BAROTROPIC VORTEX PAIRS ON A ROTATING SPHERE

Using a barotropic model in spherical geometry, we construct new solut ions for steadily travelling vortex pairs and study their stability pr operties. We consider pairs composed of both point and finite-area vor tices, and we represent the rotating background with a set of zonal st rips of uniform vorticity. After constructing the solution for a singl e point-vortex pair, we embed it in a rotating background, and determi ne the equilibrium configurations that travel at constant speed withou t changing shape. For equilibrium solutions, we find that the stabilit y depends on the relative strength (which may be positive or negative) of the vortex pair to the rotating background: eastward-travelling pa irs are always stable, while westward-travelling pairs are unstable wh en their speeds approach that of the linear Rossby-Haurwitz waves. Thi s finding also applies (with minor differences) to the case when the v ortices are of finite area; in that case we find that, in addition to the point-vortex-like instabilities, the rotating background excites s ome finite-area instabilities for vortex pairs that would otherwise be stable. As for practical applications to blocking events, for which t he slow westward pairs are relevant, our results indicate that free ba rotropic solutions are highly unstable, and thus suggest that forcing mechanisms must play an important role in maintaining atmospheric bloc king events.