Temporal variability of internal solitary waves in the northern South China Sea revealed by long-term mooring observations

The temporal variability of internal solitary waves (ISWs) in the northern South China Sea (SCS) is examined from hourly to interannual timescales based on nearly four-year-long measurements from the SCS Mooring Array between August 2010 and June 2014. On an hourly timescale, large ISWs tend to occur at the moorings around certain hours due to the concentrated occurrence of strong ebbing tidal beats in the Luzon Strait at approximately 0300 h and 1500 h (local time, UTC + 8 h) modulated by the S2 tide and the sum of the K1 and P1 tides. Intraseasonally, the mean ISW amplitudes on the 4th and 19th lunar days are twice as large as those on the 11th and 26th lunar days, corresponding to the spring-neap tidal cycles; the ISW intensity varies by as much as a factor of two in two months ascribed to the occurrence and movement of mesoscale eddies. Seasonally, ISWs are 37% weaker on average in winter than in summer over the continental shelf slope but are only 14% weaker in winter than in summer in the eastern deep basin near the Luzon Strait. The common occurrence of energetic eddies in winter significantly reduces the monthly mean ISW intensity, playing an important role in the seasonal variations of ISWs. Interannually, the monthly mean ISW intensity varies by up to 171%, which is likely associated with the year-to-year variations in mesoscale processes and tides. The observational results presented here reveal that the ISWs in the northern SCS vary significantly over multiple timescales following the trends that are shaped by tides and mesoscale processes.