Rossby (or
planetary)
waves are giant
meanders
in high-
altitude winds
that are a major influence on
weather. Their
emergence is due to
shear in
rotating
fluids, so that the
Coriolis
force changes along the sheared coordinate. In
planetary atmospheres,
they are due to the variation in the
Coriolis effect with
latitude. The waves were first identified in the
Earth's atmosphere in 1939 by
Carl-Gustaf Arvid Rossby
who went on to explain their motion. Rossby waves are a subset of
inertial waves.
Terrestrial waves
Most work on Rossby waves has been done on those in Earth's
atmosphere.
The special identifying feature of the Rossby waves is its
phase velocity (that of the wave crests)
always has a westward component. However, the wave's
group velocity (associated with the energy
flux) can be in any direction. In general: shorter waves have an
eastward group velocity and long waves a westward group
velocity.
The terms "
barotropic" and "
baroclinic" Rossby waves are used to distinguish
their vertical structure. Barotropic Rossby waves do not vary in
the vertical, and have the fastest propagation
speeds. The baroclinic wave modes are slower, with
speeds of only a few centi
metres per
second or less.
Atmospheric waves
Rossby waves in the
atmosphere are easy
to observe as (usually 4-6) large-scale meanders of the
jet stream. When these loops become very
pronounced, they detach the masses of cold, or warm, air that
become
cyclones and
anticyclones and are responsible for day-to-day
weather patterns at mid-latitudes.
The wave speed is given by
- c = u - \frac{\beta}{k^2},
where
c is the wave speed,
u is the mean westerly
flow,
\beta is the
Rossby
parameter, and
k is the total wavenumber.
Furthermore, the
Rossby parameter
is defined:
- \beta = \frac{1}{a} \frac{d}{d\phi} (2 \omega \sin\phi) =
\frac{2\omega \cos\phi}{a}
φ is the latitude,
ω is the
angular speed of the
Earth's rotation, and
a is the
mean
radius of the Earth.
Oceanic waves
Oceanic Rossby waves are thought to communicate climatic changes
due to variability in
forcing, due to both
the
wind and
buoyancy.
Both barotropic and baroclinic waves cause variations of the sea
surface height, although the length of the waves made them
difficult to detect until the advent of
satellite altimetry.
Observations by the NASA
/CNES TOPEX/Poseidon
satellite confirmed the existence of
oceanic Rossby waves.
Baroclinic waves also generate significant displacements of the
oceanic
thermocline, often of tens of
meters. Satellite observations have revealed
the stately progression of Rossby waves across all the
ocean basins, particularly at low- and
mid-latitudes.
These waves can take months or even years to
cross a basin like the Pacific
.
Rossby waves have been suggested as an important mechanism to
account for the heating of
Europa's
ocean.
Bibliography
- Rossby, Carl-Gustaf et al. (1939), Relation
between variations in the intensity of the zonal circulation of the
atmosphere and the displacements of the semi-permanent centers of
action, Journal of Marine Research, Vol. II, No. 1,
pp. 38-55
- Platzman, George W. (1968) The Rossby wave,
Quarterly Journal of the Royal Meteorological Society,
Vol. 94, No. 401, pp. 225-248
- Dickinson, Robert E. (1978) Rossby waves - long-period oscillations of
oceans and atmospheres, Annual Review of Fluid
Mechanics, Vol. 10, pp. 159-195
See also
References
External links