El
Niño events typically bring heavy, drenching rain to South
America, while Indonesia experiences drought conditions. And during
La Niña this is reversed. But their impacts can be felt even
further afield than this. Dr Mark Saunders from the University
College of London's Department of Space and Climate Physics explains.
Analyses of historical climate data over 100 years shows that
El Niño and La Niña are linked statistically to abnormal
patterns of temperature, rainfall and storminess around the globe.
These
influences are strongest in the tropics and in the pan-Pacific region,
but weak impacts extend occasionally as far as Europe. For example,
there is a link between strong ENSOs (El Niño Southern Oscillation)
and heavier than normal spring rainfall over the central European
and southern UK region.
The spring of 1998 saw rainfall which was 10-20% higher than usual,
accompanied by flooding. This coincided with the strongest El Niño
of recent times.
These remote linkages, or ‘teleconnections', arise from the disruption
ENSO causes to tropical atmospheric circulation. The warming and
cooling of air above the central east Pacific leads to the formation
of a wave-like set of stationary disturbances in the middle atmosphere.
These change the strength and position of jet streams.
As extra-tropical storms and fronts generally track these jet
streams, unusual temperatures and rain patterns may result at mid-latitudes
remote to the Pacific. Such extra-tropical teleconnections are seen
most clearly in northern hemisphere winter and spring.
One
example of ENSO’s global climate impact is its link to December-February
rainfall in Mozambique and south east Africa. During La Niña
periods, south east Africa experiences a higher likelihood of excess
rainfall, and floods. And this is exactly what happened in this
region from 1998-2001.
ENSO also has an influence on hurricane strikes on US and Caribbean
shores. During La Niña periods, hurricane and tropical storm
landfalls are twice as common over much of the Caribbean and Gulf
of Mexico. Again the recent active Atlantic hurricane seasons of
1998-2001 illustrates this.
The ability to seasonally predict ENSO several months in advance
would bring sound economic and social benefits in developing world
countries. It would also help safeguard the revenue of 70% of business
and industry in developed countries.
Efforts are underway at research centres worldwide to improve
the long-range forecasting of ENSO. Steady progress is being made
and scientists are now able to predict conditions up to six to nine
months ahead. Having said that, models failed to predict the onset
and intensity of the 1997/98 strong El Niño. Currently (Nov/Dec
2001), the consensus is for neutral ENSO conditions to persist through
to April 2002.
