Oceanography's Perspective

As it happened, much-needed help was gathering in another quarter--the ocean sciences. Without something like weather prediction as a driving force in its development, oceanography lacked the sea-going equivalent of weather balloons for monitoring the oceans. For years oceanographers had to rely on studies carried out during voyages by individual vessels. In the 1970s more systematic and broader-based efforts at monitoring the world's oceans began. Some of those programs focused on the variability of the tropical oceans and on phenomena that could shed light on El Niño.

A key contribution, confirming Bjerknes's insight that the effects of El Niño were not confined along the west coast of Peru and Ecuador, came when Klaus Wyrtki of the University of Hawaii and his colleagues collected and charted tidal records and wind patterns across the Pacific basin. In 1975 Wyrtki established that strong trade winds essentially push the warmed surface waters to the west along the equator until they pile up against the coast of Indonesia. This thickened layer of warm water, which raises the sea level in the western Pacific by as much as 18 inches, effectively depresses a layer of subsurface water called the thermocline, a kind of interface between the warm surface waters and the much colder deep ocean. In the eastern Pacific, by contrast, the warm surface layer is much thinner. As a result, the thermocline lies nearer the surface, as do cold waters welling up from the deep ocean and bringing with them the nutrients that support abundant fisheries. Wyrtki's work suggested that, when the trade winds fail, they release waves of warm water that move west to east across the Pacific Ocean, pushing the thermocline deeper in the eastern Pacific and suppressing the upwelling of cold water from the deep ocean. As a result, sea surface temperatures in the east rise, and the surface water in the eastern Pacific becomes deprived of nutrients needed to maintain certain fish populations. Because of the delayed response of the eastern Pacific to the wind changes, Wyrtki recognized the potential for predicting such events in advance.

Under normal conditions, the steady equatorial tradewinds move air westward; there warm air rises, condenses and rains heavily in the western Pacific. In El Niño conditions, lower air pressure in the east weakens the tradewinds, thus causing abnormal rainfall along the west coasts of North and South America. Temperature gradient: red, orange, and yellow (warm); aqua, green and blue (cool). (NOAA/Environmental Research Labs, Pacific Marine Environmental Laboratory)

As with everything associated with the ENSO, this redistribution of warm surface water across the Pacific displays a periodic, although irregular, character involving a complex interplay of waves, currents, and undercurrents that appear and disappear in response to changes in the winds. Oceanographers wrestling with these effects were also turning to computers for help. In the mid-1970s scientists began designing numerical models to simulate what goes on in the oceans. Using idealized computer models that treat the upper ocean as a layer of uniform temperature overlying a deep, cold ocean, they attempted to reproduce the redistribution of warm surface water. Their aim was to see what happens to the thickness of the upper layer and depth of the thermocline in response to changes in the winds. These models showed that changes in the winds over the western Pacific could indeed cause the changes in eastern Pacific sea levels associated with El Niño. In the early 1980s more realistic oceanic models were developed, in which ocean temperatures varied both horizontally and vertically. Using this model, researchers were able to reproduce the main oceanic aspects of ENSO, including sea surface temperature changes, as long as they had data about the winds for the period in question.

Any hope of documenting the actual details of the movement of warm surface water depended on continuous measurements of subsurface ocean conditions along the equator. But these measurements required maintaining moored buoys at the equator over long periods, which was considered too difficult due to strong equatorial currents. In the early 1980s David Halpern of the National Oceanic and Atmospheric Administration (NOAA) in Seattle and like-minded colleagues determined to prove conventional wisdom wrong. They pieced together funding from various programs to set up lines of moored buoys located near the equator at longitude 110° W and 140° W. Today, measurements with improved instruments continue at these and many other locations.

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