Sea Surface Observation in the Taiwan Strait Using Satellite Imager from HRPT Station

Nan-Jung Kuo and Chung-Ru Ho

Department of Oceanography, National Taiwan Ocean University, Keelung, Chinese Taipei

Abstract

A sequence of OrbView-2/SeaWiFS Chlorophyll-a (Chl-a) concentration and NOAA/AVHRR sea surface temperature (SST) images during the whole year of 1999 is used to understand the distribution of the upper ocean patterns in the Taiwan Strait (TS).  All data were obtained from HRPT (High Resolution Picture Transmission) satellite receiving station at Department of Oceanography, National Taiwan Ocean University.  It can be found that the higher Chl-a concentration with lower SST always happened along the western coast, while a wedge-like low Chl-a warmer water covered the whole southern TS and moved northward along the southeastern coast. During the summer the Chl-a shows a great contrast between central deeper water and coastal shallower water, but SST was nearly homogeneous around the whole TS.  The empirical orthogonal function (EOF) analysis is also applied in this study.  From gradient EOF mode 1 of Chl-a concentration, we can find that the bottom topography plays a very important role on its distribution; the higher Chl-a regions were located near the coastal area with the depth less than 50m. The gradient EOF mode1 from AVHRR SST images indicates that the water in the TS can be divided into two parts. One is the cold water in the west and extends to most of the northern TS, and the other one is the warmer water in the east covering most of the southern TS.  The associated temporal amplitudes show that the horizontal variation of the SST is very small in the summer, which is opposite to the Chl-a field.

Introduction

Taiwan Strait (TS) is located between Taiwan and Mainland China with the width varying from 160 km in the north to 200 km in the south (Figure 1).  It is a shallow water channel, with an average depth of about 55 m, connecting the East China Sea (ECS) and the South China Sea (SCS) in the north and the south, respectively.  The main currents in the TS are the China Coastal Current, the South China Sea Current, and the branch of the Kuroshio. The southwesterly monsoon drives the warm SCS water flowing all the way through TS during the summer, while the northeasterly monsoon drives the cold China Coast Current moves southward into the TS in winter. The warm water from the branch of the Kuroshio also moves northward into the strait during cold seasons (Fan, 1982).  The topographic change may influence the movement of the currents in the strait.  For example, Chern and Wang (2000) and Jan et al. (1998) noted that the southward flowing cold China Coastal Current always blocked by the Chang-Yuan Ridge and developed an oceanic front with the warm water of the northward moving Kuroshio branch current in the wintertime. Meanwhile, the interaction of the above two currents may induce upwelling around the Peng-Hu Islands (Fan, 1982).

Figure 1: Map of the Taiwan Strait with isobaths of 50, 100, 200, 500, and 1000m.  PI, PHC, CYR represent the Peng-Hu Islands, Peng-Hu Channel, and Chang-Yuen Ridge, respectively.

 

Satellite images can be used to detect surface patterns around the TS.  Lin et al. (1992) found two groups of shear waves in the TS from one NOAA/AVHRR image on December 12, 1989. Kuo and Ho (1996) detected the angular velocity of an anticyclonic eddy in the northern TS through two consecutive NOAA/AVHRR images. Hu et al. (2001) combined the hydrographic and NOAA/AVHRR sea surface temperature data to detect the summer upwelling in the TS. These studies emphasized the SST properties of the TS mostly, but did not consider the long-term variation of the SST field in the whole TS.  In this study, a sequence of NOAA/AVHRR images is considered to understand the spatial and temporal distributions of the thermal field in the TS during the whole year of 1999.  Meanwhile, the associated nearly simultaneous OrbView-2/SeaWiFS images are also used to see the biological variability in the upper TS.  Besides the specific patterns we can see from individual images, the EOF analysis is also be applied to decompose the time series of spatial SeaWiFS and AVHRR images into its dominant modes of variability.

Satellite observation and data analysis

All the AVHRR and SeaWiFS images in this study are received from the HRPT (High Resolution Picture Transmission) station at Department of Oceanography, National Taiwan Ocean University.  Four of selected 3-day composite SeaWiFS-derived Chl-a concentration images and their corresponding AVHRR-derived SST images are shown in Figure 2 and 3, respectively.

