The Propagation and Evolution of Cyclonic Gulf Stream Rings

Abstract

Numerical simulations of the propagation of cyclonic Gulf Stream rings are made using a primitive equation β-plane model of a flat-bottomed two-layer ocean with a rigid lid. Initially circular eddies having, upper and deep ocean maximum currents maxU₁ and maxU₂ located at radial position l from the center are allowed to evolve and four types of behavior have been discerned: 1) dispersing rings possess negligible nonlinearity and disperse rapidly; 2) barotropic rings (U₁ = U₂) are weakly dispersive, propagating recognizably for long periods of time, and, nearly barotropic eddies (U₁ ≈ U₂) slowly lose coherence in the deep ocean; 3) upper ocean rings propagate with a vortex present only in the upper ocean; and 4) eastward-traveling eddies possess circulations in the upper and lower oceans which propagate together stably to the cast. Changes in viscosity are found to be more important to the longevity of the ring than are changes in (maxU₁)/βl². Both westward and northward speeds increase with increasing maxU₂/maxU₁ and increasing (maxU₁)/βl². Speeds to the west are found to be 2–3 km day⁻¹ and those to the north are 1–9 km day⁻¹ for 3 ≲ (maxU₁)/βl² ≲ 15 and 0 ⩽ maxU₂)/maxU₁ ⩽ 1.2.