Interannual North Atlantic Sea surface height dynamics and associated predictability

• Main Author: Fraser, Robert
• Other Authors: Wilson, Chris ; Palmer, Matthew ; Zanna, Laure
• Published: University of Oxford 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.757853

A signicant degree of uncertainty is associated with interannual sea surface height (SSH) forecasts. This thesis investigates the physical origins of this uncertainty, by examining the contributions of atmospherically modulated and ocean intrinsic processes to interannual SSH dynamics and its associated interannual predictability. The analysis is based on several numerical simulations at eddy-permitting resolution, ranging in complexity from barotropic and baroclinic models, with idealised geometries and forcings, to a state-of-the-art coupled climate model. Using the output of these various simulations, I diagnosed the relevant dynamical responses of sea level variability and statistically estimated the associated predictability timescales. The three main results can be summarized as follows. (1) New insights into the mechanisms by which eddy-fluxes of momentum and buoyancy, in mid-latitude jet regions, act to modulate interannual intrinsic ocean variability. This includes the identication of interannual equatoward migrations of anomalous baroclinic eddy-driven jets arising due to a meridional asymmetry in the Eady growth rate. (2) In idealised models, the components of SSH variability associated with the intrinsic dynamical mechanisms related to eddy momentum fluxes only display predictability on sub-annual timescales. However, the aforementioned buoyancy-driven mechanism introduces multi-year predictability. (3) In the coupled climate model, the dynamical response to external forcing, modulated by modes of atmospheric variability, is shown to be predictable on interannual timescales (1-2 years) and contributes to predictable SSH signals along the US east coast and in the vicinity of the Gulf Stream. This demonstrates that the chaotic variability introduced by mesoscale eddies does not completely obscure signals of interannual SSH predictability. These results highlight the importance of ocean-atmosphere interactions in interannual SSH forecasts, and demonstrate the relevance of mesoscale eddies in modulating interannual variability in mid-latitude ocean jets.