Multisatellite observation on upwelling after the passage of Typhoon Hai-Tang in the southern East China Sea

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L03612, doi: /2007gl032858, 2008 Multisatellite observation on upwelling after the passage of Typhoon Hai-Tang in the southern East China Sea Yi Chang, 1 Hsiou-Ting Liao, 1 Ming-An Lee, 1,2 Jui-Wen Chan, 1,2 Wei-Juan Shieh, 1,2 Kuo-Tien Lee, 1 Guang-Hua Wang, 3 and Yang-Chi Lan 4 Received 1 December 2007; accepted 4 January 2008; published 9 February [1] The serial remote sensing based imageries clearly revealed large scale of upwelling within large regional enhancement of chlorophyll-a (Chl-a) concentration in the southern East China Sea (ECS) after the passage of super typhoon Hai-Tang in July After the typhoon on 22 July, the upwelling area (<26 C) expanded rapidly to 9146 km 2 on the shelf-break. The large increased upwelling persisted for more than a week. Ocean color images also reveled that high Chl-a concentration of >3.0 mg/m 3 appeared in the shelf region, where the high Chl-a pattern matched the upwelling in terms of location and time. On the other hand, a large offshore SST cooling was also observed mainly to the right of typhoon track on 20 July, it lasted in a period of 2 3 days. This paper provides clear and highresolution evidence that typhoon significant increased upwelling and Chl-a concentration in the southern ECS. Citation: Chang, Y., H.-T. Liao, M.-A. Lee, J.-W. Chan, W.-J. Shieh, K.-T. Lee, G.-H. Wang, and Y.-C. Lan (2008), Multisatellite observation on upwelling after the passage of Typhoon Hai-Tang in the southern East China Sea, Geophys. Res. Lett., 35, L03612, doi: /2007gl Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung, Taiwan. 2 Remote Sensing Laboratory of Taiwan Ocean Research Institute Keelung, Taiwan. 3 Central Weather Bureau, Taipei, Taiwan. 4 Marine Fishery Division, Fisheries Research Institute, Keelung, Taiwan. Copyright 2008 by the American Geophysical Union /08/2007GL Introduction [2] It is well documented that typhoon causes significant cooling of sea surface temperature (SST) along its path in the tropical and subtropical oceans. Typhoon also plays an important role in enhancing phytoplankton bloom and primary productivity in oligotrophic oceanic waters, especially in typhoon-dominated seas [e.g., Lin et al., 2003; Walker et al., 2005; Zheng and Tang, 2007]. The hydrographical and biochemical responses to tropical typhoon in the upper ocean are mainly due to vertical turbulent mixing and upwelling by strong wind filed [Stramma et al., 1986; Emanuel, 1999]. From numerical experiment of Sakaida et al. [1998], the remarkable sea surface cooling contrast between cold Oyashio and warm Kuroshio currents has been demonstrated in the North Pacific Ocean. Previous observations and modeling studies have shown that an asymmetrical SST response to hurricane/typhoon wind forcing is typical with the greatest cooling east of its track [Walker et al., 2005]. Although the enhancements of primary production and upwelling due to typhoon in the pelagic ocean are well known, however, influence of a passing tropical typhoon in the continental shelf region is not clear yet. [3] The year-round upwelling of Kurohsio subsurface water has been frequently observed on the shelf-break of the southern East China Sea (ECS) [e.g., Liu et al., 1992]. The interaction between the Kuroshio intrusion and monsoon forcing is very dynamic. Chen et al. [2003] indicated that this upwelling can be also enhanced under the effect of typhoon. However, the in situ hydrographic data could not provide a comprehensive vision of enhanced upwelling in the shelf region. The attempt of this study is to observe the change in upwelling and chlorophyll-a (Chl-a) concentration after the passage of Typhoon Hai-Tang in We focus on the