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Masaru Yamamoto and Naoki Hirose

Abstract

The present study examines the influence of an assimilation SST product on simulated monthly precipitation. The high-resolution SST structures located close to the oceanic front and coastal areas are important in regional atmospheric simulations over semienclosed marginal seas such as the Japan Sea. Two simulations are conducted using assimilation and interpolation SST products (experiments R and N, respectively), for January 2005. The surface heat fluxes and PBL height in experiment R are lower than those in experiment N in coastal areas and the cold tongue. A decrease of 4 K in SST leads to decreases of 120 W m−2 in surface sensible and latent fluxes and 300 m in PBL height. The precipitation in experiment R is less than that in experiment N for the sea area except at 38°N, 137°E. The cold tongue in the central Japan Sea acts to reduce moisture supply via the latent heat flux, resulting in low precipitation in coastal areas. The fact that the difference between observed and modeled precipitation in experiment R is 21% less than that in experiment N demonstrates that the assimilation of SST data leads to improved regional atmospheric simulations of monthly precipitation.

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Rui M. Ponte and Naoki Hirose

Abstract

Previous studies of barotropic ocean dynamics have shown that several global normal modes are possible at periods between 1 and 2 days. The role of these modes in diurnal tidal dynamics is well documented, but finding evidence for their direct excitation by atmospheric forcing has remained elusive. Analyses of a barotropic ocean model and a limited number of bottom pressure records in the Southern Ocean described here provide evidence for large-scale, coherent signals propagating westward around Antarctica at 1–2-day periods. Phases are roughly consistent with the fundamental Kelvin wave structure underlying a number of normal modes with significant expressions in the Southern Ocean. Model and data results suggest the weak excitation of several of these normal modes by atmospheric pressure fluctuations. Strong dissipation also contributes to their weak amplitudes and makes it difficult to detect a truly global signal in the data.

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Atsuyoshi Manda, Naoki Hirose, and Tetsuo Yanagi

Abstract

The surface restoring condition of satellite-derived sea surface temperatures (SSTs) is validated as a feasible assimilation method with an ocean circulation model that incorporates the strongly nonlinear mixed layer model. The restoring treatment is an empirical method for correcting the heat flux in order to pull the predicted SST toward the observed SST; it is referred to as the nudging method in this study. A one-dimensional experiment is conducted to evaluate the skill of the nudging method. The mixed layer model (MLM) used in the experiment is a second-order turbulence closure model that has a strong nonlinearity. The skill of the nudging method is compared with that of an ensemble Kalman filter, which is a statistically optimal method for nonlinear dynamic models. Although the nudging method is statistically suboptimal, the result of the experiment shows that the skill of this method is comparable when using an appropriate restoring time scale. A three-dimensional experiment using an ocean general circulation model (OGCM), which incorporates the same MLM as that used in the one-dimensional experiment, is also conducted to further examine the skill of the nudging method. By applying the nudging method to the OGCM, the model improves the estimated thermal structure not only near the surface, but also in the subsurface layers.

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Eunjeong Lee, Yign Noh, and Naoki Hirose

Abstract

A new method of producing sea surface temperature (SST) data for numerical weather prediction is suggested, which is obtained from the assimilation of satellite-derived SST into an atmosphere–ocean mixed layer coupled model. The Weather Research and Forecasting (WRF) Model and the Noh mixed layer model are used for the atmosphere and ocean mixed layer models, respectively. Data assimilation (DA) is carried out in two steps, based on the estimation from the covariance matching method that the daily mean SST of satellite data is more accurate than the model data, if the number of data in a grid per day is sufficiently large—that is, the daily mean SST bias correction in the first DA and the sequential SST anomaly correction in the second DA. For the second DA, the model restarts from the initial condition corrected by the first DA, and DA is applied every 30 min using the nudging method. The daily mean and the diurnal variation of satellite SST are assimilated to the bulk and skin SST, respectively. The modeled results with the new data assimilation scheme are validated by statistical comparison with independent satellite and buoy data such as correlation coefficient, root-mean-square difference, and bias. Furthermore, the sensitivity and seasonal variation of the weighting factor in the second DA are examined. The new approach illustrates the possibility of applying the atmosphere–ocean mixed layer coupled model for the production of SST data combined with the assimilation of satellite data.

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Yign Noh, Hyejin Ok, Eunjeong Lee, Takahiro Toyoda, and Naoki Hirose

Abstract

The effect of Langmuir circulation (LC) on vertical mixing is parameterized in the ocean mixed layer model (OMLM), based on the analysis of large-eddy simulation (LES) results. Parameterization of LC effects is carried out in terms of the modifications of the mixing length scale as well as the inclusion of the contribution from the Stokes force in momentum and TKE equations. The performance of the new OMLM is examined by comparing with LES results, together with sensitivity tests for empirical constants used in the parameterization. The new OMLM is then applied to the ocean general circulation model (OGCM) Meteorological Research Institute Community Ocean Model (MRI.COM), and its effect is investigated. The new OMLM helps to correct too shallow mixed layer depths (MLDs) in the high-latitude ocean, which has been a common error in most OGCMs, without making the thermocline in the tropical ocean more diffused. The parameterization of LC effects is found to affect mainly the high-latitude ocean, in which the MLD is shallow in summer and stratification is weak in winter.

