Hybrid Coordinate Ocean Model Simulations Research Articles

Nonlinear Internal Tides in a Realistically Forced Global Ocean Simulation

AbstractThe decay of the low‐mode internal tide due to the superharmonic energy cascade is investigated in a realistically forced global Hybrid Coordinate Ocean Model simulation with 1/25° (4 km) horizontal grid spacing. Time‐mean and depth‐integrated supertidal kinetic energy is found to be largest near low‐latitude internal tide generation sites, such as the Bay of Bengal, Amazon Shelf, and Mascarene Ridge. The supertidal kinetic energy can make up to 50% of the total internal tide kinetic energy several hundred kilometers from the generation sites. As opposed to the tidal flux divergence, the supertidal flux divergence does not correlate with the barotropic to baroclinic energy conversion. Instead, the time‐mean and depth‐integrated supertidal flux divergence correlates with the nonlinear kinetic energy transfers from (sub)tidal to supertidal frequency bands as estimated with a novel coarse‐graining approach. The regular spaced banding patterns of the surface‐intensified nonlinear energy transfers are attributed to semidiurnal mode 1 and mode 2 internal waves that interfere constructively at the surface. This causes patches where both surface tidal kinetic energy and nonlinear energy transfers are elevated. The simulated internal tide off the Amazon Shelf steepens significantly near these patches, generating solitary‐like waves in good agreement with Synthetic Aperture Radar imagery. Globally, we find that regions of high supertidal energy flux also show a high correlation with observed instances of internal solitary waves.

The influence of cold filaments on the evolution of dipole structures

Mesoscale eddy dipoles are oceanographic structures that can transport ocean water parcels horizontally and vertically in ways that differ from individual mesoscale eddies. The most conspicuous additional feature that presents in dipoles is the cold filament (CF) that can be spontaneously generated between a dipole’s anticyclonic eddy (AE) and cyclonic eddy (CE). A case study in this paper shows that the interaction between the CF and the CE component of the dipole is associated with the structural evolution of the dipole. This interaction is verified in synthesis and normalization studies of the CF dipoles. The CF-dipole interaction entrains CF water into the center of the dipole’ CE and leads to a cold and high chlorophyll center in the upper layer of the CE. The formation of this cold center changes the dipole’s structure by eliminating the phase difference between the thermal and dynamic centers of the dipole. The entrainment also provides a new mechanism for the development of high chlorophyll levels in the CE. In the analysis of the HYCOM (Hybrid Coordinate Ocean Model) simulation of a synthetic CF dipole, the AE has a three-zone structure while the CE only has two. The convergence of the CE’s outermost zone results in a biased interaction.

Zonal jets in the Southern Ocean: A semi-analytical model based on scale separation

A reduced-order semi-analytic model of multiple zonal jets in the Southern Ocean is proposed based on the statistical approach and scale decomposition. By introducing two dominant scales in the vorticity equation, the model describes the large-scale and mesoscale dynamics using the explicit momentum dissipation in the horizontal and vertical directions. For validation and physical insights, the results of the reduced-order model are compared with solutions of two eddy-resolving ocean models: (i) a realistic primitive-equation HYCOM (HYbrid Coordinate Ocean Model) simulation of the Southern Ocean and (ii) an idealized quasi-geostrophic model of a shear-driven channel flow.

Mechanisms of Heat Flux Across the Southern Greenland Continental Shelf in 1/10° and 1/12° Ocean/Sea Ice Simulations

AbstractThe presence of warm Atlantic water on the Greenland continental shelf has been connected to the accelerated melting of the Greenland Ice Sheet, particularly in the southwest and southeast shelf regions. Results from two high‐resolution coupled ocean‐sea ice simulations that utilized either the 1/10° Parallel Ocean Program (POP) or the 1/12° HYbrid Coordinate Ocean Model (HYCOM) are used to understand the flux of heat on and off the southern Greenland shelf. The analysis reveals that the region of greatest heat flux onto the shelf is southeast Greenland. On the southwestern shelf, heat is mainly exported from the shelf to the interior basins. We identify differences in the shelf break current structure and on‐shelf heat content between the two simulations. Just south of the Denmark Strait, there is a seasonally persistent pattern of multi‐day variability in the cross‐shelf heat flux in both simulations. In the POP simulation, this high‐frequency signal results in net on‐shore heat flux. In the HYCOM simulation, the signal is weaker and results in net off‐shelf heat flux. This variability is consistent with Denmark Strait Overflow eddies traveling along the shelf break.

