Forced resonant undulation in the deep Mascarene Basin

Current meters moored for 19.5 months at Lat. 20°S in the deep water of the western Mascarene Basin recorded a distinct, large-amplitude [O(10 cm s−1)] undulation of bimonthly period, propagating westward at 7 cm s−1. Its characteristics demonstrate that it was a barotropic Rossby wave of relatively large meridional scale. Simple theory accounts for it as having been forced by local wind-stress curl at one of the resonant frequencies of the Mascarene Basin. A sharp bimonthly peak is also prominent in spectra of TOPEX/POSEIDON sea-surface height in the Mascarene Basin, but is not seen to the eastward, as is consistent with the local generation. Fluctuations of 45-day period reported earlier in the upper ocean just northeast of Madagascar might have been generated through a similar process, but with frequency shifted by the South Equatorial Current.