 

24N

 

22N

 

23N

 

25N

 

(b)

4/17/99

 

(a)

2/23/99

 
   

        119E      120E       121E                  119E      120E       121E

22N

 

24N

 

25N

 

23N

 

(d)

10/26/99

 

(c)

8/17/99

 
    

 

Figure 2: SeaWiFS-derived Chl-a images in the Taiwan Strait.  Color bar at the bottom indicates Chl-a concentration (mg m-3) range and white pixels indicate no valid data.  

 

 

24N

 

22N

 

23N

 

25N

 

(b)

4/17/99

 

(a)

2/23/99

 
   

        119E      120E       121E                  119E      120E       121E

22N

 

24N

 

25N

 

23N

 

(d)

10/26/99

 

(c)

8/17/99

 
    

 

Figure 3: Same as Figure 2 but for AVHRR-derived SST images.  Color bar indicates SST (oC) range.

 

We can see that the high Chl-a concentration is always around the coastal areas, especially in the western coast of the TS. A wedge-like low Chl-a area always covers the whole southern TS with depth greater than 100m and along the Peng-Hu Channel near the southeastern coast to the north of about 24oN. This low Chl-a water may stretch and makes a turn northwesterly along the channel between the Peng-Hu Islands and Chang-Yuen Ridge to cover more areas deeper than 50m.  Meanwhile, the above low Chl-a water shows a much great contrast with the surrounding water during the summer.  Figure 3 shows the corresponding SST field.  We can see that the higher Chl-a water along the western TS contains lower SST, while the southeastern wedge-like low Chl-a water was always warmer.  The SST field shows a different distribution as the Chl-a field during the summer, the whole TS covers nearly homogeneous warm water except some cold spots in the western coast and south of the Peng-Hu Islands.

Empirical orthogonal function (EOF) analysis is generally regarded as a very efficient method to extract information from a large data set; it can reduce the original information to a few time-varying spatial patterns that explain most of the variance in the data.      The spatial distribution and its associated time-varying amplitudes of the first gradient EOF of SeaWiFS Chl-a images are shown in Figure 4a and 5a, respectively.  This mode contains 65.8 % of the variance and is highly related to the bottom topography. Because the time-varying amplitudes in Figure 5a are all negative, the regions with depth greater than 50m are therefore low Chl-a water. They include the large wedge-like warm water and the area along the central TS. The western coastal water including Formosa Banks and the eastern coastal area containing Chang-Yuen Ridge are all relatively high Chl-a areas. The associated time-varying amplitudes of this mode also indicate that the great spatial Chl-a difference is in the summer, while the relative low Chl-a gradient during the wintertime.    

The spatial patterns of the first covariance EOF for SeaWiFS Chl-a images are shown in Figure 4b.  It shows a similar spatial distribution to the gradient EOF mode 1 in Figure 4a. This mode includes 43.5% of the variance. The corresponding time-varying amplitudes in Figure 5b show a large and negative value in August. This tremendous temporal change will enlarge the horizontal gradient of the Chl-a concentration during the summer.  The spatial distribution of the first gradient EOF from NOAA/AVHRR SST images is shown in Figure 4c. It contains 90.6% of the variance. Because the corresponding temporal values in Figure 5c are all negative, the western region with positive spatial value would be the cold water while the eastern area with negative value would be the warm water region. Meanwhile, the coldest water is along the western coast and the warmest water covers most of the southern TS through the Peng-Hu Channel northward passing the water shallower than 100m in depth and stretching to the Chang-Yuan Ridge of about 24.3oN. Figure 5c also indicates the smallest spatial SST variation in summer while the largest spatial SST distribution in the wintertime.

The covariance EOF analysis from NOAA/AVHRR SST images is also considered in this study.  The mode 1 contains 92.5% of the variance.  Combining Figure 4d and 5d, we can see the lowest SST occurs in January and the highest SST in August or September. Meanwhile, the great seasonal SST variation occurs in the western coast of the TS while the lower temporal SST change in the eastern TS. These phenomena can be explained by the seasonal change of the flow patterns in these two distinct areas. The great seasonal SST change in the western coast area may be resulted from the steady southward cold water along the western coast of the TS from Mainland China in the winter and the warm water along the eastern strait from SCS during summer.  The lower seasonal SST change in the eastern TS is because that warm water always exists in any season, it is from SCS in summer and from Kuroshio extension in winter.

