The California Drought
The causes of the precipitation drop that drove the multiyear California drought will be examined and the relative roles of SST forcing and internal atmospheric variability discussed. An important, but not overwhelmingly dominant, role for tropical Indo-west Pacific SST anomalies will be diagnosed. It will also be shown that, from a PDSI perspective, warm temperatures which include a long term trend, notably intensified the drought. The failure of the 2015/16 El Nino to terminate or seriously alleviate the drought will be partially explained. The potential role of human-driven climate change in favoring a west coast ridge and, hence, reduced precipitation, will also be considered by examination of long term trends in Reanalyses. A case can be made that the tropical oceans have been adjusting in response to rising GHGs in a way that favors SST changes that drives a west coast ridge but this case is radical and requires a bold rejection of the CMIP5 model consensus. The quality of our observations of the climate system appear inadequate to either confirm or reject this case.