Impacts of variability and projected change in midlatitude storm tracks on the hydroclimate of the U.S. Southwest during winter

The midlatitude storm tracks made up of baroclinic waves and their associated cyclones and fronts are subseasonal variabilities that give rise to much of the precipitation over the continental U.S., especially in the cool seasons. Shifts in the storm tracks or changes in cyclone activity, such as those associated with the El Nino Southern Oscillation or global climate change, can have significant impacts on the hydroclimate of many regions in the US. It is important to quantify and understand the relationship between storm track and precipitation variability, and assess how future projected changes in storm track activity may impact water resources over these regions.

In this study, projected changes in storm track activity and precipitation over US made by models participating in phases 3 and 5 of the Coupled Model Intercomparsion Project (CMIP3 and CMIP5) have been examined. Each model's ability to simulate the relationship between storm track and precipitation variability during the historical period has been assessed based on comparisons with similar relationships derived using observed precipitation and storm track variability based on satellite and rain gauge measurements and reanalysis data. Projected changes in precipitation and storm tracks made by these models under increasing greenhouse gas forcing are then assessed based on these relationships.

In this presentation, results for winter over the U.S. Southwest will be highlighted. Previous studies have suggested that CMIP5 models exhibit a greater degree of agreement on the sign of the winter precipitation change than in CMIP3 over the US Southwest, indicating a greater portion of California falling within the increased precipitation zone. It has been hypothesized that this difference is related to changes in storm tracks responding to changes in the large scale steering flow. In this study, relationship between storm track and California precipitation variabilities simulated in models are assessed based on similar relationships found in observations and reanalysis data, and model-to-model differences in projected changes in California precipitation, as well as differences between CMIP5 and CMIP3, are examined to assess the role played by storm track variability and projected changes.