Kelvin waves from the equatorial Indian Ocean modulate the nonlinear internal waves in the Andaman Sea

In the equatorial Indian Ocean, strong westerly and easterly wind anomaly can drive eastward downwelling and upwelling Kelvin waves, respectively, which play an important role in determining the circulations and thermal structures near the equator. Kelvin waves can propagate into the Andaman Sea, a marginal sea located to the northeast of the Indian Ocean. In the Andaman Sea, nonlinear internal waves (NLIWs) that are crucial in facilitating the mixing in the ocean interior and maintaining the ecosystem are found to be extremely active. Although both equatorial Kelvin waves and NLIWs have been well known in oceanography, the influence of equatorial Kelvin waves on NLIWs in the Andaman Sea remains unclear. In this study, based on long-term mooring measurements in the southern Andaman Sea, it is found that the NLIW amplitude shows remarkable intraseasonal and seasonal variances, and these variances can be mostly explained by the occurrence of equatorial Kelvin waves. Downwelling Kelvin waves can deepen the thermocline depth by tens of meters, so that the NLIW amplitude can be reduced up to 22%. Meanwhile, upwelling Kelvin waves can notably uplift the thermocline depth and the NLIW amplitude can be enhanced up to 32%. These discoveries provide credible evidence that basin-scale waves from the open ocean can remotely modulate small-scale internal waves in marginal seas.