Seasonal-to-interannual variability of chlorophyll in central western ...

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Chinese Journal of Oceanology and Limnology Vol. 29 No. 1, P. 18-25, 2011 DOI: 10.1007/s00343-011-9931-y

Seasonal-to-interannual variability of chlorophyll in central western South China Sea extracted from SeaWiFS* QIU Fuwen (丘福文) 1, 3, FANG Wendong (方文东) 1, **, FANG Guohong (方国洪) 1, 2 1

Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology, Chinese Academy of

Sciences, Guangzhou 510031, China 2

First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China

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Graduate University of Chinese Academy of Sciences, Beijing 100039, China

Received Sept. 16, 2009; revision accepted Feb. 11, 2010 © Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2011

Abstract Using 10-year (January 1998–October 2007) dataset of Sea-viewing Wide Field-of-view Sensor (SeaWiFS), we extracted the dominant spatial patterns and temporal variations of the chlorophyll distribution in the central western South China Sea (SCS) through Empirical Orthogonal Function (EOF) analysis. The results show that the first EOF mode is characterized by a high chlorophyll concentration zone along the Vietnam coast. We found two peaks in summer (July–August) and in winter (December), respectively, in normal years. The second EOF mode is characterized by a jet-shaped tongue of high chlorophyll concentration extending seaward to the northeast in summer (July–August). To investigate the interannual variability of the chlorophyll concentration, we extracted the first non-seasonal (annual cycle removed) EOF mode, which shows high spatial variability off the southeast Vietnam coast. Both spatial pattern and time coefficients correspond well with those of sea surface temperature mode, and are closely correlated to ENSO events, with a lag of 7 months. Keyword: chlorophyll; variability; ENSO; South China Sea (SCS)

1 INTRODUCTION The South China Sea (SCS) (Fig.1) is a large semi-enclosed tropical marginal sea located between the western Pacific and the eastern Indian Ocean, with maximum depth over 5 000 m. The SCS climate is mainly governed by the East Asian monsoon system, with southwesterly winds in summer and northeasterly winds in winter. The basin scale upper ocean circulation of the SCS is mainly driven by the seasonal monsoon winds, and features two cyclonic gyres in the deep basin with an intensified southward jet along the west shore of the SCS in winter and an anti-cyclonic gyre in the southern SCS in summer with an offshore current jet on its northern fringe at a latitude of about 11°N (Wyrtki, 1961; Xu et al., 1982; Fang et al., 1998). The central western SCS is one of the most dynamically active areas in the SCS (Ho et al., 2000a). The variations of sea surface temperature (SST), sea surface height and the chlorophyll signature in this area have been revealed by previous studies, from observations and numerical models.

Studies of the seasonal and interannual variability of the SCS circulation, based on the sea surface height data either from satellite altimetry or from assimilation (Show et al., 1999; Ho et al., 2000a; Ho et al., 2000b; Wu et al., 2005; Fang et al., 2006), have shown that the circulation in the central western SCS is characterized by interannual variability that is related to ENSO events. Strong ENSO impacts on the distributions of SST in the western SCS have been revealed (Xie et al., 2003; Kuo et al., 2004; C. Wang et al., 2006). The dominant seasonal feature in the western SCS presented by previous studies is the intensive upwelling along the coast and a dipole structure with a strong offshore current jet off the western shore of the SCS in normal summer. An eastward offshore current jet with a cold filament at the inter-gyre boundary has been revealed from * Supported by the National Basic Research Program / the International Cooperative Program (Nos. 2006CB403603, 2006CB40302/05, 2006DFB21630), the National Natural Science Foundation of China (Nos. 40876008, 40520140074), and the Knowledge Innovation Program of the Chinese Academy of Sciences (No.KZCX2-YW-214) ** Corresponding author: [email protected]

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more extensive information of the spatial structure and the associated time evolution of the chlorophyll concentrations in the study area.

