Twenty Years of Eddies in Twenty Years of Eddies in the Alaska Coastal Current the Alaska Coastal Current

Albert J. Hermann Albert J. Hermann Joint Institute for the Study of the Atmosphere and the Oceans, Joint Institute for the Study of the Atmosphere and the Oceans, UW/NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115)UW/NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115)

Phyllis J. Stabeno (PMEL)Phyllis J. Stabeno (PMEL)

Michael Spillane (JISAO/NOAA/PMEL)Michael Spillane (JISAO/NOAA/PMEL)

The ProblemThe Problem

• Large interannual variability observed in Large interannual variability observed in the structure of the Alaska Coastal Current the structure of the Alaska Coastal Current (ACC) and associated fish stocks(ACC) and associated fish stocks

• Eddy statistics in the ACC (number, size, Eddy statistics in the ACC (number, size, strength) affect larval paths and may strength) affect larval paths and may affect subsequent recruitmentaffect subsequent recruitment

• Can Lagrangian/Eulerian eddy statistics of Can Lagrangian/Eulerian eddy statistics of the ACC be predicted by wind and the ACC be predicted by wind and buoyancy forcing?buoyancy forcing?

ApproachApproach

• Use primitive equation model Use primitive equation model developed for the ACC in the developed for the ACC in the northern Gulf of Alaskanorthern Gulf of Alaska

• Run the model for 20 hindcast years Run the model for 20 hindcast years (1978-1998)(1978-1998)

• Look for relations between forcing Look for relations between forcing and mesoscale response in model and mesoscale response in model outputoutput

OutlineOutline

• Overview of the regionOverview of the region

• Mesoscale physics (baroclinic Mesoscale physics (baroclinic instability)instability)

• Overview of the modelOverview of the model

• Model hindcastsModel hindcasts

• Eulerian/Lagangian Statistics Eulerian/Lagangian Statistics

• Comparisons with forcingComparisons with forcing

Overview of AreaOverview of Area

• Two major currents: Alaskan Stream and Alaska Coastal Two major currents: Alaskan Stream and Alaska Coastal CurrentCurrent

• ACC forced by downwelling-favorable winds and distributed ACC forced by downwelling-favorable winds and distributed runoffrunoff

Baroclinic Instability in the Baroclinic Instability in the ACCACC

Available Potential Energy

DownwellingWinds

Coastal Runoff

EddyKinetic Energy

x

y

light

dense

coastline

The Circulation ModelThe Circulation Model• Semispectral Semispectral

Primitive Primitive Equation Model Equation Model (SPEM)(SPEM)

• 4 km average 4 km average resolutionresolution

• Forced by local Forced by local winds and winds and upstream upstream runoffrunoff

• Validated with Validated with current meter current meter and drifter data and drifter data (Stabeno and (Stabeno and Hermann, Hermann, 1996)1996)

float release

Hindcast MoviesHindcast Movies

1987

1989

Salinity and velocity at 40 m depth

Statistical Analysis Statistical Analysis (Eulerian)(Eulerian)

• Calculate bandpass-filtered barotropic streamfunction in Calculate bandpass-filtered barotropic streamfunction in the sea valley to reveal mesoscale featuresthe sea valley to reveal mesoscale features

• Spatial variance of this filtered value is our Eulerian Spatial variance of this filtered value is our Eulerian measure of measure of EKEEKE

Statistical Analyses Statistical Analyses (Lagrangian)(Lagrangian)

• Release 100 floats in Shelikof Strait at 40 m depth in Release 100 floats in Shelikof Strait at 40 m depth in mid-May; track in three dimensionsmid-May; track in three dimensions

• Compute positions over time and subsequently Compute positions over time and subsequently calculate:calculate:

• CentroidCentroid of positions: of positions: C = <x(t)>,<y(t)>C = <x(t)>,<y(t)> ,<> = ensemble ,<> = ensemble averageaverage

• DispersionDispersion about the centroid about the centroid D = <{x(t)-<x(t)>}D = <{x(t)-<x(t)>}22 + {y(t)-<y(t)>} + {y(t)-<y(t)>}22>>

• Lagrangian decorrelation timeLagrangian decorrelation time of cross-shelf of cross-shelf velocityvelocityR(R() = <[v’(t)v’(t- ) = <[v’(t)v’(t- )]/[v’(t)v’(t)]>, [] = time average)]/[v’(t)v’(t)]>, [] = time averageTL = Integral of (R(TL = Integral of (R() d) d))

19871987

19891989

19841984

19851985

19861986

19941994

19981998

RELATE EKE TO WIND AND RUNOFF

RELATE EKE TO FORCING

RELATE DISPERSION TO WIND AND RUNOFF

RELATE DISPERSION TO FORCING

ConclusionsConclusions

• Broad range of behaviors over 20-year Broad range of behaviors over 20-year periodperiod

• ““Pulsed” baroclinic instability is observed. Pulsed” baroclinic instability is observed. Store/release APE to EKE follwing wind Store/release APE to EKE follwing wind spikes, especially in wet years spikes, especially in wet years

• Winds and runoff may be useful predictors Winds and runoff may be useful predictors of observed EKEof observed EKE

• Greater EKE does not always yield greater Greater EKE does not always yield greater dispersion! Simple shear of mean flow is dispersion! Simple shear of mean flow is also very effective.also very effective.