Teleconnection
Encyclopedia
Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of kilometers). The most emblematic teleconnection is that linking sea-level pressure
at Tahiti
and Darwin, Australia, which defines the Southern Oscillation.
between time series
of atmospheric pressure
, temperature and rainfall. They served as a building block for the understanding of climate variability, by showing that the latter was not purely random.
Indeed, the term El Niño-Southern Oscillation
(ENSO) is an implicit acknowledgment that the phenomenon underlies variability in several locations at once. It was later noticed that associated teleconnections occurred all over North America, as embodied by the Pacific-North American teleconnection pattern
.
In the 1980s, improved observations allowed to detect teleconnections at larger distances throughout the troposphere
. Concomitantly, the theory emerged that such patterns could be understood through the dispersion of Rossby waves due to the spherical geometry of the Earth. This is sometimes called the "proto-model".
Building upon the "proto-model", much of the early theory of teleconnections dealt with barotropic
, linear
ized model of atmospheric flow about a constant mean state. However, the model was soon invalidated when it was discovered that actual teleconnection patterns were nearly insensitive to the location of the forcing, in direct contradiction with the predictions offered by this simple picture.
Simmons and collaborators showed that if a more realistic background state was prescribed, it would become unstable, leading to a similar pattern regardless of the location of the forcing, in accordance to observations. This "modal" property turned out to be an artifact of the barotropicity of the model, though it has appeared for more subtle reasons in more realistic models.
More recent work has shown that most teleconnections from the tropics to the extratropics can be understood to surprising accuracy by the propagation of linear, planetary waves upon a 3-dimensional seasonally-varying basic state.. Because the patterns are persistent over time and somewhat "locked" to geographical features such as mountain ranges, these waves are called stationary
.
Another mechanism of teleconnection between tropical oceans and midlatitude regions is symmetric along latitude circles (i.e. "zonal") and between hemispheres, unlike the stationary wave mechanism. It relies on interactions between transient eddies
and the mean atmospheric flow that are mutually reinforcing (i.e. non-linear). It has been shown to explain some aspects of ENSO teleconnections in temperature and rainfall.
s are predictable up to 2 years ahead of time, knowledge of teleconnection patterns gives some amount of predictability in remote locations with an outlook sometimes as long as a few seasons.
For instance, predicting El Niño enables prediction of North American rainfall, snowfall, droughts or temperature patterns with a few weeks to months lead time. In Sir Gilbert Walker's time, A strong El Niño usually meant a weaker Indian monsoon, but this anticorrelation has weakened in the 1980s and 1990s, for controversial reasons.
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
at Tahiti
Tahiti
Tahiti is the largest island in the Windward group of French Polynesia, located in the archipelago of the Society Islands in the southern Pacific Ocean. It is the economic, cultural and political centre of French Polynesia. The island was formed from volcanic activity and is high and mountainous...
and Darwin, Australia, which defines the Southern Oscillation.
History
Teleconnections were first noted by the British meteorologist Sir Gilbert Walker in the late 19th century, through computation of the correlationCorrelation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
between time series
Time series
In statistics, signal processing, econometrics and mathematical finance, a time series is a sequence of data points, measured typically at successive times spaced at uniform time intervals. Examples of time series are the daily closing value of the Dow Jones index or the annual flow volume of the...
of atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
, temperature and rainfall. They served as a building block for the understanding of climate variability, by showing that the latter was not purely random.
Indeed, the term El Niño-Southern Oscillation
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...
(ENSO) is an implicit acknowledgment that the phenomenon underlies variability in several locations at once. It was later noticed that associated teleconnections occurred all over North America, as embodied by the Pacific-North American teleconnection pattern
Pacific-North American teleconnection pattern
The Pacific-North American teleconnection pattern is a climatological term for a large-scale weather pattern with two modes, denoted positive and negative, and which relates the atmospheric circulation pattern over the North Pacific Ocean with the one over the North American continent.The positive...
.
In the 1980s, improved observations allowed to detect teleconnections at larger distances throughout the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
. Concomitantly, the theory emerged that such patterns could be understood through the dispersion of Rossby waves due to the spherical geometry of the Earth. This is sometimes called the "proto-model".
Theory
Teleconnections within the tropical Pacific began to be understood thanks to the idealized calculations of A.E. Gill and later through more complex models.Building upon the "proto-model", much of the early theory of teleconnections dealt with barotropic
Barotropic
In meteorology, a barotropic atmosphere is one in which the pressure depends only on the density and vice versa, so that isobaric surfaces are also isopycnic surfaces . The isobaric surfaces will also be isothermal surfaces, hence the geostrophic wind is independent of height...
, linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
ized model of atmospheric flow about a constant mean state. However, the model was soon invalidated when it was discovered that actual teleconnection patterns were nearly insensitive to the location of the forcing, in direct contradiction with the predictions offered by this simple picture.
Simmons and collaborators showed that if a more realistic background state was prescribed, it would become unstable, leading to a similar pattern regardless of the location of the forcing, in accordance to observations. This "modal" property turned out to be an artifact of the barotropicity of the model, though it has appeared for more subtle reasons in more realistic models.
More recent work has shown that most teleconnections from the tropics to the extratropics can be understood to surprising accuracy by the propagation of linear, planetary waves upon a 3-dimensional seasonally-varying basic state.. Because the patterns are persistent over time and somewhat "locked" to geographical features such as mountain ranges, these waves are called stationary
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
.
Another mechanism of teleconnection between tropical oceans and midlatitude regions is symmetric along latitude circles (i.e. "zonal") and between hemispheres, unlike the stationary wave mechanism. It relies on interactions between transient eddies
Eddy (fluid dynamics)
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...
and the mean atmospheric flow that are mutually reinforcing (i.e. non-linear). It has been shown to explain some aspects of ENSO teleconnections in temperature and rainfall.
Applications
Since tropical sea surface temperatureSea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
s are predictable up to 2 years ahead of time, knowledge of teleconnection patterns gives some amount of predictability in remote locations with an outlook sometimes as long as a few seasons.
For instance, predicting El Niño enables prediction of North American rainfall, snowfall, droughts or temperature patterns with a few weeks to months lead time. In Sir Gilbert Walker's time, A strong El Niño usually meant a weaker Indian monsoon, but this anticorrelation has weakened in the 1980s and 1990s, for controversial reasons.