Quasi-geostrophic monopoles in a sheared zonal flow: Influence of the beta-effect and variable shear

This work builds on and continues a suite of earlier studies of interaction between a monopole and a sheared zonal flow in the framework of a 1.5-layer quasi-geostrophic model. In Reznik and Kravtsov (Physics of Fluids, 33, 116606, 2021; hereafter RK21), this problem was considered under an f-plane approximation for the case in which the dependence of the zonal velocity on latitude was linear. Here the conclusions stemming from that work are generalized for the case of a beta-plane and a variable shear of the background flow. Namely, numerical experiments with singular vortices using the algorithm of Kravtsov and Reznik (Physics of Fluids, 31, 066602, 2019; hereafter KR19) confirm the existence of the trapping latitude, which attracts (repels) prograde (retrograde) vortices and clarify the underlying mechanisms. Unlike in the case of a linear shear on an f-plane, the trapping latitude here does not necessarily coincide with the latitude at which the effective beta-parameter vanishes. Another important difference is that in the presence of nonzero beta, a trapped prograde vortex exhibits a near-zonal westward drift with the zonal velocity close to the phase speed of long Rossby waves and the meridional velocity at least two orders of magnitude smaller than that. On the other hand, the meridional velocity of a retrograde vortex appears to be unrestricted; such a vortex can rapidly move in any direction, including the direction across the zonal current.