Tropical Cyclone Motion
Tropical
M. D. Eastin
Outline
Tropical Cyclone Motion
• Climatology
• Environmental Steering Flow
• The Beta Effect
• Additional Influences
• Trochoidal Motions
• The Fugiwhara Effect
Tropical
M. D. Eastin
TC Motion: Climatology
Typical Tracks
L
Prevailing tracks are
shown in white for
each month
L
H
H
Main Features
Bermuda High:
Note the west – east
shift and magnitude
changes during the
season
L
L
H
H
East U.S. Trough:
Note the northwest
to southeast shift
and magnitude
changes during the
season
Tropical
L
L
H
H
M. D. Eastin
TC Motion: An Atypical Track
Tropical
M. D. Eastin
TC Motion: Steering Flow
Motion of Individual TCs:
• The deep layer environmental
flow accounts for a large fraction
(up to 80%) of TC motion
• Assumes the TC acts as a
passive vortex moving with the
speed and direction of the
mass-weighted deep layer flow
• When a deep layer estimate is
unavailable use the following:
TD and TS:
Hurricane:
700 mb flow
500 mb flow
From Velden and Leslie (1991)
Tropical
M. D. Eastin
TC Motion: The Beta Effect
Motion of Individual TCs:
• The “beta effect” accounts for 15-20%
(up to 2 m/s) of TC motion
• Results from quasi-symmetric cyclonic flow
superimposed on the north-south gradient
of the Coriolis force (β = df / dy)
• “Simple” explanation from the Cartesian
non-divergent barotropic vorticity equation
• Beta Contribution: An air parcel displaced
southward (northward) will acquire positive
(negative) relative vorticity

t
 
 

   u
v
y 
 x
Local
Vorticity
Change
 v
Advection
of Vorticity
Beta
Vorticity Generation via Beta
f3
+
-
• Results in an east-west dipole of maximum
negative-positive vorticity generation
across the cyclone
f2
f1
Initially Symmetric Cyclonic Vortex
Tropical
M. D. Eastin
TC Motion: The Beta Effect
• Advection Contribution: The resulting
cyclonic advection of the Beta-generated
vorticity produces a north-south dipole of
local vorticity change
• Their combination locally produces two
vorticity maxima, called “beta gyres”,
that induce a northwesterly component to
TC motion (in the northern hemisphere)

t
 
 

   u
v
y 
 x
Local
Vorticity
Change
 v
Advection
of Vorticity
Beta
Vorticity Generation via Beta
and Vorticity Advection
-
_
f3
-
+
f2
f1
+
+
Tropical
Initially Symmetric Cyclonic Vortex
From Holland (1983)
M. D. Eastin
TC Motion: Additional Influences
Motion of Individual TCs:
• Some storms tend to drift toward
their latent heating centroid (which
may be offset from the circulation
center due to vertical shear)
• Some storms drift toward synopticscale troughs (particularly if the
trough is deepening)
• Many storms will move toward a
weakness in a ridge (a relative low
pressure in a high pressure system)
• Common theme: TCs tend to drift
Sea-Level Pressure 06Z 0914 2006
Weakness
H
Formerly
Hurricane
Florence
L
H
Hurricane
Gordon
Forecast Track
TS Helene
toward other areas of low pressure
Tropical
M. D. Eastin
TC Motion: Trochoidal Motions
Motion of Individual TCs:
• Many hurricanes experience “wobbles”,
or oscillations, with respect to their time
averaged motion vector
Hurricane Carla (1961)
Best Track
(offset)
• This trochoidal motion is believed to result
from the co-rotation of the TC’s circulation
center with a smaller mesovortex (perhaps
generated by a deep convective burst)
• Trochoidal motions are often removed
from the official ”best” track
• Trochoidal motions are often misinterpreted
as “turns”…..forecasters beware
Actual Track
(with trochoidal
motions)
From Jarvinen et al. (1984)
Tropical
M. D. Eastin
TC Motion: The Fugiwhara Effect
Motion of Two Neighboring TCs:
• Occasionally two TCs in close
proximity will co-rotate (and in
some cases, they merge)
• This process is superimposed on
the advection by the steering flow
and the beta effect
• Named for Dr. S. Fujiwhara who
first studied the phenomenon
Earth Relative
Tracks
Centroid Relative Tracks
From Prieto et al. (2003)
Tropical
M. D. Eastin
Tropical Cyclone Motion
Summary
• TC Motion Climatology (seasonality, and large-scale forcing)
• Deep layer steering flow (function of intensity, contribution to total)
• Beta effect (physical processes, contribution to total)
• Additional Influences
• Thochoidal Motions (definition, possible causes)
• Fujiwhara Effect (definition, net result)
Tropical
M. D. Eastin
References
Holland, G. J., 1983: tropical cyclone motion: Environmental interaction plus a beta effect. J. Atmos. Sci.,
40, 328-342.
Jarvinen, B. R., C. J. Neumann, and M. A. S. Davis, 1984: A tropical cyclone data tape for the North
Atlantic basin, 1886-1983: Contents, limitations, and uses. NOAA Tech. Memo,
NWS-NHC-22, 21 pp.
Preito, R., B. D. McNoldy, S. R. Fulton, and W. H. Schubert, 2003: A classification of binary tropical
cyclone-like vortex interactions. Mon. Wea. Rev., 131, 2656-2666.
Velden, C. S., and L. L. Leslie, 1991: The basic relationship between tropical cyclone intensity and the
depth of the environmental steering layer in the Australian region. Wea. Forecasting, 6,
244-253.
Tropical
M. D. Eastin