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

• Main Author: Haase, Sabine
• Format: Thesis
• Language: English
• Published: 2014
• Subjects: Course of study: MSc Climate Physics; North Atlantic; North Atlantic oscillation
• Online Access: https://oceanrep.geomar.de/id/eprint/24964/; https://oceanrep.geomar.de/id/eprint/24964/1/MA-SHaase.pdf

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 ...