Elucidating the Character of Atlantic Tropical Cyclone Trough Interactions in the Context of Intensity Change

Since the seminal work of Merrill (1988), there has been relatively little study of tropical cyclone (TC) outflow and the upper-level TC environment in the context of TC intensity change (Rappin et al. 2011 and Emanuel 2012 notable exceptions). This is in contrast to the extensive amount of research that has related intensity change to cyclonic structural changes in the TC. In light of the much larger and higher resolution datasets now available, we seek an improved understanding of the relationship between the structure of TC outflow, the TC environment, and TC intensity change.

ERA-Interim reanalysis is used to analyze the upper troposphere around all Atlantic TCs over warm sea surface temperature (SST) during 1979-2013, yielding 3509 six-hourly samples from 299 TCs. Raw composites derived from these samples show consistencies with the observations of Merrill (1988), who composited Atlantic hurricanes from 1977-1983. Specifically, intensifying TCs exhibit more divergent outflow streamlines within 500 km of the center and more anticyclonic flow extending along the zonal axis outward from the storm. Weakening TCs exhibit a more constricted and sheared outflow pattern, with air from the primary anticyclone recirculating over the storm core, and larger values of potential vorticity (PV) to the west and east of the anticyclone. Interactions with synoptic-scale eddies in the upper troposphere are the primary modulator of these outflow patterns.

With the large available sample size, anomalies from spatially-dependent climatologies are developed in order to examine differences between strengthening and weakening TCs interacting with upper tropospheric troughs, independent of the regional differences in the outflow climatology of Atlantic TCs. Early results suggest that cases of both significant intensification and significant weakening during trough interaction share a general evolution involving a positive PV anomaly to the north of the TC, anomalously strong vertical shear near the inner core, increased radial outflow to the northeast, and increased flux convergence of angular momentum within 1000 km. However, significant differences are observed in the tilt and orientation of the PV anomaly relative to the TC, and in the development and position of the associated jet streak to the northeast of the TC.

Relative to weakening cases, intensifying cases tend to involve a more positively tilted trough that is oriented more to the north than to the west of the TC, and a jet streak that forms farther north initially before the entrance region moves within 500 km of the TC center, inducing less vertical shear over the inner core on average. These results are being examined in the context of Challa and Pfeffer (1981), McBride and Zehr (1981), Molinari and Vollaro (1990), DeMaria (1993), Challa and Pfeffer (1998), and Hanley et al. (2001). Ongoing analysis seeks to a) help qualitatively and quantitatively clarify the subtle character of TC-trough interactions that induce TC intensification (e.g. Elena 1985; Opal 1995; Charley 2004) and those that cause weakening, and b) to determine if measures of these interactions can be indicators of future short-term intensity change.