On the effect of a surface density front on the interior structure of the ventilated ocean thermocline

This study analyzes the response of the ocean thermocline to a density front at the sea surface. A set of computations has been carried out, solving the thermocline structure that results from a surface density front and analyzing the behavior of the solution as the location of the front at the sea surface varies in the meridional direction. In fact, though theoretical studies often assumes a constant SSD (Sea Surface Density) gradient, a recent observational study (Raffaele and Rudnick, 2000) suggests that much of the surface density variation, even on large scales, is gathered into frontal gradients.
This investigation is based on a model (Lionello and Pedlosky, 2000a and 2000b, here after LP model) which is an extension to a continuous fluid of the dynamics adopted in the "layer" theory of the ventilated thermocline (Luyten et al. 1983). The LP model is, in fact, the limit of a discrete many-layers model for the number of layers tending to infinity, and, consequently, their thickness tending to zero. A main result of this study is to show the effectiveness of the LP model for the analysis of the thermocline structure.

The front behavior in the interior of the thermocline depends on the latitude where it has been subducted. A SSD front located near the northern boundary of the gyre broadens while penetrating into the ocean interior, while a SSD front located in the central and southern part of the gyre shrinks. This behavior plainly reflects the behavior of the thermocline bottom. The broadening of the front is associated with the thickening of the thermocline where, in the northern portion of the gyre, its bottom deepens. The shrinking of the front is associated with the reducing thickness of the thermocline, which becomes shallow towards the equator.

Top panel: Penetration of the fronts inside the thermocline. The lower dashed line represents the bottom of the thermocline when the Sea Surface Density Distribution is linear. The curves represents the location of the five fronts analyzed in this study.
Bottom panel: The thickness of five fronts in the interior of the thermocline as function of the Coriolis parameter. Note that the vertical scale is D/50, with D scale of the thermocline thickness.