A ROBUST METHOD FOR DIAGNOSING REGIONAL SHELF CIRCULATION FROM SCATTERED DENSITY PROFILES

We present a straightforward method for estimating surface circulation on an f plane from a set of irregularly spaced vertical density profi les. The first step is to express bottom density rho(h) as the sum of a mean <(rho)over tilde (h)> for a given water depth h and an anomaly, <(rho)over tilde (h)'> = rho(h) - <(rho)over tilde (h)>. Sea level et a can then be decomposed into a dynamic height ij relative to a deep r eference level and a correction term, eta' = eta - (n) over tilde. The dynamic height is estimated using a generalization of the method of H elland-Hansen [1934] for diagnosing flow through a cross-shelf section under the assumption of zero bottom geostrophic flow. The correction eta' satisfies a two-dimensional elliptic partial differential equatio n forced by the bottom density anomaly, wind stress and the open bound ary conditions, Before calculating the density-driven component of eta ' we first test if the rho(h)' are statistically different from uncorr elated noise. If they are not, the correction associated with the bott om density anomaly is set to zero. Thus the method has a degree of rob ustness to errors in the density observations. If the rho(h)' have wel l-defined spatial structure the elliptic equation is solved for eta' a nd sea level is equated to <(rho)over tilde> + eta'. Note that even if this last step is required the only gridding of the density data is t wo-dimensional. This makes the proposed method simpler to use than man y of the existing diagnostic models which require a three-dimensional gridding of the observed density profiles, To test the method, we use it to diagnose the flow from an idealized density field overlying an i solated topographic feature. The predicted sea level and flow fields a re then compared, and shown to be in good agreement, with results from the Princeton Ocean Model, The method is then used to diagnose the wi nter surface circulation on the Scotian Shelf from observed density pr ofiles, The reliability of the diagnosed flow pattern is assessed by c omparing it against all available near-surface current measurements. T he differences between the observed and diagnosed currents are used to estimate the remotely forced circulation on the Scotian Shelf. It is shown that the main features of the circulation can be explained by gr adients in the density field, Remote forcing is important near the coa st and the shelf break, The effect of local wind is relatively weak.