Physical Mechanisms of the California Drought
At the time of writing in February 2016, California has enjoyed some heavy rain and snow, as expected given the massive El Nino of 2015 and 2016. However, California experienced less-than-normal precipitation in each of the previous four winters and, despite some relief, almost the entire state remains in drought. As a four-winter average, 2011-15 was the driest California has experienced since statewide records, as reported by the National Oceanic and Atmospheric Administration (NOAA), which began recording in 1895. There is no long-term trend, either wetting or drying, in California precipitation, but instead a tremendous amount of variability both year-to-year and decade-to-decade. For example, a silar multiyear drought occurred in the late 1980s to the early 1990s while the 1920s were an overall dry decade and the 1990s an overall wet decade. Evidence from tree rings and lake levels also indicate truly long, multi-decadal, droughts during the medieval era (Stine 1994, Cook et al. 2010).
So, given how variable California precipitation is, what caused this particular drought? Also, was human-driven climate change caused by rising greenhouse gases (GHGs) in any way involved? The results reported here largely follow from a study conducted by the NOAA Drought Task Force, a multi-institution group, and uses analysis of the instrumental record, simulations with atmosphere models forced by the observed sea surface temperature (SST) history and model simulations of the climate system response to changing atmospheric composition (e.g. GHGs) conducted for IPCC Assessment Report Five (Seager et al. 2014, 2015).
Climate variability that brings droughts and foods can arise in two fundamental ways. The first is by internal atmospheric variability in which the chaotic flow of the atmosphere can give rise to long periods of drier or wetter, or warmer or colder, than normal weather. The second method is via anomalies in the SST around the world which cause changes in the flux of heat between the ocean and atmosphere that force changes in atmospheric circulation. SST anomalies in the tropics are most effective in this regard—the El Niño-Southern Oscillation is a prime example. Coupled atmosphere-ocean interactions lead to changes in heat transport by ocean currents that generate SST anomalies that alter the intensity and location of tropical rain systems. This induces atmospheric circulation anomalies across the globe, including over North America.
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- Harvard College Review of Environment & Society