Azimuthal structure of a cyclonic Gulf Stream ring

Abstract

During the period June 8–19, 1979, an intensive expendable bathythermograph survey of a Gulf Stream ring was made. After mapping the irregularly spaced data onto a regular grid, and azimuthally decomposing the temperature field at constant depth into Fourier constituents einθ, we find the n = 2 mode to be the dominant one. We calculate the geostrophic stream function utilizing the hydrostatic equation and the measured temperature/salinity curves. The stream function is expressed in cylindrical polar coordinates (r, θ, z) and decomposed into a steady, azimuthally independent part ψ(r, z) and a temporally varying disturbance part ϕ(r, θ, z, t) with azimuthal dependence. The quasi‐geostrophic energy equation for ϕ contains source terms representing the interaction of the ψ and ϕ fields by baroclinic and barotropic processes. We find that ∼90% of the total disturbance energy flux results from baroclinic processes, and the remainder from the barotropic exchanges. The time scale of this energy flux into the disturbance ϕ is of the order of 16 days, while the time scale of an energy loss from the disturbance is ∼4.5 days.

Frictionally induced circulations and spin down of a warm‐core ring

“…The frictionally driven spin-down of a warm-core ring predicted by this model is large enough to account for the changes in thermostad depth observed during periods when the ring is not strongly interacting with the Gulf Stream. This is not to say, however, that other processes such as layering [Lambert, 1974], asymmetric perturbations [Mied et al, 1983], or Rossby-wave radiation [Flierl, 1983;McWilliams and Flierl, 1979] are not important. Indeed, it is quite likely that the horizontal Reynolds stresses may be, to a large degree, associated with the asymmetric motions driven by the beta effect.…”

On the adjustment of azimuthally perturbed vortices

“…Independent of the exact form of the mode number dependence of the decay time scale, it is clear that the rapid decrease in this time scale with increasing mode number is an additional reason why high-mode perturbations are not typically observed in ocean eddies. Mied et al [1983] discussed the variation of the azimuthal structure of a Gulf Stream ring based on three hydrographic surveys of the ring. From these data they estimated a time scale of the order of 4.5 days for the transfer of energy from the perturbation to the mean (axisymmetric) flow and a time scale of the order of 16 days for the transfer of energy from the mean flow to the perturbation.…”

Oscillations and rotations of elliptical warm‐core rings

“…1984;Kitano, 1974]. Mied et al [1983] described in detail an elongated cold-core ring. The ellipticity is probably a result of nonaxisymmetric birthing conditions and subsequent shear during the eddy's life [Nof, 1984] or inherent instability [Saunders, 1973;Griffiths and Linden, 1981;Mied et al, 1983].…”

“…Mied et al [1983] described in detail an elongated cold-core ring. The ellipticity is probably a result of nonaxisymmetric birthing conditions and subsequent shear during the eddy's life [Nof, 1984] or inherent instability [Saunders, 1973;Griffiths and Linden, 1981;Mied et al, 1983]. The noted absence of elongated elliptical rings suggests that high eccentricities lead to instability and self destruction such that only quite circular rings can survive.…”