The linear and non-linear baroclinic instability in the eastern North Pacific

Graduation date: 1987Using a linear quasi-geostrophic model of large scale spiraling flow and vertical density\ud gradient in the eastern North Pacific, we evaluate the complex dispersion relationships\ud for quasi-geostrophic waves. Our calculations indicate that the geostrophic circulation\ud of the eastern North Pacific can locally convert potential energy to mesoscale\ud kinetic energy on a scale comparable to the observed space and time scale and should\ud be a source of eddy energy, distant from the eddies spawned by the Kuroshio and near\ud the topographic features. But the local growth rates by linear stability analysis do not\ud relate to the observed features of eddy kinetic energy in the eastern ocean; eddy kinetic\ud energy increases to the south and has a maximum in the subtropical region.\ud The non-linear baroclinic instability is analyzed using a three-layer quasi-geostrophic\ud numerical model. Three experiments with different idealized initial mean\ud flow are performed. Local energetics are calculated to highlight the difference between\ud the southward return flow and westward return flow regions. It is found that the\ud boundary flux of mean to eddy kinetic energy conversion is the main differences\ud between two regions: it is large in the westward return flow region but is small in the\ud southward return flow region. Two waves with different characteristics are found: the\ud short waves (periods of 120 days), that propagate to the west and form several wave\ud trains parallel to the southern boundary, and the long waves (periods of 200 days),\ud that propagate to the south-west. These two waves are remarkably similar to the\ud measurements of open ocean eddies at 28°N and 152°W. It is shown that there are high\ud eddy activities in the southward return flow regions by influxes of eddies from other\ud areas, but the southward return flow region is vacant in eddies by outflows of the westward\ud and south-westward propagating waves