Physics-informed neural networks for predicting ocean spatio-temporal fields

Simulations of the ocean environment are important for assessing acoustic variability due to large and small scale changes in the propagation medium. By incorporating the Internal Gravity Waves (IGWs) partial differential equation (PDE) in ocean models, such as the HYbrid Coordinate Ocean Model (HYCOM) system, it is possible to model the impact of tides on acoustic propagation. This requires HYCOM simulations of the ocean environment, which are computationally expensive, especially at the high vertical resolutions required to capture IGWs. In this work, we develop a machine-learning approach to predicting the effects of IGWs without incurring the computational cost of high-resolution numerical modeling. We apply a deep-learning approach in the form of a Physics Informed Neural Network (PINN) that incorporates the IGW PDE system to make predictions about oceanographic quantities of interest with internal tides at high vertical resolution. Training data comprise HYCOM 4D fields (time, depth, latitude, and longitude) both with (IGW-HYCOM) and without tides in the Pacific subtropical/equatorial region. The developed framework is able to map a relationship between lower-resolution HYCOM inputs to highly resolved ocean fields that include internal tides.

MJO-Induced Intraseasonal Mixed Layer Depth Variability in the Equatorial Indian Ocean and Impacts on Subsurface Water Obduction

AbstractChange of oceanic surface mixed layer depth (MLD) is critical for vertical exchanges between the surface and subsurface oceans and modulates surface temperature variabilities on various time scales. In situ observations have documented prominent intraseasonal variability (ISV) of MLD with 30–105-day periods in the equatorial Indian Ocean (EIO) where the Madden–Julian oscillation (MJO) initiates. Simulation of Hybrid Coordinate Ocean Model (HYCOM) reveals a regional maximum of intraseasonal MLD variability in the EIO (70°–95°E, 3°S–3°N) with a standard deviation of ~14 m. Sensitivity experiments of HYCOM demonstrate that, among all of the MJO-related forcing effects, the wind-driven downwelling and mixing are primary causes for intraseasonal MLD deepening and explain 83.7% of the total ISV. The ISV of MLD gives rise to high-frequency entrainments of subsurface water, leading to an enhancement of the annual entrainment rate by 34%. However, only a small fraction of these entrainment events (<20%) can effectively contribute to the annual obduction rate of 1.36 Sv, a quantification for the amount of resurfacing thermocline water throughout a year that mainly (84.6%) occurs in the summer monsoon season (May–October). The ISV of MLD achieves the maximal intensity in April–May and greatly affects the subsequent obduction. Estimation based on our HYCOM simulations suggests that MJOs overall reduce the obduction rate in the summer monsoon season by as much as 53%. A conceptual schematic is proposed to demonstrate how springtime intraseasonal MLD deepening events caused by MJO winds narrow down the time window for effective entrainment and thereby suppress the obduction of thermocline water.

On the interplay between horizontal resolution and wave drag and their effect on tidal baroclinic mode waves in realistic global ocean simulations

