Study On The Near-Inertial Wave In The Northwest Pacific

Internal gravity waves are common phenomenon in the ocean.They occur in stratified waters,with the frequency in the range of f<?<N,where f is the local Coriolis frequency and N is the buoyancy frequency.According to the characteristics and formation causes of the internal gravity wave,the wave is divided into internal tide?IT?,near inertial wave?NIW?and solitary wave?SW?.As the northwestern Pacific Ocean is high-incidence of the typhoon and mesoscale eddy,scientific research here is very important.However,due to the lack of observational data,the current mechanisms of transmission of NIWs,which are affected by water feature,are still poorly understood.The mooring data in the Northwest Pacific and the lander data in the northern part of the South China Sea were applied in this paper.Combined with the temperature data,salinity data by ARGO,the sea surface altimeter data by satellites and the typhoon meteorological data,the NIW signal influenced by one single typhoon and the multiple of five typhoons.The NIW signal in the deep water are analyzed in this research.The signal characteristics of NIW in the South China Sea and the Northwest Pacific Ocean are explained.By analyzing the characteristics of the NIWs affecting the three typhoons in the western North Pacific,we find that the vertical group velocity of the three NIWs is smaller than that of the previous studies.Near inertial current has the characteristics of high vertical wavenumber.After theoretical analysis,we find that the high vertical wave number corresponds to a small vertical group velocity,which explains that the vertical group velocity is relatively small here.The three NIW signals are red-shifted.The red shift occurs in the first and second modes of EOF,and the third and fourth modes exhibit a blue-shifting.Since the NIW signals during non-typhoon are blue-shifted.Therefore,the typhoon can excite the NIW signal of the first two modes of EOF,and the near-inertial oscillation signal of the three-four mode is resonantly enhanced during the typhoon.The analysis of the NIW signal under the influence of six typhoons in this area shows that when NIW signal is not exist,near-inertial oscillations were generated two days after the date during the typhoon was closest to of the mooring.But when NIW signal is exist,the new typhoon can enhance NIW before the closest date.Compared to the effect of a single typhoon,the energy can be transmitted to a deeper position after being affected by multiple typhoons.Analysis of the lander flow field revealed that the South China Sea has a strong diurnal tide and semidiurnal tide signal.The near-inertial signal here is one order of magnitude smaller than the tidal signal,but there is a blue-shifted near-inertial oscillation signal that passes below 600 meters of water depth.The signal strength is of the same magnitude as the tidal signal and lasts from November 3 to 16.Spectral analysis revealed that the vertical flow rate exhibited five different current cores,with the strongest current cores occurring between600 and 650 meters.The near-inertial energy transmission speed is 67±5 m d^-1,and the energy dissipation from the 600-meter to the 1000-meter position is 18%.The EOF analysis results show that the NIW signal begins to dominate the first mode,and then becomes unstable to become a higher-order mode.Combined with the typhoon information,we found that the cause of the NIW signal was not caused by the typhoon.The sea surface height anomaly of the satellite data shows that the positive vorticity is favorable for the blue shift of the NIW during this time.