2 DATA

Fig.1 Geography and the bathymetry of the South China Sea. The study area is indicated by a box

observations (Kuo et al., 2000; Fang et al., 2002; Xie et al., 2003). Significant intra-seasonal variability in the cold filament and phytoplankton blooms in summer in the central western SCS has also been found (Isoguchi et al., 2006; Xie et al., 2007). The seasonal and interannual variations of the summer upwelling and the associated offshore cold water mass have been investigated based on satellite observations (Kuo et al., 2000; Xie et al., 2003; Kuo et al., 2004). These results confirm that the summer southwesterly monsoon winds play a major role in the development of the eastward jet and upwelling off the Vietnam coast. Rich nutrients induced by the upwelling in this area can support high biological productivity represented by high chlorophyll concentration in summer as reported by Tang et al. (2004). Zhao et al. (2007) found that the 1998 El Nino had a great impact on the chlorophyll distribution in this area. In the present study, we make a systematic analysis on the long-term variability of the chlorophyll concentration in the central western SCS. A new set of 10-year high resolution satellite remote sensing data is analyzed through the Empirical Orthogonal Function (EOF) decomposition to get

The study area is from 8°N to 16°N and from 108°E to 116°E (Fig.1). The monthly Level-3 Sea-viewing Wide Field-of-view Sensor (SeaWiFS) dataset from January 1998 to October 2007 is used in this study. The images are processed with the algorithm of OC4 version 4 (OC4v4) (O’Relly et al., 2000) with the SeaWiFS Data Analysis System (SeaDAS). The dataset is produced by the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) and distributed by the Distributed Active Archive Center (DAAC) with a spatial resolution of 9 km. The missing data caused by clouds are filled through weighted averaging of the values at four nearest cells. Sea surface temperature data is from the Tropical Rain Measuring Mission (TRMM) satellite’s microwave imager (TMI). The data is nearly free of cloud influence over the global tropics within 38°N/S (Wentz et al., 2000). The monthly product from January 1998 to October 2007 with a resolution of 0.25° in space is used. The Southern Oscillation Index (SOI) presented below is computed using monthly mean sea level pressure anomalies at Tahiti (T) and Darwin (D); this is an optimal index that combines the Southern Oscillation into one series. The SOI’s [T-D] used in this paper are calculated by the Climate Prediction Center and have been derived using normalization factors derived from monthly values (http://www.cpc.ncep.noaa.gov/data/indices/).

3 RESULTS OF EOF ANALYSIS The first two EOF modes of the chlorophyll concentration explain 82% of the total variance. The first EOF mode, which accounts for 72% of the total variance, is shown in Fig.2. The spatial pattern of this mode (Fig.2a) is mainly characterized by a strong seaward gradient; high over the shelf and low offshore. Maximum concentration values are located at 11°N near the Mekong River mouth and at 15°16°N. The temporal coefficients of the first EOF mode (Fig.2b) are always positive throughout the observational period. The time series shows a trough in May and two peaks in summer (July-August) and winter (December) each year except the summer of 1998. It should be noted that the highest summer peaks appear in 1999, 2006 and 2007, and the highest winter peaks appear in 1999 and 2005, indicating that

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Fig.2 First EOF mode of the surface chlorophyll concentration in mg/m3 a. Spatial pattern; b. Time coefficients

Fig.3 Second EOF mode of the surface chlorophyll concentration in mg/m3 a. Spatial pattern; b. Time coefficients

the chlorophyll concentration is especially high along the western shore of the SCS during these time periods. The second EOF mode of the chlorophyll concentration, which explains 10% of the total

variance, is shown in Fig.3. The dominant spatial pattern of this mode (Fig.3a) is characterized by an offshore jet-shaped area of high positive chlorophyll concentration centered at the Vietnam coast near 11°N. This high concentration area stretches

