Tropical Upper Ocean Unbalanced Motions: Their Observed Characteristics and Implication for the SWOT Mission

Our recent analyses of the MITgcm LLC4320 output revealed that the horizontal scale at which the dominance of balanced motions to be taken over by unbalanced motions, lengthens from mid-latitude/subtropical to tropical oceans. Specifically, a typical transition scale in the tropics can exceed 200km. To understand the processes behind this long tropical transition scale, we examined dynamical height (DH) data obtained from the available TAO/TRITON moorings that span across the tropical Pacific between 8^oS and 8^oN. A wave frequency spectral analysis reveals that the DH in the western basin has a shallow spectral slope at -1, which is shallower than that predicted by the GM theory. In the eastern basin, the spectral slope reverts to -2 and follows the GM theory. By converting the wave frequency to wave number spectra via dispersion relation, we find that the unbalanced-motion DH variance in the 15-100km range is an order-of-magnitude greater than the noise level expected from the SWOT measurement due to the shallow spectral slope in the western basin. In the eastern basin, the DH variance is found to be ~5 times greater than the expected SWOT noise level. The large sea level signals to be measured by SWOT in the 15-100km range will likely provide a good opportunity to explore the unbalanced motions in terms of their dependence on surface wind, tide and balanced-motion forcings and of their impact upon mixing in the tropical upper ocean.