The Importance of Natural and Anthropogenic Factors for the Coupling Between the Stratosphere and the Troposphere

• Main Author: Hansen, Felicitas K.
• Format: Thesis
• Language: English
• Published: 2015
• Subjects: polar night
• Online Access: https://oceanrep.geomar.de/id/eprint/30995/; https://oceanrep.geomar.de/id/eprint/30995/1/thesis_druckreif.pdf

This thesis describes several new aspects of the dynamical coupling between the stratosphere and the troposphere, and how this coupling is influenced by different natural and anthropogenic factors. A unique set of four long-term sensitivity experiments is designed to examine the importance of the Quasi-Biennial Oscillation (QBO) of equatorial stratospheric winds, modes of sea surface temperature (SST) variability like the El Ni ̃no Southern Oscillation (ENSO), anthropogenic greenhouse gases (GHGs) and ozone depleting substances (ODSs), for stratosphere-troposphere coupling. The model experiments are performed with NCAR’s CESM model with the chemistry-climate model WACCM as its atmospheric component. The number and the length of the simulations performed here, given that the model includes both an interactive ocean and an interactive chemistry module and reaches up to the thermosphere, are exceptional. Special emphasis is placed on major Stratospheric Sudden Warmings (SSWs) in the Northern hemisphere which are a prominent example of stratosphere-troposphere coupling and which can affect surface weather and climate. It is shown that the QBO strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the major SSW frequency by reducing the propagation of planetary waves into the PNJ region. Variability in SSTs weakens the PNJ and significantly increases the major SSW frequency by enhancing planetary wave forcing. Extreme climate change conditions determine the prewarming phase of major SSWs. SST variability is needed to reproduce the observed tropospheric negative Northern Annular Mode pattern after major SSWs. Further testing the sensitivity of WACCM experiments to the width of the QBO relaxation along the equator leads to a new contribution to the famous Holton-Tan mechanism in stratospheric dynamics and chemistry. The Holton-Tan mechanism, i.e., stronger zonal mean winds during QBO west phases (QBOW) in the PNJ region, is enhanced for a wider compared to a narrower QBO ...