description of high-resolution features of upper ocean response to typhoon at the shelf-break in the southern ECS from multisatellite data. 2. Data and Method [4] High-resolution satellite images were successfully captured by multisensors before and after the passage of Typhoon Hai-Tang. The data of AVHRR (Advanced Very High Resolution Radiometer) sensors are archived by the National Taiwan Ocean University. After archiving, 5-channel radiations are used to produce daily SST images through MCSST algorism [McClain et al., 1985] and validation [Lee et al., 2005]. The microwave SSTs measured by AMSR-E (Advanced Microwave Scanning Radiometer for the Earth Observing) are also used in this study. We collected Version-4 ocean products from com. The variation of Chl-a concentration was observed by both of MODIS (Moderate Resolution Imaging Spectroradiometer) and SeaWiFS (Sea-viewing Wide Field-of-view Sensor) onboard satellites. During the study period, only 25 images have been received. Because the activated coverage was low, weekly mean map are composed before and after the typhoon passage. [5] Along track Altimeter data of Jason-1 were used to plot SLA transects through the study area. These data were produced by the Colorado Center for the Global Near Realtime Data Research Group of Astrodynamics Research (CCAR) (available at Sea surface wind vector and stress were obtained from the daily QuikSCAT (Quick Scatterometer) data provided by the Remote Sensing Systems in Santa Rosa, California ( We also calculated the windinduced Ekman pumping velocity (EPV) from the wind vectors. Satellite data information is shown in Table 1. L of5

2 Table 1. Parameter and Information of Multisatellite Derived Data Variation Sensor Parameter Spatial Resolution Period Max. Min. AVHRR SST ( C) 1.1 km Jul AMSR-E SST ( C) 25 km 20, 21 Jul SeaWiFS Chl-a (mg/m 3 ) 1.1 km Jul >14.0 <0.1 MODIS Chl-a (mg/m 3 ) 1.1 km Jul >3.0 <0.3 Jason-1 SLA (cm) Along track 16, 26 Jul QuikSCAT Sea surface wind speed (m/s) 25 km 10 Jul 2 Aug [6] We then defined the study area in the northeastern shelf of Taiwan as shelf region and area off east Taiwan as the offshore region. In addition, we separate the study period as before (10 16 July) and after (22 July to 2 August) the typhoon passage due to the influence of typhoon during July. 3. Increased Upwelling and Chl-a Concentrations [7] Hai-Tang was a classical typhoon formed over the tropical Pacific Ocean on 12 July It traveled relatively slowly (4.8 m/s) on the western part of Pacific Ocean from 13 to 16 July, then moved rapidly (11.1 m/s) westwards and became a super category 5 typhoon on 15:00 UTC 16 July and kept moving westward. Upon entering the southern shelf of ECS on 21:00 UTC 17 July, Hai-Tang lingered at a near stationary slow speed (<4 m/s) on the eastern coast of Taiwan, while it reduced to a category 3 typhoon with maximum sea surface wind speed of m/s (Figure 1). [8] Figure 2 was satellite-derived SST serial images of AVHRR and AMSR-E. Before the passage of Hai-Tang on 16 July (Figure 2a), the southern ECS is characterized by warm SST predominantly above 28 C. However, two distinct cold SST (<26 C) patches were simultaneously observed on AMSR-E mean SST image (Figure 2b) 4 days after Hai-Tang s passage. One was located at the shelf region (black arrow), inshore of 200-m isobath, suggested to be an increased upwelling. The other was founded at the right side of the Hai-Tang s wake with a circle spot in the offshore region (red arrow); this cold patch is referred to as a SST cooling. In the first available AVHRR SST image after Hai-Tang on 22 July, the upwelling area (<26 C) expanded rapidly to 9146 km 2 in crescent shape (black arrow in Figure 2c). The increased upwelling weakened gradually and persisted for more than a week (Figures 2d and 2e), then disappeared on 1 August (Figure 2f) with the characteristic warm SST before Hai-Tang. On the other hand, the offshore cooling was in a large circle spot (red arrow in Figure 2b) and lasted a short period of time (2 3 days). The large scale of offshore SST cooling appeared mainly to the right of typhoon track. [9] The biological response of the southern ECS to the passing of Hai-Tang was depicted by changes in the surface distribution of ocean color images in Figure 3. Before typhoon, the MODIS and SeaWiFS composites from 10 to 16 July (Figures 3a and 3d) showed a typical summer condition of Chl-a concentrations, predominantly <0.1 mg/m 3 in the offshore, and >0.5 mg/m 3 in the shelf region, except coastal waters. After Hai-Tang s passage, the weekly composites of July (Figures 3b and 3e) revealed that Chl-a concentrations increased from 0.36 mg/m 3 (before Hai-Tang) to mg/m 3 in the shelf region (black arrows). The high Chl-a concentration pattern of >3.0 mg/m 3 was recorded roughly between the 100-m and 200-m isobaths. On the other hand,apatchofhighchl-a value (>1.0 mg/m 3 ) appeared along the typhoon track in the offshore region (red arrow in Figure 3b). After the disappearance of cooling and upwelling, the Chl-a concentration decreased to the condition of pretyphoon after 31 July (Figures 3c and 3f). 4. Oceanic Feature After the Passage of Typhoon Hai-Tang [10] Along the track T1 of Jason-1 in Figure 2a, one day before typhoon, the altimeter data showed a low SLA ( 20 cm) segment and matched with the low SST patch (<28 C) at the shelf region (arrow in Figure 4a). After the passage of Hai-Tang on 26 July (Figure 4b), SLA along the track T2 in Figure 2d was much lower ( 32 cm on average) in the shelf region. The SST fell also rapidly to 24.2 C, with a drop of 4.5 C. Crossing the offshore region, SST decreased noticeable to 26 C at 123 E where the SLA varied slightly. At typhoon center (123.8 E), another low SLA pick ( 42.8 cm) was observed with increasing SST. Figure 1. QuikSCAT averaged wind vector on 18 July The black contours indicate Hai-Tang s moving wake. Hai-Tang s 3-hour positions are depicted with black points. ECS and SCS are the East China Sea and South China Sea. 2of5

3 Figure 2. (a) AVHRR SST images on 16 July 2005, before typhoon, (b) AMSR-E mean SST image of July, after typhoon, (c) first available AVHRR SST images on 22 July after typhoon, (d) (f) weakness of cold patches. The black and red squares indicate the study area in the shelf and offshore region, respectively. The gray lines T1 and T2 are the scanning tracks of Jason-1 on 16 and 26 July, respectively. The white contours and red contour are the bottom depth and the typhoon moving track, respectively. [11] The changes in comparison with SST, sea surface wind speed, and EPV with time at the upwelling center (25.5 N, 122 E, circle in Figure 2a) are shown in Figure 4c. The pre-typhoon sea surface wind speed was <8 m/s (white bar), the EPV varied slightly, and the SST was higher than 28 C. While the typhoon was lingering on 18 July, wind speed increased to 32 m/s, and the EPV shot up to m/s (black bar). From AMSR-E SST images available after typhoon, the SST decreased drastically to 23.8 C (triangle) on 20 July. However, the minimum SST of 21 C was observed on 24 July while both wind speed and EPV decreased to the pre-typhoon condition. Then, the SST was increased to 28 C after 28 July. 5. Discussion [12] Multisatellite derived SST, Ch-a, SLA and sea surface wind measurements clearly revealed increased upwelling and biological response in shelf and offshore region. In all cloud-free SST images, increased upwelling in the shelf region extended to the northeast continuously for more than 10 days after the typhoon (Figure 2). It is evident that the upwelling was distinguished with the rabid decrease of SST Figure 3. Weekly composites of MODIS and SeaWiFS with unitary color scale on (a, d) July 2005 before typhoon, (b, e) July, 1 week after typhoon; and (c, f) 30 July to 2 August. Arrows indicate the increased Chl-a, which is higher in the MODIS images. 3of5

