  It is well known that rapid changes in tropical cyclone motion occur during
interaction with extratropical waves. While the translation speed has received
much attention in the published literature, acceleration has not. Using a large
data sample of Atlantic tropical cyclones, we formally examine the composite
synoptic-scale patterns associated with tangential and \curvature components of
their acceleration. During periods of rapid tangential acceleration, the
composite tropical cyclone moves poleward between an upstream trough and
downstream ridge of a developing extratropical wavepacket. The two systems
subsequently merge in a manner that is consistent with extratropical
transition. During rapid curvature acceleration, a prominent downstream ridge
promotes recurvature of the tropical cyclone. In contrast, during rapid
tangential or curvature deceleration, a ridge is located directly poleward of
the tropical cyclone. Locally, this arrangement takes the form of a
cyclone-anticyclone vortex pair somewhat akin to a dipole block. On average,
the tangential acceleration peaks 18 hours prior to extratropical transition
while the curvature acceleration peaks at recurvature. These findings confirm
that rapid acceleration of tropical cyclones is mediated by interaction with
extratropical baroclinic waves. Furthermore, The tails of the distribution of
acceleration and translation speed show a robust reduction over the past 5
decades. We speculate that these trends may reflect the poleward shift and
weakening of extratropical Rossby waves.
