Hong-Ryeol Shin

Seasonal variations in the low-salinity intermediate water in the region south of sub-polar front of the East Sea (Sea of Japan)

Seasonal variations in the low-salinity intermediate water (ESIW) in the region south of the sub-polar front of the East Sea were investigated by using historical hydrographic data. The salinity of the representative density (sigma-0=27.2) of the ESIW was minimal in summer and maximal in winter in the region south of the sub-polar front. The selected four subregions showed different salinity variations. In the west of Oki Spur and the Yamato Basin, salinity fluctuated similarly, with a minimum during summer. In the Ulleung Basin and northwest of Sado Island, however, variations in salinity showed two minima, one is in winter and the other is in summer. These results imply differences in the flow path of the ESIW into the region south of the sub-polar front over time.

Vertical gradient correction for the oceanographic Atlas of the East Asian Seas

Regional climatology around the East Asian Seas has been developed by an international collaboration between the National Oceanic Data Center and the Korea Oceanic Data Center. It provides reliable information on temperature and salinity climatological fields with high resolution (0.1° × 0.1° by 137 levels). However, there is a problem around near-bottom areas where topographic change is steep and observations are not available near the bottom. This study resolves this problem using a vertical gradient correction method when the profile is statically unstable. The stability is determined based on the Brunt-Vaisala frequency with individual temperature and salinity profiles. Topographic-following mapping technique employing the potential vorticity constraint term is used to construct a vertical gradient database for the temperature and salinity at every grid point. The results show that the correction is effective for eliminating large erroneous vertical gradients around near-bottom areas. In addition, we show the importance of the optimal length scale to construct a precise vertical gradient database in a particular area such as the northern shelf of Taiwan. We expect that our revised high-resolution climatological mean fields will serve as important data for relevant studies around the East Asian Seas.

The Characteristics of Coastal Currents to the Northwest of the Taean Peninsula in the Yellow Sea

To investigate the characteristics of tidal currents and water circulation in the coastal waters off the Taean Peninsula, tidal currents and sea levels were measured at the study area from 1998 to 2004. In the central waterway to the south of Changan Sand Ridge, mean speed of tidal currents and residual currents were 74.0cm/s, 17.8cm/s respectively; the dominant residual currents flowed northeastward, and the amplitudes of semi-diurnal components were larger than diurnal components . The flood and ebb tidal currents were northeastward and southwestward, respectively, and each period was about 6 hours for them, which was consistent with the period of sea levels at the study area. In the coastal region near Hakampo, Taean, mean velocities of tidal currents and residual currents were 46.1cm/s, 30.8cm/s respectively, and the dominant residual currents flowed southwestward. The amplitudes of shallow water constituents were relatively laige, which were weaker to the northeastern coastal region off Mineodo. The northeastward flow continued for about hours, while the southwestward flow continued for about hours near Hakampo during the tidal period. Tidal currents flowed northeastward in the central area of the waterway during the period from the Low Water Level (LWL) to the High Water Level (HWL). While the currents in the coastal region flowed northeastward for the first 3 hours after the LWL, southwestward counter-currents flowed between 3 and 6 hours after the LWL. During the period from the HWL to the LWL, the dominant currents flowed southwestward in the study area except to the northeastern coastal region off Mineodo. Along the shorelines, the counter-currents flowed northward between 4 and 6 hours after the HWL. It seems that the counter-currents near the coastal region are caused by the topography and the geography of the shorelines at the study area.

Analysis of Sea Surface Temperature Simulation in the Northwestern Pacific and the East Asian Marginal Seas using HadGEM2-AO

In this study, we evaluated the model performance with respect to Sea Surface Temperature (SST) and Net Heat Flux (NHF) by considering the characteristics of seasonal temperature variation and contributing factors and by analyzing heat budget terms in the Northwestern Pacific and East Asian Marginal Seas (110˚E−160˚E, 15˚N−60˚N) using the HadGEM2-AO historical run. Annual mean SST of the HadGEM2-AO is about 0.065oC higher than observations (EN3_v2a) from 1950 to 2000. Since 1960, the model has simulated well the long-term variation of SST and the increasing rate of SST in the model (0.014℃/year) is comparable with observations (0.013℃/year). Heat loss from the ocean to the atmosphere was simulated slightly higher in the HadGEM2-AO than that in the reanalysis data on the East Asian Marginal Seas and the Kuroshio region. We investigated the causes of temperature variation by calculating the heat budget equation in the two representative regions. In the central part of the Kuroshio axis (125˚E−130˚E, 25˚N−30˚N: Region A), both heat loss in the upper mixed layer by surface heat flux and vertical heat advection mainly cause the decrease of heat storage in autumn and winter. Release of latent heat flux through the heat convergence brought about by the Kuroshio contributes to the large surface net heat flux. Positive heat storage rate is mainly determined by horizontal heat advection from March to April and surface net heat flux from May to July. In the central part of the subtropical gyre (155˚E−160˚E, 22˚N−27˚N: Region B), unlike Region A, vertical heat advection predominantly causes the decrease of heat storage in autumn and winter. In spring and summer, surface heat flux contributes to the increase of heat storage in Region B and the period is two times longer than the period for Region A. In this season, shoaling of the mixed layer depth plays an important role in the increase of SST.

The Characteristics of Physical Oceanographic Environments and Bottom Currents in the KODOS Study Area of the Northeastern Tropical Pacific

Abstract : Hyrdography and deep currents were measured from 1997 to 1999 to investigate deep-seaenvironments in the KODOS (Korea Deep Ocean Study) area of the northeastern tropical Pacific. KODOSarea is located meridionally from the North Equatorial Current to the boundary between the NorthEquatorial Current and the Equatorial Counter Current. Strong thermocline exists between 10 m and 120 mdepths at the study area. Since that strong thermocline does hardly allow vertical mixing between surfaceand lower layer waters, vertical distributions of temperature, salinity, dissolved oxygen and nutrientsdrastically change near the thermocline. Salinity-minimum layer, which indicate the North PacificIntermediate Water (NPIW) and the Antartic Intermediate Water (AAIW), vertically occupies vertically atthe depths from 500 m down to 1400 m. The NPIW and the AAIW horizontally occur to the north and tothe south of 7 o N, respectively. The near-bottom water shows the physical characteristics of 1.05C and34.70 psu at the depths of 10 m to 110 m above the bottom (approximately 4000-5000 m), which wasoriginated from the Antarctic Circumpolar Water. It flows northeastwards for 2 to 4 months at the studyarea, and its mean velocity was 3.1-3.7 cm/s. Meanwhile, reverse (southwestward) currents appear for about15 days with the average of 1.0-6.1 cm/s every 1 to 6 months. Dominant direction of the bottom currentsobtained from the data for more than 6 months is northeastward with the average speeds of 1.7-2.1 cm/s.Therefore, it seems that deep waters from the Antarctica flow northwards passing through the KODOS areain the northeastern tropical Pacific.Key words : (KODOS), (the North Equatorial Current), (the EquatorialCounter Current), (thermocline), (deep waters), (the Antarctic CircumpolarWater)*Corresponding author. E-mail : hrshin@kongju.ac.kr