4 Figure 4. Jason-1 sea-level anomaly (SLA) and AVHRR SST data along the same transect (a) one day before and (b) 10 days after typhoon. (c) Changes in surface wind speed (white bar), Ekman pumping velocity (black bar), and satellite SSTs before and after Hai-Tang passage (17 21 July). and SLA after the typhoon (Figures 2b 2d and 4b). The shelf region in our study area is well-know to have a yearround upwelling due to the intrusion of Kuroshio subsurface water [Hsueh et al., 1992; Liu et al., 1992]. However, the upwelling pattern can be observed occasionally during the summer monsoon, because it is usually covered by warm water of low salinity and oligo-nutrients [Tang et al., 2000]. [13] As well as the previous studies, both SST and SLA in the shelf upwelling region were slightly lower than the surrounding waters 1 day before the passage of Hai-Tang, (Figure 4a). After the passage, however, SST and SLA decreased rapidly from 27.3 C and 20.2 cm to 25.1 C and 37.8 cm respectively (Figure 4b). During the lingering of typhoon on 18 July, the strong east/northeast sea surface winds (32 m/s) dominated at the center of upwelling and induced large EPV of m/s (Figure 4c). Hydrographic results of Chen et al. [2003] also indicated that an approaching cyclonic eddy pushed the Kuroshio towards the shelf-break. Thus we suggest that the strong surface wind and wind forced EPV reduced the cover of warm water and facilitated the inshore transport of Kuroshio subsurface water and are probably one of the factors to induce such large scale upwelling after typhoon. [14] The weekly maps of Chl-a concentration revealed that Chl-a increased significantly from 0.5 mg/m 3 to more than 3.0 mg/m 3 before and after the passage of typhoon. The high Chl-a patches matched with the upwelling in terms of location and time (Figures 3b and 3e). Chen et al. [2003] assumed that the colder and nutrient-rich water found in coastal region of Taiwan was probably originated from the Kuroshio subsurface water after Typhoon Herb in June Our results thus clearly showed significant enhancements of upwelling and Chl-a in the shelf region after typhoon passage. It is therefore, strongly suggested that the increased Chl-a anomaly is most likely a phytoplankton bloom due to the new nutrients supplied by the uplifted deep water. Apparently, the upwelling phenomenon was more pronounced and played an important role of airsea interaction after the typhoon, probably temporary. It is also noticeable that the Chl-a value of MODIS are higher than that of SeaWiFS, the validation between different sensors should be taken in future study. [15] Time-sequence SST images reveled large SST cooling 4 days after the passage of typhoon in the offshore region (red arrow in Figure 2b). The large SST cooling and high Chl-a in the offshore region appeared mainly to the right of typhoon track. It is noticeable that the large scale offshore SST cooling lasted for a short period of 2 3 days; and concentrated at a certain spot instead of along the typhoon track. Recent studies of the typhoon-forced upwelling on the open ocean in South China Sea have demonstrated that the more intense winds to the right of typhoon track will induce the vertical mixing and result in larger SST cooling [Lin et al., 2003; Walker et al., 2005; Zheng and Tang, 2007]. It may explain the phenomenon observed in this study. However, further analysis is needed to examine why the SST cooling appeared in the certain area and lasted for a short period. 