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

Abstract

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 high-impact weather, with a focus on downstream regions. It updates a previous review from 2003 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 a midlatitude Rossby wave packet, which disperses the impact of ET into 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 that may 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.

Downstream influence of mesoscale convective systems. Part 1: influence on forecast evolution

“…Keller et al . () present a review of these studies and reasons for the reduced forecast predictability. The direct impact of the transitioning tropical cyclone on the mid‐latitude flow is described as the formation of a jet streak through advection of the negative PV anomaly by the divergent outflow, and amplification of a downstream ridge.…”

“…MCSs are not the only intensely-convective weather events that impact Rossby wave structure and have been associated with reduced medium-range forecast predictability and skill -many studies have examined the impact of tropical cyclones undergoing extratropical transition. Keller et al (2019) present a review of these studies and reasons for the reduced forecast predictability. The direct impact of the transitioning tropical cyclone on the mid-latitude flow is described as the formation of a jet streak through advection of the negative PV anomaly by the divergent outflow, and amplification of a downstream ridge.…”

The extratropical transition of Hurricane Ophelia (2017) as diagnosed with a generalized omega equation and vorticity equation

“…These post-tropical storms can cause severe damage by themselves (e.g. Browning et al, 1998;Thorncroft and Jones, 2000;Feser et al, 2015), or downstream via amplification of the mid-latitude flow (Grams and Blumer, 2015;Keller et al, 2018). Moreover, due to the warming of climate, post-tropical storms impacting Europe are projected to become more frequent (Haarsma et al, 2013;Baatsen et al, 2015;Liu et al, 2017).…”

The Extratropical Transition of Hurricane Debby (1982) and the Subsequent Development of an Intense Windstorm over Finland

“…of Keller et al (2019) for an overview]. This typically manifests as ridge building poleward and downstream of the transitioning tropical cyclone, downstream trough amplification, and jet streak modifications.…”

“…Furthermore, the impact of the transition of an extratropical cyclone on the midlatitude flow can also be transmitted farther downstream by modifying Rossby wave packets and leading to the development of new cyclones and anticyclones downstream (Riemer et al 2008;Archambault et al 2013). As an example, Supertyphoon Nuri (2014) amplified the downstream ridge-trough couplet that then resulted in a new cyclone developing farther downstream, amplifying the upper-level wave pattern even more and resulting in a heat wave and a cold air outbreak far downstream from the transitioned ex-Nuri (Keller et al 2019).…”