Seasonal Variability Of Near Inertial Oscillation In South China Sea

This paper studies the seasonal variation of the near-inertial energy input from the wind field to the South China Sea through the analysis of the wind field and the slab mixed layer model from January 2000 to December 2017 in the South China Sea.The results show that the total spatial distribution of the near-inertial energy flux in the South China Sea is smaller in the north than south,and gradually increases from the northeast to the southwest.In the April and May,near-inertial energy flux is the smallest in the whole year.The near-inertial energy flux distribution from June to October is equal to the north and south.The near-inertial energy high value area in November is mainly concentrated in the middle of the South China Sea.From November to March of the next year,the near-inertial energy flux is generally higher,and its high-energy flux area is mainly concentrated in the southwestern of the South China Sea.The annual average near-inertial energy flux is 0.32 mW·m-2,and the total energy input by the wind to the ocean is 1.13GW.The total energy input by the tropical cyclone to the ocean is 0.14 GW,accounting for 12%of the total energy,of which 0.027 GW can be transmitted to the deep ocean.The spatial distribution of deep near-inertial energy in the South China Sea is significantly larger in the north than south,and the maximum is located in the western part of the Philippines,followed by the Luzon Strait and the southwestern part of the South China Sea.The basic characteristics of the near-inertial energy flux in the South China Sea are more in the south and in the north than in the summer.It is important to note that the South China Sea is one of the most active sea areas in tropical cyclones.From 2000 to 2017,a total of 47 typhoon processes stimulated near-inertial oscillations,causing the number of typhoons that generate near-inertial oscillations to increase linearly month by month.Due to the large difference in the near-inertial energy flux excited between different typhoons,the seasonal variation of the near-inertial energy flux in the South China Sea has increased a lot of uncertainty.In general,the total energy input by tropical cyclones to the ocean is 0.14 GW,accounting for 12%of the total energy.Among them,0.027 GW can be transmitted to the deep ocean,accounting for about 19%of the tropical cyclone input into the ocean.Typhoon presses has an important impact on the seasonal variation of the near-inertial oscillations in the South China Sea.In order to clarify the contribution of typhoon in the South China Sea to near-inertial energy,this paper analyzes two consecutive typhoons Nesat(1117)and Nalgae(1119)in the Xisha sea area based on the Xisha moored current records.Both typhoons generated strong near-inertial waves(NIW)as detected by a moored array,with the near-inertial velocity to the right of the typhoon path significantly larger than to the left.The estimated vertical phase and group velocities of the NIW induced by Typhoon Nesat are 0.2 cm·s-1 and 0.85 m·h-1,respectively,corresponding to a vertical wavelength of 350 m.Both the vertical phase and group velocities of the NIW induced by Typhoon Nalgae are lower than those of Typhoon Nesat,with the corresponding vertical wavelength only one-half that of Nesat.The threshold values of induced near-inertial kinetic energy(NIKE)of 5 J·m3 reach water depths of 300 m and 200 m for Typhoons Nesat and Nalgae,respectively,illustrating that the NIKE induced by Typhoon Nesat dissipated less with depth.Obvious blue-shifts in the induced NIW frequencies are also detected.The frequency of NIW induced by Typhoon Nesat significantly increases at water depths of 100-150 m because of Doppler shifting,but decreases significantly at water depths of 100-150 m for Nalgae because of the greater influence of the background vorticity during the passage of Typhoon Nalgae.19%of the near-inertial internal wave energy caused by typhoons can be transmitted to deep seawater,and the near-inertial energy in autumn is significantly greater than that of other seasons,and the main factors contributing to this phenomenon are the fall of Typhoon Nesat and Nalgae in autumn,and the near-inertial energy in spring,summer and winter is roughly the same.