Interannual and interdecadal variability in the timing and strength of the spring transitions along the United States west coast Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/79408074f

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  • Past studies of the Oregon and Washington coast have shown a rapid change in the coastal ocean conditions with the onset of upwelling in spring. This process, called the spring transition marks the change from winter to summer conditions along the west coast of Unites States. To examine the interannual and interdecadal variability of the spring transition, a state-space statistical model is applied to records of adjusted sea level (ASL) and the Coastal Upwelling Index (CUI) for the years 1971- 1998. The same model is applied to the single Crescent City sea level from 1953 to 1998 to extend the analysis to the longest period possible. The state-space model estimates nonlinear, step-like changes in the first Empirical Orthogonal Function time series of these two variables along the northern California, Oregon and Washington coast. The spring transition dates are obtained using both variables between 1971-1998. In general the spring transition dates and strengths do not show a systematic change in the mid-1970's, when other changes have been noted in the North Pacific ocean and atmosphere. Although the dates after 1977 appear more variable. The spring transitions are consistently strong for the periods 1972-1975 and 1995-1998; during 1984-1994, the transitions are more often weak and variable. Using composite maps of sea level pressure (SLP) and wind stress for years with weak and strong transitions, we present characteristic fields of surface forcing, before, during and after the transition. We conclude that: before strong transitions, there is a strong Aleutian Low, centered in the middle of the Gulf of Alaska, inducing strong downwelling winds from ~40°-50° N; during the strong transitions there is a rapid expansion of the North Pacifica High and weakening and splitting of the Aleutian Low; after strong transitions the North Pacific High creates upwelling favorable winds from 50° N to Baja California. Before weak transitions the Aleutian low is centered northeastward, inducing weaker downwelling winds along the coast north of 40° N; the North Pacific High is relative weak and an extends to -40° N; during weak transition the North Pacific High expands northwestward to -45° N and the Aleutian Low remains strong; after weak transitions, downwelling-favorable winds continue north of 45° N.
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