Upper-Tropospheric Precursors to the Formation of Subtropical Cyclones that Undergo Tropical Transition in the North Atlantic Basin

Abstract

The opportunity to examine the structure and evolution of the various upper-tropospheric precursors to the formation of North Atlantic (NATL) subtropical cyclones (STCs) that undergo tropical transition (TT) motivates this study. Intraseasonal variability associated with the location and frequency of NATL STCs forming in the presence of similar upper-tropospheric features, as well as similarities and differences in the various upper-tropospheric precursors to the formation of NATL STCs that undergo TT, are examined. NATL STCs that undergo TT are categorized according to the upper-tropospheric features associated with their formation during 1979–2010 using the 0.5° NCEP Climate Forecast System Reanalysis dataset. This categorization allows for the documentation of the location and frequency of STCs forming in the presence of similar upper-tropospheric features and for the construction of cyclone-relative composites during the five days prior to STC formation. NATL STCs that undergo TT are separated into one of three categories based on the upper-tropospheric features associated with their formation: 1) cutoff lows, 2) meridional troughs, and 3) zonal troughs. STCs included in the cutoff low and meridional trough categories typically develop poleward of ~25°N over the western, central, and eastern NATL during September–November and August–November, respectively. In contrast, STCs included in the zonal trough category typically develop equatorward of ~30°N over the western NATL during June–September. Cyclone-relative composites reveal that ~61% of the categorized NATL STCs that undergo TT form in association with an upper-tropospheric feature whose structure and evolution are linked to anticyclonic wave breaking.

Subtropical cyclones near‐term projections from an ensemble of regional climate models over the northeastern Atlantic basin

“…In case of the RCM‐modelled STCs, the distribution is similar to the observed values, as there is a distinct prevalence of STCs over winter months, although the observed March peak is not reproduced in the simulations. The higher prevalence in winter months gives support to the fact that the simulated STCs mainly develop from extratropical precursors, a relationship shown in observational studies (González‐Alemán et al, , Evans and Guishard, , Guishard et al, , Bentley et al, , ). Extratropical precursors such as upper‐level troughs and cut‐off lows are more likely to reach the subtropical domain in winter as the southern edge of the upper‐level midlatitude circulation is located at its southernmost position.…”

“…As STCs have strong connections to the atmospheric dynamics in the extratropics and subtropics (they form from extratropical precursors; Evans and Guishard, 2009; * Correspondence to: J. J. González-Alemán, Environmental Sciences Institute, University of Castilla-La Mancha, Avenida Carlos III, s/n, Toledo 45071, Spain. E-mail: juanjesus.gonzalez@uclm.es González-Alemán et al, 2015;Bentley et al, 2017) and to the ocean in their lower boundary (through sea surface temperatures (SSTs) or warm-air advections) González-Alemán et al, 2015), it is worth researching if anthropogenic climate change (ACC) could cause changes in their formation. Alterations in the atmospheric patterns over the extratropics and subtropics projected for future climate conditions (Thorne et al, 2011;Vavrus, 2012, 2015) as well as alterations in SSTs distribution (Collins et al, 2013) may lead to changes in their frequency of occurrence, location and/or intensity.…”

The Tropical Transition in the Western North Pacific: The Case of Tropical Cyclone Peipah (2007)

“…It has been reported that a cut-off low or stratospheric PV streamer in the upper troposphere associated with anticyclonic Rossby wave breaking (Martius et al, 2007;Waugh & Polvani, 2000;Wernli & Sprenger, 2007) can initiate organized deep convection (e.g., Waugh & Funatsu, 2003) and a subtropical cyclone (e.g., Galarneau et al, 2015) by destabilizing the troposphere (Juckes & Smith, 2000) or through QG forcing for ascent (Hoskins et al, 1985;Trenberth, 1978). Some of these subtropical cyclones occasionally develop into TCs as they travel across a warm SST region (Bentley et al, 2017).…”

“…Alternatively, the colder and drier air transported into the upper troposphere serves to destabilize the tropospheric column by enhancing convective available potential energy (CAPE), thus to foster convectively favorable environments (Juckes & Smith, 2000). The associated anticyclonic wave breaking (and PV streamer) over the tropical eastern Pacific and North Atlantic has been attributed to the formation of subtropical cyclones, which can subsequently undergo tropical transition (Bentley et al, 2017;Bentley & Metz, 2016;Davis & Bosart, 2003Galarneau et al, 2015).…”

Convection regimes and tropical‐midlatitude interactions over the Intra‐American Seas from May to November

“…The midlatitude wave teleconnection is stronger in regime 4 compared to regime 5, while the anomalous ridge is seen to retrograde southwestwards leading to significant positive pressure anomalies in the upper‐troposphere over the southwest United States and negative anomalies over the southeast for regime 6. This sequence of upper‐tropospheric configurations is reminescent of the behaviour of potential vorticity (PV) ‘streamers’ during anticyclonic wave breaking (Thorncroft et al ., ; Davis, ) and which have been related to subtropical cyclone formation in the North Atlantic in previous works (Bentley, ). PV streamers inject regions of relatively high upper‐tropospheric PV into the subtropical lower troposphere, which become advected westward by the mean easterlies and CLLJ during the sequence of regimes 4–6.…”