Central equatorial Pacific zonal currents. I: The Sverdrup balance, nonlinearity and tropical instability waves. Annual mean dynamics

Several recent observational studies of central Pacific equatorial current dynamics have suggested that, in the vertical integral between the surface and the thermocline, the linear equatorial Sverdrup balance holds. However, in a high vertical resolution ocean general circulation model, we find that nonlinearity is an order (1) element of the local and the vertically integrated balances on and near the equator at 140W. Although this OGCM has been used in many studies of the tropical Pacific, its equatorial zonal momentum equation balances have never been described in detail and compared with observations. We describe the annual mean balances here, identify the similarities and differences between the model balances and observational estimates of the balances, and discuss various reasons why the model and the observations may disagree in the respects that they are found to do so. The term balances vary strongly with latitude and depth; the system is nonlinear and three dimensional. There is little tendency for pairs of terms (e.g., the meridional and vertical advection terms) to balance locally or in the vertical integral. Every term in the zonal momentum equation plays a role somewhere in the analysis region discussed here. Thus the generality of point estimates of these balances is small. The Tropical Instability Wave zonal momentum flux divergence, although not an O(1) term in the balance, acts like a negative viscosity' over the upper 40 m on the equator; its tendency is to drive westward flow. If the ocean balances resemble those of the model dynamics, gaining detailed perspective on the zonal balances will require a major observational effort. Because there is strong subseasonal and interannual variability of the flows in the central equatorial Pacific, time-mean balances are not simple to estimate. Further, special attention will have to be given to resolving the shears in the upper 50 m, because it is over these depths that the model and observational results differ most strongly. We suggest that the widely used technique of extrapolating the near-surface currents based on their shears in the uppermost bins of the ADCP profiles deserves careful scrutiny; subsampling the model flow profiles in this fashion leads to important errors. Until the strong vertically sheared very near-surface current field is observed accurately it will not be possible to determine if the model results are correct, but we suggest that the existing observational results should not be regarded as definitive.