Dejun Dai

Mechanism of internal waves in the Luzon Strait

[1]xa0This study suggests a western boundary current instability generation mechanism for the ocean internal waves. A linear model describing this instability is developed, and an analytical solution is derived from a zero-order complex frequency–wavenumber relation. The real part of the relation represents a dispersion relation of the generated internal waves, and the imaginary part is an exponential growth rate. The dispersion relation describes the wave propagation characteristics, while the growth rate represents the instability properties. The theoretical results are used for the case of the Kuroshio east of the Luzon Strait. Based on the analysis, it is found that for the westward propagating disturbance, the Kuroshio west wing is unstable and the east wing is stable; while the reverse is true for the eastern propagating disturbance. The results are used to interpret satellite SAR images of the ocean internal waves, which were generated in the Luzon Strait and propagated westward. Reasonable estimates for the growth rate and propagation velocity are derived from a scale analysis of the Kuroshio east of the Luzon Strait.

Evaluating CMIP5 simulations of mixed layer depth during summer

The ability of CMIP5 models in simulating surface mixed layer depth (MLD) during summer is assessed using 45 climate models. Their ocean models differ greatly in terms of vertical mixing parameterizations and model configurations. In some models, effects of surface waves, Langmuir circulations, submesoscale eddies, as well as additional wind mixing are included to improve upper-ocean simulation. Similar to findings by previous studies, the summer MLDs are significantly underestimated in most of the models. Compared with the observation, only five of these models have deeper summer MLDs in the Southern Ocean, eight models have deeper summer MLDs in the central North Atlantic Ocean, and nine models have deeper summer MLDs in the central North Pacific Ocean. This underestimation of MLD is not caused by sea surface forcing, because most of the models tend to overestimate the surface wind stress, while they underestimate the net surface heat flux. Therefore, insufficient vertical mixing in the upper ocean may still be one of the potential reasons for this systematic underestimation of MLD in the climate models.

Wave-turbulence interaction-induced vertical mixing and its effects in ocean and climate models.

Heated from above, the oceans are stably stratified. Therefore, the performance of general ocean circulation models and climate studies through coupled atmosphere–ocean models depends critically on vertical mixing of energy and momentum in the water column. Many of the traditional general circulation models are based on total kinetic energy (TKE), in which the roles of waves are averaged out. Although theoretical calculations suggest that waves could greatly enhance coexisting turbulence, no field measurements on turbulence have ever validated this mechanism directly. To address this problem, a specially designed field experiment has been conducted. The experimental results indicate that the wave–turbulence interaction-induced enhancement of the background turbulence is indeed the predominant mechanism for turbulence generation and enhancement. Based on this understanding, we propose a new parametrization for vertical mixing as an additive part to the traditional TKE approach. This new result reconfirmed the past theoretical model that had been tested and validated in numerical model experiments and field observations. It firmly establishes the critical role of wave–turbulence interaction effects in both general ocean circulation models and atmosphere–ocean coupled models, which could greatly improve the understanding of the sea surface temperature and water column properties distributions, and hence model-based climate forecasting capability.

Field measurement of upper ocean turbulence dissipation associated with wave‐turbulence interaction in the South China Sea

[1]xa0The turbulence dissipation rate within the mixed layer was measured in the open ocean and the coastal water of the South China Sea under moderate winds of 4.7xa0∼xa08.9xa0mxa0s−1 using a free-falling MSS profiler. In the open ocean, the profile of the dissipation rate within the mixed layer exhibited an exponential decay with the depth at most stations, which was satisfactorily consistent with that predicted by the parameterization of wave-turbulence interaction presented by Huang and Qiao (2010) while deviating from that by the law of the wall. In the coastal ocean, however, both the parameterization of wave-turbulence interaction and the law of the wall can give approximate predictions to the measured dissipation rate.

A numerical study on dynamic mechanisms of seasonal temperature variability in the Yellow Sea

[1]xa0Using in situ observations and numerical modeling, this study investigates the dynamical mechanisms of seasonal variability of water temperature in the Yellow Sea (YS). Observations indicate that bottom temperature lags 3–4 months behind surface temperature in reaching a maximum in the central YS. Wave-tide-circulation coupled model simulates this time lag and indicates that the diffusion process is a key factor governing the temperature variation below the surface layer. Based on the diffusion equation of temperature, a scheme is developed to estimate the vertical diffusion coefficient. At an observation station located at 36°00′N 124°00′E, the diffusion coefficients from April to September are estimated by using the temperature data from 1954 to 1985. The mean diffusion coefficient (MDC) in the upper layer from 0 m to 15 m is almost one order of magnitude larger than those in the middle layer from 20 to 40 m, except in April. In the middle layer, the MDC is inversely proportional to the squared buoyancy frequency, and the mean value of MDC averaged from June to September is 0.28 cm2 s−1. The inverse proportionality agrees with the Osborns relation, which has been used to estimate the diapycnal diffusivity.

Three dimensional simulation of internal wave attractors in the Luzon Strait

Internal waves propagate along wave beams that are inclined with respect to the horizontal plane. It is conjectured that the internal waves generated in the Luzon Strait may be confined between the double ridges in the strait and concentrate to a closed trajectory, the so-called internal wave attractor, due to the reflection of wave beams from the lateral boundaries, sea surface and bottom. This work carried out two experiments using a three dimensional non-hydrostatic general circulation model, MITgcm, to investigate the possibility that the ridges in the Luzon Strait allows for internal wave attractors. Baroclinic current in both of the experiments demonstrate the forming of ring-like patterns in some section around 20° and 21°N, indicating that the development of the internal wave attractors are allowed in the Luzon Strait. The different resolutions and initial conditions in the two experiments also reveal that the internal-wave-attractor phenomenon is robust in this region.