Publication Date:
2022-10-25
Description:
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 relaxation. The results suggest that at least two processes are involved in transmitting the equatorial QBO signal into the polar stratosphere: first, an effect of the zero wind line in the lower stratosphere which is shifted depending on the phase of the QBO, and second, the effect of the secondary QBO circulation in the middle to upper stratosphere. Both processes affect the direction of planetary wave propagation so that these waves disturb the stratospheric polar vortex more during QBO east phases (QBOE). The first study which investigates a combined QBO-ENSO influence on the troposphere shows that large differences occur between the North Pacific and North Atlantic. The stratospheric equatorial QBO anomalies extend down to the troposphere over the North Pacific during boreal winter, but only during La Ni ̃na (not El Ni ̃no) events. The conditions for the genesis and intensification of synoptic-scale waves are improved during QBOW compared to QBOE conditions by linear QBO-ENSO interactions. In the North Atlantic, the non-linear interaction of QBOW with La Nina (QBOE with El Ni ̃no) results in a positive (negative) North Atlantic Oscillation pattern. This thesis presents an improved understanding of the physical mechanisms that couple the stratosphere and the troposphere, which has the potential to improve tropospheric weather forecasting skill and moreover highlights the importance of the stratosphere for understanding and modeling tropospheric dynamics.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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