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

Abstract

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 1ms−1 above 500m and an eastward velocity up to 0.1ms−1 below 500m. The observed structure of the vertical current was asymmetrical. For the first soliton, the downwelling region had a temporal scale of 32min and an average vertical velocity of 3cms−1, whereas the upwelling region had a temporal scale of 24min and an average vertical velocity of 2cms−1. For the second soliton, the temporal scale and average velocity were 36min and 2cms−1 in the downwelling, compared with 26min and 2cms−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 50m. Such asymmetry caused changes in temperature as high as 2.3°C, nutrient concentrations up to 12.04μmolL−1, and chlorophyll a concentration up to 0.12μgL−1 before and after the passage of the solitons.