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Western boundary currents (WBCs) have intensified and become more eddying in recent decades due to the spin-up of the ocean gyres, resulting in warmer open ocean temperatures. However, relatively l...

The baroclinic tides are a crucial source of mixing energy into the deep ocean; however, the incoherent portion of the spectrum is not well examined because it is difficult to observe. This study estimates the coherent and incoherent M2 internal tide energy fluxes in the Philippine Sea using a primitive equation model that resolves the M2 barotropic and baroclinic tides and the time-evolving atmospherically forced eddying circulation. A time-mean, incoherent, internal tide energy flux of 25% of the coherent energy flux is found to emanate eastward into the Philippine Sea from the Luzon Strait and a time-mean incoherent portion of 30% of the coherent energy flux propagates westward into the South China Sea (SCS). The incoherent internal tide energy results from baroclinic tide generation and propagation variability. Quantifying the incoherent portion estimates the energy missing from altimeter-derived or line-integral acoustic measurements and places short-lived, in situ observations in the context of variability.

Submesoscale flows dominate the vertical transport in the upper ocean and play an important role in air-sea fluxes, the distribution of nutrients and biota, the connectivity of populations. Yet submesoscale predictability is a new frontier and presents additional challenges to that of mesoscale flows. This study assesses the capacity of an operational system to predict the circulation along the landward edge of the East Australian Current where the cyclonic band of vorticity allows small scale instabilities to grow. We use a downscaling approach in which a fine-scale resolution (750–1000 m) coastal model is one-way nested within a coarser resolution (2.5–6 km) mesoscale eddy-resolving regional model that assimilates all available data, including coastal high-frequency radar radial velocities. Assimilation of the surface radial velocities into the regional model provides improved representation of the cyclonic band inshore of the current, however forecasts at this resolution perform poorly in representing this cyclonic vorticity. Downscaling to the higher resolution coastal model produces a sharper across-current vorticity gradient within the EAC and leads in turn to more intense submesoscale features growing on its inshore side. Boundary forcing error from the regional model propagates into the nested coastal model. The results motivate improved representation of fine-scale flows in regional forecasts for downscaling purposes.

This study uses a primitive equation model to estimate the time-varying M2 internal tide dissipation in the Philippine Sea in the presence of the subtidal circulation. The time-mean diapycnal diffusivity due to the M2 internal tide is estimated to be 4.0–4.8 × 10−4 m2 s−1 at the Luzon Strait and 2–9 × 10−5 m2 s−1 in the Philippine Sea basin. The variability in internal tides and their interactions with the subtidal ocean circulation results in significant spatial and temporal variability in the energy available for mixing. The subtidal circulation influences internal-tide-induced mixing in two ways: by introducing variability in internal tide generation and by increased dissipation of baroclinic energy associated with greater velocity shear. Close to the generation site, mixing is dominated by high-mode internal tide dissipation, while in the far field the influence of the mesoscale energy on internal tide dissipation is significant, resulting in increased dissipation. This study presents model-based estimates of the important and relatively unknown effect of mesoscale circulation on internal-tide-induced mixing away from internal tide generation sites in a region of high eddy kinetic energy.

This study examines the effects of the subtidal circulation on the generation and propagation of the M2 internal tide in the Philippine Sea using a primitive equation model. Barotropic to baroclinic conversion at the Luzon Strait is found to vary due to the 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 magnitude and direction of the depth-integrated baroclinic energy fluxes vary temporally, due to a combination of changing generation, propagation, and dissipation. Spatial patterns of internal tide propagation near the Luzon Strait are influenced by the locations of mesoscale eddies to the east and west of the strait. 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 traditional view of the East Australian Current (EAC), as depicted in many schematics, is of a continuous boundary current that flows along the shelf off eastern Australia, between approximately 18°S and about 32.5°S, where it separates from the coast and continues either towards New Zealand, along the Tasman Front; or towards Tasmania, as the EAC Extension. Additionally, it is widely recognised that eddies are prevalent in the EAC region – particularly south of the EAC separation. We revisit this long-standing paradigm and suggest that the EAC is perhaps better viewed as a continuous, meandering stream, flowing adjacent to the coast that “feeds” a field of mesoscale eddies. Observations show that EAC eddies are prevalent over a broad region of the western Tasman Sea, as far north as 25°S, typically with maximum intensity between 30 and 35°S. At any instant in time the EAC jet is usually evident as a poleward stream adjacent to the continental shelf edge. Other commonly accepted components of the EAC System, including the Tasman Front and EAC Extension, are rarely seen as distinct, identifiable features. Rather, these features are evident only in time-mean fields, when the eddy-variability is filtered out. It is also common for the EAC to be spatially discontinuous – due to the presence of eddies – often with multiple short streams that sometimes separate and re-attach to the coast. Recognition of the EAC as an eddy-dominated current system has seen many recent studies focus on various aspects of eddies in the EAC System, providing new insights into mesoscale ocean dynamics. Recent studies of individual eddies have shown that the circulation within eddies, including tilting and vertical motion, is more complex than previously understood. A summary of these studies, along with a review of the EAC System, particularly its path, separation, and eddy field is presented here.

