The metastability of the mid-latitude Southern Hemisphere circulation

Observed changes in the metastability of the Southern Hemisphere 500 hPa circulation are examined using non-stationary cluster analysis techniques. The cluster methodology is a purely data-driven approach for parametrisation whereby a multi-scale approximation to non-stationary dynamical processes is achieved through optimal sequences of locally stationary fast vector auto-regressive factor processes and some slow (or persistent) hidden process switching between them. Comparison is made with blocking indices commonly used in weather forecasting and climate analysis to identify dynamically relevant metastable regimes in the reanalysed 500 hPa circulation. Our analysis characterises the metastable regime in reanalysed observational data sets prior to 1978 as positive and negative phases of a hemispheric mid-latitude blocking state with the Southern Annular Mode (SAM) associated with a transition state. Post 1978, SAM emerges as a true metastable state replacing the negative phase of the hemispheric blocking pattern. Trends in the hidden state frequency of occurrences correspond to declining blocking (coherent structures) and increasing strength of the SAM (zonal flow).