Upper-Level Troughs, Jet Streaks, and TCs: An Objective Climatology and Features Associated with Rapid TC Intensity Change

Upper tropospheric disturbances on sub-synoptic to synoptic scales have been shown to influence
tropical cyclone (TC) intensity through a multitude of physical processes (e.g. Molinari and Vollaro
1989; Bosart et al. 2000; Hanley et al. 2001; Leroux et al. 2015; Peirano et al. 2016; Fischer et al.
2017). These processes include vertical wind shear and coupling of the TC secondary circulation to
environmental departures from thermal wind balance. Such imbalances can be caused by eddy
momentum fluxes into the TC vortex, environmental vorticity advection, or jet streaks. The general
concurrency of these processes makes establishing causal relationships difficult, and numerous
questions about the nature of these relationships remain.

Here, interactions of TCs with upper tropospheric potential vorticity (PV) anomalies during 1979-2015
are identified and analyzed in the context of TC intensity change. This is done independently of
historically-used eddy momentum flux thresholds for identifying TC-trough interactions, in order to
broaden the scope of analyzed flow configurations. Given the large spatial variability in climatological
flow and TC development states across ocean basins, an objective, statistical estimate of TC intensity
change climatology is developed in order to more accurately assess TC responses to these interactions
relative to typical circumstances. This also allows the mean impact of upper-level troughs and
disturbances on TCs to be explored.

Jet streaks, prevalent features near TCs in such flow configurations, are objectively identified,
structurally quantified, and examined within the broader geometric evolution of TC-trough interactions.
Dynamic forcing for ascent is typically focused on the anticyclonic shear side of the entrance regions of
such jet streaks, especially when they are anticyclonically curved (Moore and VanKnowe 1992). These
jet streaks can cooperatively interact with TCs during extratropical transition (e.g. Archambault et al.
2013; Griffin and Bosart 2014), but may do so during the TC phase as well (e.g. Hurricane Opal
[Bosart et al. 2000]). Given that jet streaks are frequently associated with upper-level troughs near TCs,
they are important to consider within the framework of TC-trough interactions. To the author's
knowledge, jets near TCs have not previously been systematically identified and analyzed in this
context.