Asymmetry of internal waves and its effects on the ecological environment observed in the northern South China Sea

•The full-depth structure of internal solitons was observed in the SCS deep basin.•The zonal velocity exceeded 1 m s−1 near surface and 0.1 m s−1 near bottom.•The asymmetry of internal solitons is studied quantitatively.•The quantitative relation between the asymmetry and water transport is studied.

Ecological parameters, including current velocity, water temperature, and nutrient and chlorophyll a concentrations, accompanied by two internal solitary waves, were observed in the South China Sea deep basin from 2 October to 4 October, 2012. Both solitons belonged to the mode-1 category, which were characterized by a westward velocity exceeding 1 m s−1 above 500 m and an eastward velocity up to 0.1 m s−1 below 500 m. The observed structure of the vertical current was asymmetrical. For the first soliton, the downwelling region had a temporal scale of 32 min and an average vertical velocity of 3 cm s−1, whereas the upwelling region had a temporal scale of 24 min and an average vertical velocity of 2 cm s−1. For the second soliton, the temporal scale and average velocity were 36 min and 2 cm s−1 in the downwelling, compared with 26 min and 2 cm s−1 in the upwelling. On the basis of the convection equation, the downward net vertical displacements of water particles ranged from nearly zero to tens of meters with the maximum value in excess of 50 m. Such asymmetry caused changes in temperature as high as 2.3 °C, nutrient concentrations up to 12.04 μmol L−1, and chlorophyll a concentration up to 0.12 μg L−1 before and after the passage of the solitons.