Northwesterly wind events along the California coast

The California coast is a windy place with strong, frequent and persistent northwesterly wind, especially in spring and summer. Wind measured at buoys 20-220 km (10-120 nm) offshore of the California coast from Monterey to the Mexican border exceed 10 m/s (20 knots) for days to weeks at a time. Wind speeds frequently exceed 15 m/s (30 knots) from the northwest producing hazardous seas that can pose risks to commercial, recreational, scientific, and naval operations. Strong and persistent surface northwesterly wind in spring and summer can be traced to large-scale forcing. Synoptic and seasonal changes in the large-scale pressure pattern often lead to an intensification of the surface pressure gradient, which drives enhanced northerly wind flow along the coast. Northwesterly wind events continue from spring through summer despite substantial changes in surface and upper atmospheric circulation patterns. In spring, cold air troughs extend south into the northeast Pacific permitting only weak surface highs to form. North to south gradients in pressure and temperature are strong over the northeast Pacific Ocean. Wind events along the California coast in spring are associated upper-level troughs along the coast of British Columbia. Relatively strong northwesterly surface wind along the California coast results from west-east sea level pressure gradients between enhanced surface high pressure in the Pacific Northwest US extending southwestward and relatively strong surface lows over the Rocky Mountain states. In summer, the eastern north pacific is dominated by relatively strong surface and upper level high pressure with weak spatial gradients in pressure and temperature. West to east coastal sea level pressure gradients between the extensive quasi-stationary high over the eastern north Pacific and thermal low situated over southwestern Arizona and the Sea of Cortez intensifies when very weak upper-level troughs set up along the California coast. These weak troughs can persist for up to 3 weeks.

In preliminary analyses of buoy data, California coastal surface wind speed exhibits strong spatial coherence. The leading empirical orthogonal function of winds at 9 buoys from Monterey to San Diego explains over 65% of variance and is highly correlated with the individual buoy records. Using National Centers of Environmental Prediction Reanalysis fields along with buoy wind observations, spatial patterns of surface and upper level fields associated with wind events are clearly highlighted. Spring and summer northwesterly wind events and their associated large-scale circulation patterns are a characteristic climatic feature of the California coast. Understanding northwesterly wind events is important in defining the climate of the eastern North Pacific and may assist in understanding regional climate variability.