The effects of horizontal resolution and wave drag damping on the semidiurnal M2 tidal energetics are studied for two realistically-forced global HYbrid Coordinate Ocean Model (HYCOM) simulations with 41 layers and horizontal resolutions of 8 km (1∕12.5∘; H12) and 4 km (1∕25∘; H25). In both simulations, the surface tidal error is minimized by tuning the strength of the linear wave drag, which is a parameterization of the surface-tide energy conversion to the unresolved baroclinic wave modes. In both simulations the M2 surface tide error with TPXO8-atlas, an altimetry constrained model, is 2.6 cm. Compared to H12, the surface tide energy conversion to the resolved vertical modes is increased by 50% in H25. This coincides with an equivalent reduction in the tuned loss of energy from the surface tide to the wave drag. For the configurations studied here, the horizontal and not the vertical resolution is the factor limiting the number of vertical modes that are resolved in most of the global ocean: modes 1–2 in H12 and modes 1–5 in H25. The wave drag also dampens the resolved internal tides. The 40% reduction in wave-drag strength does not result in a proportional increase in the mode-1 energy density in H25. In the higher-resolution simulations, topographic mode-scattering and wave–wave interactions are better resolved. This allows for an energy flux out of mode 1 to the higher modes, mitigating the need for an internal tide damping term. The HYCOM simulations are validated with analytical conversion models and altimetry-inferred sea-surface height, fluxes, and surface tide dissipation. H25 agrees best with these data sets to within ∼10%. To facilitate the comparison of stationary tide signals extracted from time series with different durations, we successfully apply a spatially-varying correction factor.

Surface and Sub‐surface Ocean Response to Tropical Cyclone Phailin: Role of Pre‐existing Oceanic Features

AbstractThe upper oceanic thermal response induced by Tropical Cyclone Phailin (9–14 October 2013) under the influence of East India Coastal Current (EICC) and a cyclonic eddy is investigated and contrasted with the response from open ocean region using a high‐resolution HYbrid Coordinate Ocean Model simulation. There is significant cooling (7° C) inside the cold core eddy and negligible cooling (0.5° C) within the EICC region characterized by the shallow and deeper thermocline, respectively. Our analysis of mixed layer heat budget terms showed that the horizontal advection plays a significant role in determining the temperature tendency for the location within the EICC, in contrary to the general dominance of vertical processes as reported in previous studies during the cyclone period. The analysis for the locations inside eddy and open ocean concurs with the previous studies showing the dominance of vertical processes toward the temperature tendency. Further, near the coast, the surface cooling is minimal compared to the subsurface cooling, dominantly seen between 50‐ and 100‐m depth. This disparity indicates that the factors responsible for the surface temperature anomalies are different from those of subsurface. Our analysis of thermal signatures after the passage of cyclone showed that the EICC and cyclonic eddy contribute to the faster advection of cold wake and recovery of sea surface temperature to the prestorm state.

The impact of ocean surface currents on global eddy kinetic energy via the wind stress formulation

A pair of 12.5-year (July 2002–December 2014) HYbrid Coordinate Ocean Model (HYCOM) simulations that only differ in the wind stress formulation are used to investigate the effect of ocean surface currents on global monthly eddy kinetic energy (EKE) variation. The model results (2004–2014) show that the global monthly mean EKE is reduced by 37%, from 1.76 EJ (1018 J) to 1.10 EJ after ocean surface currents are included in the wind stress formulation. The monthly EKE budget indicates that the shear production and buoyancy work are positive (energy source) and the eddy wind work on the geostrophic currents is negative (energy sink) in the steady state (2004–2014) for both simulations. All of these three terms are reduced in the steady state when the ocean currents are included in the wind stress formulation. The global integral of the EKE difference budget suggests that the EKE reduction is primarily due to the reduction of the buoyancy work, followed by the reduction of the wind work on the geostrophic currents and the shear production. To our knowledge this is the first study to separate the eddy wind work into the geostrophic and ageostrophic components to investigate the impact of ocean surface currents on global and depth integrated EKE via the wind stress formulation using HYCOM simulations.

