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El Niño & La Niña Forecasting
El Niño-related climate variations often have
widespread and devastating impacts. These include
the frequency and severity of storms and the
occurrence of droughts and floods. In the U.S. alone,
business losses associated with 1986-87 El Niño
amounted to $10-15 billion. Although many of the
consequences of El Niño cannot be prevented,
skillful forecasts enable resource managers in
climate-sensitive sectors to alter strategies and
reduce economic vulnerability. NOAA (National Oceanic and Atmospheric
Administration) National Centers for Environmental
Prediction (NCEP) has the mandate to provide
forecasts to the nation to reduce loss of human life
and minimize economic impacts.
Coupled ocean-atmosphere models have attained
significant forecast skill during the past decade, but
they continue to be limited by an ocean observing
system which is extraordinarily modest. Satellites
represent part of the solution. In particular, an
operational flow of altimeter data has long been
desired by the modeling community as a means of
estimating changes in upper ocean heat (to first
order, sea level is determined by the integrated
temperature of the water column). But even though
altimeters have flown nearly continuously since
1985, two challenges have stood in the way of
progress: (1) The altimeter data must be made
available fast enough (1-2 days) and with sufficient
accuracy (a few cm) to track changes in the ocean
within a tolerance that is useful for the ocean model;
(2) The assimilation method must be capable of
using a single parameter, sea level, to correct the
model temperature as a function of depth.
Using TOPEX/Poseidon altimeter data, both of
these problems have been solved through a
collaboration among NOAA, the Naval
Oceanographic Office (NAVOCEANO), and the Jet
Propulsion Laboratory (JPL). The program became
operational in March 1997, just in time to assist in
the long-range forecasts associated with the 1997-
1998 El Niño. The study by Cheney at al., showed
that predictions of Pacific sea surface temperatures,
a measure closely linked with El Niño/La Niña, that
were made with TOPEX/Poseidon have
higher skill than those made with XBT (eXpendable
BathyThermographs) and the Tropical Atmosphere
Ocean (TAO) data alone. Consequently, TOPEX/Poseidon
data analyzed within 2 days of real time have been
added to the operational assimilation scheme
implemented at NOAA NCEP.
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