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

Vasconcellos de Menezes, V (2015) The structure and dynamics of the eastward flows of the South Indian Ocean. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Ocean circulation; lndian ocean; Eastward currents; Salinity; Seasonal and interannual variability; Fronts and jets; South Indian countercurrent; Eastern Gyral current;
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Copyright 2015 the author

Date Deposited: 14 Mar 2017 22:55
Last Modified: 14 Mar 2017 22:55
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