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Northerly surface wind events over the eastern North Pacific Ocean : spatial distribution, seasonality, atmospheric circulation, and forcing


The climate of the coastal US West coast is characterized by cool, alongshore, northerly wind from the Oregon border to the Southern California Bight, south of Point Conception. New datasets, including surface wind estimates derived from satellite scatterometer and North American Regional Reanalysis fields allow for a more detailed exploration than has heretofore been possible. This thesis examines episodes of enhanced northerly surface wind to determine the spatial and temporal character of the surface wind along and offshore of the California coast and the 3-D structure and seasonality of the atmosphere associated with the wind episodes. A deterministic approach is employed to enhance understanding of fundamental processes rather than improve predictability. The methodology includes diagnostic spatial and temporal analysis and numerical sensitivity experiments. Processes known to affect coastal wind are investigated to gain a better understanding of the role of possible mechanisms driving northerly wind events. It is found that the spatial and temporal structure of wind offshore of California is characterized by episodes of enhance equatorward surface wind that often extend from Cape Mendocino to Hawaii. Wind events typically last four days and are present 20% of the time during January-August. The summer circulation structure associated with wind events is fundamentally different than winter and spring. For instance the surface and upper-air circulation patterns associated with wind events are co-located in winter and 90⁰ out-of-phase in summer. Wind events are nearly always accompanied by a strong trough in the upper-air circulation that is directly above or just east of the coast. The analysis concludes that troughs are a nearly universal forcing mechanism for wind events, while the low -level atmospheric thermal gradient across the coast also plays an important, but less consistent role in wind events. A hypothesis is developed suggesting that southeastward movement of the North Pacific High may be a sufficient forcing mechanism for wind events in summer, but a trough along the coast is a necessary ingredient in winter. Other forcing mechanisms, feedbacks, and processes may also be important in reinforcing the circulation associated with wind events

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