 

24N

 

22N

 

23N

 

25N

 

(b)

 

 

(a)

 

 
   

        119E      120E       121E                  119E      120E       121E

22N

 

24N

 

25N

 

23N

 

(d)

 

 

(c)

 

 
   

Figure 4: The mode 1 spatial patterns of (a) gradient Chl-a, (b) covariance Chl-a, (c) gradient SST, and (d) covariance SST EOF. 50 and 100m isobaths are marked.  The color bar indicates range of mg m-3 for Chl-a and o C for SST, respectively.

Figure 5: Corresponding time-varying amplitudes of the spatial patterns in Figure 4.

Summary and conclusions

In this study, a sequence of OrbView-2/SeaWiFS and NOAA/AVHRR images offers a nearly simultaneous two-dimensional comparison of biological (Chl-a concentration) and physical (SST) variability in the upper ocean of the TS during 1999.  The NOAA/AVHRR images shows that the SST distribution in the TS is mainly related to the seasonal monsoon wind prevailing.  During the winter the strong northeasterly monsoon drives the coastal cold water southward along the western TS.  Meanwhile, the branch of the warm Kuroshio also entered the south of TS and moved northward along the southeastern TS.  The SST field therefore displays a distribution of that the western coast is colder than the eastern coast and southern part is warmer than the northern area. In the summertime the southwesterly monsoon push the warm SCS water northward to make nearly homogeneous water in TS. The SeaWiFS-derived Chl-a concentration in TS shows different variations as the SST distribution. The complex bottom topography plays a very important role on the Chl-a distribution in TS.  The EOF analysis for Chl-a images shows that TS may compose two regions by depth. One is the higher Chl-a region including the western and eastern coasts and some areas in the middle TS with depth less than 50 m.  The other one is the lower Chl-a region with the depth greater than 50 m, which includes the wedge-like water from the southern TS and part of deeper water along central TS. From EOF analysis for SST images, we can find that the variation of the SST field in TS is mainly affected by the interaction between monsoon and currents. The patterns of Chl-a concentration and SST field are not the same, especially in the summertime.

Acknowledgements

This work was supported by the National Science Council (NSC) under grants NSC 90-2611-M-019-012 and NSC 90-2745-OCI-FD20-01.

References

Chern, C.-S.,Wang, J., 2000. Some aspects of the flow-topography interactions in the Taiwan Strait. Terrestrial, Atmos. and Oceanic Sciences 11(4), 861-878.

Fan, K.-L., 1982. A study of water masses in Taiwan Strait. Acta Oceanogr. Taiwanica 13, 140-153.

Ho, C.-R., Kuo, N.-J., Zheng, Q., Soong, Y.-S., 2000. Dynamically active area in the South China Sea detected from TOPEX/POSEIDON satellite altimeter data. Remote Sens. Environ., 71, 320-328.

Hu, J., Kawamura, H., Hong, H., Suetsugu, M., Lin, M., 2001. Hydrographic and satellite observations of summertime upwelling in the Taiwan Strait: A preliminary description. Terrestrial, Atmos. and Oceanic Sciences 12(2), 415-430.

Jan, S., Chern, C.-S., Wang, J., 1998. A numerical study of currents in the Taiwan Strait during winter. Terrestrial, Atmos. and Oceanic Sciences 9(4), 615-632.

Kuo, N.-J., Ho, C.-R., 1996. A satellite feature-tracking method to compute sea surface angular velocites. Acta Oceanogr. Taiwanica 35 (1), 55-64.

Lin, M., Hong, Q., Chen, S., Li, M., Lin, R., Pan, D., Zhang, H., 1992. Preliminary analysis of surface temperature field and surface current system in Taiwan Strait in early winter of 1989 by satellite remote sensing. J. Oceanogr.  in Taiwan Strait 11(3), 262-267 (in Chinese).