The role of the stratospheric state in upward wave flux prior to Sudden Stratospheric Warmings: a SNAPSI analysis

Abstract. Several studies highlight the relevance of considering polar winter stratospheric information such as the occurrence of Sudden Stratospheric Warmings (SSWs) for skillful Subseasonal to Seasonal (S2S) surface climate predictions. However, current S2S forecast systems can only predict these events about two weeks in advance. A potential way of increasing their predictability is to improve the models' representation of the triggering mechanisms of SSWs. Traditional theories indicate that SSWs follow sustained wave dissipation in the stratosphere, but the relative role of tropospheric versus stratospheric conditions in the enhancement of stratospheric wave activity remains unclear. This study aims to quantify the role of the stratospheric state in wave activity preceding SSWs by analyzing three recent SSWs: the boreal SSWs of 2018 and 2019 and the austral minor SSW of 2019, using specific sets of S2S experiments. These ensembles follow the SNAPSI (Stratospheric Nudging And Predictable Surface Impacts) guidelines and include free-evolving atmospheric runs and nudged simulations, where the zonally-symmetric stratospheric state is nudged to either observations of a certain SSW or a climatological state. Our results show that the models struggle to capture the strong enhancement of wave activity preceding the 2018 SSW, limiting predictability beyond 10 d. In contrast, both SSWs of 2019 are better predicted, consistent with a more accurate simulation of the wave activity. Nudging the zonal mean stratospheric state does not drastically influence the upward wave activity flux or tropospheric circulation anomalies prior to these SSWs, but it has some impact on the stratospheric wave activity, although this modulation depends on the event characteristics. The boreal 2019 SSW appears to be primarily driven by tropospheric processes. In contrast, stratospheric contributions may have also played an important role in triggering the boreal 2018 SSW and the austral 2019 SSW. Understanding these variations is key to improving SSW predictability in S2S models.

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Journal
Weather and Climate Dynamics
Published
2026-02-24
DOI
https://doi.org/10.5194/wcd-7-411-2026
Primary Topic
Climate variability and models
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article
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article

The role of the stratospheric state in upward wave flux prior to Sudden Stratospheric Warmings: a SNAPSI analysis

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The role of the stratospheric state in upward wave flux prior to Sudden Stratospheric Warmings: a SNAPSI analysis

Daniele Mastrangelo, Damien Specq, Hera Kim, Inna Polichtchouk, Zheng Wu, Marisol Osman, Jadwiga H. Richter, G. L. Manney, Cory Barton, Masakazu Taguchi, Blanca Ayarzagüena, Martin Jucker, Hilla Afargan‐Gerstman, Philip Rupp, Wuhan Ning, Alvaro de la Camara, Gerbrand Koren, Daniel De Maeseneire, Dong-Chan Hong, Jeff Knight, Tim Stockdale, Peter Hitchcock, Zac D. Lawrence, Piero Malguzzi, Chaim I. Garfinkel, Natalia Calvo, Isla R. Simpson, Jiyoung Oh, Yu-Kyung Hyun, Seok-Woo Son, Amy H. Butler
article en

Abstract

Abstract. Several studies highlight the relevance of considering polar winter stratospheric information such as the occurrence of Sudden Stratospheric Warmings (SSWs) for skillful Subseasonal to Seasonal (S2S) surface climate predictions. However, current S2S forecast systems can only predict these events about two weeks in advance. A potential way of increasing their predictability is to improve the models' representation of the triggering mechanisms of SSWs. Traditional theories indicate that SSWs follow sustained wave dissipation in the stratosphere, but the relative role of tropospheric versus stratospheric conditions in the enhancement of stratospheric wave activity remains unclear. This study aims to quantify the role of the stratospheric state in wave activity preceding SSWs by analyzing three recent SSWs: the boreal SSWs of 2018 and 2019 and the austral minor SSW of 2019, using specific sets of S2S experiments. These ensembles follow the SNAPSI (Stratospheric Nudging And Predictable Surface Impacts) guidelines and include free-evolving atmospheric runs and nudged simulations, where the zonally-symmetric stratospheric state is nudged to either observations of a certain SSW or a climatological state. Our results show that the models struggle to capture the strong enhancement of wave activity preceding the 2018 SSW, limiting predictability beyond 10 d. In contrast, both SSWs of 2019 are better predicted, consistent with a more accurate simulation of the wave activity. Nudging the zonal mean stratospheric state does not drastically influence the upward wave activity flux or tropospheric circulation anomalies prior to these SSWs, but it has some impact on the stratospheric wave activity, although this modulation depends on the event characteristics. The boreal 2019 SSW appears to be primarily driven by tropospheric processes. In contrast, stratospheric contributions may have also played an important role in triggering the boreal 2018 SSW and the austral 2019 SSW. Understanding these variations is key to improving SSW predictability in S2S models.

Weather and Climate DynamicsVol. 7(1)
NSF National Center for Atmospheric Research (US), University of Bern (CH), Universidad Complutense de Madrid (ES), United States Naval Research Laboratory (US), Met Office (GB), Centre National de la Recherche Scientifique (FR), Oeschger Centre for Climate Change Research (CH), National Oceanic and Atmospheric Administration (US), Korea Meteorological Administration (KR), Seoul National University (KR), European Centre for Medium-Range Weather Forecasts (GB), Cooperative Institute for Research in Environmental Sciences (US), University of Colorado Boulder (US), Utrecht University (NL), Hebrew University of Jerusalem (IL), University of Washington (US), Cornell University (US), New Mexico Institute of Mining and Technology (US), Universidad de Buenos Aires (AR), UNSW Sydney (AU), Aichi University of Education (JP), University at Albany, State University of New York (US), Institute of Atmospheric Sciences and Climate (IT), Météo-France (FR), NOAA Physical Sciences Laboratory (US), NOAA Chemical Sciences Laboratory (US), Northwest Research Associates (US), Instituto Franco-Argentino sobre Estudios de Clima y sus Impactos (AR), Fundación Ciencias Exactas y Naturales (AR), Ludwig-Maximilians-Universität München (DE)
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