An early warning indicator for blocking events

Abstract

The formation of a persistent high pressure cell in the midlatitudes that blocks the westerly flow is called a blocking event. Such events have a high impact on the general weather in these areas. In one of the theories of the transitions between zonal and blocked flow, the interaction of the atmospheric jet with the orography causes multiple equilibria in the mean state of the atmospheric flow. The main aim of this study is to develop an early warning indicator for such a transition. A new method is developed to define regime switches in the highly dimensional climate system. A barotropic vorticity model of the Northern Hemisphere, truncated at the 21st wavenumber and with realistic topography and forcing is used to simulate transitions into a blocked state of the atmosphere. The projection of all the states of the model into the plane defined by the leading Empirical Orthogonal Functions (EOF) of the streamfunction shows two attracting regions; the zonal and the blocked state of the model. The evolution in time shows preferred directions of movement in this phase-space. A complex network is built where the attracting regions are represented by highly connected nodes. We show that it is possible to identify two persistent atmospheric regimes by using a community detection algorithm on this network. Probabilities to make the transition to the blocked regime are calculated from the trajectories on this plane. From these probabilities, an early warning indicator for the transition into the blocked state can be derived that gives the most reliable results on longer timescales. Finally, a physical mechanism for the transitions to the blocked regime is found from the energetics of the system. As the system gets barotropically unstable, perturbations are enhanced by the increasing Reynolds stresses. The coupling of the flow with the topography gives rise to a loss of energy of the zonal flow towards meandering modes.