LARGE-SCALE ATMOSPHERIC FORCING OF RECENT TRENDS TOWARD EARLY SNOWMELT RUNOFF IN CALIFORNIA

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, whic h are sensitive to changes in mean winter temperatures. Such basins ha ve broad areas in which winter temperatures are near enough to freezin g that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of Calif ornia, a declining fraction of the annual runoff has come in April-Jun e. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown t rends toward warmer winters since the 1940s. A series of regression an alyses indicate that runoff timing responds equally to the observed de cadal-scale trends in winter temperature and interannual temperature v ariations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate caus e of the trend toward warmer winters in California is a concurrent, lo ng-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishabl e from natural atmospheric variability. The fluctuation began to affec t California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central Nort h Pacific Ocean that is teleconnected to California temperatures. Sinc e the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are a ssociated with concurrent shifts in both West Coast air temperatures a nd North Pacific sea surface temperatures.