Stochastic and dynamical models of sudden stratospheric warmings

• Main Author: Mester, Márton
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UCL (University College London) 2021
• Subjects: Antarctic; The Antarctic; Antarc*
• Online Access: https://discovery.ucl.ac.uk/id/eprint/10136552/1/Thesis_MM.pdf; https://discovery.ucl.ac.uk/id/eprint/10136552/

Stochastic and dynamical models are used to study resonantly excited vortex splitting sudden stratospheric warmings. Stochasticity is introduced to Kida’s vortex model in Chapter 2 to investigate the role of noise, associated to tropospheric macroturbulence, and it is illustrated that an anomalously large wave event is not necessary to trigger a sudden warming, but the history of the forcing is important, pre-conditioning the vortex into a state where it may split. In Chapter 3 a variational data assimilation method is introduced, allowing observed polar vortex evolutions to be associated with a realization of a stochastic Kida vortex from Chapter 2. The method is applied on the Antarctic polar vortex from reanalysis data, revealing key information about the stationary and unsteady parts of tropospheric wave forcing, and suggesting that the 2002 Antarctic sudden warming event was triggered by noise-induced resonance. In Chapter 4, the variational data assimilation method is applied on extrater- restrial planetary-scale vortices, including the Martian Northern Hemisphere Polar Vortex. It is demonstrated that the stationary part of the wave-forcing experienced by the Martian vortex is due to the wave-forcing generated by topography, and high ellipticity events of the Martian vortex, which are not high enough for the vortex to split, are linked to a change in the equatorial winds. It is also illustrated, that neither the topographic forcing is strong enough nor the state of the atmosphere is optimal in terms of resonance to trigger vortex splits on the Northern Hemisphere of Mars. In Chapter 5, idealized model experiments are performed to provide a link between resonant vortex states and extreme events and characterize the signals of resonance in the stratosphere. The dominant signal suggests that the equatorial stratosphere plays a key role in resonant excitation of the vortex.