Measurements of acoustic ray travel-times in the ocean provide synoptic integrals of the ocean state between source and receiver. It is known that the ray travel-time is sensitive to variations in the ocean at the transmission time, but the sensitivity of the travel-time to spatial variations in the ocean prior to the acoustic transmission have not been quantified. This study examines the sensitivity of ray travel-time to the temporally and spatially evolving ocean state in the Philippine Sea using the adjoint of a numerical model. A one year series of five day backward integrations of the adjoint model quantify the sensitivity of travel-times to varying dynamics that can alter the travel-time of a 611 km ray by 200 ms. The early evolution of the sensitivities reveals high-mode internal waves that dissipate quickly, leaving the lowest three modes, providing a connection to variations in the internal tide generation prior to the sample time. They are also strongly sensitive to advective effects that alter density along the ray path. These sensitivities reveal how travel-time measurements are affected by both nearby and distant waters. Temporal nonlinearity of the sensitivities suggests that prior knowledge of the ocean state is necessary to exploit the travel-time observations.

This study investigates the impact of remotely generated internal tides on model estimates of barotropic to baroclinic tidal conversion for two generation sites bounding the Philippine Sea: the Luzon Strait and the Mariana Island Arc. A primitive equation model is used to characterize the internal tides generated by the principal semidiurnal tide (M2) over a domain encompassing the two generation sites. Energetic internal tides are generated at the Luzon Strait where nearly 17 GW of barotropic tide energy is converted to baroclinic energy, of which 44% (4.78 GW) is radiated eastward into the Philippine Sea. From the Mariana Arc, baroclinic energy propagates westward into the Philippine Sea as a result of 3.82 GW of barotropic to baroclinic energy conversion. Simulations that focus on each generation site without influence of the other are performed, and comparisons show that remotely generated internal tides have a significant effect on local conversion at the two sites. Total conversion is greater in the absence of remotely generated internal tides at both sites: 11% greater at the Luzon Strait and 65% greater at the Mariana Arc. The first three modes of the remotely generated internal tides traverse the basin and alter the amplitude and phase of bottom pressure. The arrival of the remote internal tides varies significantly with changing stratification and mesoscale circulation. The results suggest that an important source of variability in local conversion around the globe is due to remotely generated internal tides.

Measurements of acoustic ray travel-times in the ocean provide synoptic integrals of the ocean state between source and receiver. It is known that the ray travel-time is sensitive to variations in the ocean at the transmission time, but the sensitivity of the travel-time to spatial variations in the ocean prior to the acoustic transmission have not been quantified. Using an advanced numerical model, we can identify both the ocean dynamics that control the ocean state along a ray path and quantify the informational content of the ray travel-time observation. This study examines the sensitivity of ray travel-time to the temporally and spatially evolving ocean state in the Philippine Sea over a one year experiment. The travel-times are found to be sensitive to the internal tide generation prior to the sample time and to advective effects that alter density along the ray path. Temporal nonlinearity of these sensitivities suggest that prior knowledge of the ocean state is necessary to exploit the travel-time observations. After assimilating the travel-time observations, the contribution of the travel-time information to our estimation of the ocean state is quantified and evaluated to further identify what the travel-time observation reveals about the ocean.