Observations of Upper Ocean Currents at DOMES Sites A,B, and C in the Tropical Central North Pacific Ocean During 1975 and 1976

  • David Halpern
Part of the Marine Science book series (MR, volume 9)


The time and space (vertical) scales of the upper ocean current field are described from moored current measurements made at 20 m, 50 m, 100 m, 200 m, and 300 m depths at 15°02′N, 126°01′W (Site C) during September and October 1975, at 11°45′N, 138°23′W (Site B) during March and April 1976, and at 8°27′N, 150°45′W (Site A) during August, September and October 1976. During the periods when the measurements were made, Sites A and B were located within the eastward-flowing North Equatorial Countercurrent and Site C was within the westward-flowing North Equatorial Current. At Sites A and B the vector-mean eastward currents below the thermocline at 200 m and 300 m were approximately 10 cm sec-1 and 20 cm sec-1, respectively, which were much larger than a priori expected. At Site C the vector-mean current at 200 m and 300 m was about 5 cm sec-1 towards the east, indicating the presence of a subsurface current flowing in opposite direction to the surface flow which heretofore was not previously detected. At each site the northward direction of the vector-mean meridional current component in the mixed layer at 20 m depth was consistent with the direction of wind-generated Ekman transport resulting from the Northeast Trade-winds. The standard deviations were typically larger than the mean values indicating that the upper ocean currents were extremely time-dependent. Significant current fluctuations with inertial- and semidiurnal-periods were measured at each of the depths of the current meters. The amplitude of the inertial motion was typically 2 times larger than the semidiurnal tidal internal gravity wave motion; however, the inertial oscillations were largest in the near-surface mixed layer and the internal wave amplitude was ∿ 5 m or ∿ 1.5 cm sec-1 throughout the thermocline. Fluctuations with amplitudes of about 5 cm sec-1 to 10 cm sec-1 and time-scales greater than 20-days were measured beneath the thermocline, indicating that caution must be used in regard to the representativeness of month-long averaged currents.

The wind and mixed layer currents were coupled at time-scales greater than the local inertial-period. When the monthly averaged westward wind-stress of the Northeast Tradewinds increased, the near-surface eastward current speeds in the North Equatorial Countercurrent decreased and the speed of the westward-flowing North Equatorial Current increased, and vice-versa. Good agreement was found between the monthly averaged Ekman transport produced by the zonal wind-stress and the meridional transport computed from the near-surface current measurements. At the inertial-period the wind and the 20 m current fluctuations were statistically coherent (with 95% confidence) with approximately zero phase difference. The amplitude of the near-surface inertial motion was largest at Site A where the local inertial-period was approximately equivalent to the period of the easterly waves in the atmosphere between 5°N and 10°N, indicating the possibility of a resonant air-sea interaction process.

A comparison of the zonal components of baroclinic geostrophic currents computed from hydrographic measurements obtained coincidently with the moored current observations was made with low-pass filtered currents with the fluctuations at inertial-frequency and higher frequencies removed. Above 100 m the geostrophic and observed shears were significantly different because the uppermost 100 m contained the wind-driven currents. Between 100 m and 300 m the agreement between the geostrophic and measured shears was weak. At 300 m the observed currents were larger than the geostrophic currents by approximately 15 cm sec-1 indicating that the 500 m level-of-no-motion was not deep enough. However, the geostrophic current at 500 m relative to 1000 m was small (∿ 2 cm sec-1). Calculations of the magnitudes of various terms in the equations of motion indicated the importance of the time-dependence of the long-period fluctuations in the dynamical balance of the North Equatorial Countercurrent.

The large time-variability of the current speed and direction, the large vertical shear, and the large current speeds beneath the thermocline will produce huge, variable and unpredictable current drag on the pipe and cable assembly connecting the nodule mining ship moving at slow speeds and the collector moving along bottom contours. The presence of internal waves, which have a maximum vertical velocity about 100 times larger than typical settling velocities for bottom sediments discharged at the surface during the mining operation, will cause in situ measurements of settling velocity and of the corresponding identity of the effluent to be spurious unless measurements are made for time intervals comparable to the semidiurnal-period.


Mixed Layer Internal Wave Current Speed Mooring Line Geostrophic Current 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Plenum Press, New York 1979

Authors and Affiliations

  • David Halpern
    • 1
  1. 1.NOAA Pacific Marine Environmental LaboratorySeattleUSA

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