Shih-Jen Huang

Effects of Cold Eddy on Kuroshio Meander and Its Surface Properties, East of Taiwan

Satellite remote-sensing data and glider data are used to study the Kuroshio meander and surface properties, east of Taiwan. The Kuroshio meandered eastward 13 times between 1993 and 2013 because of cold eddies propagating from the western Pacific. The maximum duration of the meanders was 80 days. The farthest eastward shift of the Kuroshio axis was approximately 270 km from its original position, depending on the size of the cold eddy. Cold eddies reduce the current speed at the Kuroshio axis to 84% of its seasonal average, which is approximately 0.75 m/s. According to glider data, isopycnal uplifting is produced when cold eddies impinge on the Kuroshio, and satellite observations show that the sea surface temperature (SST) drops 1-3 °C and that the chlorophyll-a (chl-a) concentration increases up to 0.54 mg/m3.

Analysis of chlorophyll-a concentration around the South China Sea from ocean color images

In this study, we will discuss the spatial and temporal variation of the chlorophyll-a concentration (Chl-a) around the SCS. All Chl-a data are derived from Aqua satellite MODIS (Moderate Resolution Imaging Spectroradiometer) visible ocean color data during the period from 2002 to 2006. The Empirical Orthogonal Function (EOF) analysis is applied to see the dominant patterns of the Chl-a variation in the study area. From EOF mode 1 results of Chl-a, we find that the higher Chl-a is mainly along the coastal area of the SCS and rapidly decreasing offshore. The Chl-a EOF mode 2 indicates a higher Chl-a jet along the Vietnam coast in summer, and a high Chl-a meandering off the northwestern Luzon in winter. From Chl-a EOF mode 2 results we can see a clear Chl-a vibration between the eastern and western coast of the SCS. The Chl-a along the western coast enhances in summer but decays in winter, the reversed situation happens in the eastern coast. The covariance Chl-a EOF1 results emphasize again a clear high Chl-a plume along the Vietnam coast in summer. Furthermore, we also can retrieve sea surface temperature (SST) data from Aqua satellite MODIS thermal infrared bands. The gradient SST EOF mode 1 results show the colder water is mainly along the northwestern SCS. Meanwhile, the warm Kuroshio water intrusion to the Luzon Strait happens mainly in winter. The gradient SST EOF2 results indicate two circulation gyres in the SCS, one nearly covers the whole northern SCS and the other smaller one is in the southern SCS. These two gyres are all warmer in summer and cooler in winter. The spatial-mean removed joint EOF results of Chl-a and SST indicate the SST pattern is much clear than the Chl-a pattern in the SCS, it is also found the Kuroshio intrusion to the Luzon Strait in winter and two circulation gyres in the northern and southern SCS.

Variability of chlorophyll-a concentration and sea surface wind in the South China Sea associated with the El Niño-Southern Oscillation

In this paper, the seasonal and interannual variability of the chlorophyll-a concentration (Chl-a) and sea surface wind (SSW) fields in the South China Sea (SCS) are analyzed using Orbview-2/SeaWiFS, and ERS-1/2 and QuikSCAT satellite data. The mode 1 results of the empirical orthogonal function (EOF) analysis for Chl-a indicate the higher Chl-a is mainly along the coastal areas in the SCS, while the EOF mode 2 results show a summer-winter oscillation of the Chl-a distribution in the SCS. Through the comparison between EOF mode 2 results of the Chl-a anomaly and Nino 3.4 index, it is found that the Chl-a variation in the SCS is highly related to the El Nino-Southern Oscillation (ENSO) events. The SSW EOF mode 1 results indicate that the northeasterly monsoon in winter is much stronger than the southwesterly monsoon in summer. The SSW EOF mode 2 results show the zonal line near 15o N can be a boundary to separate the wind pattern into two parts in fall, the northeasterly wind is in its north and southwesterly wind in its south. The EOF results of the SSW anomaly show the weaker monsoon occurring during the 1997/1998 El Nino winter. Comparing Chl-a EOF mode 2 and SSW EOF mode 1 results, we can find that Chl-a in the SCS is mainly influenced by the monsoon wind, wind-induced coastal upwelling makes the Chl-a increasing along the western SCS in summer but along the eastern SCS in winter.

