The propagation of “streamers” along the periphery of warm-core rings

Abstract

The collision between the Gulf Stream and warm-core rings usually involves narrow bands (∼5 km) of Gulf Stream water that envelop the colliding rings. Such bands (“streamers”) propagate along the ring's edge in a clockwise manner; they sometimes lead to the recapture of the rings by the Gulf Stream. A nonlinear layered model is used to examine the streamers' propagation rate. The streamers are viewed as an intrusion of light Gulf Stream water along a curved surface that contains the slightly heavier ring water. Their propagation rate is determined by (i) transforming the governing equations to a coordinate system that rotates steadily with the intrusion's nose, (ii) using the fact that the fluid must stagnate at the intrusion's nose, and (iii) considering the integrated torque of the intrusion. Analytical solutions are constructed by using a perturbation scheme in μ, the ratio of the streamer's width to the ring's radius. Surprisingly, it is found that, no matter what the radius of the ring is, the propagation rate is always one half of the particle speed along the upstream front of the Gulf Stream. For most situations, this gives about 0.5–1.0 m s −1 as a typical propagation rate. Such a relatively fast advancement rate is of the same order as the ring's orbital speed even though the two are not directly related. In contrast to intuition, the streamers are a result of the geometrical structure of the ring rather than advection by the fluid circulating clockwise within the ring. In addition, unlike most other intrusions, the propagation speed is not related to the phase speed of a Kelvin wave (based on the streamer's depth).