Including Tides Improves Subtidal Prediction in a Region of Strong Surface and Internal Tides and Energetic Mesoscale Circulation

Model predictions of the oceanic subtidal circulation have typically excluded the tides, assuming that tides have little influence on subtidal flows. The deterministic surface (barotropic) tide can be removed from observations for assimilation, yet the surface tides may have important impacts on subtidal circulation in shelf regions with flow-on effects to the deep ocean. Internal (baroclinic) tides are non-deterministic so must be included as observation error for assimilation if they are not resolved by the model. Using Twin Experiments with 4-D Variational Data Assimilation, we examine the influence of the principal semidiurnal (M-2) tide on the predictability of the subtidal ocean circulation in the South China and Philippine Seas. We find that including tides in our model significantly improved subtidal predictions across the region. This results from both the influence of tidal dynamics on the subtidal circulation, and from higher prior errors prescribed to the observations when the model does not resolve the internal tide signal. The tides are particularly important for predictions of temperature in the South China Sea and Kuroshio Current where tidal mixing influences water mass properties. In the Philippine Sea deep basin, improved predictability results from resolving the internal tides allowing smaller prior errors to be prescribed to the assimilated Sea Surface Height and subsurface observations. This study illustrates the importance of including tides to accurately predict subtidal circulation in a region of strong surface and internal tides and energetic mesoscale circulation and including a significant Western Boundary Current region.