Spatial And Temporal Variations Of Thermocline Water Masses In The Western Tropical Pacific Ocean And The Effect Of Mesoscale And Finescale Oceanic Processes

The western tropical Pacific Ocean is an interesting and important regionfor Ocean/Climate research, because its dynamic and thermal variations areclosely related with not only climatic modulations in tropical oceans, but alsoweather/climate, environment, fishery, and military of China. This region isof the most complicated circulation pattern in the world ocean, includingequatorial currents, western boundary currents, and through-flows to theIndian Ocean. Currents bring subsurface water masses from different origins,which are intensively mixed and subsequently exported to various importantdownstream areas, like equatorial Pacific, tropical Indian Ocean, and ChinaSeas, marking the western tropical Pacific Ocean as a crossroads forthermocline water masses. Therefore, our work focusing on thermoclinewater masses in the western tropical Pacific Ocean is important not only in aphysical oceanographic sense, but also for better understanding thelow-frequency climate changes of tropical oceans and China.In this study, various existing oceanic observations, including Argo floatprofiles, in-situ measurements of recent research cruises, historicalhydrographic records, are used, combining with altimetric sea surface height(SSH) and wind stress data, to investigate the spatial-temporal variations ofthermocline water masses in the western tropical Pacific Ocean. During theinvestigation, the description mainly focuses on several interestingphenomena and emphasizes the roles of mesoscale and fine-scale oceanicprocesses which are less explored in previous water mass researches.Spreading and salinity change of the North Pacific Tropical Water(NPTW) in the Philippine Sea (PS) is at first explored. The spreading of NPTW in the PS is closely related with NMK current system, with theKuroshio and the Mindanao Current (MC) transporting comparable amountof NPTW to the subtropics and tropics, respectively. Estimated for subsurfacewater with S>34.8psu, the southward (northward) geostrophic transport ofNPTW by the MC (Kuroshio) at8°N (18°N) is about4.4(5.7) Sv (1Sv=106m3s-1), which is not sensitive to reference level choice. Fields of subsurfacesalinity maximum (Smax), current, sea level variation, and potential vorticity(PV) suggest that the equatorward spreading of NPTW to the tropics isprimarily afforded by the MC, whereas its poleward spreading is achieved byboth the Kuroshio transport along the coast and open-ocean mesoscale eddyfluxes. The NPTW also undergoes a prominent freshening in the PS. Lyingbeneath fresh surface water, salinity decreases quicker in the upper part ofNPTW, which gradually lower the Smaxof NPTW to denser isopycnals.Salinity decrease is especially fast in the MC, with along-path decreasing ratereaching O(10-7psu s-1). Both diapycnal and isopycnal mixing effects areshown to be elevated in the MC due to enhanced salinity gradients near theMindanao Eddy.We then address the interannual variations of NPTW in the PS, whichshows an out-of-phase salinity changes between the northern and southern PS(NPS and SPS), with peak-to-peak amplitudes exceeding0.1psu. Thesesalinity anomalies are mainly generated locally by anomalous cross-frontgeostrophic advections. In2003, an anomalous cyclonic circulation developsin the PS, transporting more (less) than normal high-salinity NPTW to theNPS (SPS) and produces positive (negative) salinity anomalies there. In2009,an anomalous anticyclone emerges, which produces negative (positive)salinity anomalies in the NPS (SPS). These year-to-year variations are closelyassociated with ENSO cycle. During strong El Ni o (La Ni a) episodes,positive (negative) wind stress curl anomalies between8°and18°N evoke westward propagating upwelling (downwelling) Rossby waves in the centralPacific and positive (negative) anomalous Ekman pumping in the westernPacific, resulting in the anomalous cyclonic (anti-cyclonic) circulation in thePS. Therefore, the observed current variations and resultant salinity changesin NPTW layer is mainly oceanic response in the PS to strong ENSO events.The next problem focused is the climatologic characteristics ofthermocline water in the North Pacific tropical gyre (referred to as the TSSW),which exists as a vertical maximum and lateral minimum in salinity fieldbetween130°E-150°W,5°N-10°N,22.5-25.5σθ. Comparing with NPTW, theTSSW are of lower salinity, lower oxygen, and much higher nutrientconcentration, which is primarily due to atmospheric wind/freshwaterforcing in the tropical gyre. In the western Pacific, Smaxof TSSW is mainlyformed by the southward intrusion of NPTW. Intensive diapycnal mixingwith fresh surface water reduce its salinity and lower its Smaxto denserisopycnals. For The TSSW in the western Pacific can be regarded as a dilutedportion of NPTW. For TSSW in the central Pacific, influences of NPTW, SouthPacific Tropical Water (SPTW), and East Pacific fresh water are all important.It can be regarded as a mixture of various origins. The salinity of TSSW showswell defined annual cycle, which is controlled by both fresh water fluxes byprecipitation and cross-front salinity advection by current variations. Thesouthern (northern) boundary of TSSW corresponds to the wave guide ofequatorial (off-equatorial) Rossby waves. Westward propagating1st-orderbaroclinic annual Rossby waves are the main cause of current changesassociated with TSSW annual variations.Another point addressed in this study is the characteristics ofthermohaline intrusions in the western tropical Pacific Ocean and their role inlarge-scale water mass conversion. Using hydrographic measurements of three recent cruises, we have revealed the existence and richness ofthermohaline finestructure in the subsurface and intermediate layers off thenorthern Philippine coast, which are shown to be mainly thermohalineintrusions. Also shown is that the intrusions in NPTW layer is mainly drivenby double diffusion. The westward extension of NPTW salty tongue produceslateral salinity gradients in the NPS which is enhanced by the stirring ofactive mesoscale eddies in this region. Driven by double diffusion, intrusionsdevelop at these gradients and significantly enhance lateral mixing whichaccelerates the property change of water masses like NPTW. By linkinglarge-scale water mass changes and mesoscale eddy stirring to micro-scalediffusion, fine-scale thermohaline intrusions play an important role in theocean energy cascade of this region.Using existing high-resolution hydrographic data, spatial distributionand temporal variations of intrusions are investigated in the western tropicalPacific Ocean. Our results show that, in the upper thermocline, intensiveintrusions mainly exist at the equatorial front (EF), which is produced by theinterleaving of water masses from southern and northern hemispheres. In thelower thermocline, strong intrusions show a 'C'-shape distribution, residingin the vicinity of the EF, tropical front (TF), and MC, i.e., the westernboundary pathway of the North Pacific subtropical cell (STC). Synopticsection analysis reveals that intrusions are more prominent on thewarm/salty flank of the fronts, implying more cross-front tongues ofcold/fresh water. Among the intrusions, those at the EF are with best lateralcoherence which implies a unique driving mechanism involving near-inertialvelocity perturbations near the equator. At JMA-137E section, intrusions atlow latitudes show prominent temporal variations. The meridionaldisplacement of the North Equatorial Countercurrent (NECC) at seasonal andinterannual time scales results in pronounced changes in intrusion strength between4°-6°N, which show a well defined seasonal cycle and prominentinterannual fluctuations associated with ENSO events.