Models of upper-ocean fronts

Macvean, Malcolm Kevin (1977) Models of upper-ocean fronts. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A two-dimensional model of synoptic-scale frontogenesis, forced by a horizontal stretching deformation field, is developed. Turbulent mixing is neglected and the long-front flow is assumed geostrophic. aInitial conditions modelling a pycnocline outcropping at the surfaceand a mixed layer overlying a pycnocline are considered. As frontogenesis proceeds, the surface horizontal gradients increase non-linearly and become infinite, after a finite time; in reality, we expect turbulent mixing to become important locally before this occurs, preventing the formation of a singularity but not affecting the qualitative validity of the solutions. The frontogenetic time scale predicted by the model is 3 - 4 days if, as suggested, 10_5s_1 is arealistic value for the near-surface deformation rate; thus horizontal Istretching deformation may be an important frontogenetic mechanism in 3 the upper ocean.Below the top of the pycnocline, the flow is approximately parallel to the isopycnals. If the isotherms are inclined to the isopycnals, this flow will be more effective in increasing the temperature gradient along isopycnals than in increasing their slope. The effect of frontogenesis on the temperature field is shown to vary significantly with the thermoclinicity. In the cases considered, frontogenesis reduces the ratio of the scales of variation of temperature and pressure along isopycnals of an order of magnitude.The existing data is inadequate to permit objective comparison with the model predictions but certain broad similarities are noted. It is hoped that the model will form a basis for the design of-future experiments to test its predictions.A model, designed to investigate the finite-amplitude structure of waves on fronts is outlined; the variation in the long-front direction is represented by a single Fourier component while finite differences are used in the other directions. Reliable solutions.have not yet been obtained.

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