Fields of Resources, Energy, the Environment, and Social Infrastructure
Achievement
Establishment of a scientific foundation for understanding and predicting extreme weather events(UK)
Born: May 17, 1945 (Age: 78)
Professor, Department of Meteorology,
University of Reading

Teleconnection – an essential element of
numerical weather and climate prediction
One of the most important achievements made byProf. Sir Brian Hoskins and Prof. John Wallace is theirclarification of the conditions and mechanisms underlying“teleconnection,” which explain how abnormal weather
in one region affects weather in distant regions. Figure 1
illustrates how distinct weather trends are produced fromhighly persistent fluctuations mainly in the tropical atmosphere-ocean coupled system to affect winter and summer
in the mid-high latitudes in the northern hemisphere.
The “westerlies” are prevailing winds that blow from thewest at mid-latitudes (shown by the dashed yellow linewith arrows), and their continuous meandering leads toabnormal weather.
When using numerical weather and climate prediction,
it is necessary to understand that atmospheric conditions
in one region are constantly susceptible to fluctuations andchanges due to the influence of atmospheric, oceanic, andsea ice conditions in distant regions. This is particularly
true for seasonal weather prediction in the mid-latitudes
on scales greater than two weeks, as it is understood thatsuch predictions can be based upon teleconnection effects
from highly persistent climatic fluctuations such as theEl Niño/La Niña phenomena – in which trade winds andoceanic changes interact with each other in the equatorial
Pacific – and decadal fluctuations in the Pacific Ocean.
Those persistent fluctuations in the tropics can therefore
act as sources of predictability of mid-latitude weather
conditions on seasonal scales via “teleconnection”.

(USA)
Born: October 28, 1940 (Age: 83)
Professor Emeritus, Department of Atmospheric Sciences,
University of Washington

Data-based and theoretical explanation
for the Pacific-North American PatternFigure 2 provides a more detailed look of the left sideof Figure 1, and shows Wallace’s 1981 discovery of the“Pacific-North American Pattern,” which arises out of theteleconnection effects of El Niño. That same year, Hoskins
provided a theoretical explanation of the dynamic processes that form such fluctuations in atmospheric patterns.
During El Niño, the water becomes warmer than usual
across an area of the equatorial Pacific Ocean from aroundthe International Date Line to the Peruvian coast of SouthAmerica. Precipitation there increases, becoming a sourceof heat and generating large-scale atmospheric waves
called “Rossby waves.” Figure 2 illustrates how suchwaves result in alternating anticyclonic (H– clockwise in
the northern hemisphere) and cyclonic (L – counterclockwise in the northern hemisphere) atmospheric circulation
anomalies thousands of kilometers wide.
Subtropical and mid-latitude westerlies (or jet streams)
play an important role in forming these patterns. The anticyclonic and cyclonic patterns generated by Rossby waves
tend to move westwards, but strong westerly winds in winterand other periods halt that westward movement and holding
those patterns in place, as is seen in the Pacific-North American Pattern. When this occurs, energy propagates withthe pattern, resulting in the effects of El Niño being felt
throughout North America and as far as the North Atlantic
Ocean. The westerlies (red arrows in Figure 2) meander toa great extent due to their overlapping with Rossby waves,thereby resulting in abnormal weather that is persistent.

Making weather and climate prediction possible
Hoskins numerically calculated how stagnant Rossby
waves are excited when a simulated heat source withincreased precipitation is placed near 15 degrees northlatitude, and how that will lead to anticyclonic (H) andcyclonic (L) circulation anomalies forming in the upper
atmosphere (see left side of Figure 3).
Meanwhile, Wallace conducted a statistical analysis ofroughly fifteen years of objective weather observation datafrom various regions in the northern hemisphere that had
been gathered to provide initial values for numerical weather forecasts. In that data, he discovered several types ofatmospheric pressure fluctuation patterns with alternating
and persistent anticyclonic and cyclonic circulation anomalies.
The righthand illustration of Figure 3 depicts the typical
distribution of pressure anomalies that eventually appear in
the middle troposphere after the arrival of an often-observed
teleconnection pattern in the western Atlantic. This patterncoincides well with Hoskins’ numerical calculations (lefthandLL

66

HHL
HL

H134

LH235

L

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illustration of Figure 3), and it was an extremely importantdiscovery that greatly advanced the predictability of persistent
weather anomalies.

The increasing importance of numerical weather
and climate prediction
The primary success of the work of these two researchers
was their discovery of global-scale fluctuations in thewesterlies driven by El Niño, and this has become one of
the foundations of seasonal weather prediction around theworld. However, their work resulted in more advancementsthat could be listed here, but included the identificationof mechanisms through which extratropical cyclones aregenerated and the regions in which they are active, both ofare essential for daily weather forecasting; the dynamics
behind the formation of blocking anticyclones (or blockinghighs) which cause abnormal weather; the discovery ofhemisphere-scale fluctuations in the westerlies (the ArcticOscillation); and the identification of decadal fluctuations in
the Pacific atmosphere-ocean coupled system that accelerateand decelerate increases in the global temperature. Many ofthese successes were achieved through Hoskins constructing
theoretical models and Wallace understanding actual conditions using observational data, and their work has become
the cornerstone of useful, practical, and promising numerical
weather and climate prediction. It also led to the use of atmospheric reanalysis data in monitoring climate change.
In addition, their work is also useful in studying weather
anomalies arising from global warming, a subject that is ofgrowing concern, by showing how regional weather effects
can be described using the knowledge of meteorology andclimate dynamics that has been accumulated over the years.
Numerical weather and climate prediction will become anincreasingly important part of the social infrastructure that
protects human lives.