Hong Sik Min

Large Eddy Simulation of the Ocean Mixed Layer: The Effects of Wave Breaking and Langmuir Circulation

Abstract Large eddy simulation (LES) of the ocean mixed layer was performed in which both wave breaking and Langmuir circulation are realized. Wave breaking was represented by random forcing consistent with the observed near-surface turbulence, and Langmuir circulation was realized by the Craig–Leibovich vortex force. High- resolution simulations were carried out using parallel computing with or without each contribution, wave breaking and Langmuir circulation, with an aim to clarify their respective roles in the ocean mixed layer. The effects of wave breaking were found to be mainly limited to the near-surface zone of the upper few meters. Langmuir circulations below it are not significantly modified, although they become somewhat weakened and less coherent. Under the influence of wave breaking, however, the turbulence production in the upper-ocean mixed layer becomes dominated by the turbulent kinetic energy flux, contrary to the case of the atmospheric boundary layer where it is dominated by shear prod...

Influence of the Surface Heating on Langmuir Circulation

Large-eddy simulation of the oceanic mixed layer showed that Langmuir circulation (LC) is weakened under the surface heating and is ultimately broken down if the intensity of the surface heating becomes sufficiently strong. The critical condition for the breakdown of LC was mainly determined by the Hoenikker number Ho, and the transition occurs in the range Ho ; 1‐2. The breakdown of LC leads to a drastic change in the characteristics of the oceanic mixed layer, such as the variation of the rms horizontal velocities with time, the ratio of the horizontal spectra of vertical velocity field, and the pitch. The stability condition for LC suggested by Leibovich was still observed in this simulation. Furthermore, it was found that LC is largely responsible for the formation of a thermocline and the maintenance of a well-mixed layer above it, and the depth of a thermocline was estimated in that case.

Pacific Decadal Oscillation and its relation to the extratropical atmospheric variation in CMIP5

Abstract It is investigated how the simulated Pacific Decadal Oscillation (PDO) differs among various coupled general circulation models (CGCMs), and how it is related to the simulated atmospheric variation in the North Pacific. The dataset of the historical runs of the 26 CGCMs reported to the Coupled Model Intercomparison Project Phase 5 are used. It is shown that the differences in the PDO pattern among 26 CGCMs are closely related to diverse displacements of the simulated Aleutian Low (AL), that is, the PDO pattern is highly dependent on longitudinal and latitudinal position of the anomalous AL associated with the PDO (PDO-related AL; ALPDO). In addition, it is demonstrated that in the models that have the southward shift of the ALPDO, the North Pacific sea surface temperature anomalies (SSTAs) associated with the PDO tend to modulate the atmospheric circulation strongly, implying strong two-way feedback between ocean and atmosphere. Therefore, this gives relatively strong persistency of the PDO. It is also found that the PDO in these models is highly correlated with the variation of the Kuroshio Extension current, implying that ocean dynamical processes play an important role in developing SSTAs.

Inter-model diversity in jet stream changes and its relation to Arctic climate in CMIP5

We examined how coupled general circulation models (CGCMs) simulate changes in the jet stream differently under greenhouse warming, and how this inter-model diversity is related to the simulated Arctic climate changes by analyzing the simulation of the Coupled Model Intercomparison Project Phase 5. Although the jet stream in the multi-model ensemble mean shifts poleward, a considerable diversity exists among the 34 CGCMs. We found that inter-model differences in zonal wind responses, especially in terms of meridional shift of the midlatitude jet, are highly dependent on Arctic surface warming and lower stratospheric cooling. Specifically, the midlatitude jet tends to shift relatively equatorward (poleward) in the models with stronger (weaker) Arctic surface warming, whereas the jet tends to shift relatively poleward (equatorward) in the models with stronger (weaker) Arctic lower stratospheric cooling.

Ocean mixed layer processes in the Pacific Decadal Oscillation in coupled general circulation models

It is investigated how the Pacific Decadal Oscillation (PDO) is simulated differently among various coupled general circulation models (CGCMs), and how it is related to the heat budget of the simulated ocean mixed layer, which includes the surface heat flux and ocean heat transport. For this purpose the dataset of the climate of the 20th Century experiment (20C3M) from nine CGCMs reported to IPCC’s AR4 are used, while the MRI and MIROC models are examined in detail. Detailed analyses of these two CGCMs reveal that the PDO is mainly affected by ocean heat transport rather than surface heat flux, in particular in the MRI model which has a larger contribution of ocean heat transport to the heat budget. It is found that the ocean heat transport due to Ekman advection versus geostrophic advection contributes differently to the PDO in the western and central North Pacific. Specifically, the strength of PDO tends to be larger for CGCMs with a larger ocean heat transport in the region.

