Quasi-biennial Rossby and Kelvin waves in the South Indian Ocean: Tropical and subtropical modes and the Indian Ocean Dipole

The quasi-biennial (QB) band (1.2 yr <= T < 3 yr) accounts for large part of the interannual variability in the South Indian Ocean (SIO). Despite that, it did not receive much attention to date, especially in the subtropical region. In the present work, we investigate the QB variability in sea surface height, the relationship between the tropics and the subtropics in this period band and the influence of the Indian Ocean Dipole (IOD). We use 24 years of altimeter-derived sea surface height anomalies and the MERRA-2 reanalysis winds. We find QB signals propagating westward in the tropical and subtropical SIO and eastward along the Equator; they are very likely to be expressions of Rossby and Kelvin-like waves in the QB frequency. A Complex Empirical Orthogonal Function (CEOF) analysis reveals two distinct reconstructed modes in terms of spatial distribution, which we refer as the tropical and the subtropical modes. The tropical mode explains about half of the QB variance and is given by the leading CEOF; the subtropical mode explains the QB variance in the subtropics and is formed by the superposition of the second to fifth CEOFs. In the subtropics, we show that the vertical structure of the background eastward-flowing South Indian Countercurrent affects the QB signal propagation. The tropical mode contains clear IOD spatial fingerprints, its temporal amplitudes increase in IOD years and has a high correlation with the Dipole Mode Index. Most importantly, the QB tropical waves that emerge in IOD years also appear in non-IOD years albeit with weaker amplitudes. An intriguing finding is the QB eastward propagation along the Equator, resembling a Kelvin wave. This eastward equatorial QB wave also has the highest amplitude in IOD-years, and the Maldives island chain at 73 degrees E seems to disturb its propagation.