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Variability of boundary currents in low latitude regions: the northern Indian Ocean and southern Pacific Ocean


This thesis examines the low-latitude western boundary current in the northern Indian Ocean, to understand its role in inter-basin salinity exchange, and that in the southern Pacific Ocean to understand its forcing mechanisms. Continuous volume transports along Sri Lankan eastern and southern coasts are inferred from in-situ bottom pressure and acoustic travel-time measurements. Together with remotely-sensed surface salinity, eddy salt flux is estimated; the flux produced by the boundary currents along Sri Lankan eastern (southern) coasts accounts for 11% (6%) of the salt input required to maintain salinity in the Bay of Bengal. In addition, glider measurements, historical hydrographic data, and Ocean General Circulation Models (OGCMs) are analyzed to better understand the variability of flow below 200 m depth and the associated eddy salt flux. Both observations and the OGCMs reveal the presence of an opposing undercurrent along the Sri Lankan eastern coast during the spring monsoon transition and southwest monsoon. However, the subsurface salinity range near the coast is small, resulting in low eddy salt flux produced by the undercurrent.

In the southern Pacific Ocean, flow derived from end-point dynamic heights and bottom pressures at the Solomon Sea southern entrance fluctuates greatly on subseasonal, seasonal, and interannual timescales. The subseasonal variability of flow in the 0-500 m depth layer (up to 25 Sv in one week) is partly forced by local and remote winds. The seasonal and interannual fluctuations (+/-10 Sv) are mainly influenced by westward-propagating Rossby waves from the interior of the Pacific Ocean. Anomalously high (low) equatorward volume transport during El Ni˜no (La Ni˜na) conditions is accompanied by a suppressed (enhanced) seasonal cycle. At the

Solomon Sea exits, continuous mooring measurements of the flow are also available; thus, the relationship between the Solomon Sea inflow and outflow is investigated over the overlapping period of July 2012 - February 2014. Variability of the inflow and outflow compares well, with mean volume transport above the 26.7 isopycnal at the exit of 17.3+/-2.1 Sv and that at the entrance of 13.8+/-3.3 Sv. The difference is likely contributed from flow through the unobserved Jomard Channel at the southwest of the basin.

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