Seasonal variation of semidiurnal internal tides in the East/Japan Sea

The seasonal variation of semidiurnal internal tides in the East/Japan Sea was investigated using 25 month long output from a real-time ocean forecasting system. The z coordinate eddy-resolving high-resolution numerical model, called the RIAM ocean model, incorporates data assimilation that nudges temperature and salinity fields together with volume transport through the Korea Strait to produce realistic oceanic currents and stratification. In addition to atmospheric forcing, it includes tidal forcing of 16 major components along open boundaries. The model generates energetic semidiurnal internal tides around the northern entrance of the Korea Strait. Energy conversion from barotropic to baroclinic (internal) tides varies seasonally with maxima in September (ranging 0.48–0.52 GW) and minima in March (ranging 0.11–0.16 GW). This seasonal variation is induced by the seasonality in stratification near the southwestern East/Japan Sea. The propagation distance of the internal tides is associated with generation intensity and wavelength. From late summer to early winter, the semidiurnal internal tides travel relatively far from the generation region due to stratification changes; its energy dissipates less as a result of longer wavelengths. Our results suggest that spatiotemporal variation of internal-tide-induced mixing due to the seasonality in the generation, propagation, and dissipation of internal tides should be considered for a more realistic simulation of water masses and circulation in models of the East/Japan Sea.

Sensitivity of Arctic warming to sea ice concentration

We examine the sensitivity of Arctic amplification (AA) to background sea ice concentration (SIC) under greenhouse warming by analyzing the data sets of the historical and Representative Concentration Pathway 8.5 runs of the Coupled Model Intercomparison Project Phase 5. To determine whether the sensitivity of AA for a given radiative forcing depends on background SIC state, we examine the relationship between the AA trend and mean SIC on moving 30 year windows from 1960 to 2100. It is found that the annual mean AA trend varies depending on the mean SIC condition. In particular, some models show a highly variable AA trend in relation to the mean SIC clearly. In these models, the AA trend tends to increase until the mean SIC reaches a critical level (i.e., 20–30%), and the maximum AA trend is almost 3 to 5 times larger than the trend in the early stage of global warming (i.e., 50–60%, 60–70%). However, the AA trend tends to decrease after that. Further analysis shows that the sensitivity of AA trend to mean SIC condition is closely related to the feedback processes associated with summer surface albedo and winter turbulent heat flux in the Arctic Ocean.

Second-mode semidiurnal internal tides on the continental slope of the southwestern East/Japan Sea

ABSTRACT Lee, H.Y.; Park, J.H.; Jeon, C.; Seo, S.; Kim D.G.; Park, Y.G.; Min H.S., and Kim, S.D., 2016. Second-mode semidiurnal internal tides on the continental slope of the southwestern East/Japan Sea. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1387 - 1391. Coconut Creek (Florida), ISSN 0749-0208. Energetic second-mode semidiurnal internal tides with a three-layer-like structure were observed from time series of 26-hour-long 2 hourly velocity profiles collected on the continental shelf-slope of the southwestern East/Japan Sea during a spring tidal period in early November 2013. Modal-fitted baroclinic semidiurnal signals using a least-squared method demonstrate that the kinetic energy of second-mode semidiurnal internal tides is comparable to the first-mode ones. In order to understand physical processes affecting the enhanced generation of the second-mode internal tides and their spatio-temporal variations, we utilize 30-month long numerical simulation outputs from a real-time ocean forecasting system of the East/Japan Sea. The model includes realistic oceanic circulation and stratification together with tides of 16 major components. The analysis results reveal that the most energetic second-mode semidiurnal internal tides generate during fall months when the main thermocline was located near the mid-depth (100–150 m) over the continental shelf-slope. It appears that the deepened mixed layer together with the enhanced mid-depth stratification induced by the increased flow through the western channel of the Korea Strait provide a second-mode favorable stratification condition near the internal tide generation region. This study has an implication that the seasonally-varying second-mode semidiurnal internal tides would have an impact on the tide-induced ocean mixing in the southwestern East/Japan Sea.