Mechanisms of the Internally Generated Decadal-to-Multidecadal Variability in the Atlantic

Abstract

The thesis investigates the physical question of mechanisms of the Atlantic multidecadal variability (AMV) in a new way, using the weather noise forced interactive ensemble. It includes two distinct but closely related components. 1) Verification of the interactive ensemble strategy and justification for the noise forcing. In order to separate the noise from the SST forced response, the SST forced response in the atmospheric GCM (AGCM), forced by the SST from the coupled GCM (CGCM), has to be the same as in the CGCM. To be consistent, the noise should also be the same statistically in the CGCM and AGCM. Comparison of the CGCM and AGCM ensemble shows that these conditions are satisfied. Therefore, the interactive ensemble is an appropriate tool for the investigation, and the "noise" that is diagnosed and used as forcing is appropriate. Our results apply not just to the interactive ensemble, but also have broader implications important for the design of a wide range of climate modeling experiments. 2) Diagnosis of the multidecadal variability in the CGCM simulation. The diagnosis is done using the interactive ensemble CGCM, in which the heat flux, wind stress and fresh water flux weather noise components are applied at the ocean surface in different regions and in different combinations. The interactive ensemble simulations prove that the model's climate variability is predominantly forced by weather noise. The local weather noise forcing is found to be responsible for the SST variability in the Atlantic, with the noise heat flux and noise wind stress playing a critical role, while the noise fresh water flux has negligible impact. The North Atlantic Oscillation (NAO) pattern in the atmosphere, dominated by weather noise, forces the AMV 45-year mode through the noise heat flux and noise wind stress, with the former important in the eastern North Atlantic and the latter along the separated Gulf Stream. The noise wind stress forces the AMV 45-year mode through ocean dynamics, including Rossby waves, ocean gyres and the Atlantic Meridional Overturning Circulation. The atmospheric response to SST, including the SST-forced heat flux and SST-forced wind stress, acts as a damping on the AMV 45-year mode.