Quantification of the impact of extratropical transition on the midlatitude flow: From case studies to a composite view

As a tropical cyclone (TC) recurves and undergoes extratropical transition (ET), its diabatically enhanced outflow may interact with the extratropical flow such that a jet streak forms and an upper-level ridge amplifies. In some instances, the TC–extratropical flow interaction can amplify a Rossby wave train that disperses far downstream of the TC. Therefore, ET may cause high-impact weather in the vicinity of the transitioning TC as well as in downstream regions such as North America or Europe.

Previous studies have quantified the impact of ET either for individual storms or in an idealized framework. Furthermore, composite analyses of recurving western North Pacific (WNP) TCs during 1979–2009 indicate that Rossby wave amplification is a general characteristic of ET. In this study we unite the case study approach with composite analysis to quantify the integrated impact of ET and to elucidate the general characteristics of ET.

Recurving WNP TCs are composited based upon the strength of the TC–extratropical flow interaction (Archambault et al. 2013). Time-lagged composites relative to the TC recurvature point are constructed from 0.5° NCEP Climate Forecast System Reanalysis data. Then a potential vorticity (PV) surgery technique based on PV inversion is employed to remove the TC from the composite fields (Grams et al. 2013) two days prior to recurvature. These modified composite fields serve as initial conditions for the limited-area model COSMO to simulate the midlatitude flow evolution in the absence of ET. Comparing this "no TC" simulation against a "control" simulation initialized from the recurvature relative composite fields in the large scale and a TC relative composite vortex allows for a quantification of the impact of ET on the midlatitude circulation.

The results of the study corroborate previous findings that a general characteristic of ET is downstream ridge amplification and jet streak intensification. Lagrangian and Eulerian analyses of the control simulation together indicate that diabatic PV reduction and isentropic transport of low-PV air by warm conveyor belt-like outflow of the TC undergoing ET are key physical processes that amplify the downstream ridge. Furthermore, in the control simulation, the initial amplification of the flow pattern disperses downstream, resulting in a second even stronger ridge over the eastern North Pacific and western North America. In contrast, in the no TC simulation, only a weak Rossby wave train is present over the North Pacific. Thus, based on this study, ET over the WNP significantly influences the flow pattern downstream over North America.

Archambault, H.M., Bosart ,L.F., Keyser, D., Cordeira, J.M., 2013: A climatological analysis of the extratropical flow response to recurving western North Pacific Tropical Cyclones. Mon. Wea, Rev., 141, 2325–2346.

Grams, C. M., Jones, S.C., Davis, C.A., Harr, P.A., Weissmann, M., 2013a: The impact of Typhoon Jangmi (2008) on the midlatitude flow. Part I: upper-level ridgebuilding and modification of the jet. Quart. J. Roy. Meteor. Soc., in press. doi:10.1002/qj.891