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

Abstract 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 Tmin and maxima temperature Tmax 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 Tave values. Results for all 253 California National Weather Service (NWS) Cooperative Observer Program (COOP) sites together showed increased Tave values (0.23°C decade−1); asymmetric warming, as Tmin values increase faster than Tmax values (0.27° versus 0.04°C decade−1) and thus decreased daily temperature range (DTR) values (0.15°C decade−1). The spatial distribution of observed SoCAB and SFBA Tmax values exhibited a complex pattern, with cooling (−0.30°C decade−1) in low-elevation coastal areas open to marine air penetration and warming (0.32°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°C decade−1) combined with large decreases (−0.58°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 Tmax 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 Tave warming decreased from 0.13°C decade−1 at inland sites to 0.08°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.