Solution of the linear thermocline equations driven by wind stress and thermohaline forcing

In this paper, new steady-state solutions of the linearized thermocline equ ations satisfying prescribed fluxes of heat and salt at the base of the sur face Ekman layer, are presented for a semi-infinite ocean of constant depth . A decomposition into vertical modes is used to solve the problem. The sol ution is first determined in terms of a derivative of the unknown density a t the surface and this derivative is then determined from an integral equat ion arising from applying the surface thermohaline boundary conditions. Sol utions forced by wind stress alone, and by wind stress and thermohaline for cing are considered. The wind-driven solution exhibits a temperature field with many realistic f eatures, such as largest meridional gradients in the sub-polar gyre, and th e latitudinal spreading of isotherms towards the eastern boundary. The wind -driven salinity field increases towards the poles, contrary to the observe d annual mean salinity field. The stability of the sub-tropical gyre is enh anced, whilst the sub-polar gyre is de-stabilized. With the addition of the thermohaline forcing the deficiencies of the salinity field associated wit h the wind-driven solution are largely corrected, whilst the solution retai ns a reasonable representation of the climatological temperature field. Tem perature and salinity anomaly fields relative to the Levitus climatology, c alculated from the Met. Office Forecasting Ocean Assimilation Model, are sh own to be qualitatively similar to the anomaly fields dervied from the mode l discussed in this paper. This result serves to underline the message that the combination of wind and surface buoyancy forcing are essential when mo delling the large-scale temperature and salinity fields using the thermocli ne equations.