Importance of the Stratosphere for North Atlantic Climate: A Model Study.

Haase, Sabine (2014) Importance of the Stratosphere for North Atlantic Climate: A Model Study. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 97 pp.

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Abstract

Anomalies of the winter stratospheric polar vortex can propagate down to tropospheric levels and modulate variability patterns, such as the North Atlantic Oscillation (NAO), which is the leading mode of variability in the North Atlantic (NA) sector during boreal winter. Not only is the NAO important for European winter weather conditions, but the NAO related heat and freshwater fluxes, and the associated changes in westerly wind over the NA region, also influence the formation of deep water masses in the NA basin and can thereby influence the variability of the Atlantic meridional overturning circulation (AMOC). The northward transport of heat by the AMOC is very important for European climate and the variability of the AMOC is therefore of great interest. To investigate the role of the stratosphere for variability over the North Atlantic sector, two state-of-the-art ocean-atmosphere general circulation models are used: a high-top model (CESM1(WACCM)) and a low-top model (CCSM4). For each model, a Control simulation is analyzed and compared to a simulation under the Intergovernmental Panel on Climate Change (IPCC)’s RCP8.5 scenario, which represents the worst case scenario of greehouse gas (GHG) emissions. Strong and weak vortex events are defined using the Northern Annular Mode (NAM), which is also used to describe the downward propagation of these anomalies. In the low-top model the downward propagation of stratospheric NAM anomalies to the surface is not well captured, but it is very well represented in the high-top model. This simulated difference in stratosphere-troposphere coupling is also reflected in the simulated effects of the stratosphere on the surface atmosphere and ocean parameters. While stratospheric vortex events in the high-top model are connected to NAO-like anomalies at the surface (in sea level pressure, turbulent heat flux and surface wind stress), in the low-top model this connection is less pronounced. No significant changes in mixed layer depth (MLD), which is used as an indicator for deep water formation, are found in the low-top model. The high-top model, on the other hand, shows a strong connection between stratospheric polar vortex events and MLD anomalies (strong (weak) vortex events are connected to deeper (shallower) MLDs), especially in the Labrador Sea, which is an important area of deep water formation in the NA region. A cross-correlation analysis of the NAM/NAO and AMOC shows that the NAO leads the AMOC by about 4 years in both, the high and low-top model Control simulations. While the stratospheric NAM is also highly correlated with the AMOC in the high-top model (peaking when the NAM leads the AMOC by 2 years), there is no resonable correlation between NAM and AMOC in the low-top model. Under global warming the correlation between the AMOC and NAO decreases for both models. In the case of the high-top model, the NAM and AMOC are more strongly correlated than the NAO and AMOC under the GHG scenario.

Document Type: Thesis (Master thesis)
Thesis Advisor: Matthes, Katja and Latif, Mojib
Subjects: Course of study: MSc Climate Physics
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-ME Maritime Meteorology
Open Access Journal?: Yes
Date Deposited: 26 Jun 2014 07:17
Last Modified: 28 Jul 2023 11:36
URI: https://oceanrep.geomar.de/id/eprint/24964

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