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Contribution to the SSL Synthesis Paper

A. J. Miller, D. J. Neilson, E. Di Lorenzo, H-J.Kim

A distinct change in the ocean circulation of the Gulf of Alaska after the 1976-77 climate shift is studied in an eddy-permitting primitive equation model forced by observed wind stresses from 1951-1999. After the Aleutian Low strengthens, mean velocities of the Alaska Stream increase to the northeast of Kodiak Island and decrease to the southwest of it. Mesoscale eddy variance likewise increased to the northeast of Kodiak and weakened to the southwest. Since mesoscale eddies provide a mechanism for transporting nutrient rich open-ocean waters to the productive nearshore shelf region, the fundamental flow of energy through the food web may have been altered due to this physical oceanographic change. This mechanism may help to explain the changes in forage fish quality in the diets of the Steller sea lions whose populations have declined precipitously since the mid-1970’s.

An eddy-permitting ocean model is forced over the 1950-1999 time period by monthly-mean wind stresses taken from the NCEP/NCAR re-analysis (Miller et al., 2004). After the shift, the Alaska Stream is strengthened considerably in the northwest part of the GoA and weakened in the southwestern domain. The increase in the strength of the Alaska Stream is consistent with coarse resolution model results of Capotondi et al. (2004). This change in the mean strength of the Alaska Stream over decadal timescales alters the stability properties of the flow field and changes the mesoscale eddy variance distribution. Figure 1AJM shows the surface current velocity variance for two 10-yr epochs, along with the difference in variance, before and after the climate shift. Before the shift, mesoscale eddy variance is highest southeast of Kodiak Island and along the Alaska Stream to the southwest of Kodiak. After the shift, mesoscale eddy variance increases precipitously in the northwestern GoA and decreases precipitously to the south and west of Kodiak Island. The consequences of this change will include altering the cross-shelf/slope mixing of water masses of the open ocean and shelf regions.

The mean flows of the Alaska Current in the eastern GoA , in contrast, are nearly unchanged after the shift. Likewise, the surface velocity variance is only weakly altered, being reduced slightly compared to pre-shift conditions. Hence, an east-west asymmetry occurs in the GoA circulation response to the strengthening Aleutian Low.

References

Capotondi, A., M. A. Alexander, C. Deser, A. J. Miller, Low-frequency pycnocline variability in the Northeast Pacific, J. Phys. Oceanogr., sub judice 2004.

Miller, A. J., D. J. Neilson, E. Di Lorenzo, H.-J. Kim, A. Capotondi, M. A. Alexander, S. J. Bograd, F. B. Schwing, R. Mendelssohn, K. Hedstrom, and D. L. Muscgrave, Interdecadal changes in mesoscale eddy variance in the Gulf of Alaska circulation: Implications for the Steller sea lion decline, Geophys. Res. Lett., sub judice, 2004.

Figure AJM. Variance of the monthly mean surface currents for the 10-year epochs 1967-1976 (top) and 1979-1988 (middle), and the difference between the two epochs (bottom). Variance of the current is contoured at 100cm2/s2 intervals.