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Several recent studies have suggested that the stratosphere can be a source of subseasonal‐to‐seasonal predictability of Southern Hemisphere circulation during the austral spring and early summer seasons, through its influence on the zonal‐mean eddy‐driven jet. We exploit the large sample size afforded by the hindcasts from the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System to address a number of unanswered questions. It is shown that the picture of coherent seasonal variability of the coupled stratosphere‐troposphere system apparent from the reanalysis record during the spring/early summer period is robust to sampling uncertainty and that there is evidence of nonlinearity in the case of the most extreme variations. The effect of El Niño–Southern Oscillation on the eddy‐driven jet during this time of year is found to occur via the stratosphere, with no evidence of a direct tropospheric pathway. A simple two‐state statistical model of the stratospheric vortex is introduced to estimate the subseasonal‐to‐seasonal predictability associated with shifts of the seasonal cycle in the Southern Hemisphere extratropical atmosphere. This simple model, along with a more general model, is subsequently used to interpret skill scores associated with hindcasts made using the full seasonal forecast system. Together, the results provide evidence of tropospheric predictability on subseasonal‐to‐seasonal timescales during this time of year from at least as early as 1 August and show no evidence of a "signal‐to‐noise paradox" between the hindcasts and the reanalysis. Key Points: Southern Hemisphere midlatitude jet variations in spring and early summer are predictable several months aheadThis subseasonal‐to‐seasonal predictability of the Southern Hemisphere jet comes via the stratospheric polar vortexThe observed influence of ENSO on the jet during this time is via this stratospheric pathway [ABSTRACT FROM AUTHOR]

Abstract: The impact of parametrized nonorographic gravity wave drag (NOGWD) on the stratosphere‐troposphere dynamical coupling in atmospheric models is relatively unexplored. Using the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System, we find that changes in NOGWD strength have a substantial impact on the tropospheric eddy‐driven jet (EDJ) in both hemispheres, but the sense of the impact is opposite in the two hemispheres. In the Northern Hemisphere the impact occurs via changes in the amplitude and persistence of stratospheric anomalies. In the Southern Hemisphere it occurs instead via differences in the sensitivity of the EDJ to a given stratospheric anomaly, arising from changes in the seasonal cycle leading up to the polar vortex breakdown. Increasing NOGWD eliminates the springtime phase of the Southern Hemisphere tropospheric semiannual oscillation, resulting in a more equatorward annual‐mean EDJ and showing that the semiannual oscillation cannot be explained entirely from tropospheric mechanisms. [ABSTRACT FROM AUTHOR]

Previous studies have highlighted an important organizing influence of the seasonal Southern Hemisphere stratospheric vortex breakdown on the large-scale stratospheric and tropospheric circulation. The present study extends this work by considering the statistical predictability of the stratospheric vortex breakdown event, using reanalysis data. Perturbations to the winter stratospheric vortex are shown to persist into austral spring and to lead to a shift in the statistics of the breakdown event during austral summer. This is interpreted as evidence for the potential for seasonal predictability of the vortex breakdown event in the stratosphere. Coupled variability between the stratosphere and troposphere is then considered. The semiannual oscillation of the tropospheric midlatitude jet is discussed, and evidence for a connection between this behavior and variations in the stratosphere is presented. Based on this connection, an argument is made for the concomitant potential for seasonal predictability in the troposphere, assuming knowledge of the stratospheric initial state. Combining these various results, a nonstationary, regime-based perspective of large-scale extratropical Southern Hemisphere circulation variability between late winter and summer is proposed. The implications of this perspective for some previous studies involving annular modes of the circulation are discussed. In particular, the long annular mode time scales during austral spring and summer should not be interpreted as an increased persistence of perturbations to some slowly varying seasonal cycle, but instead as a reflection of a phase shift of the seasonal cycle induced by stratospheric variability. [ABSTRACT FROM AUTHOR]

Statistical models of climate generally regard climate variability as anomalies about a climatological seasonal cycle, which are treated as a stationary stochastic process plus a long-term seasonally dependent trend. However, the climate system has deterministic aspects apart from the climatological seasonal cycle and long-term trends, and the assumption of stationary statistics is only an approximation. The variability of the Southern Hemisphere zonal-mean circulation in the period encompassing late spring and summer is an important climate phenomenon and has been the subject of numerous studies. It is shown here, using reanalysis data, that this variability is rendered highly nonstationary by the organizing influence of the seasonal breakdown of the stratospheric polar vortex, which breaks time symmetry. It is argued that the zonal-mean tropospheric circulation variability during this period is best viewed as interannual variability in the transition between the springtime and summertime regimes induced by variability in the vortex breakdown. In particular, the apparent long-term poleward jet shift during the early-summer season can be more simply understood as a delay in the equatorward shift associated with this regime transition. The implications of such a perspective for various open questions are discussed. [ABSTRACT FROM AUTHOR]