Changes in the Metastability of the Midlatitude Southern Hemisphere Circulation and the Utility of Nonstationary Cluster Analysis and Split-Flow Blocking Indices as Diagnostic Tools

Abstract

Changes in the metastability of the Southern Hemisphere 500-hPa circulation are examined using both cluster analysis techniques and split-flow blocking indices. The cluster methodology is a purely data-driven approach for parameterization whereby a multiscale approximation to nonstationary dynamical processes is achieved through optimal sequences of locally stationary fast vector autoregressive factor (VARX) 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 500-hPa circulation in both reanalysis and Atmospheric Model Intercomparison Project (AMIP) datasets. The analysis characterizes the metastable regime in both reanalysis and model datasets prior to 1978 as positive and negative phases of a hemispheric midlatitude blocking state with the southern annular mode (SAM) associated with a transition state. Post-1978, the SAM emerges as a true metastable state replacing the negative phase of the hemispheric blocking pattern. The hidden state frequency of occurrences exhibits strong trends. The blocking pattern dominates in the early 1980s, and then gradually decreases. There is a corresponding increase in the SAM frequency of occurrence. This trend is largely evident in the reanalysis summer and spring but was not evident in the AMIP dataset. Further comparison with the splitflow blocking indices reveals a superficial correspondence between the cluster hidden state frequency of occurrences and split-flow indices. Examination of composite states shows that the blocking indices capture splitting of the zonal flow whereas the cluster composites reflect coherent block formation. Differences in blocking climatologies from the respective methods are discussed.

Sources of heterogeneous variability and trends in Antarctic sea-ice

“…The ability of synoptic weather to generate multi-decadal trends in sea-ice duration arises through ocean-sea-ice coupling 31 . The recent analysis of mid-latitude tropospheric circulation reveals multi-year variability in the persistence and frequency of atmospheric blocking 36 . Such changes alter synoptic weather and the wind forcing of the ocean by the atmosphere, introducing another way for winds to generate multi-decadal changes in sea-ice.…”

Decadal Patterns of Westerly Winds, Temperatures, Ocean Gyre Circulations and Fish Abundance: A Review

“…The temperature contrast and mixing in equatorial thermocline waters modulates El Nino activity. After the late 1970s regime shift a high positive Southern Annular Mode dominated resulting in strong zonal winds [66]. Upwelling anomalies from the South Pacific perturbed the equatorial thermocline seven years later resulting in a series of strong El Nino events in the late 20th century [67].…”

The metastability of the mid-latitude Southern Hemisphere circulation

“…In the context of non-stationary inference, it is appropriate to use the aic to determine the right order parameters of the varx model, that is the memory depth m, the number of clusters K and the optimal bv-persistency C [11,15]. To select the proper order parameters (and the optimal functional for external factors φ 2 (u t ) in (2)) for a given persistency parameter value [11] the aic is defined as…”

“…Large values indicate strong high and low latitude westerly winds or that the mid-latitude westerly flow is weak. O'Kane et al [15] considered a related measure of sh blocking, originally developed by Tibaldi et al [20], which depends on mid-and high latitude geo-potential height gradients.…”

“…Here we focus on the fem-bv-varx (finite element-bounded variation-vector auto-regressive factor) method [10,11] in combination with the Akaike information criterion (aic) to determine the optimal cluster state, as described in Section 2. O'Kane et al [15] were the first to examine the relative merits of split-flow blocking indices and non-stationary cluster analysis for the sh.…”