Abstract
This work investigates the effect of the Gulf Stream on wave breaks in the mid-latitude jet stream, commonly known as atmospheric blocking. Firstly, the connection between atmospheric blocking over the North Atlantic and the diabatic influence of the Gulf Stream is explored using potential vorticity diagnostics from the ERA5 reanalysis data set during winter (1979-2020). Consistent with previous research, the findings highlight the significant role of turbulent heat fluxes over the Gulf Stream and its extension in the induction and maintenance of atmospheric blocks. These heat fluxes generate negative potential vorticity (PV) air masses in the atmospheric boundary layer, which then ascend through the warm conveyor belt, contributing to the block’s negative PV anomaly at upper levels. The block’s size and frequency are shown to be partially dependent on oceanic preconditioning via anomalous ocean heat transport and heat content before the blocking event, both of which enhance turbulent heat fluxes.
Building on this, the impact of oceanic moisture fluxes on atmospheric blocks using the ECMWF IFS model were tested. Artificial suppression of surface latent heat flux over the Gulf Stream region results in a reduction in blocking frequency across the Northern Hemisphere by up to 30%. Affected blocks exhibit a shorter lifespan (-6%), smaller spatial extent (-10%), and reduced intensity (-0.4%), while the number of individual blocking anticyclones increases (+17%). The analysis shows a consistent response across all model resolutions, with Tco639 (~18 km) showing the largest statistically significant changes in blocking characteristics. Exploring the broader Rossby wave pattern, it is observed that diminished moisture fluxes favour eastward propagation and higher zonal wavenumbers, while air‐sea interactions promote stationary and westward‐propagating waves with zonal wavenumber 3.
Finally a simple heuristic three-box blocking model was derived to mimic this phenomenon. This model consists of an oceanic mixed layer, coupled to an atmospheric boundary layer through air-sea heat fluxes, which is further coupled to the upper troposphere via convection. The model mimics the real-world oceanic pathway to atmospheric blocking, displaying positive heat anomalies in the ocean preceding a block and negative anomalies afterward. The minimum upper-level PV anomaly was directly correlated with the minimum PV anomaly in the atmospheric boundary layer and the maximum ocean temperature prior to the blocking event. However, neither of the lower domain extrema exhibited a clear relationship with the upper-level PV anomaly after the blocking event, suggesting that the causal signal primarily propagates upwards. Modifications to the control parameters resulted in a Hopf bifurcation, followed by a chaotic regime. Further changes to the control parameters led to a generally increasing Lyapunov exponent, indicating reduced predictability.
These results underscore the importance of understanding the interaction between oceanic and atmospheric processes from a coupled perspective, as neglecting this could result in overlooking emergent properties of the system as a whole.