The Structure And Variability Of Subthermocline Gyre And Water Mass Exchange East Of Mindanao

The thesis focuses on the variability of Mindanao Undercurrent(MUC), structure and variability of subthermocline anticyclonic gyre(ACG) related to the MUC, and the process/conveyer of semiannual water mass exchange east of Mindanao where the MUC locates.The northwest tropical Pacific is the important region for the circulation variability. Surface circulation could be investigated by many methods, such as satellite or buoy measurements. However, the subthermocline circulation is seldom described because of low covered observation. The state of the art ocean general circulation model for the Earth Simulator(OFES), with spatial resolution of 0.1×0.1° and time resolution of 1 month, makes it available to explore the structure and variability of the quasi-permanent ACG, which is a dominant feature of the sub-thermocline circulation in the southern Philippine Sea and is believed to be closely associated with the continuous northward flow of the Mindanao Undercurrent(MUC) east of Mindanao Island. Between 26.8-27.3 σθ, the ACG covers a large area. Its anticyclonic structure is however disconnected from the surface Halmahera Eddy(HE). Further analysis suggests that temporal variations of the ACG are linked to the variations of the northward transport of the MUC along the Mindanao coast. At seasonal timescale, stronger(weaker) ACGs during late winter-spring(summer) lead to larger(smaller) MUC transport. At the interannual timescale, the SACG(NACG) shows a power peak at 4-8(3-5) years. Strength of ACGs is significantly correlated with the MUC transport. Lead-lag correlation analysis indicates that interannual variations of the ACGs and the MUC transport are not strongly linked with the El Ni?o-South Oscillation(ENSO). Possible causes for the ACG variability are discussed.Intermediate water exchange in northwest tropical Pacific is primily based on synoptic or climatologic observations. Few attempts have been made to confirm relationships between water mass and current, either through measurements combining water properties and velocities, or through long-term continuous time series to determine any temporal vairiablity. Here we explored intermediate water exchange through analysis of a ~4 years of synchronous temperature, salinity and velocity records in the upper 800 m, obtained since late 2010 by a mooring system at 8°N, 127.05°E. For the first time, we report prominent semi-annual variability(SAV; ~ 187 days) of intermediate-layer current and a significant correlation with salinity, providing direct evidence for the alternating transports of South and North sourced intermediate waters by northward and southward undercurrents, respectively. Using the OFES outputs with resolution of 3-day, we demonstrate that the SAV is generated locally near the western boundary, manifesting as large-scale subthermocline eddies and leading to alternating northward and southward flows near the Mindanao coast, which plays an efficient role in the intermediate water exchange in northwest tropical Pacific.