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James S. Risbey
and
Peter H. Stone

Abstract

Data on zonally averaged atmospheric angular momentum and high cloud cover percent are analyzed for the periods April–October 1979 and November 1982–October 1983. The dominant periodicity in both momentum and cloud datasets was the so called “30–60 day atmospheric oscillation” in tropical and subtropical belts. In lag correlations between high cloud belts, both the periodicity and a latitudinally varying correlation structure were evident. In the 1979 period (Northern Hemisphere summer) the cloud-cloud correlations had nodes near 17°S, 5°N, 24°N and 36°N, i.e., anomalously high/low zonal mean convection between 5 and 24°N coincided with anomalously low/high zonal mean convection between 17°S and 5°N, and between 24 and 36°N. In April–October 1983, a similar periodicity and phase structure were present, but not as well defined. The principal node in the northern Hemisphere summer, near 5°N, appears to lie between the belt of maximum cloud cover for the period (which is between 5 and 9°N) and the equator. In an analysis of the period November 1982–April 1983 (Southern Hemisphere summer), the principal node was located in the Southern Hemisphere. Lag correlations between high cloud belts and momentum belts showed strong correlations with the 30–60 day oscillation present. Anomalously high/low zonal mean high cloudiness in the tropics is accompanied by anomalously high/low zonal mean momentum in the tropics, with the latter anomalies subsequently propagating into midlatitudes.

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Terence J. O’Kane
,
James S. Risbey
,
Christian Franzke
,
Illia Horenko
, and
Didier P. Monselesan

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 split-flow 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.

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James S. Risbey
,
Terence J. O’Kane
,
Didier P. Monselesan
,
Christian Franzke
, and
Illia Horenko

Abstract

This study applies a finite-element, bounded-variation, vector autoregressive method to assess midtropospheric flow regimes characterized by regime switches between metastable states. The flow is assessed in reanalysis data from three different reanalysis sets assimilating surface data only; surface and upper-air data; and ocean, surface, and upper-air data. Results are generally consistent across the reanalyses and confirm the utility of surface-only reanalyses for capturing midtropospheric variability. The method is applied to a set of regional domains in the Northern Hemisphere and for the full-hemispheric domain. Composites of the metastable states for each region yield structures that are consistent with the well-documented teleconnection modes: the North Atlantic Oscillation in the Atlantic Ocean, the Pacific–North America pattern (PNA) in the Pacific Ocean, and Scandinavian blocking over Eurasia. The PNA mode includes a clear waveguide structure in midlatitudes. The Northern Hemisphere domain yields a state composite that reflects aspects of an annular mode (Arctic Oscillation), where the annular component in midlatitudes comprises a circumglobal waveguide. The Northern Hemisphere waveguide is characterized by wavenumber 5. Some of the nodes in this circumglobal waveguide manifest as part of regional dipole structures like the PNA. This situation contrasts with the Southern Hemisphere, where the circumglobal waveguide exhibits wavenumbers 3 and 5 and is monopolar. For each of the regions and modes examined, the annual time series of residence percent in each state displays prominent decadal variability and provides a clear means of identifying regimes of the major teleconnection modes.

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