Annual variations of sea surface elevation and currents over the Scotian Shelf and Slope

Abstract

TOPEX/Poseidon (T/P) altimeter data over the period 1992-98 have been analyzed to examine annual variability of sea surface elevation and currents over the Scotian Shelf and Slope. A modified orthogonal response analysis is used to derive the annual cycle while simultaneously removing the residual tides and other dynamical processes at the appropriate T/P alias periods. An evaluation of the M2 and K1 alias variations is carried out, suggesting notable tidal correction errors off Cape Cod and over Georges Bank. The along-track sea surface slopes, which represent surface geostrophic current components normal to the track, are estimated on selected T/P ascending and descending ground tracks. The annual altimetric sea level harmonic is compared with steric height anomalies and wind-driven setup. The comparison indicates that the altimetric sea surface elevation variability is dominated by the baroclinic (and associated barotropic) component and supplemented by the wind-driven and remotely forced components. Altimetric elevations agree favorably with tide-gauge data at Halifax. Nova Scotia, and well with those at St. John's, Newfoundland. Wintertime intensification of the shelf-break flows is indicated in the altimetric surface currents, consistent with the solutions of regional diagnostic model forced by baroclinicity and boundary flows. Altimetric results clearly demonstrate seasonal variability of north-eastward slope current stronger in fall and winter and weaker in spring and summer, which is less well resolved in the model. Assimilation of altimetric data into regional circulation models could help improve their prognostic ability to hindcast and nowcast seasonal variability of shelf-edge and slope water circulation. This study also implies a demand for better shelf tidal models to detide altimetric data for extraction of semiannual and shorter-period processes.