Klaus Wyrtki

El Niño—The Dynamic Response of the Equatorial Pacific Oceanto Atmospheric Forcing

Abstract El Nino 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 Nino is presented. It isshown that El Nino is not due to a weakening of the southeast trades over the waters off Peru, but that during the two years preceding El Nino, 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 Nino is theresult of the response of the equatorial Pacific Ocea...

Physical Oceanography of the Indian Ocean

During the preparation of the Atlas on the physical oceanography of the Indian Ocean (WYRTKI, 1971) a comprehensive analysis of its oceanographic conditions had to be undertaken to construct meaningful maps, sections and diagrams, and to interpret the structure and circulation of this ocean, which is in so many ways different from the other oceans. On a large scale 3 distinct circulation systems can be delineated. These are: I. the seasonally changing monsoon gyre II. the south hemispheric subtropical anticyclonic gyre III. the Antarctic waters with the Circumpolar Current

An Estimate of Equatorial Upwelling in the Pacific

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...

Mean water and current structure during the Hawaii-to-Tahiti shuttle experiment

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.

The oxygen minima in relation to ocean circulation

Abstract The distribution of oxygen in the ocea, as it is influenced by consumption and circulation, is discussed, and discrepancies in earlier theories are resolved. Curves of vertical distribution of oxygen in the oxygen minimum layer are calculated and compare well with the observations. It is concluded that the oxygen minimum layer is caused by the biochemical process of oxygen consumption, that its position and distribution are determined by circulation, and that the shape of the oxygen profile, especially the high position of its minimum, is due to the exponential decrease of oxygen consumption.

Fluctuations of the Dynamic Topography in the Pacific Ocean

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 v...

The Trade Wind Field Over the Pacific Ocean

Abstract The trade wind field over the Pacific Ocean between 30°N and 30°S has been studied on the basis of five million wind observations made from ships. Data were sorted by quadrangles of 2° latitude and 10° longitude to resolve north-south gradients in the wind field adequately. Maps of the surface wind stress vector for February and August are presented and the development of the field throughout the year is discussed. The trade wind regime in each hemisphere is largest and strongest during the respective winter and spring. The area covered by northeast trades is smaller than the area covered by southeast trades, but the northeast trades have a stronger mean wind stress and a larger annual variation both in area and mean stress. The computed divergence of the wind velocity revealed a little known area of convergence in the southeastern Pacific near the equator. The curl of the wind stress and the meridional profile of zonal wind stress vary considerably during the year. The minimum in zonal stress be...

Equatorial Currents in the Pacific 1950 to 1970 and Their Relations to the Trade Winds

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 n...

Equatorial Undercurrent Disappears During 1982-1983 El Niño

The equatorial undercurrent at 159�W decayed during August 1982, partially reversed during September, and rapidly reappeared in January 1983. The virtual disappearance is consistent with the basin-wide adjustment of sea surface slope to the strong westerly winds in the western and central Pacific that caused the 1982-1983 El Ni�o event.

Sea Level and the Seasonal Fluctuations of the Equatorial Currents in the Western Pacific Ocean

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.