Assessment of shelf sea tides and tidal mixing fronts in a global ocean model

Tidal mixing fronts, which represent boundaries between stratified and tidally mixed waters, are locations of enhanced biological activity. They occur in summer shelf seas when, in the presence of strong tidal currents, mixing due to bottom friction balances buoyancy production due to seasonal heat flux. In this paper we examine the occurrence and fidelity of tidal mixing fronts in shelf seas generated within a global 3-dimensional simulation of the HYbrid Coordinate Ocean Model (HYCOM) that is simultaneously forced by atmospheric fields and the astronomical tidal potential. We perform a first order assessment of shelf sea tides in global HYCOM through comparison of sea surface temperature, sea surface tidal elevations, and tidal currents with observations. HYCOM was tuned to minimize errors in M2 sea surface heights in deep water. Over the global coastal and shelf seas (depths <200 m) the area-weighted root mean square error of the M2 sea surface amplitude in HYCOM represents 35% of the 50 cm root mean squared M2 sea surface amplitude when compared to satellite constrained models TPXO8 and FES2014. HYCOM and the altimeter constrained tidal models TPXO8 and FES2014 exhibit similar skill in reproducing barotropic tidal currents estimated from in-situ current meter observations. Through comparison of a global HYCOM simulation with tidal forcing to a global HYCOM simulation with no tides, and also to previous regional studies of tidal mixing fronts in shelf seas, we demonstrate that HYCOM with embedded tides exhibits quite high skill in reproducing known tidal mixing fronts in shelf seas. Our results indicate that the amount of variability in the location of the tidal mixing fronts in HYCOM, estimated using the Simpson-Hunter parameter, is consistent with previous studies when the differences in the net downward heat flux, on a global scale, are taken into account. We also provide evidence of tidal mixing fronts on the North West Australian Shelf for which we have been unable to find references in the existing scientific literature.

Southern Ocean Biogeochemical Float Deployment Strategy, With Example From the Greenwich Meridian Line (GO-SHIP A12).

Biogeochemical Argo floats, profiling to 2,000‐m depth, are being deployed throughout the Southern Ocean by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM). The goal is 200 floats by 2020, to provide the first full set of annual cycles of carbon, oxygen, nitrate, and optical properties across multiple oceanographic regimes. Building from no prior coverage to a sparse array, deployments are based on prior knowledge of water mass properties, mean frontal locations, mean circulation and eddy variability, winds, air‐sea heat/freshwater/carbon exchange, prior Argo trajectories, and float simulations in the Southern Ocean State Estimate and Hybrid Coordinate Ocean Model (HYCOM). Twelve floats deployed from the 2014–2015 Polarstern cruise from South Africa to Antarctica are used as a test case to evaluate the deployment strategy adopted for SOCCOM's 20 deployment cruises and 126 floats to date. After several years, these floats continue to represent the deployment zones targeted in advance: (1) Weddell Gyre sea ice zone, observing the Antarctic Slope Front, and a decadally‐rare polynya over Maud Rise; (2) Antarctic Circumpolar Current (ACC) including the topographically steered Southern Zone chimney where upwelling carbon/nutrient‐rich deep waters produce surprisingly large carbon dioxide outgassing; (3) Subantarctic and Subtropical zones between the ACC and Africa; and (4) Cape Basin. Argo floats and eddy‐resolving HYCOM simulations were the best predictors of individual SOCCOM float pathways, with uncertainty after 2 years of order 1,000 km in the sea ice zone and more than double that in and north of the ACC.

The Impact of Ocean Surface Currents on Sverdrup Transport in the Midlatitude North Pacific via the Wind Stress Formulation

AbstractA more complete wind stress τn formulation takes into account the ocean surface currents Vo, while the conventional wind stress τc popularly used in ocean circulation models is only a function of 10-m winds V10. An analytical solution is derived for the difference of Sverdrup transport induced by using τn instead of τc. A scaling analysis of the analytical solution indicates a 6% reduction of the Sverdrup transport in the North Pacific (i.e., the Kuroshio transport in the East China Sea) when Ekman velocity dominates the ocean surface currents. Because of the quadratic nature of wind stress, four nonlinear terms contribute equally to this difference: two vorticity torque terms and two speed gradient torque terms. A pair of 12.5-yr (July 2002–14) Hybrid Coordinate Ocean Model simulations that only differ in the wind stress formulation are used to test the analytical solution. The model results (2004–14) confirm that using τn instead of τc reduces the Sverdrup transport in the North Pacific by 8%–17% between 23° and 32°N. The reduction rate of the simulated 11-yr mean Kuroshio transport through the East Taiwan Channel and Tokara Strait is 8.0% (−2.5 Sv; 1 Sv ≡ 106 m3 s−1) and 12.8% (−4.8 Sv), respectively, in good agreement with the Sverdrup transport reduction rate, which is 7.4% (−2.6 Sv) and 15.4% (−6.3 Sv) at the corresponding latitude. The local effect of changing wind stress/wind work and Ekman transport due to the inclusion of Vo in the wind stress formulation is negligible compared to the Kuroshio volume transport change estimated in this study.

