||Water temperature affects marine ecosystems, primary productivity, weather forecast and underwater acoustic communication. When internal waves in the South China Sea pass through the Dongsha Atoll, there will be a significant temperature drop with the nutrient in the cold deep water upwelling to the shallow water. The vertical mixing of seawater will increase, too.|
To understand the change of flow field and the effect of cold water uplifting to shallow sea when internal waves passing through the Dongsha Atoll, we deployed thermometer and ocean current meter on the seabed to measure the water current and temperature at the ocean bottom. Instruments were placed at nine stations with depth of 15m to 20m around the Dongsha atoll. At each station, two thermometers and two ocean current meter were set to record the in-situ data. The measurement extends from 7/16 to 7/23, but the eight instruments at East-northeast (ENE) Station lasted for 33 days from July to August in 2014.
Then a detailed analysis, the records show that the temperature drop to the east of Dongsha atoll is highest, and the temperature drop decreases for west side stations. It is difficult to define whether the drop is due to an internal wave.However, then filtering out the background oscillation, it is concluded that the temperature drop, flow rate and cooling rate in the southern part of the Atoll are all higher than that of the northern part.
The order of arrival and the extension time of each cooling event are not regular in the eastern part of Dongsha atoll. However, it can be concluded that faster flow rate and shorter extension time can be expected in the southern part. On the other hand, the averaged extension time is longest for the ENE station, but both the number of events and the flow speed are low.
The correlation between the flow rate and flow direction is low. Flow at the Southwest (SW) station located at the southern waterway is basically tidal current. Tidal ellipses at both North (N) and Northwest (NW) stations are parallel to the coral crest.