Sea level variabilities in the Gulf Stream between Cape Hatteras and 50°W: A Geosat study

Abstract

Sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 80°W and 50°W longitude) are studied using Geosat altimetry between November 1986 and December 1988. Spectral slopes computed at four nominal locations along the Gulf Stream (72°W, 65°W, 60°W, and 55°W) range between −1.5 and −0.9, significantly less steep than the −2.5 spectral slope found at Bermuda. Because of increased short‐wavelength energy, the spectra tend to become more “white” going eastward, but this trend is not statistically significant. Energy at the annual cycle decreases eastward from around 40% of the total variance at 72°W to 20% of the total variance at 55°W. Maximum residual geostrophic sea surface velocities associated with the annual cycle were of the order of 70 cm/s. Comparison between the Gulf Stream path from sea level residual maps and the north wall edge observed from NOAA infrared (IR) imagery shows an rms difference of 49 km between 75°W and 60°W longitude, but not all sea level maps show a clearly defined Gulf Stream. Using the IR estimates for Gulf Stream position, sea level residuals were modeled as the meandering of a sea level front shaped as 0.5 m · tanh {(X–X0)/[50 km · sin (α)]}, with α the angle between the Geosat track and the Gulf Stream edge and X0 is based on the observed IR north wall positions. This showed that the annual signal, in terms of the location and phase of its maxima, is dominated by the meandering of the Gulf Stream.