Observed 1970-2005 Cooling of Summer Daytime Temperatures in Coastal California

This study evaluated 1950-2005 summer [June-August (JJA)] mean monthly air temperatures for two California air basins: the South Coast Air Basin (SoCAB) and the San Francisco Bay Area (SFBA). The study focuses on the more rapid post-1970 warming period, and its daily minima temperature T-min and maxima temperature T-max values were used to produce average monthly values and spatial distributions of trends for each air basin. Additional analyses included concurrent SSTs, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) sea level coastal pressure gradients, and GCM-downscaled average temperature T-ave values. Results for all 253 California National Weather Service (NWS) Cooperative Observer Program (COOP) sites together showed increased T-ave values (0.238 degrees C decade(-1)); asymmetric warming, as T-min values increase faster than T-max values (0.27 degrees versus 0.04 degrees C decade(-1)) and thus decreased daily temperature range (DTR) values (0.15 degrees C decade(-1)). The spatial distribution of observed SoCAB and SFBA T-max values exhibited a complex pattern, with cooling (-0.30 degrees C decade(-1)) in low-elevation coastal areas open to marine air penetration and warming (0.32 degrees C decade(-1)) in inland areas. Results also showed that decreased DTR values in the basins arose from small increases at inland sites (0.16 degrees C decade(-1)) combined with large decreases (20.58 degrees C decade(-1)) at coastal sites. It is also possible that some of the current observed temperature trends could be associated with low-frequency decadal variability, expected even with a constant radiative forcing. Previous studies suggest that cooling JJA T-max values in coastal California were a result of increased irrigation, coastal upwelling, or cloud cover. The current hypothesis is that they arise (as a possible "reverse reaction'') from the global warming of inland areas, which results in increased sea-breeze flow activity. GCM model T-ave warming decreased from 0.13 degrees C decade(-1) at inland sites to 0.08 degrees C decade(-1) in coastal areas. Sea level pressure increased in the Pacific high and decreased in the thermal low. The corresponding gradient thus showed a trend of 0.04 hPa 100 km(-1) decade(-1), supportive of the hypothesis of increased sea-breeze activity.