On the Dynamics of Southern Hemisphere Sudden Stratospheric Warmings

• Main Author: Nordstrom, Viktoria Jody
• Other Authors: Annika , Seppala
• Format: Thesis
• Language: English
• Published: University of Otago 2021
• Subjects: Sudden; stratospheric; warming; 2019; Antarctica; Polar; Physics; Antarctic; Arctic; Merra; The Antarctic; Antarc*
• Online Access: http://hdl.handle.net/10523/12373

Above both poles harsh temperature gradients fuel strong westerlies. These winds form a vortex, which will breakdown in spring. During sudden stratospheric warmings (SSW), however, the polar vortex is dramatically disrupted earlier than expected. The Arctic experiences one SSW every two years, however, the Antarctic has only had two. The most recent Antarctic SSW occurred in 2019, here, temperatures in the stratosphere rose by 50 K, and the mesospheric winds reversed. The anomalies induced by the SSWs influence the neighbouring atmosphere. The 2019 SSW contributed to the dry spring conditions over Australia, and the ozone hole was extraordinarily small. Currently, SSWs are believed to occur as planetary waves interact with the polar atmospheric flow. Our understanding, however, remains incomplete. This thesis investigated the dynamics of the 2019 Antarctic SSW, using MERRA-2 reanalysis data, first locally around the pole, then extending towards the equator. Later analysis was expanded to consider the 2002 SH SSW and other years with similar equatorial flow. Using geopotenital height, zonal wind and potential vorticity, we found that the mesospheric vortex was displaced and weakened - well before the SSW onset date. This motivated us to peer into the upper atmosphere, and extend our analysis to the equator. We found an intersection between two equatorial atmospheric modes, the quasi-biennial oscillation (QBO) and the semiannual oscillation (SAO), during the SH winters of 2019 and 2002. In early winter the two modes merge over the equator, during their easterly phases. Together they form a zero-wind line that stretches from the lower stratosphere into the mesosphere. This influences the meridional wave guide, and easterly momentum is 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, 1988 and 2017, which have dynamical disruptions later in the spring. The timing and magnitude of the SAO-QBO intersection, in the years with SH SSW, was unique when compared to the years with a similar QBO phase. We theorise that this early winter behaviour could be a key physical processes that decelerates the mesospheric winds, which may precondition the Southern Hemisphere for a SSW. Hence, the early winter equatorial upper stratosphere-mesosphere together with the polar mesosphere may provide critical early clues to an imminent SH SSW - potentially extending the current predictability from ∼ 10 days to multiple months.