1. Tropical Cyclone Motion
M. D. Eastin

2. Outline Tropical Cyclone Motion Climatology
Environmental Steering Flow
The Beta Effect
Additional Influences
Trochoidal Motions
The Fugiwhara Effect
Tropical
M. D. Eastin

3. TC Motion: Climatology
Typical Tracks
Prevailing tracks are
shown in white for
each month
Main Features
Bermuda High:
Note the west – east
shift and magnitude
changes during the
season
East U.S. Trough:
Note the northwest
to southeast shift
and magnitude L L H H L L H H L L H H
Tropical
M. D. Eastin

4. TC Motion: An Atypical Track
Tropical
M. D. Eastin

5. From Velden and Leslie (1991)
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: 700 mb flow
Hurricane: 500 mb flow
From Velden and Leslie (1991)
Tropical
M. D. Eastin

6. Vorticity Generation via Beta Initially Symmetric Cyclonic Vortex
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
Results in an east-west dipole of maximum
negative-positive vorticity generation
across the cyclone
Local
Vorticity
Change
Advection
of Vorticity
Beta
Vorticity Generation via Beta f3 + - f2 f1
Initially Symmetric Cyclonic Vortex
Tropical
M. D. Eastin

7. _ + 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)
Local
Vorticity
Change
Advection
of Vorticity
Beta
Vorticity Generation via Beta
and Vorticity Advection - _ f3 + - f2 f1 + +
Initially Symmetric Cyclonic Vortex
From Holland (1983)
Tropical
M. D. Eastin

8. 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 synoptic-
scale 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
toward other areas of low pressure
Sea-Level Pressure 06Z
Formerly
Hurricane
Florence L
Weakness H H
Hurricane
Gordon
Forecast Track
TS Helene
Tropical
M. D. Eastin

9. (with trochoidal motions)
TC Motion: Trochoidal Motions
Motion of Individual TCs:
Many hurricanes experience “wobbles”,
or oscillations, with respect to their time
averaged motion vector
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
Hurricane Carla (1961)
Best Track
(offset)
Actual Track
(with trochoidal motions)
From Jarvinen et al. (1984)
Tropical
M. D. Eastin

10. Centroid Relative Tracks
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

11. 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

12. References
Holland, G. J., 1983: tropical cyclone motion: Environmental interaction plus a beta effect. J. Atmos. Sci.,
40,
Jarvinen, B. R., C. J. Neumann, and M. A. S. Davis, 1984: A tropical cyclone data tape for the North
Atlantic basin, : 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,
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,
Tropical
M. D. Eastin