NASA sponsored scientists have discovered by knowing
the salt content of the ocean’s surface, they may be able to
improve the ability to predict El Nino events. Scientists,
studying the western Pacific Ocean, find regional changes in
the saltiness of surface ocean water correspond to changes
in upper ocean heat content in the months preceding an El
Nino event. Knowing the distribution of surface salinity may
help predict events.
Salinity and temperature combine to dictate the ocean’s
density. Greater salinity, like colder temperatures, results
in an increase in ocean density with a corresponding
depression of the sea surface height. In warmer, fresher
waters, the density is lower resulting in an elevation of
the sea surface. These ocean height differences are related
to the circulation of the ocean.
The surface salinity in two regions contributes to El Nino
events: an area of warmer temperatures and lower salinity in
the western Pacific, and the higher salinity and cooler
temperatures in the eastern Pacific. Differences in surface
salinity are related to changes in temperature and upper
ocean heat content, which are part of the El Nino
phenomenon. They have the potential to influence the Earth’s
climate through air-sea interaction at the ocean’s surface.
The study, conducted for NASA by University of Maryland
researchers Joaquim Ballabrera, Tony Busalacchi, and Ragu
Murtugudde, is one of the first to look at ocean salinity in
El Nino, Southern Oscillation (ENSO) predictions and their
relationship to tropical sea surface temperatures, sea
level, winds, and fresh water from rain. Results of the
study are in the latest issue of the Journal of Geophysical
Research – Oceans.
Ballabrera and his colleagues looked at data, from 1980 to
1995, about sea surface temperatures, winds, rainfall,
evaporation, sea surface height, and latent heat, the energy
released when water vapor condenses into droplets.
Using computer models, they performed a series of
statistical predictions of the El Nino events for such a
period. The results indicate short-term predictions only
require monitoring sea surface temperatures, while
predictions over a season require the observation of sea
level. They concluded observations of salinity significantly
improve predictions. When changes in salinity occur, they
affect the El Nino event for the next six to 12 months. In
this lag time, salinity changes have the potential to modify
the layers of the ocean and affect the heat content of the
western Pacific Ocean; the region where the unusual
atmospheric and oceanic behavior associated to El Nino first
develops. “As a result, when changes in ocean saltiness are
considered, improvements are found in El Nino forecasts six
to 12 months in advance,” Ballabrera said.
“This research holds tremendous potential for the NASA
Aquarius mission to monitor the surface salinity of the
global ocean,” Busalacchi said. Aquarius is scheduled for
launch during 2006-2007. Aquarius will provide the first
global maps of salt concentration on the ocean surface. Salt
concentration is a key area of scientific uncertainty in the
oceans’ capacity to store and transport heat, which in turn
affects Earth’s climate and water cycle.
By using remote sensing data from satellites, scientists
will be able to see changes in ocean salinity. Knowing the
lag time factor, computer models simulating the movement of
the atmosphere may be able to accurately predict El Nino
episodes. This may lead to longer lead-time for predictions
of ENSO events.
Florida State University, the National Center for
Environmental Prediction, National Center for Atmospheric
Research and the Etudes Climatiques de l’Ocean Pacifique
tropical program at Institut de Recherche pour le
Developpement, Centre de Noumea contributed ocean and
atmosphere data to this study.
For more information and images, see:
http://www.gsfc.nasa.gov/topstory/2003/0114salt.html
http://essic.umd.edu/~joaquim/salinity/
The National Oceanic and Atmospheric Administration’s El
Nino Web Page:
