Separating and quantifying the distinct impacts of El Niño and sudden stratospheric warmings on North Atlantic and Eurasian wintertime climate

Oehrlein, Jessica; Chiodo, Gabriel; Polvani, Lorenzo M.

doi:10.1002/asl.923

Cited by 16 publications

(15 citation statements)

References 41 publications

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“…The warm anomalies are centered over Southern Greenland and from the central Sahara northeastward across Saudi Arabia and Iran. These SAT impacts in WACCM are consistent with previous model‐based studies (e.g., Oehrlein et al., 2019). Composites of SAT anomalies following SPVs (Figures 6b and 6g) show largely similar patterns but of opposite sign as those following SSWs.…”

Section: Resultssupporting

confidence: 92%

The Atlantic Jet Response to Stratospheric Events: A Regime Perspective

et al. 2021

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The tropospheric response to Sudden Stratospheric Warmings (SSWs) is associated with an equatorward shift in the midlatitude jet and associated storm tracks, while Strong Polar Vortex (SPV) events elicit a contrasting response. Recent analyses of the North Atlantic jet using probability density functions of a jet latitude index have identified three preferred jet latitudes, raising the question of whether the tropospheric response to SSWs and SPVs results from a change in relative frequencies of these preferred jet regimes rather than a systematic jet shift. We explore this question using atmospheric reanalysis data from 1979 to 2018 (26 SSWs and 33 SPVs), and a 202‐years integration of the Whole Atmosphere Community Climate Model (92 SSWs and 68 SPVs). Following SSWs, the northern jet regime becomes less common and the central and southern regimes become more common. These changes occur almost immediately following “split” vortex events, but are more delayed following “displacement” events. In contrast, the northern regime becomes more frequent and the southern regime less frequent following SPV events. Following SSWs, composites of 500‐hPa geopotential heights, surface air temperatures, and precipitation most closely resemble composites of the southern jet regime, and are generally opposite in sign to the composites of the northern jet regime. These comparisons are reversed following SPVs. Thus, one possible interpretation is that the two southernmost regimes appear to be favored following SSWs, while the southernmost regime becomes less common following SPVs.

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Section: Resultssupporting

confidence: 92%

The Atlantic Jet Response to Stratospheric Events: A Regime Perspective

et al. 2021

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“…The zonal wind anomaly at 60°N is also positive throughout the atmospheric column in AMIP, but not CESM‐LE, showing stronger stratosphere‐troposphere coupling in the AMIP runs that combine stratospheric and El Niño forcing. The mechanism for this is unclear, as previous work has suggested that El Niños are associated with negative AO and NAO indices, not positive ones (Oehrlein et al., 2019).…”

Section: Resultsmentioning

confidence: 92%

“…Oehrlein et al. (2019) showed that El Niños typically are associated with a negative mode of the NAO in the troposphere, but that is in the absence of strong stratospheric forcing. A tropospheric mechanism connecting equatorial Pacific warming and negative NAO‐like variability in the North Atlantic was also proposed by Mezzina et al.…”

Section: Resultsmentioning

confidence: 99%

The Influence of Stratospheric Soot and Sulfate Aerosols on the Northern Hemisphere Wintertime Atmospheric Circulation

Coupe

Robock

2021

JGR Atmospheres

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The Northern Hemisphere wintertime circulation response to volcanic eruptions has been explored extensively using general circulation models. In observations and some models, the response is characterized by an enhanced stratospheric polar vortex (SPV), a positive mode of the North Atlantic Oscillation (NAO), and warm surface temperatures during the winter over North America and Eurasia. A weak surface air temperature signal in previous studies has led to conflicting conclusions on the robustness of the response. Here, we use simulations with the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) of six nuclear war scenarios to present a new perspective on the connection between stratospheric aerosol heating, the SPV, and the surface temperature response. We show that stratospheric aerosol heating by soot is the primary contributor to the SPV response in nuclear war simulations, which is coupled to the troposphere and projects as a positive mode of the NAO at the surface. Winter warming is observed across northern Eurasia, albeit poleward of the response after volcanic eruptions. We compare the results to simulations of volcanic eruptions and find that observed Eurasian warming in the first winter after the 1963 Agung, 1982 El Chichón, and 1991 Pinatubo volcanic eruptions is simulated with the NCAR CAM5 climate model only when tropical sea surface temperatures, including the observed El Niño, are specified along with the volcanic aerosols. This suggests an undiagnosed tropospheric mechanism connecting the tropics and high latitudes, as without specifying sea surface temperatures, internal variability dominates the simulated winter warming response after historical volcanic eruptions.

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“…The relationship with the La Niña teleconnection is more obscure, and overall their implication during ENSO events is not clear, as other studies consider SSWs and ENSO as distinct sources of variability in the North Atlantic (e.g. Polvani et al 2017;Oehrlein et al 2019). Using the same set of sensitivity experiments as in the present study, Palmeiro et al (2021a) have concluded that while ENSO modifies the seasonal-mean state of the polar vortex by conditioning it to be more (less) easily perturbed during El Niño (La Niña), the actual triggering of SSWs (at higher-frequency time-scales) is largely unrelated to ENSO.…”

Section: Introductionmentioning

confidence: 94%

Multi-model assessment of the late-winter stratospheric response to El Niño and La Niña

et al. 2021

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The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January–March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and shrinking/stretching around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking.

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Separating and quantifying the distinct impacts of El Niño and sudden stratospheric warmings on North Atlantic and Eurasian wintertime climate

Cited by 16 publications

References 41 publications

The Atlantic Jet Response to Stratospheric Events: A Regime Perspective

The Atlantic Jet Response to Stratospheric Events: A Regime Perspective

The Influence of Stratospheric Soot and Sulfate Aerosols on the Northern Hemisphere Wintertime Atmospheric Circulation

Multi-model assessment of the late-winter stratospheric response to El Niño and La Niña

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