Three-dimensional Structures of Internal Solitary Waves in the Northern South China Sea Revealed by Mooring Array Observations

The data collected from an array of five subsurface moorings, which are deployed along the direction of internal solitary wave (ISW) crests in the northern South China Sea (SCS) from October 2013 to June 2014, are used to investigate the three-dimensional structures of ISWs and their temporal variabilities. The measurements reveal that the ISWs are asymmetric along their crests, with the average amplitude in the southern portion being 70% larger than that in the northern portion. The observed three-dimensionally integrated energy and flux of ISWs are accurately calculated for the first time, reaching 53 TJ and 0.82 GW, respectively, on average. Over the whole observation period, the pattern of ISW crests was dominantly convex, accounting for 76.2% of all observed episodes. However, due to the changes in propagation speeds along the direction of ISW crests caused by either single or combined effects of mesoscale eddies and intruded Kuroshio, the ISW crests could deform into S-shaped and concave patterns, which accounted for 19.6% and 4.2%, respectively. Moreover, during November and December, the positions of the largest amplitude along ISW crests mostly shifted from the southern portion to the northern portion due to the energy refraction caused by mesoscale eddies, leading to the increase/decrease in wave intensity in the northern/southern portion. In early February, the intruded Kuroshio remarkably shifted ISW energy southward, which increased the wave amplitude in the southern portion by as much as 96%. These results clearly demonstrate that the intensities of three-dimensional ISWs could temporally vary out of phase between the northern and southern portions of crests due to the remarkable modulations by intense background processes.