Predictability in a region of strong internal tides and dynamic mesoscale circulation: the Philippine Sea

Abstract

This study uses a modeling approach to examine the interactions between the principal semidiurnal (M2) internal tides and the atmospherically-forced eddying ocean circulation in the Philippine Sea, a region of energetic internal tides and dynamic mesoscale circulation. Remotely generated internal tides are found to have a significant effect on local generation at the two generation sites bounding the region, the Luzon Strait and the Mariana Island Arc, by altering the amplitude and phase of bottom pressure. Internal tide generation at the Luzon Strait is found to vary due to background circulation changes over the generation site and the changing influence of remotely generated internal tides from the Mariana Arc. The varying effect of remotely generated waves results from both changing generation energy levels at the Mariana Arc and variability in the propagation of the internal tides across the Philippine Sea. The interactions with the subtidal ocean circulation result in significant spatial and temporal variability in internal tide propagation and dissipation, and hence the energy available for mixing. Close to the generation site, mixing is dominated by high-mode internal tide dissipation, while in far-field the influence of the mesoscale energy on internal tide dissipation is significant. The results provide insight into the mechanisms of variability of the baroclinic tides and highlight the importance of considering both the remotely generated internal tides and the subtidal dynamics to estimate internal tide energetics. The strong and highly variable internal tides are found to have a significant impact on state estimates and predictions of the subtidal circulation in the region. Using 4-D Variational Data Assimilation, we find that including the tides in our model provides significantly improved subtidal predictions across the Philippine Sea. This result is two-fold; firstly, high error bars must be applied to the observations if the model does not include the tides to account for the unresolved internal tide signal, and secondly, the tidal dynamics influence the subtidal circulation. Predictions from a model that does not resolve the tides are particularly poor in the Kuroshio and South China Sea regions where tidal dynamics influence water mass properties. Overall, this study makes an important contribution towards understanding the dynamic connection between internal tides and the eddying ocean circulation and the implications for predictability in regions that contain significant energy at both dynamic scales.