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No. 99: May-Jun 1995

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90-DAY SEA-LEVEL OSCILLATION AT WAKE ISLAND

Most North Americans are familiar with rather powerful diurnal tides. The oceans, however, also move in ponderous cycles that beachcombers can never appreciate. Thanks to data from Geosat's precision altimeter, geophysicists can now discern some of these long-period moving patterns on the oceans' surfaces.

"Energetic 90-day oscillations of sea levels have been intermittently observed at Wake Island in the western tropical Pacific during the past 2 decades. The oscillations tend to occur about 1.5 years after El NinoSouthern Oscillation events, to have amplitudes of 10-15 cm, and to persist for about 1 year. Sea-surface heights from the Geosat altimeter are used to establish that these signals take the form of Rossby waves and have an energy source near the Big Island of Hawaii, which lies 40 of longitude to the east. Sea-level and upper-layer currents from an eddy-resolving numerical model are examined and suggest that the energy source is eddies generated off the Big Island of Hawaii. These eddies appear to be associated with westward currents that intermittently impinge on the island."

(Mitchum, Gary T.; "The Source of 90-Day Oscillations at Wake Island," Journal of Geophysical Research, 100:2459, 1995.)

Comment. Such eddies would have to persist for long periods to survive the long trip to Wake Island some 2500 miles away. In this, they must be like the current rings that break off from the Gulf Stream in the Atlantic.

From Science Frontiers #99, MAY-JUN 1995. 1995-2000 William R. Corliss