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The US Navy had never seen this signal before in its decades of listening under the waves, according to a 2002 New Scientist article. Upsweep was heard straight across the Pacific, ruling out certain localized or small-scale sources.
The sound persisted, with a peak in strength in 1994. Since then, it has been subsiding, though it remains audible, reports the National Oceanic and Atmospheric Administration (NOAA). It seems to peak in the spring and fall. This could be related to the source of the sound, or it may simply be that the sound travels better in the aquatic environment at these times for whatever reason, says NOAA.
The sound is uniform overall, unlike the varied sounds of most volcanic activity and unlike the varied intonations of whale communications. But Emile Okal of Northwestern University in Chicago and Jacques Talandier, formerly with the French Atomic Energy Agency, suggested a particular type of volcanic activity that may be the cause.
Between the two trade-wind belts is a region of generally light winds, known as the Doldrums. This allows water, which would otherwise pile up against the western boundary of the ocean in the Equatorial currents, to flow back eastwards in the surface Equatorial Countercurrent. There is also an eastward-flowing Equatorial Undercurrent, which forms a jet within the thermocline , driven by the horizontal pressure gradient . This system of eastward-flowing and westward-flowing currents is found in the upper 200 meters (650 feet) in all three oceans, although their distribution may change seasonally depending on the wind forcing. The equatorial undercurrents are much stronger than the surface currents at the equator, and can have flows of more than 50 Sv.