An Analysis Of The Effects Of Wave-induced Mixing On Hydrological Structure In East China Sea During Typhoon Mstsa

MASNUM wave-tide-circulation coupled numerical model is employed to simulate the temperature and currents fields in the Yellow Sea (YS), Bohai Sea, and East China Sea (ECS) in the summer of 2005. The model has a horizontal resolution of (1/8)°by (1/8)°, and 21 sigma levels in the vertical. The temperature patterns from the model are basically consistent with the observation. The model also satisfactorily reproduces the three-dimensional circulation structure in the YS. In this paper, we focus on the ocean responses to typhoon Mstsa from 31 July to 9 August in the presence of wave-induced mixing (WIM). The effects of WIM on the hydrological structure in ECS are analyzed in detail. The main conclusions are as follows.1. The sea temperature responses to the typhoon include two stages: rising and decreasing. Before the onset of the maximum wind speed, the upper water is forced onshore, accumulating and downwelling in vertical, and the nearshore sea temperature goes up. The increase of temperature in upper 50 m is about 2-3℃. We find a temperature rise of about 0.2℃even at 200 m depth. After the rising phase, the strong mixing and upwelling induced by typhoon result in sea surface temperature (SST) decreasing. SST cooling in the right side of the typhoon wake is far more intense than that in the left. Numerical experiments indicate that wave mixing, tide mixing, and upwelling contribute 21%, 5%, and 8% to SST cooling, respectively.2. The thermocline responses to typhoon can be separated into 3 steps. Wave-induced mixing always plays a very important role in deepening the mixed layer (ML) in all stages. (1) In the typhoon forcing stage, the average ML depth increase quickly from ~30 m to ~50 m. (2)During the 3 days after typhoon forcing stage, the thermocline moves upward slowly. In this period, the role of wind is next only to wave-induced mixing in affecting the thermocline: wind induces upwelling which lifts up the thermocline as a major dynamic mechanism. (3) In step 3, the shoaling trend of ML continues. The difference from step 2 is that upwelling decreases and even die away, and tide-induced mixing takes the place to be the main drive of raising the thermocline. Gradually, the thermocline restores its normal state after the typhoon process, and the typical time scale is 5-10 days.3. Driven by typhoon, the upper water flows as an anti-clockwise grye. Strong current occurs in the right region of the typhoon track with maximum speed of 1.2 m/s, and the velocity slows down with the increase of water depth. After the landing of typhoon, the surface current velocity decreases naturally. However, the currents below 30 m intensify gradually with the downward propagation of upper signals. The maximum speed reaches 0.4 m/s at 70 m depth 12 hours after the landing of Mstsa.