Estimation of the Effect of Eddies on Coastal El Niño Flows Using along-Track Satellite Altimeter Data

During an El Niño anomalous westerly winds in the central equatorial Pacific push the equatorial water eastward, raising the sea level in the eastern equatorial Pacific and lowering it in the western equatorial Pacific. In the western equatorial Pacific, this sea level signal leaks through gaps in the western equatorial Pacific boundary, lowering sea level all around the western and southern coasts of Australia. Opposite anomalies occur during La Niña when the equatorial winds are easterly. Assuming geostrophic balance and no normal flow into the boundary at interannual frequencies, we expect the sea level anomaly to be spatially constant all along the western and southern Australian coasts, but instead it is only spatially constant along the boundary north of about 22˚S. South of this latitude, in the region of the Leeuwin Current, the amplitude of the interannual signal decreases. This unexpected drop is not explained by local wind stress, bottom friction, or Bernoulli conservation of energy of the large-scale flow. Theory is developed to show how this drop in interannual sea level may be explained by energetic, high frequency, small scale Leeuwin Current eddies that interannually dissipate the El Niño sea level signal along the coast. This theory is tested with coastal sea level station measurements and along-track altimetry data from TOPEX/Poseidon, Jason-1, and OSTM/Jason-2 satellites. The interannual variation in divergence of the eddy field momentum flux is calculated over a box along the southwest coast of Australia and found to be correlated with the interannual coastal flow through the box and the southward along-shelf sea level decay. The surprisingly short dissipation time scale is only about 2 days, much shorter than the interannual time scale. The theory is also tested off the western coast of South America where the original coastal El Niño flow was named. Interannual sea level decay along the shelf edge is observed, and the interannual southward flow along the shelf edge is found to be highly positively correlated with the along-shelf sea level decay with a dissipation time scale of a few days. Dynamics similar to the Australian case likely apply.