Nonstationarity in Southern Hemisphere Climate Variability Associated with the Seasonal Breakdown of the Stratospheric Polar Vortex

Byrne, Nicholas; Shepherd, Theodore G.; Woollings, Tim; Plumb, R. Alan

doi:10.1175/jcli-d-17-0097.1

Cited by 33 publications

(52 citation statements)

References 36 publications

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“…This effect is consistent with the downward propagation of stratospheric vortex anomalies into the troposphere on interannual time scales as found in reanalysis data (Baldwin & Dunkerton, 2001;Black & McDaniel, 2007;Byrne et al, 2017;Byrne & Shepherd, 2018;Thompson et al, 2005). This effect is consistent with the downward propagation of stratospheric vortex anomalies into the troposphere on interannual time scales as found in reanalysis data (Baldwin & Dunkerton, 2001;Black & McDaniel, 2007;Byrne et al, 2017;Byrne & Shepherd, 2018;Thompson et al, 2005).…”

Section: Summary and Discussionsupporting

confidence: 90%

“…We include the following 22 models in our analysis: As in previous literature, we define the vortex strength using the daily mean zonal-mean zonal wind at 50 hPa and 60 • S (Black & McDaniel, 2007;Byrne & Shepherd, 2018;Byrne et al, 2017;Sheshadri et al, 2015). We define the RCP8.5 response as the difference between years 2080-2099 of RCP8.5 and a 20-year climatology from piControl.…”

Section: Methodsmentioning

confidence: 99%

“…On interannual time scales, variability in the seasonal breakdown of the stratospheric polar vortex has been shown to modulate the transition from a poleward jet regime to a more equatorward one between late spring and early summer (Byrne et al, 2017;Byrne & Shepherd, 2018). On interannual time scales, variability in the seasonal breakdown of the stratospheric polar vortex has been shown to modulate the transition from a poleward jet regime to a more equatorward one between late spring and early summer (Byrne et al, 2017;Byrne & Shepherd, 2018).…”

Section: Introductionmentioning

confidence: 99%

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The Role of the Stratospheric Polar Vortex for the Austral Jet Response to Greenhouse Gas Forcing

Ceppi

Shepherd

2019

Geophysical Research Letters

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Future shifts of the austral midlatitude jet are subject to large uncertainties in climate model projections. Here we show that, in addition to other previously identified sources of intermodel uncertainty, changes in the timing of the stratospheric polar vortex breakdown modulate the austral jet response to greenhouse gas forcing during summertime (December–February). The relationship is such that a larger delay in vortex breakdown favors a more poleward jet shift, with an estimated 0.7–0.8° increase in jet shift per 10‐day delay in vortex breakdown. The causality of the link between the timing of the vortex breakdown and the tropospheric jet response is demonstrated through climate modeling experiments with imposed changes in the seasonality of the stratospheric polar vortex. The vortex response is estimated to account for about 30% of the intermodel variance in the shift of the summertime austral jet and about 45% of the mean jet shift.

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Section: Summary and Discussionsupporting

confidence: 90%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Role of the Stratospheric Polar Vortex for the Austral Jet Response to Greenhouse Gas Forcing

Ceppi

Shepherd

2019

Geophysical Research Letters

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“…Figure focuses on the equatorward transition of the EDJ in early summer using hindcasts initialized on 1 November, so that the hindcasts are as close to observations as possible during the vortex breakdown period. The timing of the equatorward transition of the EDJ has been found to be closely coupled to the SPV breakdown date in the reanalysis (Byrne et al, ). In particular, years with a later than average SPV breakdown date are associated with a later than average equatorward transition of the EDJ, with opposite behavior for earlier than average years.…”

Section: Role Of Sampling Uncertaintymentioning

confidence: 84%

“…The strength of the SPV during this lead‐up period has a strong influence on the timing of the breakdown event, as well as on the latitude of the EDJ in the troposphere (Byrne & Shepherd, ). In addition, the SPV breakdown event itself typically precedes an equatorward shift of the EDJ (Byrne et al, ). This close relationship between the SPV and the EDJ in the SH can be parsimoniously viewed as a continuous shift of the seasonal cycle during this time of year (Byrne & Shepherd, ).…”

Section: Introductionmentioning

confidence: 99%

Subseasonal‐to‐Seasonal Predictability of the Southern Hemisphere Eddy‐Driven Jet During Austral Spring and Early Summer

2019

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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.

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Storyline description of Southern Hemisphere midlatitude circulation and precipitation response to greenhouse gas forcing

et al. 2020

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As evidence of climate change strengthens, knowledge of its regional implications becomes an urgent need for decision making. Current understanding of regional precipitation changes is substantially limited by our understanding of the atmospheric circulation response to climate change, which to a high degree remains uncertain. This uncertainty is reflected in the wide spread in atmospheric circulation changes projected in multimodel ensembles, which cannot be directly interpreted in a probabilistic sense. The uncertainty can instead be represented by studying a discrete set of physically plausible storylines of atmospheric circulation changes. By mining CMIP5 model output, here we take this broader perspective and develop storylines for Southern Hemisphere (SH) midlatitude circulation changes, conditioned on the degree of global-mean warming, based on the climate responses of two remote drivers: the enhanced warming of the tropical upper troposphere and the strengthening of the stratospheric polar vortex. For the three continental domains in the SH, we analyse the precipitation changes under each storyline. To allow comparison with previous studies, we also link both circulation and precipitation changes with those of the Southern Annular Mode. Our results show that the response to tropical warming leads to a strengthening of the midlatitude westerly winds, whilst the response to a delayed breakdown (for DJF) or strengthening (for JJA) of the stratospheric vortex leads to a poleward shift of the westerly winds and the storm tracks. However, the circulation response is not zonally symmetric and the regional precipitation storylines for South America, South Africa, South of Australia and New Zealand exhibit quite specific dependencies on the two remote drivers, which are not well represented by changes in the Southern Annular Mode.

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Nonstationarity in Southern Hemisphere Climate Variability Associated with the Seasonal Breakdown of the Stratospheric Polar Vortex

Cited by 33 publications

References 36 publications

The Role of the Stratospheric Polar Vortex for the Austral Jet Response to Greenhouse Gas Forcing

The Role of the Stratospheric Polar Vortex for the Austral Jet Response to Greenhouse Gas Forcing

Subseasonal‐to‐Seasonal Predictability of the Southern Hemisphere Eddy‐Driven Jet During Austral Spring and Early Summer

Storyline description of Southern Hemisphere midlatitude circulation and precipitation response to greenhouse gas forcing

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