Inter-comparison studies between high-resolution HYCOM simulation and observational data: The South Atlantic and the Agulhas leakage system

Statistical analyses and model-data inter-comparisons are performed to evaluate the model's ability to reproduce the dynamics in the upper layers (<2000m) of the South Atlantic ocean. Outputs of an eddy-resolving Ocean General Circulation Model (OGCM) are analyzed and compared with observed data. The model, a 1/12-degree, 22-layer implementation of the Hybrid Coordinate Ocean Model (HYCOM) to the South Atlantic, was forced with monthly-mean products from the NCEP Reanalysis for the period 1960 to 2010. The numerical experiment was capable to reproduce the large scale and the mesoscale dynamic in the South Atlantic and in the Agulhas region. The vertical structure is in agreement with in situ data, the model has lower skill when compared with PIRATA lower temperatures, and is able to capture the seasonal and annual variability in the tropical Atlantic. Furthermore, sensitive change until 2007 is clear in the vertical structure, at 4°N–38°W; 0°–35°W and 10°S–10°W, suggesting an important change in the stratification. The primary results concern a significant change in the decadal anomalies of the temperatures and salinity, which exhibit a warmer and saltier water in the southeastern Atlantic. Furthermore, linear trends found in the transport time-series in the North Brazil Current, and the South Equatorial Current were seen to correspond with increasing trends of the warmer water from the Agulhas Current into South Atlantic. The integrated transport during the period 1960–2010, shows an increase in westward changes in the large-scale circulation south of Africa are show in the negative trends indicate a widening of the “Agulhas gap” and increase in westward volume transport since 1980. It is therefore suggested that variability in the Agulhas System on the last two decades is affecting the dynamic in the South Atlantic, namely the temperature and the volume transport, reach the tropical region of the Atlantic.

Modeling Salinity Exchanges Between the Equatorial Indian Ocean and the Bay of Bengal

Abstract : With a focus on the Bay of Bengal, models ranging from a 1/12.5 global ocean model to a 1/4 regional Indian Ocean model to a 2 km local high-resolution coupled model are used to simulate salinity exchanges in the Indian Ocean. Global Hybrid Coordinate Ocean Model simulations show a surprisingly large persistent flow of high-salinity water from the equatorial Indian Ocean into the Bay of Bengal during the northeast monsoon, although it is weaker than during the southwest monsoon. On average, salt is transported into the Bay of Bengal between 83E and 95E, and low-salinity water flows southward near the east coast of Sri Lanka and east of 95E. The Regional Ocean Modeling System shows that knowledge of river input of freshwater is essential for modeling surface salinities correctly in the Bay of Bengal. High-resolution coupled model simulations are in agreement with recent observations and show that a strong subsurface current with a speed of about 1 m s -1 intrudes into the Bay of Bengal beneath southward-flowing low-salinity water during the northeast monsoon. The subsurface high-salinity water, which originates in the northern Arabian Sea, spreads northward into the Bay of Bengal and downward along constant density surfaces. North of 10N, the model simulation implies that mixing takes place on density surfaces at depths of 100 -150 m after advection of cold, low-salinity water from the north, and subsequent stirring of the two water masses. Vertical diffusion plays an insignificant role in this mixing.

