The Response of the Atlantic Meridional Overturning Circulation to heat flux forcing in the Kiel Climate Model

• Main Author: Reintges, Annika
• Format: Thesis
• Language: English
• Published: 2014
• Subjects: Course of study: MSc Climate Physics; North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://oceanrep.geomar.de/id/eprint/26597/; https://oceanrep.geomar.de/id/eprint/26597/1/master_thesis_areintges.pdf

The response of the Atlantic Meridional Overturning Circulation (AMOC) to heat flux forcing of the North Atlantic Oscillation (NAO) is analyzed. This is done with the Kiel Climate Model (KCM), a coupled ocean-atmosphere-sea ice model. The dynamical links of the atmosphere-ocean interaction in this experiment agree with other studies and theoretical considerations: In a positive (negative) NAO phase the enhanced (reduced) heat loss from the subpolar North Atlantic to the atmosphere leads to a deepening (shallowing) of the mixed layer, especially over the center of the subpolar gyre. This causes increased (decreased) convection and after 3 to 14 years a stronger (weaker) AMOC at 30°N. The NAO forcing has a slightly red spectrum. The AMOC, however, responds with lower frequencies. For the AMOC index at 30°N two modes of variability can be identified through Singular Spectrum Analysis (SSA): one with a period of 93 years and one with a period of 36 years. However, general differences of these two modes in the spatial structure of the overturning streamfunction cannot be identified. Though, the longer mode might be related to anomalies in the mixed layer depth and the strength of the subpolar gyre, which exhibit variability on similar time scales. To compare the conditions of an unforced simulation, a control experiment is also investigated. Here, the relation between the NAO and the AMOC is not as clear. The largest correlation is found when the AMOC 30°N index leads the NAO index by 1 year. Additionally, the temporal variability of the AMOC modes differs in this experiment. A low-frequency but unstable variability mode of about 120 years period is detected mainly for the first half of the simulation period. All other modes of the control experiment were interannual to decadal. This disagrees with a different control experiment version of the KCM, where a multi-centennial, a quasi-centennial, and a multi-decadal mode were dominant. Furthermore, one must bear in mind that the KCM sea surface temperatures of the North ...