# Presentation: 344020

## 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

- Available
-  Potential Energy

- Downwelling
- Winds

- Coastal Runoff

- Eddy
- Kinetic Energy

- x

- y

- light

- dense

*coastline*

## 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

- 1987

- 1989

- Salinity and velocity at 40 m depth

## 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() = <[v’(t)v’(t- )]/[v’(t)v’(t)]>, [] = time average
- 	TL = Integral of (R() d)

## 1987

**Notes:**
Wet and windy year, big EKE but slow dispersion

## 1989

**Notes:**
Dry and calm year most of the time

## 1984

**Notes:**
Slow advection, loopy in the sea valley, fast decorrelation

## 1985

**Notes:**
Strong advection, smooth in sea valley, slow decorrelation

## 1986

**Notes:**
Coast hugger, loopy on shelf, fast decorrelation

## 1994

**Notes:**
Loopy in sea valley; fast decorrelation

## 1998

**Notes:**
Strong advdection, smooth in sea valley, slow decorrelation

## 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.