Rossby wave

Rossby waves, also known as planetary waves, are a natural phenomenon in the atmosphere and oceans of planets that largely owe their properties to rotation. Rossby waves are a subset of inertial waves.

Atmospheric Rossby waves on Earth are giant meanders in high-altitude winds that have a major influence on weather. These Rossby waves are associated with pressure systems and the jet stream.[1] Oceanic Rossby waves move along the thermocline: the boundary between the warm upper layer and the cold deeper part of the ocean.

Rossby wave types

Atmospheric waves

Meanders of the northern hemisphere's jet stream developing (a, b) and finally detaching a "drop" of cold air (c). Orange: warmer masses of air; pink: jet stream.

Atmospheric Rossby waves are the result of the viscosity of air and the resulting shear forces, of rotating fluids, and of the deviation of those fluid motions caused by the Coriolis acceleration. A fluid, on the Earth, that moves toward the pole will deviate toward the east; a fluid moving toward the equator will deviate toward the west (true in either hemisphere). The deviations are caused by the Coriolis force, which is a manifestation of conservation of angular momentum. In planetary atmospheres, Rossby waves in particular, 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.

One can identify a terrestrial Rossby wave as its phase velocity, marked by its wave crest, always has a westward component. However, the collected set of Rossby waves may appear to move in either direction with what is known as its group velocity. In general, shorter waves have an eastward group velocity and long waves a westward group velocity.

The terms "barotropic" and "baroclinic", are used to distinguish the vertical structure of Rossby waves. 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 centimeters per second or less.

Most investigations of Rossby waves has been done on those in Earth's atmosphere. Rossby waves in the Earth's atmosphere are easy to observe as (usually 4-6) large-scale meanders of the jet stream. When these deviations become very pronounced, masses of cold or warm air detach, and become low strength anticyclones and cyclones, respectively, and are responsible for day-to-day weather patterns at mid-latitudes. Rossby waves may be partly responsible for the fact that eastern continental edges, such as the Northeast United States and Eastern Canada, are colder than Western Europe at the same latitudes.[2]

Poleward-propagating atmospheric waves

Deep convection (heat transfer) to the troposphere is enhanced over very warm sea surfaces in the tropics, such as during El Niño events. This tropical forcing generates atmospheric Rossby waves that have a poleward and eastward migration.

Poleward-propagating Rossby waves explain many of the observed statistical connections between low and high latitude climates.[3] One such phenomenon is sudden stratospheric warming. Poleward-propagating Rossby waves are an important and unambiguous part of the variability in the Northern Hemisphere, as expressed in the Pacific North America pattern. Similar mechanisms apply in the Southern Hemisphere and partly explain the strong variability in the Amundsen Sea region of Antarctica.[4] In 2011, a Nature Geoscience study using general circulation models linked Pacific Rossby waves generated by increasing central tropical Pacific temperatures to warming of the Amundsen Sea region, leading to winter and spring continental warming of Ellsworth Land and Marie Byrd Land in West Antarctica via an increase in advection.[5]

Oceanic waves

Oceanic Rossby waves are large-scale waves within an ocean basin. They have a low amplitude, on the order of centimetres (at the surface) to metres (at the thermocline), compared to a very long wavelength, on the order of hundreds of kilometres of atmospheric Rossby waves. They may take months to cross an ocean basin. They gain momentum from wind stress at the ocean surface layer and 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. Satellite observations have confirmed the existence of oceanic Rossby waves.[6]

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 the ocean on Europa, a moon of Jupiter.[7]

Waves in astrophysical discs

Rossby wave instabilities are also thought to be found in astrophysical discs, for example, around newly forming stars. [8] [9]

Definitions

Free barotropic Rossby waves under a zonal flow with linearized vorticity equation

To start with, a zonal mean flow, "U", can be considered to be perturbed where "U" is constant in time and space. Let \vec{u} = <u, v> be the total horizontal wind field, where "u" and "v" are the components of the wind in the x- and y- directions, respectively. The total wind field can be written as a mean flow, "U", with a small superimposed perturbation, "u'" and "v'".

u = U + u'(t,x,y)\!
v = v'(t,x,y)\!

The perturbation is assumed to be much smaller than the mean zonal flow.

U \gg u',v'\!

Relative Vorticity \eta, u and v can be written in terms of the stream function \psi (assuming non-divergent flow, for which the stream function completely describes the flow):

u' = \frac{\partial \psi}{\partial y}
v' = -\frac{\partial \psi}{\partial x}
\eta = \nabla \times (u' \mathbf{\hat{\boldsymbol{\imath}}} + v' \mathbf{\hat{\boldsymbol{\jmath}}}) = -\nabla^2 \psi

Considering a parcel of air that has no relative vorticity before perturbation (uniform U has no vorticity) but with planetary vorticity f as a function of the latitude, perturbation will lead to a slight change of latitude, so the perturbed relative vorticity must change in order to conserve potential vorticity. Also the above approximation U >> u' ensures that the perturbation flow does not advect relative vorticity.

\frac{d (\eta + f) }{dt} = 0 = \frac{\partial \eta}{\partial t} + U \frac{\partial \eta}{\partial x} + \beta v'

with \beta = \frac{\partial f}{\partial y} . Plug in the definition of stream function to obtain:

0 = \frac{\partial \nabla^2 \psi}{\partial t} + U \frac{\partial \nabla^2 \psi}{\partial x} + \beta \frac{\partial \psi}{\partial x}

Using the Method of undetermined coefficients one can consider a traveling wave solution with zonal and meridional wavenumbers k and l, respectively, and frequency \omega:

\psi = \psi_0 e^{i(kx+ly-\omega t)}\!

