Mechanisms for a sharpened seasonal cycle over California under global warming

Wednesday, December 11, 2019 - 11:05
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There is an increasing demand for projections of precipitation changes under warming in California with a growing population and economy. California has a Mediterranean-type climate with temperate wet winter and warm dry summer, lying in the transitional latitudes between the subtropical dry climate and the midlatitude, storm track-dominated climate. Hence, future changes in California precipitation are more sensitive to the influence of circulation shift as the Earth warms. Understanding potential future changes in seasonal precipitation under warming in California can help predict wildfires, droughts, and floods. Here we explore the characteristics of seasonal precipitation changes by comparing the contributions of extreme and non-extreme precipitation and investigate the underlying mechanisms with a focus on clarifying the roles of moisture (thermodynamic) and circulation (dynamic).

The Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model simulations show a sharpened seasonal cycle marked by a stronger and narrower wet season over California. Both extreme and non-extreme precipitation contribute to the sharpened seasonal cycle. In particular, increased extreme precipitation due to enhanced extreme intensity and more extreme days dominates the wetter winter, while decreased non-extreme precipitation due to fewer wet days induces the dryer spring and fall.

Moisture budget analysis shows that mean zonal moisture advection due to seasonally-dependent changes in land-sea moisture contrast originating from the nonlinear Clausius–Clapeyron relation dominates the sharpened wet season over California. More specifically, the stronger climatological land-sea thermal contrast in winter with warmer ocean than land results in more moisture increase over ocean than land under warming and hence, wet advection to California. However, in fall and spring, the future change of land-sea thermal contrast with larger warming over land than ocean induces an opposite moisture gradient and hence, dry advection to California. These results have important implications for projecting changes in the hydrological cycle of California.

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