The Role Of Surface Winds In The seasonal And Interannual
Variability Of Large-Scale Eddies In The Gulf Of Alaska

Vincent Combes and Emanuele Di Lorenzo
Georgia Institute of Technology, Atlanta, GA, USA


Eddies in the open ocean are primarily generated through instability processes associated with regions of strong horizontal shear or upper ocean baroclinicity. However in the Gulf of Alaska interior, the largest (D ≥ 200km) and longer lived (T ≥ 4 yr) eddies originate in coastal areas along the eastern boundary, where the generating dynamics are strongly dependent on the interaction between coastal flows and the complex geometry.
In this study, a high resolution regional oceanic model is used to explore the seasonal and interannual variability of eddy statistics. It is shown that most of the variance in the Gulf of Alaska basin is explained by interannual variability associated with the eddy field. During El Niño the eddy activity in the eastern basin is stronger. For example, consistent with observational records, the model captures a strong Haïda eddy in the winter 1982-83. However in contrast with previous findings by Melsom et al. (1999), who report on the important role of equatorial coastally trapped waves and enhanced baroclinic instability during El Nino years, we find that the baroclinicity of the flow field is not stronger during El Nino years and that local changes in the wind field, typical of the El Nino atmospheric teleconnection, play a bigger role in the generation of stronger eddy fields. Specifically along the eastern basin the eddy field results from a large scale geostrophic adjustment of the coastal flow field in response to a positive perturbation in sea surface height forced locally by poleward winds. The positive sea level anomaly corresponds to enhanced poleward buoyancy flow around the cape. The interplay of the coastal buoyancy flow with the complex coastal geometry is shown to be important in the generation and detachment of the larger eddies. The initial adjustment involves the excitation of nonlinear Rossby waves.
This study leads to a better understanding of the exchanges between inshore and offshore waters in regional oceans characterized by complex geometry at seasonal and interannual time scale. A correct representation of cross-shelf mixing is relevant for biogeochemical cycle in climate models. Also, given the long lived life of eddies generated along the eastern boundary, we hypostatize that these coastal dynamics may play a more important role in basin wide climate variability than previously recognized.