Pacific Decadal Oscillation

While change may be on the way, the Pacific is still dominated by the strong,
larger-than- El Nièo/La Nièa pattern called the Pacific Decadal Oscillation (PDO),
according to the latest data from the U.S.-French TOPEX/Poseidon satellite mission,
managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif. The PDO
is a long-term ocean temperature fluctuation of the Pacific Ocean that waxes
and wanes approximately every 10 to 20 years. "This continuing PDO pattern of the
past three years signals more of the unusually dry conditions that have afflicted
the North American west coast," said JPL oceanographer Dr. William Patzert.

The data, shown above, were taken during a 10-day collection cycle ending June 11, 2001.
Purple and blue indicate lower than normal sea level, green indicates normal
sea level, and yellow, red, and white indicate higher than normal sea level.

The new satellite image also shows a pulse of warm water traveling
toward South America, a reminder that another El Nièo is due in the next year or so.
El Nièos generally return every two to seven years; the last one occurred in 1997.
This equatorial, eastward-traveling Kelvin wave (a bulge of warm water)
is headed toward South America at about 140 degrees West longitude.
In late July, when this wave arrives at the west coast of South America, there should be a
modest warming of the eastern Pacific. Kelvin waves, often seen before an El Nièo develops,
are triggered by westerly wind bursts (i.e., winds blowing in the opposite direction from the
normal easterly trade winds) in the western Pacific. Also, the strength of El Nièo's
next appearance could depend on how much the PDO dominates ocean circulation and
temperature patterns over the next few years.