6. Conclusion [16] Based on the synergy of multisatellite data sets, our results provide a rare opportunity to observe the air-sea interaction and biological response to strong typhoon in the southern ECS. Evidence of high-resolution satellite data is provided to show that strong winds of Typhoon Hai-Tang increased upwelling and Chl-a in the shelf region lasted for more than 10 days. The large upwelling water was likely due to the EPV, which reduced the covered warm water; and the wind-driven Kuroshio moved more shoreward increased its intrusion on the shelf-break. On the other hand, the large offshore SST cooling immediately after the passage of typhoon was due to the intense winds right to the typhoon track. However, how did typhoon trigger such a drastic 4of5

5 response in the southern ECS? As a final point, modeling simulations of the mechanism are required to investigate the interaction among upwelling, wind field and traveling/ lingering speed of typhoons, topographies and oceanic currents. [17] Acknowledgments. This study was supported by a research grant ( E8) awarded by the Center for Marine Bioscience and Biotechnology, NTOU, and another grant ( M ) provided by the National Science Council, Taiwan. The authors would like to thank F. Sakaida and H. Kawamura of the Center for Atmospheric and Oceanic Studies, Tohoku University, Japan, for helpful discussions. The authors would also express their appreciation to Chang-Tai Shih of NTOU for his helpful advice on this manuscript. References Chen, C. T., C. Liu, W. Chuang, Y. Yang, F. K. Shiah, T. Tang, and S. Chung (2003), Enhanced buoyancy and hence upwelling of subsurface Kuroshio waters after a typhoon in the southern East China Sea, J. Mar. Syst., 42, Emanuel, K. A. (1999), Thermodynamic control of hurricane intensity, Nature, 401, Hsueh, Y., J. Wang, and C. S. Chern (1992), The intrusion of the Kuroshio across the continental shelf northeast Taiwan, J. Geophys. Res., 97, 14,323 14,330. Lee, M. A., Y. Chang, F. Sakaida, H. Kawamura, C. H. Cheng, J. W. Chan, and I. Huang (2005), Validation of satellite-derived sea surface temperatures for waters around Taiwan, TAO: Terr. Atmos. Oceanic Sci., 16, Lin, I. I., W. T. Liu, C. C. Wu, G. T. Wong, C. Hu, Z. Chen, W. D. Liang, Y. Yang, and K. K. Liu (2003), New evidence for enhanced ocean primary production triggered by tropical cyclone, Geophys. Res. Lett., 30(13), 1718, doi: /2003gl Liu, K. K., G. C. Gong, S. Lin, C. Y. Wang, C. L. Wei, and S. Y. Chao (1992), The year-round upwelling at the shelf break near the northern tip of Taiwan as evidenced by chemical hydrography, TAO: Terr. Atmos. Oceanic Sci., 3, McClain, E. P., W. G. Pichel, and C. C. Walton (1985), Comparative performance of AVHRR-based multichannel sea surface temperatures, J. Geophys. Res., 90, 11,587 11,601. Sakaida, F., H. Kawamura, and Y. Toba (1998), Sea surface cooling caused by typhoons in the Tohoku Area in August 1989, J. Geophys. Res., 103, Stramma, L., P. Cornillon, and J. F. Price (1986), Satellite observations of sea surface cooling by hurricanes, J. Geophys. Res., 91, Tang, T. Y., J. H. Tai, and Y. J. Yang (2000), The flow pattern north of Taiwan and the migration of the Kuroshio, Cont. Shelf Res., 20, Walker, N. D., R. R. Leben, and S. Balasubramanian (2005), Hurricaneforced upwelling and chlorophyll a enhancement within cold-core cyclones in Gulf of Mexico, Geophys. Res. Lett., 32, L18610, doi: / 2005GL Zheng, G. M., and D. L. Tang (2007), Multi-sensor remote sensing of two episodic phytoplankton blooms triggered by one typhoon in the South China Sea, Mar. Ecol. Prog. Ser., 333, J.-W. Chan, Y. Chang, K.-T. Lee, M.-A. Lee, H.-T. Liao, and W.-J. Shieh, Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University., 2 Pei-Ning Rd., Keelung 20224, Taiwan. (malee@mail.ntou.edu.tw) G.-H. Wang, Central Weather Bureau, Gongyuan Road, Taipei 10048, Taiwan. Y.-C. Lan, Main Fishery Division, Fisheries Research Institute, Keelung 20246, Taiwan. 5of5