Preprints
https://doi.org/10.5194/acp-2020-1237
https://doi.org/10.5194/acp-2020-1237

  12 Jan 2021

12 Jan 2021

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

Does the coupling of the mesospheric semiannual oscillation with the quasi-biennial oscillation provide predictability of Antarctic sudden stratospheric warmings?

Viktoria J. Nordström and Annika Seppälä Viktoria J. Nordström and Annika Seppälä
  • Department of Physics, University of Otago, Dunedin, New Zealand

Abstract. During September 2019 there was a sudden stratospheric warming over Antarctica, which brought disruption to the usually stable winter vortex. The mesospheric winds reversed and temperatures in the stratosphere rose by over 50 K. Whilst this was only the second SSW in the Southern Hemisphere (SH), the other having occurred in 2002, its Northern counterpart experiences about six per decade. Currently, an amplification of atmospheric waves during winter is thought to trigger SSWs. However, our understanding remains incomplete, especially in regards to its occurrence in the SH. Here, we investigate the interaction of two equatorial atmospheric modes, the Quasi Biennial Oscillation (QBO) and the Semiannual Oscillation (SAO) during the SH winters of 2019 and 2002. Using MERRA-2 reanalysis data we find that the two modes interact at low latitudes during their easterly phases in the early winter, forming a zero wind line that stretches from the lower stratosphere into the mesosphere. This influences the meridional wave guide, resulting in easterly momentum being deposited in the mesosphere throughout the polar winter, reducing the magnitude of the westerly winds. As the winter progresses these features descend into the stratosphere, until SSW conditions are reached. We find similar behaviour in two other years leading to delayed dynamical disruptions later in the spring. The timing and magnitude of the SAO and the extent of the upper stratospheric easterly QBO signal, that results in the SAO-QBO interaction, was found to be unique in these years, when compared to the years with a similar QBO phase. We propose that this early winter behaviour may be a key physical process in decelerating the mesospheric winds which may precondition the Southern atmosphere for a SSW. Thus the early winter equatorial upper stratosphere-mesosphere together with the polar mesosphere may provide critical early clues to an imminent SH SSW.

Viktoria J. Nordström and Annika Seppälä

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1237', Anonymous Referee #4, 23 Jan 2021
  • RC2: 'Comment on acp-2020-1237', Anonymous Referee #3, 01 Feb 2021
  • RC3: 'Comment on acp-2020-1237', Anonymous Referee #1, 05 Feb 2021
  • RC4: 'Comment on acp-2020-1237', Anonymous Referee #2, 08 Feb 2021

Viktoria J. Nordström and Annika Seppälä

Viktoria J. Nordström and Annika Seppälä

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Short summary
The winter winds over Antarctica form a stable vortex. However, in 2019 the vortex was disrupted and the temperature in the polar stratosphere rose by 50 °C. This event, called a sudden stratospheric warming, was only the second to occur over the south pole, the other was in 2002. The 2019 reoccurrence helps us to unravel its causes, which remain largely unknown. We have discovered a unique behaviour of the equatorial winds in 2002 and 2019 which may signal an impending SH SSW.
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