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Twenty Years of Eddies in the Alaska Coastal Current

Twenty Years of Eddies in the Alaska Coastal Current. Albert J. Hermann Joint Institute for the Study of the Atmosphere and the Oceans, UW/NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115) Phyllis J. Stabeno (PMEL) Michael Spillane (JISAO/NOAA/PMEL). The Problem.

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Twenty Years of Eddies in the Alaska Coastal Current

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  1. Twenty Years of Eddies in the Alaska Coastal Current Albert J. Hermann Joint Institute for the Study of the Atmosphere and the Oceans, UW/NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115) Phyllis J. Stabeno (PMEL) Michael Spillane (JISAO/NOAA/PMEL)

  2. The Problem • Large interannual variability observed in the structure of the Alaska Coastal Current (ACC) and associated fish stocks • Eddy statistics in the ACC (number, size, strength) affect larval paths and may affect subsequent recruitment • Can Lagrangian/Eulerian eddy statistics of the ACC be predicted by wind and buoyancy forcing?

  3. Approach • Use primitive equation model developed for the ACC in the northern Gulf of Alaska • Run the model for 20 hindcast years (1978-1998) • Look for relations between forcing and mesoscale response in model output

  4. Outline • Overview of the region • Mesoscale physics (baroclinic instability) • Overview of the model • Model hindcasts • Eulerian/Lagangian Statistics • Comparisons with forcing

  5. Overview of Area • Two major currents: Alaskan Stream and Alaska Coastal Current • ACC forced by downwelling-favorable winds and distributed runoff

  6. Baroclinic Instability in the ACC Downwelling Winds Available Potential Energy Eddy Kinetic Energy Coastal Runoff coastline light y dense x

  7. The Circulation Model • Semispectral Primitive Equation Model (SPEM) • 4 km average resolution • Forced by local winds and upstream runoff • Validated with current meter and drifter data (Stabeno and Hermann, 1996)

  8. float release

  9. Hindcast Movies Salinity and velocity at 40 m depth 1987 1989

  10. Statistical Analysis (Eulerian) • Calculate bandpass-filtered barotropic streamfunction in the sea valley to reveal mesoscale features • Spatial variance of this filtered value is our Eulerian measure of EKE

  11. Statistical Analyses (Lagrangian) • Release 100 floats in Shelikof Strait at 40 m depth in mid-May; track in three dimensions • Compute positions over time and subsequently calculate: • Centroid of positions: C = <x(t)>,<y(t)> ,<> = ensemble average • Dispersion about the centroid D = <{x(t)-<x(t)>}2 + {y(t)-<y(t)>}2> • Lagrangian decorrelation time of cross-shelf velocity R(t) = <[v’(t)v’(t- t)]/[v’(t)v’(t)]>, [] = time average TL = Integral of (R(t) dt)

  12. 1987

  13. 1989

  14. 1984

  15. 1985

  16. 1986

  17. 1994

  18. 1998

  19. RELATE EKE TO WIND AND RUNOFF

  20. RELATE EKE TO FORCING

  21. RELATE DISPERSION TO WIND AND RUNOFF

  22. RELATE DISPERSION TO FORCING

  23. Conclusions • Broad range of behaviors over 20-year period • “Pulsed” baroclinic instability is observed. Store/release APE to EKE follwing wind spikes, especially in wet years • Winds and runoff may be useful predictors of observed EKE • Greater EKE does not always yield greater dispersion! Simple shear of mean flow is also very effective.

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