Envisat altimeter watches Pacific for cold tongue of La Nina(Forwarded)

ESA News
http://www.esa.int

3 March 2006

Envisat altimeter watches Pacific for cold tongue of La Nina

Satellite measurements of a steep difference in sea surface height between
the western and eastern tropical Pacific support predictions that a La
Nina event is in the offing. El Nino's chillier sister, La Nina is linked
to opposing but equally wide-ranging shifts in weather patterns.

Hundreds of years ago fishermen off the west coast of Peru noted how
periodically around Christmas time the waters grew unusually warm and fish
became scarce: a phenomenon they called 'the Christ Child' -- El Nino. It
begins when a mass of warmer water from the western Pacific moves east,
displacing cooler, nutrient-rich waters in the vicinity. This warmer water
adds moisture to the atmosphere, raises rainfall levels and disrupts
atmospheric circulation on a global basis.

La Nina is an equivalent cooling event during which the warm waters shift
westwards to induce upwelling of cold water, reducing rainfall in the
eastern equatorial Pacific but increasing it in the west. Researchers now
recognise that these twin extremes of El Nino and La Nina are ocean
components of a larger phenomenon that extends to the atmosphere, called
the El Nino Southern Oscillation (ENSO).

Back in the 1920s meteorologist Sir Gilbert Walker noticed seasonal
fluctuations in the air pressure difference across the equatorial Pacific,
which he called the Southern Oscillation, and in the 1960s the realisation
came this was linked to El Nino and La Nina events. Inter-annual ENSO
variations can influence weather patterns worldwide, and researcher seek
to combine all available data for enhanced understanding and forecasting.

So today, as the Pacific warm pool shifts westward and La Nina's 'cold
tongue' of cool water extends across the eastern Pacific, it is being
monitored via a global ocean observing system that includes an important
space element.

Sea surface height (SSH) is not constant but varies across the global
ocean, with vertical expansion due to increased water temperature being
one of the main reasons why: warm water masses can stand up to a metre
higher than the surrounding sea. Satellite radar altimeters measure sea
surface height down to a maximum accuracy of two centimetres.

Instruments such as the Radar Altimeter-2 (RA-2) on ESA's Envisat bounce
1800 radar pulses per second off the Earth's surface, measuring their
return time to the nanosecond to calculate the precise signal distance
travelled. The data returned over the open ocean helps to chart changes in
sea surface temperature (SST). Altimetry data from Envisat and its
predecessor mission ERS-2 are made available in near-real time, then
blended with altimetry results from the French/US Jason-1 mission and the
US GFO to ensure the best possible global ocean coverage.

Current altimetry-derived sea level anomaly measurements show differences
in SSH of 60 cm between the west and east Pacific. This wide-area view
provided by altimetry-derived sea level anomaly data complements other
in-situ sources, including the 70 buoys of the Tropical Ocean Atmosphere
(TAO) array across the equatorial Pacific, operated by the US National
Oceanic and Atmospheric Administration (NOAA), the TRIangle Trans-Ocean
buoy Network (TRITON) array operated by Japan's Ministry of Education,
Culture, Sports and Technology and also the more than 2000 global Argo
profiling floats, which provide temperature and salinity profiles at
various depths across the global ocean.

These results are then assimilated into sophisticated numerical models by
weather centres worldwide, including the Reading-based European Centre for
Medium Range Weather Forecasting (ECMWF) to help provide 'initial state'
information to make seasonal forecasts of ocean states. The US NOAA
Climate Prediction Center at the start of February announced that the
conditions for a weak La Nina are in place -- with central Pacific SST
departing more than -0.5 C for the last three months -- and the event is
likely to last into late spring and possibly summer.

"The development of a negative anomaly does seem to be well in hand,"
agrees altimetry expert Christophe Maes of the Institut de Recherche pour
le Développement (IRD) in New Caledonia. "It will still take a few months
for the scientific community to better comprehend what is going on because
the system has often surprised us in the past.

"If it is necessary to be careful in forecasting El Nino then it is all
the more necessary for La Nina -- not due to any feminine mystique but
simply due to our lack of experience. La Nina events are simply less
frequent than their male equivalents. This asymmetry has yet to be
explained from a theoretical point of view at the current time."

