California's Summer Coastal Cooling: Global-Warming Counter-Reaction and/or Large-scale Oceanic Forcing?

Spatial distribution of 273 National weather Service (NWS) observed summer max temperatures in California during the last four decades (1970-2010) showed a complex pattern, in which cooling trends were found at low elevation coastal areas open to marine air penetration and warming trends at inland and high elevation coastal areas. These trends were found at the South Coast Air Basin (SoCAB) and San Francisco Bay Area (SFBA) by Lebassi et al. (2009), as well as the San Diego Air Basin and San Luis Obispo Air Basin (SLOAB) by Sequera et al. (2013). The authors hypothesized that this coastal cooling was the consequence of an increase of sea-breeze activity during the period, reflected by the increase in sea-land pressure gradients in the region calculated, as from ECMWF ERA-40 data from 1979-2002. This work looks further at the two factors that could induce the increase in pressure gradients that generates increase sea breeze activity: (a) warming of inland areas, as a consequence of large-scale warming changes and (b) cooling of SSTs, controlled by the Pacific Decadal Oscillation (PDO). Many observations support the first hypothesis: average inland Tmax increases, coastal Tmax decreases, and sea level pressure increases for 1980-2010. These findings, however, could be modulated by the PDO phases in which they took place. These past observations took place during negative PDO trends (1977-2010). Closer examination of coastal and inland Tmax trends during positive PDO trend (1950-1998) follow a different temporal pattern, with slight increases in coastal Tmax and inland Tmax decreasing. These observations along with consistent positive linear correlation of PDO vs. coastal Tmax trends and negative linear correlation with inland Tmax trends, independent of the PDO phase, suggest that PDO may play a more important role in the coastal cooling process. This presentation will review these recent findings and will expand the discussion on which mechanism, i.e., increased thermal gradient attributed to global warming or PDO, has a stronger influence on the reported California coastal cooling.