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northeastward from the SCS western coast to about 114°E. In addition, the spatial pattern also shows a high negative chlorophyll concentration belt along the Vietnam coast north of 12°N. The coefficients of this mode (Fig.3b) generally have a peak in summer each year. The highest peaks appear in August of 1999 and 2007, indicating that the jet-shaped high chlorophyll concentration is the most remarkable feature during those periods. An exception is that the second EOF coefficient is nearly zero in summer 1998, implying that the jet-shaped high chlorophyll concentration almost disappears during that period. This feature is consistent with the study of Zhao and Tang (2007). On the other hand, the lowest troughs (negative values) are present in winter 1998 and 1999, suggesting that the high chlorophyll concentration appears along the Vietnam coast north of 12°N during those periods. From Figs.2b and 3b we can see that interannual signals exist in the chlorophyll concentration fields in the central western SCS. In order to distinguish the interannual variability from the seasonal variability, the climatologically monthly means from January to December during 1998/01–2007/10 are calculated and removed from the monthly values to obtain the monthly anomalies (hereafter referred as nonseasonal values) from the climatology. We then perform EOF analysis on the non-seasonal fields. Before performing this analysis, the monthly values are logarithmically transformed to respect the lognormal distribution of the chlorophyll (Campbell, 1995). The first EOF mode of the chlorophyll concentration anomalies explains 23% of the total variance. Its spatial pattern and time coefficients are shown in Fig.4. Interannual variability is highlighted by removing the seasonal cycles and the results of this non-seasonal mode demonstrate obvious year-to-year variations in the chlorophyll concentration in the central western SCS. The spatial pattern (Fig.4a) shows great variability off the southeast Vietnam coast with the greatest centered at 110°E, 11°N. The time coefficients (Fig.4b) have peaks appearing in the summers of 1999, 2000, 2001, 2002, 2004 and 2006, and have a deep trough in summer 1998 and shallow troughs in 2003 and 2005. The lagged regression analysis shows that the time coefficients of the first EOF of the chlorophyll concentration are best correlated with the southern oscillation index (SOI) at a lag of 7 months, with a correlation coefficient equal to 0.64. The SOI is also plotted in Fig.4b for comparison with the time

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coefficients of the first EOF of the chlorophyll concentration. It can be seen that the chlorophyll EOF peaks in the summers of 1999, 2000, and 2001 correspond well with the strong La Nina events occurring in the winters of 19981999, 19992000 and 20002001. Following the weak La Nina events in winter 20012002 and early 2006, the first chlorophyll EOF exhibit positive peaks in the summers of 2002 and 2006. In these summers, higher than normal chlorophyll concentrations appeared in the greatest area of the central western SCS, and slightly lower than normal chlorophyll concentration appeared off the Vietnam coast near 14N. In contrast, the deep trough of the first chlorophyll EOF in summer 1998 corresponds well to the strong El Nino in winter 1997-1998; following the weak El Nino events in winters 20022003, 20042005 and 20062007 (McPhaden, 2008), the first chlorophyll EOF has shallow troughs in 2003, 2005 and 2007.

4 DISCUSSION Together, the leading two EOF modes of the chlorophyll concentration account for 82% of the total variance, representing the major feature of the seasonal variability of the distribution of the phytoplankton in the western SCS. From the spatial pattern of the first EOF mode, we can see that high chlorophyll variation areas are located in the near-shore area within the 200 m isobath along the western shore of the SCS, and the maximum chlorophyll concentration appears in the shallow waters of less than 50 m depth. The most noticeable feature is that the time coefficients of this mode have two positive peaks, one in summer each year (except 1998) and one in winter each year. The high chlorophyll concentration in summer may be related to wind-induced upwelling. Previous studies have indicated that intensive costal upwelling (Fang et al., 1998; Kuo et al., 2000; Fang et al., 2002; Xie et al., 2003) and large regional increases in chlorophyll concentration (Tang et al., 2004) occur along the western shore of the central SCS during the southwesterly monsoon periods. Increased upwelling induced by southwest winds along the western coast of the SCS in summer can increase nutrient flux into the euphotic zone to support positive chlorophyll anomalies there, because nutrient supply is one of the most important factors controlling chlorophyll production. Although the upwelling plays a major role in the increase of chlorophyll concentration in summer, it cannot explain the wintertime high

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Fig.4 First non-seasonal EOF of the surface chlorophyll concentration in mg/m3 a. Spatial pattern; b. Time coefficients. The blue heavy solid curve indicates the time coefficients smoothed with a 5-point running mean filter. Red curve indicates the Southern Oscillation Index (SOI) scaled by SOI/CHL, with SOI and CHL representing standard deviations of SOI and the first EOF time coefficients of chlorophyll, respectively. SOI curve is shifted rightward by 7 months

Fig.5 First non-seasonal EOF of the sea surface temperature (SST) in °C a. Spatial pattern; b. time coefficients. The blue heavy solid curve indicates the time coefficients smoothed with a 5-point running mean filter. Red curve is the first EOF time coefficients of chlorophyll scaled by CHL/SST, with SST representing the standard deviations of the first EOF time coefficients of SST

