Deep Eddy Energy and Topographic Rossby Waves in the Gulf of Mexico

Abstract

Observations suggest the hypothesis that deep eddy kinetic energy (EKE) in the Gulf of Mexico can be accounted for by topographic Rossby waves (TRWs). It is presumed that the TRWs are forced by Loop Current (LC) pulsation, Loop Current eddy (LCE) shedding, and perhaps also by LCE itself. Although the hypothesis is supported by model results, such as those presented in Oey, the existence of TRWs in the model and how they can be forced by larger-scale LC and LCEs with longer-period vacillations have not been clarified. In this paper, results from a 10-yr simulation of LC and LCEs, with double the resolution of that used by Oey, are analyzed to isolate the TRWs. It is shown that along an east-to-west band across the gulf, approximately over the 3000-m isobath, significant EKE that accounts for over one-half of the total spectrum is contained in the 20–100-day periods. Bottom energy intensification exists in this east–west band with vertical decay scales of about 600–300 m decreasing westward. The decrease agrees with the TRW solution. The band is also located within the region where TRWs can be supported by the topographic slope and stratification used in the model and where wavenumber and frequency estimates are consistent with the TRW dispersion relation. The analysis indicates significant correlation between pairs of east–west stations, over distances of approximately 400 km. Contours of lag times suggest offshore (i.e., downslope) phase propagation, and thus the east–west band indicates nearly parabathic and upslope energy propagation. Ray tracing utilizing the TRW dispersion relation and with and without (for periods >43 days) ambient deep currents shows that TRW energy paths coincide with the above east–west high-energy band. It also explains that the band is a result of TRW refraction by an escarpment (with increased topographic gradient) across the central gulf north of the 3000-m isobath, and also by deep current and its cyclonic shear, and that ray convergence results in localized EKE maxima near 91°W and 94°–95°W. Escarpment and cyclonic current shear also shorten TRW wavelengths. Westward deep currents increase TRW group speeds, by about 2–3 km day−1 according to the model, and this and ray confinement by current shear may impose sufficient constraints to aid in inferring deep flows. Model results and ray paths suggest that the deep EKE east of about the 91°W originates from under the LC while farther west the EKE also originates from southwestward propagating LCEs. The near-bottom current fluctuations at these source regions derive their energy from short-period (<100 days) and short-wavelength (<200 km) near-surface fluctuations that propagate around the LC during its northward extrusion phase and also around LCEs as they migrate southwestward in the model.