Title

Dynamical Aspects of the Norwegian Coastal Current: A Numerical Study Using a Hybrid Approach

Date of Award

2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Science

First Advisor

Vladimir Kamenkovich

Advisor Department

Marine Science

Abstract

A 1/12°, 26 layer Ocean General Circulation Model (OGCM), the HYbrid Coordinate Ocean Model (HYCOM), has been applied for parts of the Norwegian Sea and adjacent North Sea and Norwegian coast in order to study the dynamics of the frontal zone separating Atlantic Water from Coastal Water. Special attention was given on how varying freshwater input affected the circulation pattern of the Norwegian Coastal Current and how this impacted the generation of eddies on the frontal zone. In all 9 two year (2000-2001) simulations were run using realistic bathymetry, river runoff along the coast of Norway for 53 rivers and 6 hourly atmospheric forcing. A period of 14 model days was given special attention. During this period the frontal zone goes from having no mesoscale activity to becoming very turbulent. Using transport calculations for the upper 50m and the total water column, the triggering mechanism for this transition was revealed as a combination of strong outflow from the Skagerrak, increased horizontal current shear and vortex stretching. As the instabilities develop a tilted vertical velocity structure develops, indicative of baroclinic instabilities. Three experiments with variable freshwater input were conducted. All three experiments showed instabilities being generated in the same period over the same area. The intensity of the instabilities, however, was different. The experiment containing the least amount of freshwater had the weakest horizontal currents associated with the instabilities, while the experiment with the largest freshwater input had the strongest horizontal currents associated with the instabilities. Differences were also observed in the vertical extent of the instabilities, where a large amount of freshwater efficiently prevented the vertical extension of the eddies. Nondeterministic behavior of the flow instabilities was also examined. A method for separating a model variable into a nondeterministic and deterministic response to forcing, the first one indicative of mesoscale flow instabilities and the latter a response to atmospheric and boundary forcing, was applied. A ensemble of six simulations were run, only the initial state separating them. The frontal zone displayed little or no nondeterministic behavior, indicating little randomness in the generation pattern of the eddies. Transport calculations for the western coast of Norway also reveal that the eddies play a role in the exchange of water across the Norwegian Trench in the upper 50m of the water column.