Extreme air-sea interactions over the Gulf Stream

The ocean carries more heat poleward than the atmosphere at low latitudes, whilst the reverse occurs at high latitudes. In the Northern Hemisphere, the largest ocean-atmosphere heat fluxes occur over the Gulf Stream, suggesting that an ocean-atmosphere “relay” is active at mid-latitudes. This thesis is concerned with the significance of the extremes in air-sea heat fluxes over the Gulf Stream. In the first research chapter, the direct interaction between the ocean and the atmosphere is examined in the ERA-Interim dataset. Based on Lagrangian trajectory calculations, the most extreme air-sea heat flux events are found to be associated entirely with air of continental origin. The subsequent heat gain in the overlying air is caused almost completely by surface heat fluxes. For average air-sea heat fluxes, the associated air is both continental and maritime in origin, with a noticeable contribution to the heat content of the air parcels from entrainment at the top of the boundary layer. The second research chapter determines the causes for variations in surface heat flux in the ERA-Interim dataset. Roughly 90% of the time, one observes a baroclinic waveguide of varying strength over the Gulf Stream, setting the intensity of the air-sea heat exchange and the mean state in precipitation and tropospheric wind divergence. A potential mechanism whereby a change in sea-surface temperature gradient could cause an alteration of these mean patterns is discussed. Finally, the link between sea-surface temperature gradients and atmospheric fronts is explored in model simulations. A smoothing in the sea-surface temperature gradient is found to broadly reduce front intensity over the Gulf Stream. Increases in front intensity are shown to be consistent with a thermal damping mechanism. A significant effect is also observed on the regional precipitation and tropospheric vertical velocity, as well as on the direction of frontal propagation.Open Acces