Northwest atlantic surface circulation from multi-satellite observations

Abstract

The advent of satellite remote sensing has led to new ways of monitoring and studying the nearsurface ocean circulation, but its use has often been limited to the deep ocean. Here we present and validate a method of reconstructing the near-surface currents from multi-sensor satellite data off Atlantic Canada with a resolution of 0.25°, for the period from October 1992 to September 2006. The reconstructed ocean circulation consists of a superposition of 1) depth-variable wind-induced Ekman currents and surface wave-induced Stokes drift calculated from scatterometer-measured wind velocities using a theoretical formulation; 2) depth-invariant geostrophic surface current anomalies calculated from altimetric sea surface height anomalies; and 3) mean currents that are a combination of model-simulated currents for the Newfoundland and southern Labrador Shelves and satelliteobserved currents for the rest of the study region. The method features two novel aspects: the integration of simulated and satellite-observed mean circulation fields, and the consideration of the surface wave-induced Stokes drift. Individual weekly-mean composite current fields show considerable temporal variation and spatial structure. Inclusion of the numerically simulated mean circulation field leads to better agreement between the composite currents and buoy-observed currents at 15 m depth, compared to using the satellite-derived mean current field alone. There is also fair agreement between the composite currents and instantaneous currents observed at various depths by shipboard acoustic Doppler current profiler (ADCP) data, with a correlation coefficient of 0.59 (a 95% confidence interval of 0.49-0.68) and a vector difference ratio of 0.61. The 15 m depth composite currents agree better with buoy-observed currents than a similar near-surface current product with coarser horizontal resolution. At the sea surface, composite currents calculated with Stokes drift have a mean speed of 0.35 m s-1, similar to currents observed by drifters (0.36 m s -1), but exclusion of the Stokes drift leads to an underestimation of the current speed. The present study demonstrates that an appropriate integration of multi-sensor satellite data can provide approximate near-surface current information and could be used for routine monitoring of the shelf circulation at synoptic scales, complementing in situ observations and numerical ocean modelling.