Detecting Internal Waves From Satellite Ocean Color Imagery

Internal waves have been observed by lots of high resolution satellite images, such as Synthetic Aperture Radar (SAR) and optical images of SPOT and Landsat. These images are usually expensive. In this study, some free but lower spatial resolution satellite images are applied to observe the internal wave phenomena. The internal waves in the Sulu Sea are detected from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) onboard the Orbview-2 satellite. The SeaWiFS image has a spatial resolution of 1.1 km. It is acceptable to observe the internal wave phenomena while the soliton width is larger than the image resolution. The results show that the internal solitary in the Sulu Sea can be observed successfully with SeaWiFS chlorophyll images. The internal waves in the Sulu Sea have amplitudes of 10 to 90 m and wavelengths of 5 to 16 km. The large-amplitude internal solitary waves may significantly influence the near-surface chlorophyll concentration. The chlorophyll concentration would be lower when the depression internal waves passed through. A theoretic model is proposed and tested to estimate the amplitudes of internal waves from chlorophyll concentration images.© 2006 ASME

The influence of tide on sea surface temperature in the marginal sea of northwest Pacific Ocean

Tide gauge data provided by the University of Hawaii Sea Level Center and daily sea surface temperature (SST) data from the Moderate Resolution Imaging Spectroradiometer (MODIS) product are used in this study to analyze the influence of tide on the SST in the seas of Northwestern Pacific. In the marginal region, the climatology SST is lower in the northwestern area than that in the southeastern area. In the coastal region, the SST at spring tide is higher than that at neap tide in winter, but it is lower in other seasons. In the adjacent waters of East China Sea and Yellow Sea, the SST at spring tide is higher than that at neap tide in winter and summer but it is lower in spring and autumn. In the open ocean region, the SST at spring tide is higher than that at neap tide in winter, but it is lower in other seasons. In conclusion, not only the river discharge and topography, but also tides could influence the SST variations, especially in the open ocean region.

Satellite observations of rainfall effect on sea surface salinity in the waters adjacent to Taiwan

Changes of oceanic salinity are highly related to the variations of evaporation and precipitation. To understand the influence of rainfall on the sea surface salinity (SSS) in the waters adjacent to Taiwan, satellite remote sensing data from the year of 2012 to 2014 are employed in this study. The daily rain rate data obtained from Special Sensor Microwave Imager (SSM/I), Tropical Rainfall Measuring Mission’s Microwave Imager (TRMM/TMI), Advanced Microwave Scanning Radiometer (AMSR), and WindSat Polarimetric Radiometer. The SSS data was derived from the measurements of radiometer instruments onboard the Aquarius satellite. The results show the average values of SSS in east of Taiwan, east of Luzon and South China Sea are 33.83 psu, 34.05 psu, and 32.84 psu, respectively, in the condition of daily rain rate higher than 1 mm/hr. In contrast to the rainfall condition, the average values of SSS are 34.07 psu, 34.26 psu, and 33.09 psu in the three areas, respectively at no rain condition (rain rate less than 1 mm/hr). During the cases of heavy rainfall caused by spiral rain bands of typhoon, the SSS is diluted with an average value of -0.78 psu when the average rain rate is higher than 4 mm/hr. However, the SSS was increased after temporarily decreased during the typhoon cases. A possible reason to explain this phenomenon is that the heavy rainfall caused by the spiral rain bands of typhoon may dilute the sea surface water, but the strong winds can uplift the higher salinity of subsurface water to the sea surface.

The coastal sea surface temperature changes near the nuclear power plants of northern Taiwan observed from satellite images