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chlorophyll concentration in this region. Our explanation is as follows: The strong northeasterly monsoon winds in winter may strengthen the mixing of the seawater in the western SCS, especially in the shallow water along the coast. The vertical mixing brings nutrients from lower layers into the euphotic zone to promote the growth of chlorophyll. In addition, the sea surface temperature in winter is also suitable for the growth of the chlorophyll in the western SCS. This explanation can be confirmed by previous studies. Chu et al. (1997) revealed a large SST anomaly off the southeast Vietnam coast during winter months (December-February). Based on monsoon-forced numerical model, Liu et al. (2002) indicated that there is a weak chlorophyll peak in winter along the western coast of the SCS which is not driven by the local upwelling. The possible influence of colored dissolved organic matter (originating in the Mekong River outflow or other coastal regions) on the chlorophyll peak in winter along the western coast of the SCS also deserves future study. The second EOF mode of the chlorophyll concentration is mainly characterized by the jet-shaped high chlorophyll concentration off the southeast Vietnam coast. This offshore jet-shaped high-chlorophyll tongue occurs during July-August each year except 1998. Previous hydrographic observations reveal that there is an intensive wind-induced coastal upwelling associated with the southeast Vietnam offshore current (Fang et al., 1998, 2002). Kuo et al. (2000) observed the coastal upwelling along the western shore of the SCS using satellite SST images. Their results show that this coastal upwelling is induced by the southwesterly monsoon winds and modified by the currents in this region. Tang et al. (2004) suggested that the circulation and currents may play an important role in the distribution of the chlorophyll in the western SCS. Nutrient-rich water can be brought into the euphotic zone by the dynamical upwelling. This causes a chlorophyll concentration maximum there. The nutrient-rich water can also be transported by the offshore current into the deep basin, resulting in high chlorophyll concentration in the offshore area. The first non-seasonal EOF mode of the chlorophyll concentration is dominated by a strong offshore jet-shaped feature that reveals high chlorophyll variability off the southeast Vietnam coast. A particularly prominent negative anomaly of the chlorophyll concentration is present there in summer 1998. This feature agrees with the findings

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of previous studies (Xie et al., 2003; Zhao et al., 2007). The time coefficients of this mode, however, further exhibit positive anomalies during the summers (July-August) of 19992002, implying that the jet-shaped high chlorophyll concentration is the dominant feature during those periods. In order to investigate the relationship between the chlorophyll concentration and the SST, we derived the non-seasonal SST EOF modes in the study area from January 1998 to October 2007 by the same method as the chlorophyll concentration. Most of the non-seasonal SST variability is captured by the first non-seasonal EOF mode of the residual SST, which accounts for 66% of the total variance. The spatial pattern and the time coefficients are presented in Fig.5. This mode is characterized by an intensified sea surface cooling off the southeast Vietnam coast, which coincides very well with the first non-seasonal EOF mode of the chlorophyll concentration in terms of shape, timing and location. As the values of the spatial pattern are negative, the troughs in the time coefficients of this SST non-seasonal EOF mode correspond to warm anomalies in the area. Note that there are several extreme warm anomalies appearing in summers of 1998, 2003, and 2007 and winter 2002. On the contrary, cooling anomalies occur from summer to early fall in 19992002, and an especial cooling persists throughout the year of 2004. The time coefficients of the chlorophyll non-seasonal EOF mode are plotted in Fig.5 for comparison. It is obvious that these two time coefficient curves coincide very well with each other. The correlation coefficient of the non-seasonal chlorophyll EOF mode and non-seasonal SST EOF mode is 0.81. The warm anomalies correspond to low chlorophyll concentration while cool anomalies correspond to high chlorophyll concentration. This seems to indicate that both the interannual variations of the SST and the chlorophyll concentration in the central western SCS are closely related to the upwelling in this area. Previous studies indicated that the most notable interannual variability in the SCS is the weakened wind-driven circulation gyres and upwelling, resulted from weakened monsoon winds during ENSO events (Wu et al., 1999; Xie et al., 2003; Wang et al., 2006). The time coefficients of both SST and chlorophyll show large negative anomalies in summer 1998, indicating that interannual variations are particularly noticeable after the 1997–1998 ENSO event, as shown by Wu et al. (2005). The result of the present study further confirms the large anomaly in summer 1998 as