Variation of flow properties during a collision event of two mesoscale eddies in the Gulf Stream region from numerical simulation

The temporal variation of the flow structure and consequent mixing process during the collision of two counter-rotating mesoscale eddies are investigated by analyzing the HYbrid Coordinate Ocean Model simulation for the Gulf Stream region using Eulerian parameters—Okubo-Weiss parameters and horizontal kinetic energy (KE)—and Lagrangian parameters—finite-size Lyapunov exponent (FSLE) and relative dispersion coefficients (Kr). During the collision process, a transport barrier constructed by FSLE ridges develops between the two eddies and hyperbolic points are formed at both ends of the barrier. High values of the shear components of strain (> mean + standard deviation) are observed around the hyperbolic points, indicating possible deformation of the eddy. The magnitudes of spatially averaged KE and FSLE values increase (~20% and ~25%, respectively) during the collision as the flows around the main eddy become more energetic and dispersive. The Eulerian measures—the relative vorticity and the shear components of strain—show different temporal evolutions. The former does not significantly vary (~3%) while the latter has a peak value (~34%) at the time of maximum impact of the collision. In contrast, the Lagrangian measures show a similar pattern of temporal variations as both FSLE and Kr values generally increase (~25% and ~35%, respectively) during the collision, which indicates increased mixing due to the collision.

How Does Drifter Position Uncertainty Affect Ocean Dispersion Estimates?

AbstractTo develop methodologies to maximize the information content of Lagrangian data subject to position errors, synthetic trajectories produced by both a large-eddy simulation (LES) of an idealized submesoscale flow field and a high-resolution Hybrid Coordinate Ocean Model simulation of the North Atlantic circulation are analyzed. Scale-dependent Lagrangian measures of two-particle dispersion, mainly the finite-scale Lyapunov exponent [FSLE; λ(δ)], are used as metrics to determine the effects of position uncertainty on the observed dispersion regimes. It is found that the cumulative effect of position uncertainty on λ(δ) may extend to scales 20–60 times larger than the position uncertainty. The range of separation scales affected by a given level of position uncertainty depends upon the slope of the true FSLE distribution at the scale of the uncertainty. Low-pass filtering or temporal subsampling of the trajectories reduces the effective noise amplitudes at the smallest spatial scales at the expense of limiting the maximum computable value of λ. An adaptive time-filtering approach is proposed as a means of extracting the true FSLE signal from data with uncertain position measurements. Application of this filtering process to the drifters with the Argos positioning system released during the LatMix: Studies of Submesoscale Stirring and Mixing (2011) indicates that the measurement noise dominates the dispersion regime in λ for separation scales δ &lt; 3 km. An expression is provided to estimate position errors that can be afforded depending on the expected maximum λ in the submesoscale regime.

Estimation of the barrier layer thickness in the Indian Ocean using Aquarius Salinity

AbstractMonthly barrier layer thickness (BLT) estimates are derived from satellite measurements using a multilinear regression model (MRM) within the Indian Ocean. Sea surface salinity (SSS) from the recently launched Soil Moisture and Ocean Salinity (SMOS) and Aquarius SAC‐D salinity missions are utilized to estimate the BLT. The MRM relates BLT to sea surface salinity (SSS), sea surface temperature (SST), and sea surface height anomalies (SSHA). Three regions where the BLT variability is most rigorous are selected to evaluate the performance of the MRM for 2012; the Southeast Arabian Sea (SEAS), Bay of Bengal (BoB), and Eastern Equatorial Indian Ocean (EEIO). The MRM derived BLT estimates are compared to gridded Argo and Hybrid Coordinate Ocean Model (HYCOM) BLTs. It is shown that different mechanisms are important for sustaining the BLT variability in each of the selected regions. Sensitivity tests show that SSS is the primary driver of the BLT within the MRM. Results suggest that salinity measurements obtained from Aquarius and SMOS can be useful for tracking and predicting the BLT in the Indian Ocean. Largest MRM errors occur along coastlines and near islands where land contamination skews the satellite SSS retrievals. The BLT evolution during 2012, as well as the advantages and disadvantages of the current model are discussed. BLT estimations using HYCOM simulations display large errors that are related to model layer structure and the selected BLT methodology.

