Linking Gulf Stream Air-Sea Interactions to the exceptional blocking episode in February 2019: A Lagrangian Perspective.

The development of atmospheric blocks over the North Atlantic European region can lead to extreme weather events like heatwaves or cold air outbreaks. Despite their potential severe impact on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction (NWP) models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air (cold air outbreaks, CAOs) from the Arctic over the North Atlantic, and associated air-sea interactions over the Gulf Stream are key processes responsible for the onset, maintenance, and persistence of such flow regimes. In order to establish how air mass transformations over the ocean, and in particular over the Gulf Stream, affect the large-scale flow, we focus on an episode between 20 and 27 of February 2019, when a quasi-stationary upper-level ridge established over western Europe accompanied by an intensified storm track in the Northwestern North Atlantic. During that time a record-breaking warm spell occurred over Western Europe bringing temperatures above 20 °C to the United Kingdom, the Netherlands, and Northern France. The event was preceded and accompanied by the development of several, rapidly intensifying cyclones originating in the Gulf Stream region and traversing the North Atlantic. To explore the mechanistic linkage between the formation of this block and air-sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using backward and forward kinematic trajectories. This allows us to study the pathways and transformations of air masses forming the upper-level potential vorticity anomaly and interacting with the ocean front. We establish that more than one-fifth of these air masses interact with the Gulf Stream in the lower troposphere, experiencing intense heating and moistening over the region, due to the frequent occurrence of CAOs behind the cold front of the cyclones. Trajectories moistened within the cold sector of one cyclone, later ascent into the upper troposphere with the ascending air stream of a consecutive cyclone, fueled by the strong surface fluxes. These findings highlight the importance of CAOs in the Gulf Stream region with their intense coupling between the ocean and atmosphere for blocking development, and provide a mechanistic pathway linking air-sea interactions in the lower troposphere and the upper-level flow. [ABSTRACT FROM AUTHOR]