Statistics and dynamics of blockings with a point vortex model

Abstract

We investigate a reduced point vortex model for statistical and dynamical analyses of atmospheric blocking phenomena. Thereby, we consider High-over-low and Omega blocking as relative equilibria of two and three point vortices. Under certain conditions, such point vortex systems move westward opposing the mean westerly flow and hence can become stationary. Based on the kinematic vorticity number, two novel, independent methods, the contour and the trapezoid method, are introduced in order to identify the vortices that form the blocking pattern, their local positions and circulation magnitudes. While the contour method takes into account the observed stationarity of blocking, the trapezoid method minimizes the total circulation of the vortex system following point vortex theory. Using an instantaneous blocking index, a total number of 347 blocking periods were identified in NCEP-NCAR Reanalysis data for the Euro-Atlantic region during the time period 1990–2012. This procedure provides the basis to corroborate the applicability of the point vortex model to atmospheric blocking in a statistical framework. The calculated translation speed of point vortex systems associated with the atmospheric blocking appears to match the zonal mean velocity reasonably well. This model explains the stationary behaviour of blocking patterns. A comparison between the theoretical and a statistical model further reveals that the circulation of the blocking high follows the principles of the point vortex model to a large extent. However, the low-pressure systems behave more variable. Moreover, the stability of point vortex equilibria is analysed regarding the relative distances by considering linear stability analysis and simulations. This reveals that the point vortex blocking model corresponds to an unstable saddle point. Furthermore, we take viscosity and a Brownian motion into account to simulate the influence of the smaller, subgrid-scale disturbances. As a result, a clustering near the equilibrium state emerges indicating the persistence of the atmospheric blocking pattern.