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described by previous studies: the southwesterly monsoon winds weakened, the mid-summer sea surface cooling never took place and the jet-shaped high chlorophyll concentration off the western shore of the SCS almost disappeared (Xie et al., 2003; Kuo et al., 2004; Zhao et al., 2007). In addition, one can also see troughs (negative peaks) in December 2002 and in June 2007, which reflects that the weak ENSO events can also affect the distribution of phytoplankton and SST in those years. From the images of SST and chlorophyll concentration (not shown), we can see that there are particularly warm SST and low chlorophyll concentration in December 2002 and June 2007. The time coefficients of the non-seasonal EOF modes of both the chlorophyll concentration and SST are correlated with the Southern Oscillation Index at a 7-month lag, with correlation coefficients of -0.64 and -0.51, respectively. This implies that the interannual variations of the chlorophyll concentration and SST in the central western SCS are both modulated by even weak ENSO events.

5 SUMMARY In this study, we use SeaWiFS chlorophyll concentration and the TMI SST satellite data to investigate the variability of the chlorophyll in the central western SCS in the period from January 1998 to October 2007 with emphasis on seasonal and interannual scales. Similar to the previous studies, which are limited to some special cases or climatological mean field analysis, this study provides a further understanding of the central western SCS chlorophyll variability in terms of long-term continuous behaviors presented in EOF modes. The major results are as follows: 1) The first EOF mode of the chlorophyll concentration, which accounts for 72% of the total variance, clearly shows that a high chlorophyll variability zone exists along the Vietnam coast. It is found for the first time that two peaks appear both in summer (July-August) and in winter (December) during normal years. The second EOF mode of the chlorophyll concentration, which explains 10% of the total variance, reveals a jet-shaped high chlorophyll tongue extending northeastward in normal summer off the western coast of the SCS. 2) The dominate pattern of the non-seasonal (annual cycle removed) EOF mode of chlorophyll exhibits a high variability area southeast of the Vietnam coast, and coincides with the non-seasonal SST in terms of shape, timing and location in the

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central western SCS. The correlation coefficient between these two non-seasonal EOF modes is 0.81. Both of them are related to the offshore upwelling induced by the southeast Vietnam offshore current. The upwelling brings cool and nutrient-rich water from the lower layers into the euphotic zone to promote the growth of the chlorophyll in this area. 3) The distribution of chlorophyll in the central western SCS is an obvious response to the ENSO events, especially in summer 1998. The time coefficients of the EOF modes of the chlorophyll concentration show remarkable anomalies several months after the ENSO events. During these periods, the upwelling and the jet-shaped high chlorophyll weaken and even disappear. The non-seasonal EOF modes of the SST and the chlorophyll concentration further confirm that in summer 1998, the sea surface cooling did not occur and the high-chlorophyll tongue off the southeast Vietnam coast almost disappeared.

6 ACKNOWLEDGMENT We thank two anonymous reviewers for their valuable comments. We also thank ZHANG Haigang for helpful discussions. References Campbell J. 1995. The lognormal distribution as a model for bio-optical variability in the sea. J. Geophys. Res., 100(C7): 13 237-13 254. Chu P C, Lu S, Chen Y. 1997. Temporal and spatial variability of the South China Sea surface temperature anomaly. J. Geophys. Res., 102: 20 6955-20 937. Fang G, Fang W, Fang Y, Wang K. 1998. A survey of studies on the South China Sea upper ocean circulation. Acta Oceanogr. Taiwan, 37(1): 1-16. Fang W, Fang G, Shi P, Huang Q, Xie Q. 2002. Seasonal structures of upper layer circulation in the South China Sea from in situ observations. J. Geophys. Res., 107(C11): 3 202. Fang W, Guo Z, Huang Y. 1998. Observational study of the circulation in the southern South China Sea. Chin. Sci. Bull., 43: 898-905. Fang W, Guo J, Shi P, Mao Q. 2006. Low frequency variability of South China Sea surface circulation from 11 years of satellite altimeter data. Geophys. Res. Lett., 33: L22 612. Ho C R, Kuo N J, Zheng Q, Soong Y S. 2000a. Dynamically active areas in the South China Sea detected from TOPEX/POSEIDON satellite altimeter data. Remote Sens. Environ., 71: 320-328. Ho C R, Zheng Q, Soong Y S, Kuo N J, Hu J J. 2000b.

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