-atmosphere interactions in the south Asian monsoon system, and can be formulated as follows: an anomalously large snowpack requires a large amount of energy to melt and evaporate, resulting in a delayed seasonal warming of the land mass and a reduction inlarge-scale thermal contrasts between the warm land mass and the cooler adjacent oceans. This weaker temperature gradient is associated with a delay in the onset and a reduction in the intensity of the summertime monsoon circulation. Opposing arguments can be made to explain the response of monsoonal circulation to negative snowpack anomalies.

To address snow-monsoon interactions over North America, we first characterize the climatological characteristics and variability in the NAMS. A network of 6822 NOAA cooperative observing stations west of the Mississippi River are used to identify spatial variations in the onset date of the NAMS and total summer (July-August) precipitation. Coherent sub-regional modes of variability within the North American monsoon region are identified using empirical orthogonal function (EOF) analysis, and fields of geopotential height, diabatic heating, convective available potential energy and moisture fluxes from the NCEP/NCAR Reanalysis are examined to identify differences in monsoon development and intensity in each sub-region. Finally, anomalies in springtime snow water equivalent and snow depth, snow extent, and the date of disappearance of snow cover are examined throughout the western U.S. to assess regional effects of snow cover on monsoon development.