The role of seasonal circulation in the variability of dynamic parameters of internal solitary waves in the Sulu Sea

In-situ observations of the Sulu Sea internal solitary waves (ISWs) are too limited to reveal their variability. In this study, based on verified model reanalysis data, we study the variability of three basic dynamic parameters of ISWs, i.e., linear phase speed, nonlinear and dispersive coefficients, affected by the Sulu Sea circulations. Overall, the linear speed and dispersive coefficient of ISWs modulated by the annual-mean Sulu Sea circulation decrease, while the nonlinear coefficient increases, and these variations are largely attributed to the variable stratification rather than current. Particularly, at the deep basin and near the western boundary of the Sulu Sea, the modulated climatological annual-mean linear phase speed reduces at least -10% of that resulted from only topography, while at the deep basin the modulated dispersive (nonlinear) coefficient has a significant decrease (increase) up to approximately 25-30% (20-30%) relative to that resulted from only topography. Moreover, the first EOF mode of the three ISW dynamic parameters, dominant in winter and summer, reflects the modulation of the Sulu Sea basin-scale circulation, while the second EOF mode, dominant in spring and autumn, illustrates the modulation associated with the meander flow along the western boundary. Specifically, it is found that the seasonal cycles of linear phase speed (nonlinear and dispersive coefficients) are largely attributed to the variable current (stratification) induced by the seasonal circulations. The circulation-induced peak-to-peak fluctuations of linear speed, nonlinear and dispersive coefficients reach up to -15%, -30% and -20%, respectively, of those resulted from only topography.