The coming of La Nina is marked by shifts in global atmospheric
circulation patterns, as colder than normal waters in the eastern tropical
Pacific suppress rainfall in the vicinity, at the same time as increasing
it to the west.

During the northern hemisphere winter, wetter than normal conditions are
found over northern Australia and Indonesia. They also occur over the
Philippines during the northern hemisphere summer, and Indian monsoon
rainfall also increases, particularly in northwest India. Meanwhile South
America's tropical west coast undergoes drier than normal winters.

In turn these shifts affect the position and weaken the intensity of jet
streams and the behaviour of storms occurring beyond the tropics in both
hemispheres, including a summer decrease in hurricane activity in the
eastern tropical North Pacific and a corresponding increase in the number
of hurricanes in the tropical North Atlantic. The last La Nina took place
during 2000-2001, a weak event compared to the previous La Nina in
1998-2000 -- it, and the 1988-89 event together setting the terms of
reference for a 'standard' La Nina.

Each week radar altimetry adds a new frame to the ongoing film being shot
of the Pacific Ocean. Other satellite instrument types can gather
additional detail on La Nina as it occurs: Envisat's Advanced Along Track
Scanning Radiometer (AATSR) can measure sea surface temperature (SST) to
an accuracy of 0.2 C while the Medium Resolution Imaging Spectrometer on
the same spacecraft monitors changes in ocean colour due to associated
shifts in marine phytoplankton concentrations.

Through a lack of observational data, the scientific community found
itself blindsided by the 1982-1983 El Nino, then regarded as the 'El Nino
of the century'. Researchers' predictive ability was improved by the
international Tropical Ocean-Global Atmosphere (TOGA) programme of
1985-1989, which set up the first in-situ observing system -- including
the array of buoys that currently make up the TAO-TRITON network.

However this array was mostly concentrated in the equatorial Pacific, so
from the early 1990s onward, radar altimeters flown on ERS-1 and
TOPEX/Poseidon began to prove themselves valuable means of monitoring and
analysing ENSO events on a wider scale. In particular, researchers are
utilising altimetry data to monitor the exchange of warm water across the
Pacific in order to evaluate different theories of the build-up to ENSO
events.

ESA and altimetry

ESA has been flying radar altimeters in space ever since the launch of
ERS-1 in 1991, amassing a continuous 15 year dataset that covers not only
the global ocean but also freshwater bodies, land surfaces and the
cryosphere. This archive will extend further into the future, with a radar
altimeter set to be included on the Sentinel-3 mission planned to support
operational oceanography services developed as part of the Global
Monitoring for Environment and Security (GMES) initiative of the European
Union and ESA.

A major scientific symposium concerning '15 Years of Progress in Radar
Altimetry' is taking place in Venice Lido from 13 to 18 March 2006.
Jointly sponsored by ESA and the French space agency CNES, the event will
include discussion of current and future scientific and operational
applications of radar altimetry, and how altimetry data can be used in
synergy with other satellite and in-situ results as well as computer
modelling.

Related news

* Taking measure of the world: radar altimetry in spotlight at Venice
event
http://www.esa.int/esaEO/SEMPI66Y3EE_index_0.html
* ERS-2 data used in El Nino animation
http://www.esa.int/esaCP/SEMY0J2A6BD_Protecting_0.html

Related missions

* Envisat overview
http://www.esa.int/esaEO/SEMWYN2VQUD_index_0_m.html
* ERS overview
http://www.esa.int/esaEO/SEMGWH2VQUD_index_0_m.html

In depth

* 15 Years of Progress in Radar Altimetry
http://earth.esa.int/venice06/
* Radar Altimetry Symposium
http://earth.esa.int/venice06/
* GMES
http://www.esa.int/esaLP/LPgmes.html

Related links

* CNES
http://www.cnes.fr/html/_455_.php
* NOAA
http://www.noaa.gov/
* ECMWF
http://www.ecmwf.int/

[NOTE: Images supporting this release are avaiable at
http://www.esa.int/esaCP/SEMBFVMVGJE_index_1.html ]