Investigation of the Relationship Between the Yucatan Channel Transport and the Loop Current Area in a Multidecadal Numerical Simulation

Abstract A hypothesis by Maul (1977), stating the rate of change of loop current (LC) volume is related to deep Yucatan Channel (YC) transport, is tested with a continuous 54-year simulation of the Gulf of Mexico (GoM) using a regional 1/25° resolution Hybrid Coordinate Ocean Model (HYCOM) configuration. The hypothesis states that the imbalance of transport between the upper YC and the Florida Straits controls the rate of change of the LC volume and that the imbalance is compensated by transport through the deep YC. Bunge et al. (2002) found a strong relationship between the deep YC transport and the LC area using 7.5 months of data from a mooring array in the YC, but the observational record length was relatively short compared to the time scale of LC variability. The 54-year HYCOM simulation provides a much longer and spatially complete data set to study the LC variability. Results show that the time evolution of the LC between two shedding events can be viewed as a combination of relatively high-frequency fluctuations superimposed on a low-frequency trend. The high-frequency variability of the LC area time derivative and the deep YC transport are related. The low-frequency variability is examined by comparing the LC area time series with time-integrated transport in the deep YC, and statistically similar trends are identified, supporting the Maul (1977) theory.

Effects of Kuroshio intrusions on nonlinear internal waves in the South China Sea during winter

During winter the Kuroshio tends to cross Luzon Strait, penetrating the northeastern South China Sea where it forms energetic mesoscale structures. Luzon Strait is also a site where westward‐propagating large‐amplitude internal waves are generated. We describe observations of these waves acquired in the deep basin of the South China Sea during the winter of 2010−2011, with the goal of assessing the influence of mesoscale variability on their properties. Combining tidal current simulations with an internal wave generation and evolution model, we obtain time series of deviations between our observations and the model simulations. These deviations are analyzed in terms of mesoscale variability based on the data‐assimilated HYbrid Coordinate Ocean Model (HYCOM) simulations. We find that simplified models of nonlinear internal wave response to changes in horizontal stratification gradients and vertical shear provide at best weak simulations of amplitude modulation. In contrast to these results, deviations of internal wave arrival time, occurring up to 2 h early during Kuroshio inflows, are quite well simulated when derived from integration of the first internal mode phase speed along two‐dimensional ray‐traced paths using HYCOM simulations of velocity and density fields together with bathymetry. Refraction of the internal waves by Kuroshio intrusions can lead to substantial distortion of the paths, sufficient to provide a potential explanation for the apparent suppression of waves during previous winter measurements. Our results suggest that the internal wave field can be a sensitive indicator of mesoscale variability in Luzon Strait and the northeastern South China Sea.

Seasonal Variation of the Indonesian Throughflow in Makassar Strait

Abstract The seasonal variation of Indonesian Throughflow (ITF) transport is investigated using ocean general circulation model experiments with the Hybrid Coordinate Ocean Model (HYCOM). Twenty-eight years (1981–2008) of ⅓° Indo-Pacific basin HYCOM simulations and three years (2004–06) from a global HYCOM simulation are analyzed. Both models are able to simulate the seasonal variation of upper-ocean currents and the total transport through Makassar Strait measured by International Nusantara Stratification and Transport (INSTANT) moorings reasonably well. The annual cycle of upper-ocean currents is then calculated from the Indo-Pacific HYCOM simulation. The reduction of southward currents at Makassar Strait during April–May and October–November is evident, consistent with the INSTANT observations. Analysis of the upper-ocean currents suggests that the reduction in ITF transport during April–May and October–November results from the wind variation in the tropical Indian Ocean through the generation of a Wyrtki jet and the propagation of coastal Kelvin waves, while the subsequent recovery during January–March originates from upper-ocean variability associated with annual Rossby waves in the Pacific that are enhanced by western Pacific winds. These processes are also found in the global HYCOM simulation during the period of the INSTANT observations. The model experiments forced with annual-mean climatological wind stress in the Pacific and 3-day mean wind stress in the Indian Ocean show the reduction of southward currents at Makassar Strait during October–November but no subsequent recovery during January–March, confirming the relative importance of wind variations in the Pacific and Indian Oceans for the ITF transport in each season.