American Geophysical Union 2024
Date:
Year-long study on the behavior and seasonality of Internal Gravity Waves in the South China Sea using Satellite and Ocean Bottom Seismic data
Oceans are stratified by density gradients resulting from vertical variations in temperature and salinity, creating a medium for internal gravity waves. At the pycnocline, where the shallow water column exhibits the most significant density gradient, nonlinear internal waves (NLIWs) can form and propagate hundreds of kilometers before breaking in shallow water. NLIW activity is modulated by climate change and plays a critical role in numerous oceanic processes, including vertical thermal mixing, nutrient supply, sediment transport, and acoustic transmission. Despite their importance, NLIWs remain difficult to detect over large spatial and temporal scales. In this study, we perform a rare year-long analysis of NLIWs using remote sensing and geophysical methods. We leverage the Himawari-8 geostationary satellite data from the Japan Meteorological Agency (JMA) to detect internal waves near Dongsha Atoll in the South China Sea, a region known for the largest amplitude NLIWs in the world. By selecting days with optimal visibility and manually tracing the waves at 10-minute intervals from November 2019 to December 2020, we calculate the velocity and back azimuth of NLIWs approaching Dongsha Atoll from the eastern Luzon Strait generation site. We examine the seasonality of the waves in terms of velocity and propagation direction. Our preliminary findings indicate that most NLIWs approach from the east-southeast, consistent with the eastern generation site, and that NLIWs are fastest from late June through August 2020 and generally approach from a more southerly direction. Interestingly, the best-observed waves occur from late May to late June 2020 and have an anomalous back azimuth almost due east, gradually shifting southward over the month. This could have implications for changes in stratification over the year. Using an ocean bottom seismometer (OBS) deployed offshore Dongsha Atoll during the study period, we compare the seismic amplitudes during times with the highest and lowest NLIW satellite-calculated velocities and find that velocity does not correlate with seismic amplitude. These annual records provide systematic information on how internal waves respond to a known forcing and can inform our anticipation of how they will react to future climate.
For more details, visit my Internal Wave repository.