Air-sea interactions and diabatic processes in the Gulf Stream region and their role in the life-cycle of a blocking anticyclone: a case study of European Blocking in Feb 2019.

The variability of the large-scale atmospheric circulation over the North Atlantic is governed by persistent and recurrent flow patterns, so-called weather regimes. Weather regimes describe preferential quasi-stationary states of the atmosphere that are characterised by persistent configurations of the large-scale flow patterns and the occurrence of synoptic weather systems in specific parts of the North Atlantic and western Europe. Increasing evidence suggests that the key processes responsible for the onset and persistence of such weather regimes are 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. However, how air mass transformations over the ocean, and the Gulf Stream, in particular, affect the large-scale flow and their role in the development of specific weather regimes are not fully understood.

Here, we focus on an episode of European blocking - a weather regime associated with an anticyclone over the British Isles - in the period between 20 and 27 of February 2019, which was accompanied by a record-breaking warm spell bringing temperatures above 20°C to the United Kingdom, Netherlands, and France. This particular event has been chosen due to its strong connection with air-sea interactions over the Gulf Stream region. Specifically, the formation of the anticyclone was preceded by several, rapidly intensifying cyclones originating in the Gulf Stream region and traversing the North Atlantic. These cyclones contributed to the establishment of the anticyclone over Europe by injection of low potential vorticity air into the upper troposphere. In addition, they were accompanied by multiple CAOs over the area of the strong sea surface temperature (SST) gradient associated with the Gulf Stream, resulting in intense upward fluxes of sensible and latent heat.  In order to quantify the dynamical linkage between the formation of this block and air-sea interactions over the Gulf Stream SST front, we adopt a Lagrangian perspective, using backward and forward kinematic trajectories to study the pathways of air masses forming the upper-level potential vorticity anomaly and interacting with the ocean front. The detailed investigation of the evolution of potential temperature, moisture and other variables along the trajectories, as well as of surface fluxes, SST and SST gradient underneath the trajectory path is carried out to examine the nature of the processes involved in upper-tropospheric flow variability. Determining the exact geographical location of moisture uptakes as well as their environment allows us to link air-sea interaction processes and the dynamical evolution of the flow. Thereby, we address the hypothesis that air-sea interaction processes along the Gulf Stream front, in particular during CAOs, is of fundamental importance for the maintenance of favourable conditions for cyclone intensification and the formation of European blocking.