Anthropogenic Reduction of Santa Ana Winds

The frequency of Santa Ana wind events is investigated within a high-resolution downscaling of the ERA40 Reanalysis to 6-km resolution over Southern California. In this climate reconstruction, the number of Santa Ana days per winter season declines significantly over the 44-year reanalysis period, resulting in over 30% fewer events per year over the final decade of the reconstruction (1991–2001) compared to the first decade (1959–1969). We investigate this signal further in late-20th and mid-21st century realizations of the NCAR CCSM3 global climate change scenario run downscaled to 12-km resolution over California. The reduction in events per year in the mid-21st century compared with the late-20th century is similar to that seen in the ERA40 downscaling, suggesting the cause of the decrease is a change in the climate due to anthropogenic forcing. A regression model is used to reproduce the Santa Ana time series based on two previously-documented forcing mechanisms: synoptically-forced strong offshore winds at the mountain tops which transport offshore momentum to the surface, and a local desert-ocean temperature gradient causing katabatic-like winds as the cold desert air pours down the coastal topography. Both past and future climate simulations show a large reduction in the contribution of the local thermodynamic forcing. This reduction is due to the differential warming that occurs during transient climate change conditions, with more warming in the desert interior than over the ocean. Thus the mechanism responsible for the decrease in Santa Ana frequency originates from a well-known aspect of the climate response to increasing greenhouse gases, but cannot be understood or simulated without mesoscale atmospheric dynamics.