Near Inertial Oscillations In The Northern Southern South Chinas Sea

The South China Sea (SCS) is the largest marginal sea of the western north Pacific.Due to the special geographic location and complex topography, many scales ofmotion are active in the SCS, including large scale circulation, mesoscale eddies andstrong internal tides etc. The SCS climate is part of the East Asian monsoon systemand has nearly the most frequently typhoons in the world associated with the westernnorth Pacific, hence energetic near-inertial oscillations (NIO) are generated locally bystrong winds. However, due to the lack of field observations, little was known aboutthe near-inertial response to typhoons or seasonal variations of NIW in the SCS. In thepresent study, the upper ocean near-inertial response to super typhoon Megi, thenonlinear interactions between NIO and internal tides, as well as the seasonalvariations of NIO are carefully examined, based on continuous current or temperatureprofiles observed by7subsurface moorings, in addition with satellite remote sensingdata and model experiments.Super typhoon Megi passed between two subsurface moorings in the northern SCSon22October2010and the upper ocean thermal and dynamical response was firstexamined in detail with strong internal tides present. Intense sea surface temperaturecooling was induced by Megi, and the maximum cooling was6.6°C. The wholeobserved water column (60-360m) was cooled due to strong Ekman-pumpedupwelling (up to50m in the thermocline) by Megi, with maximum cooling of4.2°Coccurring in thermocline (located at about100m). The upper ocean near-inertialresponse was significantly modulated by the strong background internal tides: arelatively weak (max amplitude0.4m s-1, approximately1/4under hurricane Franceswith the same storm intensity in the Atlantic) and quickly damped (e-foldingtimescale2inertial periods) near-inertial oscillation (NIO) was observed in the mixed layer. This weak and quickly damped NIO response was attributed to the enhancednonlinear wave-wave interaction between typhoon-generated NIO (f) and backgrounddiurnal tides (D1) during and after Megi. Numerical experiments suggested thatenergy transfer to fD1from NIO via nonlinear interaction between f and D1may havelimited the growth and speeded up the damping of mixed layer NIO generated byMegi. The vertical nonlinear momentum term, associated with the vertical shear ofNIO and vertical velocity of D1or vertical shear of D1and vertical velocity of NIOwas responsible for forcing fD1. After Megi, surface-layer diurnal energy wasenhanced by up to100%, attributed to the combined effect of increased barotropictide at Luzon Strait, increased surface-layer stratification and additional Megi-forceddiurnal current. Two cold core eddies appeared along the track of Megi aftertyphoon. The surface chlorophyll-a concentration was increased by more than10times by typhoon, which was related to strong upwelling and enhance diapycnalmixing. The diapycnal diffusivity, parameterized by high-resolution temperature andsalinity profiles, was enhanced more than10times.The nonlinear wave-wave interactions between NIO and internal tides weresummarized using nearly8-month current and temperature datasets. The kinetic andshear spectrum in the northern SCS was dominated NIO (f), diurnal (D1) andsemidiurnal (D2) tides, with the most intense shear at the inertial band. Significantspectra peaks appeared at the sum/difference frequencies of f, D1and D2, indicatingthe active nonlinear interactions between NIO and internal tides. Energy peaks werealso found at exactly sub-(M1) or supper-harmonics (D3etc.) of semidiurnal tides.Secondary waves fD1, fD2and D2f were highly correlated with NIOs. Throughhaving smaller current amplitudes, fD1, fD2and D2f had more intense shears than D3,implying NIO may have significant potential influence on diapycnal mixing in theNorthern SCS.Based on nearly1-year ADCP current datasets from4moorings, the seasonalvariations of NIO in the upper and deep layer of Northern SCS was examined.Vertical wavenumber spectrum indicated that the near-inertial energy concentrated atwavelengths larger than100m and mainly propagated downward. A winter enhancement of near-inertial kinetic energy relative to that in summer was observedby about30-106%in the deep basin of northern SCS and7%at Luzon Strait. Basedon measurements from4deep ocean current meters, obvious near-inertial signalappeared and the maximum horizontal current amplitude was about5.7-7cm s-1, withpeak frequency blueshift about5-7%. As with that in the upper ocean, near-inertialkinetic was18-88%stronger than winter.