The Effects of Wind Forcing and Pycnocline Stresses on Zonal Currents in the Western Equatorial Pacific

Sean C. Kennan Nova Southeastern University Oceanographic Center, Dania Beach, Florida

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Pearn P. Niiler Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

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Abstract

Simultaneous moored temperature, salinity, velocity, and wind measurements from the equator at 157.5°E, during 10 May–21 December 1992, are combined with a no-stress-level boundary condition in the Equatorial Undercurrent core to estimate the total zonal pressure gradient force and subgrid-scale residuals of the momentum balance. Estimates are made of the depth of wind stress penetration, momentum depth, distribution of subgrid-scale stresses, and balance of forcing terms in the surface layer and pycnocline. Westerly winds of greater than 5 m s−1 in September 1992 coincided with the appearance of an eastward surface Yoshida jet and subsurface westward (Hisard) jet on the equator. The momentum depth increased with successive wind events, eroding the shallow halocline until it merged with the permanent thermocline. Wind-induced stresses were not restricted to the depth of density homogenization. The record-length-averaged pressure gradient force was westward and was balanced by downstream accelerations and stress drag. However, time-dependent accelerations were balanced by vertical divergence of the stresses. The pressure gradient dominated decelerations of the surface flows and played a lesser role in accelerating subsurface currents. The force balance was consistent with the concept of wind-driven surface flow above the momentum depth; in the pycnocline it implied forcing of the mean zonal currents via the eddy and turbulent momentum flux divergences. The results indicate that steady-state theories do not explain the existence of subsurface zonal currents on the equator. Time-dependent forcing in the equatorial pycnocline includes significant transfers of zonal momentum by submesoscale processes.

Corresponding author address: S. Kennan, NSU Oceanographic Center, 8000 N. Ocean Dr., Dania Beach, FL 33004. Email: skennan@nova.edu

Abstract

Simultaneous moored temperature, salinity, velocity, and wind measurements from the equator at 157.5°E, during 10 May–21 December 1992, are combined with a no-stress-level boundary condition in the Equatorial Undercurrent core to estimate the total zonal pressure gradient force and subgrid-scale residuals of the momentum balance. Estimates are made of the depth of wind stress penetration, momentum depth, distribution of subgrid-scale stresses, and balance of forcing terms in the surface layer and pycnocline. Westerly winds of greater than 5 m s−1 in September 1992 coincided with the appearance of an eastward surface Yoshida jet and subsurface westward (Hisard) jet on the equator. The momentum depth increased with successive wind events, eroding the shallow halocline until it merged with the permanent thermocline. Wind-induced stresses were not restricted to the depth of density homogenization. The record-length-averaged pressure gradient force was westward and was balanced by downstream accelerations and stress drag. However, time-dependent accelerations were balanced by vertical divergence of the stresses. The pressure gradient dominated decelerations of the surface flows and played a lesser role in accelerating subsurface currents. The force balance was consistent with the concept of wind-driven surface flow above the momentum depth; in the pycnocline it implied forcing of the mean zonal currents via the eddy and turbulent momentum flux divergences. The results indicate that steady-state theories do not explain the existence of subsurface zonal currents on the equator. Time-dependent forcing in the equatorial pycnocline includes significant transfers of zonal momentum by submesoscale processes.

Corresponding author address: S. Kennan, NSU Oceanographic Center, 8000 N. Ocean Dr., Dania Beach, FL 33004. Email: skennan@nova.edu

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