Coastal-cooling of observed daytime average and extreme temperatures in the LA basin: a reverse reaction to GHG induced warming?

Analysis of long-term (1969-2005) air temperatures in California (CA) during summer (June-August) previously showed an aggregate CA asymmetric warming, as daily minimum temperatures increased faster than daily maximum values Tmax. The spatial distributions of daily Tmax temperatures in the heavily urbanized South Coast and San Francisco Bay Area air basins were more complex pattern, with cooling at low-elevation coastal-areas and warming at inland areas. Our hypothesis was that this temperature pattern arose from a “reverse-reaction” to greenhouse gas induced global-warming, in that the global warming of inland areas resulted in an increased (cooling) sea breeze activity in coastal areas. These results appeared in the July 2009 issue of the J. of Climate.

Extension of analysis over Los Angeles urbanized basin over the entire year now shows that the cooling trend in average Tmax values occurred during several warm season months (and unexpectedly in February, with warming trends during the colder months. The largest rate of cooling, however, occurred in June, indicating that an earlier initiation of sea breeze activity may be the most important cooling factor, relative to increases in its intensity, duration, and/or penetration. Possible beneficial effects of the cooling include decreased maximum O3 and human thermal-stress levels, but as these impacts would occur during periods of maximum Tmax values, the previous analysis was thus expanded to include trends in the CFDs of Tmax values. Results showed that frequencies of high values were decreasing and increasing for low values. These results thus show that coastal cooling should in fact reduce the frequency and severity of high O3 and thermal stress events.