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Klaus Wyrtki

Abstract

The response of sea level in the equatorial Pacific Ocean during the 1976 El Niño event is analyzed and compared with the response during the stronger El Niño event in 1972. Monthly mean maps of sea surface topography illustrate the large horizontal scale of the internal wave associated with El Niño. Strong equatorial trade winds in 1975 increase sea level in the western Pacific. The relaxation of the wind in January 1976 allows an internal wave to form and proceed eastward, raising sea level along the eastern side of the ocean. This is followed by a year-long decline of sea level in the west, by an intensification of the North Equatorial Countercurrent and by a slackening of the South Equatorial Current. Strengthening of the winds in January 1977 terminates the 1976 event very rapidly. An estimate of the volume change of the warm upper layer gives a rate of 27 × 1061 m3 s−1 for the draining of warm water from the western Pacific Ocean over a one-year period. The 1976 El Niño event is concentrated predominantly north of the equator, in contrast to the 1972 event, and is terminated early by a renewed increase in the winds.

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Klaus Wyrtki

Abstract

Sea level records at many island and coastal stations in the equatorial Pacific Ocean have been used to study its response and that of the associated equatorial circulation to the 1972 El Niño. The response can be divided into five phases. Preceding El Niño, stronger than normal equatorial trade winds cause a buildup of sea level in the western Pacific in 1970 and 1971. After the wind strength peaks, sea level begins to drop slowly in the western Pacific. The first collapse of the wind field is followed by high sea level along the eastern border of the ocean, actually initiating El Niño off Peru. The initial oceanic response seems to consist of an equatorial Kelvin wave, which has been successfully modeled by others, and a strong reduction of the South Equatorial Current. During the third phase sea level drops very rapidly in the western Pacific, the equatorial trough is being filled, the South Equatorial Current retreats to the south of the equator, and the Countercurrent intensifies. Then follows a second peak of sea level along the eastern side of the ocean including the coast of Central America and extremely low sea level in the western Pacific. Thereafter, conditions return to normal, and sea level changes are even more rapid than at the onset of El Niño. The development of a weaker event in 1975 is also analyzed, and it is shown that such an event terminates after the second phase.

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Klaus Wyrtki

Abstract

Long equatorial waves recently discovered in the Pacific Ocean on satellite photographs are being linked to oscillations in sea level at Fanning Island and to the oscillatory trajectory of a drifting buoy. The drift pattern of the buoy suggests that lateral oscillations of the Equatorial Countercurrent with a period of about 34 days are responsible for the observed variations of sea level.

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Klaus Wyrtki

Abstract

Observations taken during the Hawaii to Tahiti Shuttle Experiment allow identification of eddies drifting westward with the North Equatorial Current. During 17 months only three eddies passed through the area and their individual residence time is about four months. They probably represent non-dispersive first-mode baroclinic Rossby waves. The eddies represent no real problem for monitoring the strength of the North Equatorial Current since they can be easily recognized in temperature sections and are located near the center of the current.

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Klaus Wyrtki

Abstract

Upwelling in the equatorial Pacific Ocean manifests itself by a tongue of cool water stretching from the Galapagos Islands to the date line. To estimate the rate of upwelling, the mass, heat and salt budgets of the tongue are investigated. The Ekman divergence is determined from wind stress as 84 × 106 m3 s−1. It is compensated by geostrophic convergence of the same magnitude as determined from the zonal pressure gradient. Since the vertical distribution of the two meridional flows is different, a strong vertical circulation results, which leads to upwelling at a rate of ∼50 × 106 m3 s−1. A consideration of the heat budget leads to the conclusion that horizontal advection in the South Equatorial Current does contribute to the cool tongue, but that the contribution of upwelling is much larger. The heat budget also indicates that upwelling comes from depths above the core of the undercurrent and that the source water has temperatures only ∼3°C less than the water flowing out laterally. The seasonal variation of all properties associated with the cool tongue is strong and produces a cross-equatorial flow of water from the summer to the winter hemisphere of ∼20 × 106 m3 s−16.

