The gulf stream as a modulator for atmospheric blocking

This work examines the influence of the Gulf Stream on wave breaks in the mid-latitude jet stream, known as atmospheric blocking. Chapter 3 explores the connection between North Atlantic blocking and the Gulf Stream’s diabatic effects using ERA5 reanalysis (1979-2020). Turbulent heat fluxes over the Gulf Stream generate negative potential vorticity (PV) in the atmospheric boundary layer, which ascends via the warm conveyor belt to strengthen the block’s upper-level PV anomaly. Blocking size and frequency are linked to oceanic preconditioning, with anomalous heat transport and content enhancing turbulent heat fluxes before blocking events. In Chapter 4, the ECMWF IFS model tests this theory by suppressing surface latent heat flux over the Gulf Stream. This reduces Northern Hemisphere blocking frequency by up to 30\%, with shorter lifespans (-6\%), smaller spatial extents (-10\%), and reduced intensities (-0.4\%), though the number of individual blocking anticyclones increases (+17\%). Responses are consistent across model resolutions, with Tco639 (~18 km) showing the largest changes. Diminished moisture fluxes favour eastward Rossby wave propagation and higher zonal wavenumbers, while air-sea interactions promote stationary, westward-propagating waves with zonal wavenumber 3. Chapter 5 abstracts these interactions by deriving a simple heuristic three-box blocking model which consists of an oceanic mixed layer coupled to the atmospheric boundary layer through air-sea heat fluxes, which is further coupled to the upper troposphere via convection. The model reproduces key features of blocking, including positive ocean heat anomalies before a block and negative anomalies afterward. Upper-level PV anomalies correlate with boundary-layer PV and ocean temperature prior to the event, suggesting upward causal propagation. Parameter modifications reveal a Hopf bifurcation and chaotic regime, leading to a generally increasing Lyapunov exponent, indicating reduced predictability. These findings highlight the importance of coupled ocean-atmosphere interactions, as neglecting them risks overlooking critical system dynamics and emergent properties.