Observed and simulated California climate trends due to concurrent long-term inland-warming and coastal-cooling

The current observational study reports on analyses of 2 m land and sea-surface temperatures from about 300 Co-op sites throughout CA for the last 60 years. Results for all of CA showed daily min temperatures with a significant upward trend, but corresponding max values with a smaller upward trend. Sub-area analyses showed these same results at inland Central Valley (CV) sites, but while coastal-plain areas in the San Francisco Bay Area (SFBA) and South Coast Air Basin (SCAB) generally also showed increasing min values, they showed decreasing max temperatures. In addition, concurrent trends in station pressure at several NWS air port sites, and long term NCEP sea level pressure trends in CA have also shown evidence of increased coastal-inland pressure gradients. Thus analysis of the data for surface temperatures and pressure gradients in summer time leads to hypothesize that the combined effects of global warming and regional land use are increasing summer daytime horizontal temperature and pressure gradients across the state, which is resulting in increased strength and frequency of cool summer marine sea-breeze flows into heavily populated and highly polluted CA coastal plains. This hypothesis is further evaluated by dew point temperature increases at coastal San Francisco Airport which was also observed to be much larger than that at inland Sacramento Executive Airport.

The hypothesis is further investigated using mesoscale simulations (RAMS) simulations, with a horizontal grid resolution of 4 km on an innermost grid over SFBA. Two simulations of the “present” and “past (potential natural vegetation)” land-use conditions were undertaken for the month of July. Results showed that monthly averaged 00 Z temperature changes (i.e., past minus current) in the eastern CV would warm, while those in its western half (as well as in the low elevation parts of the SFBA) would cool. The warming results from the reduced CV soil moisture, while the cooling results from an increased sea-breeze flow. Concurrent wind speeds increased mainly in the low-level sea-breeze flow corridor from the Pacific Ocean, through the SFBA, and into the CV (via the Carquinez Straight).

These results are thus providing an increased understanding of past and present trends in summer time temperatures, impacts of global and mesoscale (e.g., marine, land use, and topographic) physical processes in coastal environments.