Although prior studies have established that the extratropical flow pattern often amplifies downstream of recurving tropical cyclones (TCs), the extratropical flow response to recurving TCs has not to the authors' knowledge been systematically examined from a climatological perspective. In this study, a climatology of the extratropical flow response to recurving western North Pacific TCs is constructed from 292 cases of TC recurvature during 1979-2009. The extratropical flow response to TC recurvature is evaluated based on a timelagged composite time series of an index of the North Pacific meridional flow surrounding TC recurvature. Similar time series are constructed for recurving TCs stratified by characteristics of the large-scale flow pattern, the TC, and the phasing between the TC and the extratropical flow to assess factors influencing the extratropical flow response to TC recurvature. Results reveal that following TC recurvature, significantly amplified flow develops over the North Pacific and persists for ;4 days. The tendency for significantly amplified North Pacific flow to develop following TC recurvature is sensitive to the strength of the TC-extratropical flow interaction (the phasing between the TC and the extratropical flow), which is based on the negative potential vorticity advection by the divergent outflow of the TC. In contrast, the tendency for significantly amplified North Pacific flow to develop following TC recurvature is relatively insensitive to the intensity or size of the recurving TC, or whether it subsequently reintensifies after becoming extratropical.

Extratropical transition (ET) is the process by which a tropical cyclone, upon encountering a baroclinic environment and reduced sea surface temperature at higher latitudes, transforms into an extratropical cyclone. This process is influenced by, and influences, phenomena from the tropics to the midlatitudes and from the meso- to the planetary scales to extents that vary between individual events. Motivated in part by recent high-impact and/or extensively observed events such as North Atlantic Hurricane Sandy in 2012 and western North Pacific Typhoon Sinlaku in 2008, this review details advances in understanding and predicting ET since the publication of an earlier review in 2003. Methods for diagnosing ET in reanalysis, observational, and model-forecast datasets are discussed. New climatologies for the eastern North Pacific and southwest Indian Oceans are presented alongside updates to western North Pacific and North Atlantic Ocean climatologies. Advances in understanding and, in some cases, modeling the direct impacts of ET-related wind, waves, and precipitation are noted. Improved understanding of structural evolution throughout the transformation stage of ET fostered in large part by novel aircraft observations collected in several recent ET events is highlighted. Predictive skill for operational and numerical model ET-related forecasts is discussed along with environmental factors influencing posttransition cyclone structure and evolution. Operational ET forecast and analysis practices and challenges are detailed. In particular, some challenges of effective hazard communication for the evolving threats posed by a tropical cyclone during and after transition are introduced. This review concludes with recommendations for future work to further improve understanding, forecasts, and hazard communication.

This study investigates the composite extratropical flow response to recurving western North Pacific tropical cyclones (WNP TCs), and the dependence of this response on the strength of the TC-extratropical flow interaction as defined by the negative potential vorticity advection (PV) by the irrotational wind associated with the TC. The 2.58 NCEP-NCAR reanalysis is used to construct composite analyses of all 1979-2009 recurving WNP TCs and of subsets that undergo strong and weak TC-extratropical flow interactions.Findings indicate that recurving WNP TCs are associated with the amplification of a preexisting Rossby wave train (RWT) that disperses downstream and modifies the large-scale flow pattern over North America. This RWT affects approximately 2408 of longitude and persists for approximately 10 days. Recurving TCs associated with strong TC-extratropical flow interactions are associated with a stronger extratropical flow response than those associated with weak TC-extratropical flow interactions. Compared with weak interactions, strong interactions feature a more distinct upstream trough, stronger and broader divergent outflow associated with stronger midlevel frontogenesis and forcing for ascent over and northeast of the TC, and stronger upper-level PV frontogenesis that promotes more pronounced jet streak intensification. During strong interactions, divergent outflow helps anchor and amplify a downstream ridge, thereby amplifying a preexisting RWT from Asia that disperses downstream to North America. In contrast, during weak interactions, divergent outflow weakly amplifies a downstream ridge, such that a RWT briefly amplifies in situ before dissipating over the western-central North Pacific.

Recurving tropical cyclones (TCs) undergoing extratropical transition (ET) may substantially modify the large-scale midlatitude flow pattern. This study highlights the role of diabatic outflow in midlatitude flow amplification within the context of a review of the physical and dynamical processes involved in ET. Composite fields of 12 western North Pacific ET cases are used as initial and boundary conditions for high-resolution numerical simulations of the North Pacific-North American sector with and without the TC present. It is demonstrated that a three-stage sequence of diabatic outflow associated with different weather systems is involved in triggering a highly amplified midlatitude flow pattern: 1) preconditioning by a predecessor rain event (PRE), 2) TC-extratropical flow interaction, and 3) downstream flow amplification by a downstream warm conveyor belt (WCB). An ensemble of perturbed simulations demonstrates the robustness of these stages. Beyond earlier studies investigating PREs, recurving TCs, and WCBs individually, here the fact that each impacts the midlatitude flow through a similar sequence of processes surrounding ET is highlighted. Latent heat release in rapidly ascending air leads to a net transport of low-PV air into the upper troposphere. Negative PV advection by the diabatically driven outflow initiates ridge building, accelerates and anchors a midlatitude jet streak, and overall amplifies the upper-level Rossby wave pattern. However, the three weather systems markedly differ in terms of the character of diabatic heating and associated outflow height, with the TC outflow reaching highest and the downstream WCB outflow producing the strongest negative PV anomaly.

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.