Group seminar on 10. October, 10:15 CET

Master Thesis Defence: The influence of the sea surface temperature gradient in the Gulf Stream region on cyclone development, warm conveyor belt ascent, and downstream ridge formation during the European winter 'heat wave' in February 2019

Svenja Christ

An atmospheric blocking event in February 2019 facilitated record-breaking winter heat in western Europe with anomalously high temperatures. The blocking was preceded and accompanied by several, rapidly intensifying extratropical cyclones originating in the Gulf Stream region and crossing the North Atlantic. Those cyclones and their associated warm conveyor belts (WCBS) were important for the development of a downstream upper-level ridge and the blocking event. WCBs are fast ascending air streams and essentially contribute to ridge amplification by divergent outflow and upper-level potential vorticity anomalies. These WCBs gained moisture during a cold air outbreak located over the Gulf Stream region associated with large sea surface temperature (SST) gradients and strong air-sea interactions.   The primary hypothesis of this master thesis is that air-sea interactions in the Gulf Stream region can influence the formation of the anticyclonic blocking in February 2019. To evaluate the impact of air-sea interactions on cyclone development, WCB ascent, and downstream ridge formation, we run five high-resolution global ICON simulations with artificially idealized, weakened, and warmed SST gradients in the Gulf Stream region. To address the impact of SST and SST gradient on WCBs, forward trajectories from the Gulf Stream region are calculated using LAGRANTO. Using an Eulerian and Lagrangian perspective, our results demonstrate that the SST gradient in the Gulf Stream region indeed influences air-sea interactions and, in addition, impacts moisture availability and air temperature in the WCB inflow region as well as, low-level baroclinicity. The modified environment where extratropical cyclones intensify presumably influences cyclone depending rates. Moreover, the amount of detected WCB trajectories reflects the intensity of the cyclones, i.e., more WCB trajectories and deeper cyclones are found in the experiments with warmed SST in the Gulf Stream region where an increased specific humidity and warmer air temperatures are observed. In contrast, decreased WCB trajectory numbers and weaker cyclones are found in the experiment with weaker SST gradients in the Gulf Stream region. Results suggest moisture availability in the WCB inflow region is essential in regulating cyclone intensification and diabatic heating by cloud formation. Diabatic heating, along with the associated cross-isentropic WCB ascent, determines the outflow characteristics of WCBs and their capacity to sustain the downstream ridge. With the results of this master's thesis, the pathway through which the SST in the Gulf Stream region is linked to the downstream upper-level flow evolution in the North Atlantic is demonstrated.