30 Sep 2021

30 Sep 2021

Review status: this preprint is currently under review for the journal WCD.

Sudden stratospheric warmings during El Niño and La Niña: sensitivity to model biases

Nicholas L. Tyrrell1, Juho M. Koskentausta1,2, and Alexey Yu. Karpechko1 Nicholas L. Tyrrell et al.
  • 1Meteorological Research Unit, Finnish Meteorological Institute, Helsinki, 00500, Finland
  • 2Faculty of Science, University of Helsinki, Helsinki, Finland

Abstract. The number of sudden stratospheric warmings (SSWs) per year is affected by the phase of the El Niño–Southern Oscillation (ENSO), yet there are discrepancies between the observed and modeled relationship. We investigate how systematic model biases may affect the ENSO-SSW connection. A two-step bias-correction process is applied to the troposphere, stratosphere or full atmosphere of an atmospheric general circulation model. ENSO type sensitivity experiments are then performed to reveal the impact of differing climatologies on the ENSO–SSW teleconnection.

The number of SSWs per year is overestimated in the control run, and this statistic is improved when stratospheric biases are reduced. The seasonal cycle of SSWs is also improved by the bias corrections. The composite SSW responses in the stratospheric zonal wind, geopotential height and surface response are well represented in both the control and bias corrected runs. The model response of SSWs to ENSO phase is more linear than in observations, in line with previous modelling studies, and this is not changed by the reduced biases. However, the trend of more wave-1 events during El Niño years than La Niña years is improved in the bias corrected runs.

Nicholas L. Tyrrell et al.

Status: open (until 17 Nov 2021)

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Nicholas L. Tyrrell et al.

Nicholas L. Tyrrell et al.


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Short summary
El Niño events are known to effect the variability of the wintertime stratospheric polar vortex. The observed relationship differs from what is seen in climate models. Climate models have errors in their average winds and temperature, and in this work we artificially reduce those errors to see how that changes the communication of El Niño events to the polar stratosphere. We find reducing errors improves stratospheric variability, but does not explain the differences with observations.