In order to estimate the thermal plumes discharged from Chinshan and Kuosheng nuclear power plants on the coast of north Taiwan, this study uses the thermal infrared data from Landsat 7 ETM+(Enhanced Thematic Mapper Surface Temperature) to contrast with the in-situ SST measurement for the intake/discharge ports of the nuclear power plants. The near-infrared (band 4) data of Landsat 7 ETM+ are firstly applied to distinguish ocean and land, and then the thermal infrared (band 6) data are used to estimate SST. The algorithm of SST on north Taiwan is established in this study by the contrast between the in-situ SST data of the two nuclear power plants and the thermal infrared data of Landsat 7 ETM+. The standard deviation of SST retrieved through this algorithm is estimated to be 3.1°C, but the mean difference is near 0. According to the retrieved SST from the satellite data, the warm-plume (>4°C than offshore SST) discharge of Chinshan nuclear power plant reaches 540-1080 m far from its discharge port, but for Kuosheng Nuclear Power Plant, the farthest of the warm-plume discharge from the discharge port is 390-900 m. The retrieved SST gradually gets cooler by diffusion from the discharge port to the offshore. Apparently, the Landsat 7 ETM+ can be applied to measure the special variance of SST. The result also shows the area of significant thermal plume (>4°C than offshore SST) are about 0.01-1.3 km2 and 0.09-8.53 km2 for the Chinshan and Kuosheng nuclear plants respectively. Moreover, the significant thermal plume area is affected by tides. During the flood tide, the warm-plume discharge gets close to the coast, and it will make the significant thermal plume area increase. Besides, the second significant thermal plume (>2°C than offshore SST) is also increased during the ebb tide because the thermal plume may be taken away and diluted from the discharged port. However, due to different topographies, the area of thermal plume of the Kuosheng is broader than that of Chinshan nuclear power plant.

Typhoon Eye Observations Using SAR and MTSAT

Typhoon eyes have been delineated from the smoother area in the Radarsat Synthetic Aperture Radar (SAR) images of ocean surface roughness and from the warmer area in the Multi-functional Transport Satellite (MTSAT) infrared images by using wavelet analysis. Case studies for different typhoons and environment have been investigated to demonstrate that SAR can be a powerful tool to help in typhoon tracking and prediction, especially at the ocean surface. It is found that the distance between the center locations of these typhoon’s eyes, as determined by SAR and MTSAT, respectively, is quite significant (14–26 km) for all five cases. The result of large center distance between typhoon eyes at the cloud level from MTSAT data and on the ocean surface from SAR data implies that the eyewall shaft may be highly tilted and the vertical wind shear profile is more complex than generally expected. Some of the issues concerning the definition of typhoon eye and typhoon tracking/prediction have been identified and compared with other data sets. Also, the tilted structure and associated vertical wind shear, especially during typhoon turning and staggering, may be caused by the ocean feedback or island blocking effects.

Estimation of Tropical Cyclone Heat Potential From Oceanic Measurements

The primary purpose of this study is to estimate the tropical cyclone heat potential (TCHP) in the western North Pacific Ocean using in-situ measurements and satellite remote sensing data, as well as to explore the influence of TCHP on the genesis and intensification of typhoon in this region. The TCHP is defined as the integration of heat content from the depth of 26°C isotherm to the sea surface. Sea surface height and sea surface temperature data are used to estimate the TCHP based on a two-layer reduced gravity model. Totally 35 typhoons from 2006 to 2009 are analyzed in this study. The result shows that the typhoon is dramatically developed when it goes through the area with more TCHP than surroundings. From the result of regression analysis, the correlation coefficient between typhoon intensity and accumulated TCHP is above 0.8. It implies that the typhoon intensity changes might be controlled by the TCHP conditions of tropical cyclone formation region.Copyright

Detecting the Phytoplankton Bloom From Satellite Images

Phytoplankton is the base of the marine food web. However, the phytoplankton bloom may deplete the dissolved oxygen and shade aquatic life, and even damages the marine environment. In this study, the satellite-derived sea surface chlorophyll-a concentration (Chl-a) and sea surface temperature (SST) are considered to examine the behavior of the phytoplankton bloom in the sea around the western side of the Luzon Strait in the northeastern South China Sea (SCS). Meanwhile, the related sea surface wind (SSW) is also included to understand the possible mechanism to induce the high Chl-a plume around there. The results indicate that the clear high Chl-a water is mainly around the northeastern SCS in winter, and spreads out westward from the northern edge of the Luzon island, Philippines. The external force of this blooming is probably from monsoon wind driving, the strong and diverse winter monsoon in the Luzon Strait can develop a very clear positive wind stress curl in the northwestern side of Luzon island, and then induce the cold and high nutrient water upward. Meanwhile, the strong winter monsoon can also spread this cold water plume out to develop a very clear and large phytoplankton blooming around there.Copyright