Observations on the long‐period variability of the Gulf Stream downstream of Cape Hatteras

Abstract

To examine the long‐period variability of the Gulf Stream, sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 75°W and 60°W longitude) are used. Residuals, as derived from Geosat altimetry between November 1986 and December 1988, were gridded in space and time at a temporal resolution of 10 days and spatial resolution of 1/4°. Complex empirical orthogonal function (CEOF) analysis was applied to the data set to extract the spatially correlated signal with the original data sub sampled to 1/2°. In addition to determining the space‐time scales and propagation characteristics of the different modes, wavenumber‐frequency spectral techniques were used to separate the variability into propagating and stationary components. The CEOF technique applied to the data set indicated that the first four CEOF modes accounted for 60% of the variability and were found to be above the noise level 99% of the time. CEOF 1 was associated with westward propagation at 5 km/d at a wavelength of 2000 km and eastward propagation at 1–2 km/d centered at a 500‐km wavelength. This first CEOF is in good agreement with thin‐jet equivalent barotropic models which predict westward propagation for wavelengths greater than 1130 km. A deflection of the wavelike pattern at 65°W also indicates a possible topographic effect. A simple scaling of the effect of topography indicates that for length scales longer than the internal Rossby radius of deformation, the topographic term is at least of the same order of magnitude as the beta effect. The second CEOF was more broadbanded in wavenumber space, with eastward propagation occurring in a wavenumber‐frequency band between 300 and 1400 km and 0.5 and 2.0 cycles/yr. The third CEOF is similar in structure to the first, but with less energy. CEOF 4 was clearly identifiable with higher frequencies than the first three with westward propagation at 4 km/d. The spatial location of this mode along with the westward propagation indicates possible influences from eddy‐stream interactions. Thus topography, Rossby wave dynamics, and eddy‐stream interactions all appear to have a significant role in determining the space‐time scales and propagation properties of the long‐period response of sea level in the Gulf Stream.