Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic Tropical Cyclones: Factors Determining Post-Transition Evolution

A 34-member ensemble-mean trajectory through the cyclone phase space (CPS) is calculated using Navy NOGAPS analyses for North Atlantic tropical cyclones (TCs) undergoing extratropical transition (ET). Synoptic composites at four ET milestones are examined: 24hr prior to the beginning of ET (TB-24), the beginning of ET (TB), the end of ET (TE), and 24hr after the end of ET (TE+24). Variability from this mean trajectory is small in the tropical phase, but increases dramatically once ET has begun, and continues to increase after ET. While the extratropically transitioning TC structure is tightly constrained in its tropical phase, it has a variety of evolutions after TE: cold-core (intensification or decay), merger, warm-seclusion, or it may regain TC status. Partitioning the ensemble based upon post-ET intensity change or structure discriminates between significant ET precursor conditions.

A TC that intensifies post-transition (i.e. t>TE) begins transition (t=TB) with a negatively tilted trough 1000km upstream; the transitioning cyclone is in the right entrance region of the attendant jet streak, where the mesoscale ascent enhances the convective instability of the TC remnant. An intense temperature gradient immediately poleward and downstream of the TC leads to isentropic lift and further enhances the convective potential in this area. Conversely, a post-transition weakener commences transition 2000km east of a positively tilted trough. The weakener is in the right exit region of the jet streak, an area of low convective potential. The TC has a weak large-scale poleward temperature gradient.

In six of the 34 cases (18%), the post-tropical cold-core cyclone develops a warm-seclusion structure, rather than remaining cold-core. Anticipation of this warm-seclusion evolution is critical to a forecast since it represents a dramatically increased risk of middle to high-latitude wind and wave damage. We show here that warm-seclusion evolution is most favored when the scale of the interacting trough closely matches the scale of the transitioned tropical cyclone and discuss the impacts of this scale matching on the subsequent storm evolution. The sensitivity of the post-ET evolution of the TC is demonstrated here to be critically dependent on the timing of interaction between the TC and the attendant midlatitude trough, as well as the characteristics of each system and of the broader synoptic environment. These results give insight into the degradation in the forecast accuracy of global models in the middle latitudes during a pending ET event.