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 • 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?

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

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

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

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

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)

float release

Hindcast Movies Salinity and velocity at 40 m depth 1987 1989

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

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)

1987

1989

1984

1985

1986

1994

1998

RELATE EKE TO WIND AND RUNOFF

RELATE EKE TO FORCING

RELATE DISPERSION TO WIND AND RUNOFF

RELATE DISPERSION TO FORCING

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.

Twenty Years of Eddies in the Alaska Coastal Current

Twenty Years of Eddies in the Alaska Coastal Current

Presentation Transcript

Chemical Engineering: the next twenty years

2007 Alaska Coastal Conference

5 YEARS ALASKA

Influence of mesoscale eddies on ichthyoplankton assemblages in the Gulf of Alaska

Leeuwin Current Eddies

Twenty Pearls Long, Twenty Years Strong

“After Twenty Years”

After Twenty Years O.Henry

Twenty-Five Years of “Thinking Green”

Twenty Years of Acceleration Measurements in Low-G Environments

Seasonal, Interannual, and Decadal Scale Freshwater Variability in the Alaska Coastal Current

Twenty Years of Microlensing Observations From the

On the role of eddies for coastal productivity and carbon cycling

Twenty years of an integrated aging community in Japan

Simulating the variability of Florida Current frontal eddies

In Twenty Years

My in Twenty Years Page

Me in Twenty Years Page

THE MARINE LAYER AND COASTAL EDDIES

The Alaskan Coastal Current

Twenty Years of Diffraction at the Tevatron

After Twenty Years