Gwang-Ho Seo

Long-term comparison of satellite and in-situ sea surface temperatures around the Korean Peninsula

Satellite remotely sensed sea surface temperature (SST) was compared with in-situ SST in the seas around the Korean Peninsula from 1984 to 2013. A matchup dataset between satellite and in-situ SSTs was used. The root mean square error (RMSE) between satellite and in-situ SSTs was approximately 1°C in the offshore area and 2~3°C in the coastal area. The satellite SST exhibits a cold bias of 1°C or less in the offshore area and a warm bias of 1~3°C in the coastal area. The satellite SSTs generally agree with the in-situ data in the East/Japan Sea (EJS) better than in the South Sea and the Yellow Sea (YS). The RMSE between the two SSTs in the South Sea (SS) is 1~2°C. In-situ and satellite SST analyses respectively indicate a warming trend of 0.024°C/year and 0.011°C/year for the study period in the seas around the Korean Peninsula. The difference in the long-term trends from the two data sources is mainly due to the difference in the YS. The satellite SST showed a warm bias of 0.5~1.0°C in the early 1980’s and a cold bias of 0.5°C in the early 2010’s, which should be carefully considered in studying long-term trends with satellite SST data.

Evolution of wind‐driven flows in the Yellow Sea during winter

To examine the evolution of the wind-driven flows in the Yellow Sea (YS) during winter, ocean circulation was simulated using a three-dimensional ocean model with realistic topography and atmospheric forcing. The simulated sea surface temperature, ocean currents, and path of the Yellow Sea Warm Current (YSWC) agreed with observations. Southward currents along the Korean coast and the Chinese coast in winter were also effectively identified. Spectra of the daily mean winds and the YSWC velocities in the subsurface layer had dominant peaks at 12 and 20 day periods. Time-lagged correlation analysis suggested that the downwind flow in the surface layer reacts concurrently to the northwesterly wind in winter whereas the subsurface layer responds with a delay. One day after the wind burst, an upwind current in the subsurface layer appeared in the center of the trough, whereas the downwind flow in the surface layer decreased significantly. Two days later, the upwind flow in the subsurface layer shifted to the west of the trough while the downwind flow along the Korean coast strengthened. These flow responses to the wind variations resulted in a clockwise circulation in the YS during winter.

Evaluation of a regional ocean reanalysis system for the East Asian Marginal Seas based on the ensemble Kalman filter

This study introduces the East Asian Marginal Seas (EAMS) reanalysis system and evaluates its products from 1982 to 2006. The EAMS reanalysis system consists of an ocean circulation model with 0.25° horizontal grid spacing and a data assimilation module. Temperature profiles taken by ship and Argo floats as well as satellite-borne sea surface temperature (SST) were assimilated into the model by applying the ensemble Kalman filter every 7 days. The reanalyzed oceanic fields were compared with ones by the control run without data assimilation to assess the impact of the data assimilation. The assimilative model significantly improved the horizontal structures of SST, sea surface height (SSH), vertical structure of temperature, and volume transports through the major straits. Root-mean-square error (RMSE) of SST decreased from 1.0 to 0.6°C. Horizontal and vertical distribution of subsurface temperature was corrected close to the observed values. High SSH variability in the south of Japan and in the Kuroshio extension region was partially restored. Overshooting of the Kuroshio Current and the East Korea Warm Current was suppressed to the south by the data assimilation. SSH and surface circulation in the Oyashio region and in the East Sea (also known as the Sea of Japan) recovered, which corrected the sea level difference between the East Sea and the Pacific Ocean and produced realistic transport through the Korea Strait. The reanalysis well resolved transport through the Korea Strait. The correlation coefficient with the transport based on an Acoustic Doppler Current Profiler measurement was 0.70 and the RMSE was 0.37 Sv (106 m3/s), whereas those were 0.59 and 0.43 Sv in the control run, respectively. The EAMS reanalysis dataset may help us to understand circulation variations in the marginal seas and to investigate factors controlling volume and heat transport.

Climate change projection in the Northwest Pacific marginal seas through dynamic downscaling

This study presents future climate change projections in the Northwest Pacific (NWP) marginal seas using dynamic downscaling from global climate models (GCMs). A regional climate model (RCM) for the Northwest Pacific Ocean was setup and integrated over the period from 2001 to 2100. The model used forcing fields from three different GCM simulations to downscale the effect of global climate change. MIROC, ECHAM, and HADCM were selected to provide climate change signals for the RCM. These signals were calculated from the GCMs using Cyclostationary Empirical Orthogonal Function analysis and added to the present lateral open boundary and the surface forcing. The RCM was validated by comparing hindcast result with the observation. It was able to project detailed regional climate change processes that GCMs were not able to resolve. A relatively large increases of water temperature were found in the marginal seas. However, only a marginal change was found along the Kuroshio path. Heat supply to the atmosphere decreases in most study areas due to a slower warming of the sea surface compared to the atmosphere. The RCM projection suggests that the temperature of the Yellow Sea Bottom Cold Water will gradually increase by 2100. Volume transports through major straits except the Taiwan Strait in the marginal seas are projected to increase slightly in future. Increased northeasterly wind stress in the East China Sea may also result in the transport change.