Leading-Mode Connections of the Interannual Variability in Upper-Ocean Salinity in the Tropical Indian Ocean

Leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO) and their connections were studied based on 17 years (2002-18) of oceanic historical and reanalysis data. Empirical orthogonal function (EOF) analysis depicted the dominant roles of the first two leading modes in salinity variability in the TIO over a wide range of interannual time scales. Among the rich oscillations of the leading EOF modes, a coherent near-biennial band was identified with basinwide loading of sea surface salinity anomalies (SSSa) (EOF1) leading/lagging the northeast-southwest dipolar mode of SSSa (EOF2) by around 4 months across the TIO, with southwestward migration of SSSa center. The spatial loadings of the SSSa leading modes in the TIO were strongly shaped by sea surface temperature-related freshwater fluxes and wind-driven regional ocean circulation on a near-biennial time scale. Composite analysis of the mixed layer salinity budget reflected characteristic features of basin-scale ocean-atmosphere coupling, both temporally and regionally during the life cycle of the near-biennial fluctuation in anomalous salinity in the TIO. Consistent with the intrinsic oscillation paradigm in the observed Indian Ocean dipole (IOD) variation, the dynamic and thermodynamic feedbacks associated with switches from the positive to negative IOD modes provided the phase-connection mechanisms for the SSSa leading-mode displacement over the TIO. SIGNIFICANCE STATEMENT: This study investigates the leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO). The intrinsic oscillation and associated dynamic and thermodynamic feedbacks over the TIO drive the basinwide connections of upper-ocean salinity variability. Our results show that a coherent near-biennial band is identifiable within the leading modes of sea surface salinity anomalies (SSSa), in which the wind-induced horizontal advections and evaporation-minus-precipitation anomalies associated with the switches from positive to negative Indian Ocean dipole modes mainly provide the phase-transition mechanism of SSSa. This research illustrates substantial evidence for the displacement of basin-scale sea surface temperature anomalies modulating the structures of SSSa and inducing the dynamical connections of leading modes of SSSa on the near-biennial time scale.