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Klaus Wyrtki

Abstract

The seasonal variations of the dynamic topography relative to 500 decibars in the western equatorial Pacific, using data from 6900 hydrographic stations, are compared with the seasonal fluctuations of sea level observed at eleven islands. It is shown that the changes in the meridional profile of sea level correlate with changes in the strength of the major ocean currents measured by their speed or by geostrophic transports. The strength of the North Equatorial Current and that of the Countercurrent vary synchronously, both being strong in fall and weak in spring. The South Equatorial Current varies exactly out of phase with the two Northern Hemisphere currents, but is apparently in phase with the Undercurrent. Investigation of the particular example indicates that large anomalies seem to behave in the same fashion as the seasonal fluctuations of these currents.

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Klaus Wyrtki

Abstract

The mean meridional profile of dynamic topography across the zonal currents of the equatorial Pacific Ocean is disturbed by the observed deviation of monthly sea level at island stations from the long-term mean. From these monthly profiles the sea level difference across the major zonal currents for the period 1950–70 can be derived. A strong seasonal signal is apparent in the intensity of the currents, as well as large short- and long-term anomalies. Most of the energy of the currents is in the low frequencies. The North Equatorial Current and the Countercurrent fluctuate synchronously and in opposition to the South Equatorial Current. The fluctuations of the currents are related to the trade winds and are more strongly influenced by the position of the trade winds than by their strength. When the northeast trades are strong and in a southerly position during the first half of the year, both the North Equatorial Current and the Counter-current are weak; when the trades are weaker and in a more northerly position during the second half of the year, both currents are strong.

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Klaus Wyrtki

Abstract

El Niño is the occasional appearance of warm water off the coast of Peru; its presence results in catastrophic consequences in the fishing industry. A new theory for the occurrence of El Niño is presented. It isshown that El Niño is not due to a weakening of the southeast trades over the waters off Peru, but that during the two years preceding El Niño, excessively strong southeast trades are present in the central Pacific.These strong southeast trades intensify the subtropical gyre of the South Pacific, strengthen the SouthEquatorial Current, and increase the east-west slope of sea level by building up water in the western equatorial Pacific. As soon as. the wind stress in the central Pacific relaxes, the accumulated water flows eastward,probably in the form of an internal equatorial Kelvin wave. This wave leads to the accumulation of warmwater off Ecuador and Peru and to a depression of the usually shallow thermocline. In total, El Niño is theresult of the response of the equatorial Pacific Ocean to atmospheric forcing by the trade winds.

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Klaus Wyrtki

Abstract

The dynamic topography of the Pacific Ocean has been mapped and its mean annual and random variability has been investigated using approximately 66,600 hydrographic stations. Largest annual fluctuations are associated with the equatorial current system and are probably due to vertical displacements of the thermocline in response to the changing wind field. In the subtropical gyres and in higher latitudes, dynamic topography varies seasonally in response to the annual cycle of surface layer temperature, but the pattern of dynamic height does not change much between summer and winter. In the western portion of the North Pacific anticyclonic gyre, a U-shaped ridge is developed with its open end pointing east, which separates the western boundary current from the interior of the gyre and links the north equatorial ridge with a ridge at the right flank of the Kuroshio. The variability of dynamic height given by its standard deviation increases from east to west across the Pacific Ocean. Most of this variability is due to fluctuations in the position of currents, their meandering, and the presence of eddies.

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Klaus Wyrtki and Bernard Kilonsky

Abstract

Mean sections based on data from a discrete one-year period during the Hawaii-to-Tahiti Shuttle Experiment are used to describe and analyse the equatorial current system, its water masses and the transverse circulation in the central Pacific Ocean. Computations of mean geostrophic transports for the individual currents identify the south equatorial current as the strongest flow, transporting 55 × 106 m3 s−1. Information derived from the water mass and nutrient distributions is used to develop a consistent picture of the transverse circulation and of the role of the different divergences and convergences.

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