The Extratropical Transition of Tropical Cyclones. Part II: Interaction with the Midlatitude Flow, Downstream Impacts, and Implications for Predictability

The article of record as published may be found at http://dx.doi.org/10.1175/MWR-D-17-0329.1 This review was partly initiated at the World Meteorological Organization’s Eighth International Workshop on Tropical Cyclones in 2014. The extratropical transition (ET) of tropical cyclones often has an important impact on the nature and predictability of the midlatitude flow. This review synthesizes the current understanding of the dynamical and physical processes that govern this impact and highlights the relationship of downstream development during ET to highimpact weather, with a focus on downstreamregions. It updates a previous review from2003 and identifies new and emerging challenges and future research needs. First, the mechanisms through which the transitioning cyclone impacts the midlatitude flow in its immediate vicinity are discussed. This ‘‘direct impact’’manifests in the formation of a jet streak and the amplification of a ridge directly downstream of the cyclone. This initial flow modification triggers or amplifies amidlatitude Rossby wave packet,which disperses the impact ofETinto downstream regions (downstream impact) and may contribute to the formation of high-impact weather. Details are provided concerning the impact of ET on forecast uncertainty in downstream regions and on the impact of observations on forecast skill. The sources and characteristics of the following key features and processes thatmay determine the manifestation of the impact of ET on the midlatitude flow are discussed: the upper-tropospheric divergent outflow, mainly associated with latent heat release in the troposphere below, and the phasing between the transitioning cyclone and the midlatitude wave pattern. Improving the representation of diabatic processes during ET in models and a climatological assessment of the ET’s impact on downstream high-impact weather are examples for future research directions. German Science Foundation (DFG) Swiss National Science Foundation (SNSF) PZ00P2_148177/1 Helmholtz Association VH-NG-1243 Transregional Collaborative Research Center SFB/TRR 165 NSF AGS-1240502 NSF AGS- 1355960 NRL Base Program PE 0601153N ONR PE 0602435N Australian Research Council Centre of Excellence CE110001028 NSF ATM-1461753 ONR N000140910526