This yields the dispersion relation:

\omega = Uk - \beta \frac {k}{k^2+l^2}

The zonal (x-direction) phase speed and group velocity of the Rossby wave are then given by

c \ \equiv\ \frac {\omega}{k} = U - \frac{\beta}{(k^2+l^2)},
c_g \ \equiv\ \frac{\partial \omega}{\partial k}\ = U - \frac{\beta (l^2-k^2)}{(k^2+l^2)^2},

where c is the phase speed, c_g is the group speed, U is the mean westerly flow, \beta is the Rossby parameter, k is the zonal wavenumber, and "l" is the meridional wavenumber. It is noted that the zonal phase speed of Rossby waves is always westward (traveling east to west) relative to mean flow "U", but the zonal group speed of Rossby waves can be eastward or westward depending on wavenumber.

Meaning of Beta

The Rossby parameter is defined:

\beta = \frac{\partial f}{\partial y} = \frac{1}{a} \frac{d}{d\phi} (2 \omega \sin\phi) = \frac{2\omega \cos\phi}{a}

\phi is the latitude, ω is the angular speed of the Earth's rotation, and a is the mean radius of the Earth.

If \beta = 0, there will be no Rossby Waves; Rossby Waves owe their origin to the gradient of the tangential speed of the planetary rotation (planetary vorticity). A "cylinder" planet has no Rossby Waves. It also means that at the equator of any rotating, sphere-like planet, including Earth, one will still have Rossby Waves, despite the fact that f = 0, because \beta > 0. (Equatorial Rossby wave).

Quasiresonant amplification of Rossby waves

It has been proposed that a number of regional weather extremes in the Northern Hemisphere associated with blocked atmospheric circulation patterns may have been caused by quasiresonant amplification of Rossby waves.[10] Examples include the 2013 European floods, the 2012 China floods, the 2010 Russian heat wave, the 2010 Pakistan floods and the 2003 European heat wave. Even taking global warming into account, the 2003 heat wave would have been highly unlikely without such a mechanism.

Normally freely travelling synoptic-scale Rossby waves and quasistationary planetary-scale Rossby waves exist in the mid-latitudes with only weak interactions. The hypothesis, proposed by Vladimir Petoukhov, Stefan Rahmstorf, Stefan Petri, and Hans Joachim Schellnhuber, is that under some circumstances these waves interact to produce the static pattern. For this to happen, they suggest, the zonal (east-west) wave number of both types of wave should be in the range 6-8, the synoptic waves should be arrested within the troposphere (so that energy does not escape to the stratosphere) and mid-latitude waveguides should trap the quasistationary components of the synoptic waves. In this case the planetary-scale waves may respond unusually strongly to orography and thermal sources and sinks because of "quasiresonance".

It is also suggested that the phenomenon is made more likely by anthropogenic global warming, but the EEA has cautioned that more data would be needed to confirm that specific events such as flooding were caused by global warming.[11]

See also

References

  1. ↑ Holton, James R. (2004). Dynamic Meteorology. Elsevier. p. 347. ISBN 0-12-354015-1.
  2. ↑ Kaspi, Yohai; Schneider, Tapio (2011). "Winter cold of eastern continental boundaries induced by warm ocean waters". Nature 471 (7340): 621–4. Bibcode:2011Natur.471..621K. doi:10.1038/nature09924. PMID 21455177.
  3. ↑ Hoskins, Brian J.; Karoly, David J. (1981). "The Steady Linear Response of a Spherical Atmosphere to Thermal and Orographic Forcing". Journal of the Atmospheric Sciences 38 (6): 1179. Bibcode:1981JAtS...38.1179H. doi:10.1175/1520-0469(1981)038<1179:TSLROA>2.0.CO;2.
  4. ↑ Lachlan-Cope, Tom; Connolley, William (2006). "Teleconnections between the tropical Pacific and the Amundsen-Bellinghausens Sea: Role of the El Niño/Southern Oscillation". Journal of Geophysical Research 111. Bibcode:2006JGRD..11123101L. doi:10.1029/2005JD006386.
  5. ↑ Ding, Qinghua; Steig, Eric J.; Battisti, David S.; Küttel, Marcel (2011). "Winter warming in West Antarctica caused by central tropical Pacific warming". Nature Geoscience 4 (6): 398. Bibcode:2011NatGe...4..398D. doi:10.1038/ngeo1129.
  6. ↑ Chelton, D. B.; Schlax, M. G. (1996). "Global Observations of Oceanic Rossby Waves". Science 272 (5259): 234. Bibcode:1996Sci...272..234C. doi:10.1126/science.272.5259.234.
  7. ↑ Tyler, Robert H. (2008). "Strong ocean tidal flow and heating on moons of the outer planets". Nature 456 (7223): 770–2. Bibcode:2008Natur.456..770T. doi:10.1038/nature07571. PMID 19079055.
  8. ↑ Lovelace, R.V.E., Li, H., Colgate, S.A., \& Nelson, A.F. 1999, "Rossby Wave Instability of Keplerian Accretion Disks", ApJ, 513, 805-810,http://arxiv.org/abs/astro-ph/9809321
  9. ↑ Li, H., Finn, J.M., Lovelace, R.V.E., \& Colgate, S.A. 2000, ``Rossby Wave Instability of Thin Accretion Disks. II. Detailed Linear Theory, ApJ, 533, 1023-1034, http://arxiv.org/abs/astro-ph/9907279
  10. ↑ Petoukhov, Vladimir; Rahmstorf, Stefan; Petri, Stefan; Schellnhuber, Hans Joachim (16 Jan 2013). "Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes". www.pnas.org. PNAS. Retrieved 1 Jan 2015.
  11. ↑ "Climate and land use: Europe’s floods raise questions". Inquirer News (Agence France-Presse). 5 June 2013. Retrieved 8 June 2013.

Bibliography

External links

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