Was California’s winter 2013-14 really the warmest on record?

Wednesday, April 22, 2015
Kim Wang, University of California Los Angeles, Department of Civil and Environmental Engineering, Los Angeles, United States, Yixin Mao, University of Washington Seattle Campus, Seattle, WA, United States and Dennis P Lettenmaier, University of California Los Angeles, Department of Geography, Los Angeles, CA, United States
Many statements have been made to the effect that California’s winter 2013-14 (variously taken as Nov 1- Feb 28 and Nov 1 – Mar 31) was the warmest on record. These statements appear to be based on the National Climatic Data Center’s gridded monthly climate data for the U.S. (http://www.ncdc.noaa.gov/cag/time-series/us; hereafter NCDC). These data show, when aggregated (by NCDC) a trend of about 1.5 oC/century from 1920-2014. We also examine the trend in the gridded (1/16 degree) West Coast Surface Water Monitor (SWM) (http://www.hydro.washington.edu/forecast/monitor_cali/index.shtml) also averaged over California, and the station average of the U.S. Historical Climatology Network (HCN) averaged over the same period.. The SWM data set is based on about 100 stations within California, which were selected to have long and consistent records, and to report in near-real time. The HCN stations (of which 54 are in California) were selected specifically to represent long-term trends (slightly less than half of the HCN stations are also used in SWM). We find that the trend magnitude for both SWM and HCN over the last century is less than half that of NCDC. Furthermore, the trends in SWM and HCN for daily temperature minima are quite consistent; whereas SWM has almost no trend in the daily temperature maxima, while NCDC has an upward trend that is similar in magnitude to its daily temperature minima. Therefore, almost all of the difference in the trend in the two data sets is attributable to the large trend in the daily maxima in NCDC. Also, most of the difference in the two data sets arises after about 1950. We conduct various diagnostics of the stations and methods used in the two data sets in an attempt to resolve the discrepancy. Due almost entirely to the difference in trends, winter 2013-14 is the warmest on record in NCDC, but is the third warmest (after 1934 and 1996) in SWM. However, both data sets agree that winter 2014-15 was the warmest on record.