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Naoki Hirose, Hyun-Chul Lee, and Jong-Hwan Yoon

Abstract

Climatological monthly mean variations of the surface heat fluxes over the East China Sea and the Yellow Sea are calculated by both a data analysis and a numerical simulation. The result of the data analysis based on the empirical/bulk method agrees well with the directly observed solar radiation and several previous studies of the surface heat fluxes. An adjustment in the formation of the Haney-type heat flux is presented by comparing to the result of the bulk method. The numerical simulation of these seas using an ocean general circulation model demonstrates the success of the improved Haney-type condition over the original one in simulating sea surface temperature. The surface heat flux simulated in the Yellow Sea is more reasonable than by the data analysis considering the total heat budget of this sea.

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Naoki Hirose, Tianran Liu, Katsumi Takayama, Katsuto Uehara, Takeshi Taneda, and Young Ho Kim

Abstract

This study clarifies the necessity of an extraordinary large coefficient of vertical viscosity for dynamical ocean modeling in a shallow and narrow strait with complex bathymetry. Sensitivity experiments and objective analyses imply that background momentum viscosity is on the order of 100 cm2 s−1, while tracer diffusivity estimates are on the order of 0.1 cm2 s−1. The physical interpretation of these estimates is also discussed in the last part of this paper. To obtain reliable solutions, this study introduces cyclic application of the dynamical response to each parameter to minimize the number of long-term sensitivity experiments. The recycling Green’s function method yields weaker bottom friction and enhanced latent heat flux simultaneously with the increased viscosity in high-resolution modeling of the Tsushima/Korea Strait.

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Jae-Hong Moon, Naoki Hirose, Jong-Hwan Yoon, and Ig-Chan Pang

Abstract

A patchlike structure of low-salinity water detached from the Chanjiang “Diluted Water” (CDW) is frequently observed in the East China Sea (ECS). In this study, the offshore detachment process of CDW into the ECS is examined using a three-dimensional numerical model. The model results show that low-salinity water is detached from the CDW plume by the intense tide-induced vertical mixing during the spring tide period when the tidal current becomes stronger. During the spring tide, thickness of the bottom mixed layer in the sloping bottom around Changjiang Bank reaches the mean water depth, implying that the stratification is completely destroyed in the entire water column. As a result, the offshore detachment of CDW occurs in the sloping side of the bank where the tidal energy dissipation is strong enough to overcome the buoyancy effect during this period. On the other hand, the surface stratification is retrieved during the neap tide period, because the tidal current becomes substantially weaker than that in the spring tide. Wind forcing over the ECS as well as tidal mixing is a critical factor for the detachment process because the surface wind primarily induces a northeastward CDW transport across the shelf region where tide-induced vertical mixing is strong. Moreover, the wind-enhanced cross-isobath transport of CDW causes a larger offshore low-salinity patch, indicating that the freshwater volume of the low-salinity patch closely depends on the wind magnitude.

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Shin’ichiro Kako, Tomofumi Nakagawa, Katsumi Takayama, Naoki Hirose, and Atsuhiko Isobe

Abstract

This study investigated how the Changjiang River discharge (CRD) emptying into the East China Sea (ECS) affects the upper-ocean stratification [hence, sea surface temperature (SST) changes], based on ocean general circulation modeling with and without CRD. A new finding in this study is that CRD contributes significantly to a reduction in summer SST in the ECS. Comparison between the two model runs revealed that vertical one-dimensional processes contribute considerably to SST warming in the ECS, while horizontal advection plays an important role in lowering SST in summer. The results of a particle-tracking experiment suggested that the cold water mass formed along the Chinese coast during the previous winter contributes to the SST reduction in the following summer. From the end of the summer monsoon season, the less saline CRD advected toward the Chinese coast generates a shallow mixed layer (ML), which inhibits heat exchange between the ML and thermocline. In winter, heat loss of the ML through the sea surface results in a reduction in SST over a broad region. Water exchange through the bottom of the ML is relatively suppressed by robust stratification, which prevents cooling of the thermocline and leads to a temperature inversion. The northeastward ocean current associated with the summer monsoon carries the cold water mass in the ML across the ECS; therefore, SST decreases during the following season. These results suggest that CRD has a critical role on both the ocean circulation system and the coupled air–sea interactions in the ECS.

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