The region of large sea surface height variability in the southeast Pacific Ocean

Satellite altimeter observations of sea surface height (SSH) show an extensive triangular region of high variability in the southeastern Pacific. The region is unusual in two ways. First, it appears to have a horizontal correlation scale that is much larger than that associated with the mesoscale eddy-rich regions of the deep ocean. Second, the feature appears to reflect the shape of the underlying topography. Previous computer studies have suggested that it is a Rossby wave trapped by topography. Here detailed results are presented from a study that used a high-resolution global ocean model forced by 6-hourly ECMWF winds. The empirical orthogonal modes of the region are calculated, and it is shown that 29% of the SSH variance is due to a single mode with a similar shape to the region of high variability. The time series of the mode is correlated against the local Ekman pumping, and it is shown that the largest correlation is obtained when the lag is approximately 24 hours and the wind curl is averaged over a region roughly the size of the mode itself. The response function relating the amplitude of the mode time series to the amplitude of the forcing is calculated and is found to be dominated by a resonance at zero frequency and a decay time of 2.7 days. The results indicate that the feature is a highly damped geostrophic mode of the ocean.