The impact of the planetary scale on the decay of blocking and the use of phase diagrams and Lyapunov exponents as a diagnostic

Many studies have observed that the growth and decay of blocking events, and have shown that the transition from a non-blocked to a blocking state, and vice-versa, can happen on the synoptic time-scales even though blocking events are a large-scale phenomenon in time and space. The onset of blocking has been studied extensively over the last decade, while the decay of blocking events has not received nearly as much attention. Some studies have suggested that blocking events decay when there is no longer upstream support from synoptic-scale transients. Other studies have shown that a change in the planetary-scale flow regime can result in the abrupt end of blocking episodes. Using PV diagnostics and phase diagrams, several Southern Hemisphere blocking events will be examined in order to assess the role of the planetary-scale in the decay phase of these events. The use of phase diagrams in examining the behavior of the large-scale flow is not new, and it will be shown that substantial changes in the large-scale flow can be identified using these diagrams. Additionally, observed fields can be used in order to calculate a fluid-trapping diagnostic following Cohen and Schultz (2005) and this can be related to the Lyapunov exponent of the flow regime. The predictability of blocking will also be examined using observational data and calculating the Lyapunov exponent following Dymnikov (1993).