Ocean-atmosphere interaction at the Gulf Stream sea surface temperature front: variability and impacts on midlatitude atmospheric circulation

Sea surface temperature (SST) gradients associated with western boundary currents affect the atmospheric circulation across a range of spatial and temporal scales.

Yet, several aspects of ocean-atmosphere interactions linked to oceanic fronts remain unclear.

This PhD thesis analyses such interactions for the Gulf Stream SST front (GSF).

The first part assesses the atmospheric response to the interannual GSF meridional shifts and its dependence on model horizontal resolution, using ERA5 reanalysis and atmosphere-only simulations forced by observed SST.

Results show that the response is strongly resolution dependent, with only simulations finer than 50km resembling observed anomalies.

Locally, diabatic heating near the GSF is mainly balanced by vertical motion and transient eddy heat transport.

At large-scale, the GSF shifts is associated with a homo-directional shift in the North Atlantic eddy-driven jet and storm track, mediated by changes in low-level baroclinicity.

The second part assesses the North Atlantic Oscillation (NAO)-GSF interaction and the mechanisms through which the NAO forces the GSF shifts on decadal timescale, using atmosphere and ocean reanalyses.

The NAO and GSF covary on decadal timescales only during 1972-2018.

This non-stationarity is also reflected in their lead-lag relationship: the NAO leads the GSF shifts by 3 years during 1972-1990 and by 2 years during 1990-2018.

The lag is interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the lagged response of deep oceanic circulation, and the propagation of Rossby waves.

However, Rossby wave propagation is evident only before 1990, suggesting that its non-stationarity may explain the different NAO-GSF time lag before and after 1990.

Overall, the thesis improves understanding of GSF variability and its role in North Atlantic and extratropical climate variability.