Observations of second baroclinic mode internal solitary waves on the continental slope of the northern South China Sea

Abstract

A temperature and current velocity mooring, located on the upper continental slope of the northern South China Sea, recorded a number of second baroclinic mode (mode 2) internal solitary waves (ISWs). These types of waves are seldom observed in nature. The mode 2 ISWs typically showed upward (downward) displacement of isotherms in the upper (lower) water column and three layers of eastward, westward, and eastward current from the uppermost to bottommost portions of a wave. In summer, westward‐propagating mode 2 ISWs were observed only occasionally. These waves generally appeared after mode 1 ISWs, a feature that may relate to the diurnal tide with a period of approximately 24 hours. The displacement of isotherms induced by mode 2 ISWs was 20 ± 14 m at 75 m and −22 ± 15 m at 240 m, and the characteristic time scale was approximately 8.0 ± 4.3 min. In winter, mode 2 ISWs were more active but mode 1 ISWs were rarely observed. Isotherm displacement by mode 2 ISWs in winter was 30 ± 18 m at 75 m and −26 ± 16 m at 240 m, and the average characteristic time scale was 6.9 ± 4.6 min. The mode 2 ISWs thus had larger amplitudes and smaller time scales in winter than they did in summer. The observed vertical temperature profile also showed notable seasonal change. The thermocline was shallow in summer and deep in winter. In winter, vertical temperature profiles indicated that the main thermocline was located near middepth over the upper continental slope near the 350 m isobath. Mode 1 ISWs were more active in summer than in winter, reflecting the larger Ursell numbers for mode 1 ISWs in summer. Among mode 2 ISWs in summer, 90% appeared after mode 1 ISWs. These results suggest that mode 2 ISWs could be related to mode 1 ISWs. In contrast, mode 2 ISWs were more active in winter than in summer, with larger mode 2 Ursell numbers also found in winter. Among winter mode 2 ISWs, 72% appeared without mode 1 ISWs. Mode 2 ISWs in winter could be related to the main thermocline being located near middepth. These seasonal variations of mode 2 ISWs were correlated with the seasonal change of local stratification. Further study on the different generating mechanisms of mode 2 ISWs in summer and winter is needed.