Synoptic and mesoscale processes supporting vertical superposition of the polar and subtropical jets in two contrasting cases

Abstract

Observational studies have shown that the tropopause characteristically exhibits a three‐step Pole‐to‐Equator structure, with each break between steps in the tropopause height associated with a jet stream. While the two jet streams, the polar and subtropical jets, typically occupy different latitude bands, their separation can occasionally vanish, resulting in a relatively rare vertical superposition of the two jets. A cursory examination of several historical and recent high‐impact weather events over North America and the North Atlantic suggests that superposed jets are a component of their evolution. This study examines the processes that support the production of a polar/subtropical jet superposition during two such events; the 18–20 December 2009 Mid‐Atlantic Blizzard and the 1–3 May 2010 Nashville Flood. Given that ageostrophic transverse circulations and convection have both been shown to be capable of restructuring the tropopause within a single jet environment, the analysis focuses on the role these same processes play within the more complex double‐jet environment. The results demonstrate that ageostrophic transverse circulations play a primary role in the production of a superposition during the December 2009 case by placing subsidence, and a downward protrusion of high potential vorticity (PV) air, between the two jet cores, thereby contributing to the production of the single, steep tropopause wall characteristic of the superposed jet environment. Furthermore, convection fundamentally influences the existence and structure of the subtropical jet stream in both cases, through its associated latent heat release and irrotational outflow as well as the geostrophic adjustment process that responds to upper‐tropospheric mass deposition from convection on the anticyclonic shear side of the jet.

Rossby Wave Initiation by Recurving Tropical Cyclones in the Western North Pacific

“…Recurving TCs can act as a source of kinetic energy for the midlatitude circulation (Harr and Dea 2009;Hart 2011;Keller et al 2014) and promote meridional displacements of the jet stream axis (Klein et al 2002), especially during the process of extratropical transition (ET; e.g., Harr and Archambault 2016). Latent heat release in the region of the transitioning TC allows low-PV air to move from the lower to the upper troposphere (Grams et al 2013a;Winters and Martin 2016), thereby reducing static stability and PV at the equatorward side of the jet. This can promote frontogenesis at the level of the tropopause (Lang and Martin 2013).…”

Kinematic processes contributing to the intensification of anomalously strong North Atlantic jets

“…Finally, the steep, vertical tropopause structure in the Full Physics simulation resembles a polar–subtropical jet superposition event (e.g., Winters and Martin, 2014; 2016; 2017; Christenson et al ., 2017; Winters et al ., 2020a; 2020b). The stark contrast between the tropopause structure in the Full Physics and No LH simulations suggests that latent heating may be a process that discriminates between the formation of a jet superposition and a split jet.…”

“…The steeper slope in the Full Physics simulation can be attributed to stronger negative PV advection by V a in the vicinity of the tropopause in that simulation (compare Figure 14c,d with Figure 14g,h), as well as the diabatic erosion of upper‐tropospheric PV on the equatorward side of the jet due to latent heating. Notably, the steep slope of the tropopause in the Full Physics simulation resembles the structure of a polar–subtropical jet superposition event (e.g., Winters and Martin, 2014; 2016; 2017; Christenson et al ., 2017; Winters et al ., 2020a; 2020b). Therefore, the omission of latent heating may be a discriminating factor with respect to the formation of jet superpositions.…”

“…Heretofore, the synoptic‐dynamic characteristics of anomalously strong North Atlantic jets have been examined predominantly via observed or idealized case studies. These studies demonstrate that strong jets often develop in association with recurving tropical cyclones (e.g., Atallah and Bosart, 2003; McTaggart‐Cowan et al ., 2007; Grams et al ., 2011; 2013; Grams and Blumer, 2015; Torn and Hakim, 2015; Keller et al ., 2019; Harvey et al ., 2020) and extratropical cyclogenesis events (e.g., Hoskins and Berrisford, 1988; Pyle et al ., 2004; Torn and Hakim, 2015; Winters and Martin, 2016; Berman and Torn, 2019). The influence of latent heating within the near‐jet environment is frequently identified as a contributing factor for flow amplification and jet intensification in these studies (e.g., Riemer et al ., 2008; Riemer and Jones, 2010; Grams et al ., 2011; 2013; Archambault et al ., 2015; Röthlisberger et al ., 2016; 2018; Berman and Torn, 2019; Pohorsky et al ., 2019; Harvey et al ., 2020).…”

The influence of temperature on #ClimateChange and #GlobalWarming discourses on Twitter

“…To address RQ2, we use case studies focusing on two events, a heat wave (March 2012) and a cold surge (January 2014). Case studies have been used by atmospheric scientists who aim to investigate relationships between a weather event and its associated atmospheric and/or societal response [e.g., Mohri, 1953;Hakim, Keyser and Bosart, 1996;Winters and Martin, 2016;Bosart et al, 1996]. While the results of case studies are not generalizable, such analyses allow us to observe interesting trends in tweets and extreme temperatures, which can be combined with statistical analyses to further our understanding of relationships of interest.…”