Possible causes for the recent droughts over California

Abstract:

Rainfall over California and snow pack were below normal for the recent three years from 2012-2014.. There is evidence of changes in the hydroclimate over the western United States over the last half century. For example: snow has declined, surface temperatures have warmed and runoff has occurring much earlier in the year. Do these changes link to the ‘climate change’ ?

There are large uncertainties in projection. While most CMIP models project dryness over the Southwest, the changes of drought frequency over California very from one model to another. However, all models indicate the increases of surface temperature. The warm winter conditions will reduce the snow pack.

California rainfall also has large interannual variability. The cold ENSO events with cold sea surface temperature anomalies (SSTAs) and suppressed convection over the tropical Pacific will cause dryness over California. In addition to ENSO, rainfall over California is also modulated by SSTAs in the North Pacific. Warm SSTAs in the North Pacific provide more favorable conditions for drought to occur over California. The 2012 winter was influenced by a cold ENSO, but there were warm SSTAs in the North Pacific and that could cause droughts over California.