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Influence of Rocky Mountain Snowpack on Subsequent North American Monsoon

Collaborators: Azar Zarrin

Funding: NOAA CPPA

Publications:

Notaro, M., and A. Zarrin, 2011: Sensitivity of the North American monsoon to antecedent Rocky Mountain
snowpack. Geophysical Research Letters, 38, L17403, doi: 10.1029/2011GL048803.

Methods: While the inverse relationship between antecedent snowpack in the Himalayas and Tibetan Plateau and subsequent monsoon rainfall in South and East Asia has been extensively studied and established, the potential for such a relationship between Rocky Mountain snowpack and the North American monsoon remains uncertain. The present study represents the first modeling assessment of this vital predictability issue, going beyond simple observational correlations, which fail to identify causality or a mechanism, by applying a regional climate model (ICTP RegCM4) to demonstrate that a deep Rocky Mountain snowpack tends to hinder the poleward advance of the subtropical ridge and associated monsoon rainfall into the Southwest United States. Ensemble experiments are developed, with either the snowpack over the Rocky Mountains doubled or completely removed on March 1 of different years, and compared to the control simulation.

Key finding: A deep, extensive snowpack increases the surface albedo and provides an abundant surge of soil moisture, both of which reduce tropospheric temperatures in spring and early summer, weakening the land-ocean thermal gradient and related monsoon system. This snow-monsoon relationship, which may serve as a vital prediction tool of summer rainfall in the semi-arid Southwest United States, has direct implications for regional water supply, streamflow, and biodiversity.

Mean difference (MoreSnow-CONTROL) in (a,d) 200-hPa and (b,e) 700-hPa geopotential heights (m) and (c,f) precipitable water (kg/m2) for (a-c) July and (d-f) August in 1991, 1994, 1997, and 1998, shown in color shading (Notaro and Zarrin 2011). Contours in (a,b,d,e) show the mean height field from CONTROL. Simulations are produced using ICTP RegCM4 regional climate model. In MoreSnow, the snowpack over the Rocky Mountains is doubled in depth in spring. The effect of a deep, extensive snowpack over the Rocky Mountains in spring is to weaken the subsequent North American monsoon, including the monsoon's northward advance.