The structure and dynamics of the eastward flows of the South Indian Ocean

This thesis investigates the structure of the broad near-surface eastward flow that dominates the circulation of the South Indian Ocean (SIO). This eastward flow extends from Madagascar to western Australia between 20°S and 30°S and is now known as the South Indian Countercurrent (SICC). The study also includes the tropical Eastern Gyral Current (EGC), located east of 90°E, spanning between 15°S and 20°S. Both currents advect waters into the Leeuwin Current, the only poleward-flowing eastern boundary current of the global ocean. A particularly interesting feature of these eastward currents is that they flow in a direction opposite to that predicted by the classical theories of wind-driven circulation. In a broad sense, this study aims to build a detailed picture of these quasizonal currents that are still poorly known. Specific goals of this thesis include: i) to characterize the mean structure of the SICC and associated fronts; ii) to investigate whether the genesis of the SICC can be explained by the Subtropical Countercurrent-Potential Vorticity (STCC-PV) paradigm, as suggested in the literature; iii) to analyse the SICC variability across interannual time scales; iv) to explore newly available salinity data and to verify if the salinity pattern is a key factor in understanding the peculiar distribution of the SIO eastward currents. These goals are accomplished by thorough analyses of several in situ and remote sensing datasets, reanalysis products, and high-resolution simulations that only recently became available. Several diagnostic quantities are derived from these datasets to help understand the physics of the eastward currents. Diverse tools of analysis (e.g. Empirical Orthogonal Functions(EOF), Complex EOF, Singular Spectrum Analysis and Wavelet) are used to extract the maximum possible information from these large datasets. The thesis is organized into four self-contained papers, each one studying different aspects of the goals described above. A first major finding is that the tropical eastward Eastern Gyral Current is very likely a salinity-driven current. In the EGC region, salinity overwhelms the temperature contribution to density gradients, generating a near-surface eastward geostrophic vertical shear that forms the EGC. It is shown that without the strong salinity front between the fresh waters of the Indonesian Throughflow and the salty subtropical waters, the EGC cannot be maintained. Analysis of the new Aquarius satellite sea surface salinity and Argo floats data led to the second major finding of this research. The annual cycle of sea surface salinity (SSS) in the SIO is characterized by propagating features that can be interpreted as a superposition of propagating planetary wave modes (Kelvin and Rossby waves). This is the first work in the literature to detect signatures of Rossby waves in observed SSS fields. Previously, Rossby wave signatures in SSS have been described only from high-resolution model runs (HYCOM). The SIO salinity waves are shown to be strikingly different from the sea surface height waves obtained by altimetry. The third major finding is the multiple jet structure of the SICC. Although the SICC and the EGC look similar in a circulation map, they are dynamically different. The SICC vertical shear arises from thermal meridional gradients while the EGC is generated by salinity gradients. The STCC-PV theory only explains the existence of the southern SICC jet. The SICC multiple jet structure seems to be more related to the formation of PV staircases in the SIO. This result suggests that the dynamics of the SICC jets is basically related to wave-mean flow interaction processes. The SICC interannual variance is dominated by the quasi-biennial band. This band has two distinct spectral peaks, with the main peak in the 1.5-1.8 yr interval and a secondary (weaker) peak centred at 2.1-2.5 yr, the latter mostly occurring west of 80°E. Interannual and decadal-type modulations of the quasibiennial signal are also identified. Within the quasi-biennial band the SICC flow presents two main configurations with a multiple jet structure. One pattern is characterized by a robust northern jet, while in the other the central jet is more well developed. The quasi-biennial signal propagates westward with phase speeds characteristic of Rossby waves. Interestingly, in the southern SICC jet domain the observed quasi-biennial Rossby wave is 2 to 5 times faster than that predicted by the standard linear Rossby wave theory. The brief analysis of the quasi-biennial Rossby wave stresses the importance of taking into account the vertical structure of the SICC in order to fully understand the interannual Rossby wave propagation characteristics in the South Indian Ocean.