The impact of extratropical transition on the downstream flow: An idealized modelling study with a straight jet

The interaction of a tropical cyclone undergoing extratropical transition (ET) with the midlatitude synoptic-scale flow is investigated using full-physics numerical experiments with idealized initial conditions. The emphasis is on the impact on the midlatitude flow downstream of the ET event. The midlatitude flow is represented by a balanced straight jet stream. As the tropical cyclone approaches the jet, a ridge-trough couplet and a distinct jet streak form in the upper-level flow. A midlatitude cyclone develops rapidly downstream of the ET system and the further evolution is characterized by downstream baroclinic development. Based on Hovmöller diagrams, the upper-level development is interpreted as the excitation and subsequent dispersion of a Rossby wave train on the potential vorticity gradient associated with the jet. The characteristics of this wave train are sensitive to the structure of the jet and to moist processes in the midlatitudes. The tropical cyclone undergoing ET acts as a sustained forcing for the wave train and the structure of the ET system impacts the development most significantly one to two wavelengths downstream of ET. Piecewise inversion of potential vorticity, complemented by the partitioning of the flow into its rotational and divergent parts, is applied to assess the impact of the ET system quantitatively. Both the cyclonic circulation and the outflow of the tropical cyclone are important contributors to the formation and amplification of the ridge-trough couplet. The outflow anomaly reduces the eastward motion of the ridge-trough couplet significantly and thus promotes phase-locking between the tropical cyclone and the upper-level pattern.