Upwelling Variability along the Southern Coast of Bali and in Nusa Tenggara Waters

Ocean Sci. J. (2013) 48(1):49-57 http://dx.doi.org/10.1007/s12601-013-0004-3 Available online at http://link.springer.com Article Upwelling Variability along the Southern Coast of Bali and in Nusa Tenggara Waters Nining Sari Ningsih 1 *, Noviani Rakhmaputeri 2, and Agung B. Harto 3 1 Research Group of Oceanography, Faculty of Earth Sciences and Technology, Bandung Institute of Technology (ITB), Indonesia 2 United Surveyors Pte Ltd., 1092 Lower Delta Road, 169203, Singapore 3 Research Group of Remote Sensing and Geographical Information Sciences, Faculty of Earth Sciences and Technology, Bandung Institute of Technology (ITB), Indonesia Received 2 March 2012; Revised 12 June 2012; Accepted 5 February 2013 KSO, KIOST and Springer 2013 Abstract Spatial and temporal variation of upwelling along the southern coast of Bali and in the Nusa Tenggara waters - Indonesia was studied by using satellite image data of sea surface temperatures and chlorophyll-a from September 1997 to December 2008. This study clearly reveals annual upwelling in the regions from June to October, associated with the southeast monsoon cycle, with the sea surface temperature (chlorophyll-a concentration) being colder (higher) than that during the northwest monsoon. In addition, this study also shows that the upwelling strength is controlled remotely by ENSO and IOD climate phenomena. During El Niño/positive IOD (La Niña/negative IOD) periods, the Bali - Nusa Tenggara upwelling strength increases (decreases). Key words upwelling, monsoon, ENSO, IOD, Bali-Nusa Tenggara 1. Introduction Based on the long history of fish stock abundance in tropical and temperate marine waters such as in the Indonesian waters, it has been recognized that there exists an important relationship between oceanographic factors (e.g. temperature, salinity, and currents) and fish stock abundance (Sitepu and Dahuri 1995). Previous scientists have documented that the oceanographic factors in tropical regions, especially in Indonesia, are influenced by the monsoon system (Wyrtki 1962; Miyama et al. 1995; Susanto et al. 2001; Moore et al. 2003), El Niño-Southern Oscillation (ENSO) (Meyers 1996; Susanto and Marra 2005), and Indian Ocean Dipole (IOD) climate phenomena (Saji et al. *Corresponding author. E-mail: nining@fitb.itb.ac.id 1999; Webster et al. 1999). The monsoon, ENSO, IOD, and the complex interplay among them, play important roles in the circulation of Indonesian seas. The physical consequences of the monsoon system on oceanographic factors are the occurrence of the upwelling phenomenon which in turn supports the biological productivity of marine waters where the upwelling takes place (Sitepu and Dahuri 1995). The upwelling phenomenon takes place to compensate for the divergence of the horizontal flow and produces upward flow, carrying cold and nutrient-rich deep waters to the surface. This, in turn, will also increase biological activities such as the abundance of fish stock. Therefore, understanding the spatial and temporal evolution of upwelling is important, especially for coastal fisheries, as it can be used as a predictive tool, which is quite valuable for designing proper fishery management plans. In the Indonesian waters, there are several upwelling regions, indicated by lower temperature and higher salinity and nutrients than those of surrounding regions, and their existence has been known about, such as in the eastern tropical Indian Ocean along the southern coasts of Java and Sumatra, Banda Sea, and southern of Papua (Wyrtki 1962; Qu et al. 2005). Several studies of this upwelling mechanism and its relation to the monsoon, especially along the southern coast of Java and its surrounding area, have been carried out by previous investigators based on field measurements of physical and chemical sea water properties (Wyrtki 1962; Purba 1995), satellite image analyses of sea surface temperatures (SST) and Chlorophyll-a (Susanto et al. 2001;

50 Ningsih, N. S. et al. Siswanto and Suratno 2005), and numerical models (Murtugudde et al. 2000; Ningsih et al. 2002). Along the southern coasts of Java and Sumatra, it has been known that during the southeast (SE) monsoon (June to August), southeasterly wind generates upwelling, bringing cooler waters and nutrients to the surface, whereas conditions are reversed during the northwest (NW) monsoon (December to February). The reversal of winds associated with the onset of the NW monsoon reduces the upwelling intensity (November). Upwelling eventually disappears during the NW monsoon (December - February) and the transitional period of the monsoon (March). Furthermore, Susanto et al. (2001) reported that upwelling along the coasts of Sumatera and Java is forced not only locally by the alongshore wind associated with the SE monsoon but also remotely by atmosphere-ocean circulation associated with ENSO. During El Niño (La Niña) periods, the Java-Sumatera upwelling strength increases (reduces) due to shallowing (deepening) thermocline depths. In addition, Susanto and Marra (2005) have investigated both spatial and temporal variability of chlorophyll-a along the south coasts of Java and Sumatra from September 1997 to December 2003 using SeaWiFS (the Sea-viewing Wide-field-of-view Sensor) data. They reported that the strong 1997/1998 El Niño event, coinciding with positive IOD, produced a significant departure of chlorophyll-a concentrations along the southern coasts of Java and Sumatra. The basic feature and existence of upwelling along the southern coasts of Sumatera and Java have been extensively studied by the previous investigators mentioned above. However, this important subject that is primarily related to upwelling variation and its response to the monsoon, ENSO, and IOD, has so far not been extensively studied along the southern coast of Bali and in both West and East Nusa Tenggara waters. In these regions there is the Sawu Sea, which is a small sea, and it is bounded by Sawu Island to the south (marked by Su in Fig. 1(d)), the Islands of Rote and Timor to the east (denoted by R and T in Fig. 1(d), respectively), Flores and Alor Archipelago to the north/ northwest (marked by F and A in Fig. 1(d), respectively), and the Island of Sumba to the south and west (denoted by S in Fig. 1(d)). The Sawu Sea is known among environmentalists as a part of a coral triangle, which is also recognized as one of the richest marine biodiversity areas in the world, containing whales, dugong, sea turtles, dolphins, and coral reef. Kahn and Subijanto (2009) reported that upwelling Fig. 1. Study Area. The names of islands (denoted by alphabet letter) are Bali (B), Lombok (L), Sumbawa (Sw), Flores (F), Alor (A), Sumba (S), Sawu (Su), Rote (R), and Timor (T). Meanwhile, C-C is a track along the Bali - Alor coasts affects marine biodiversity in the region. In addition, this Sawu Sea is also a critical area for yellow-fin tuna, which is one of the most important fish resources to the fisheries industry (Maarif 2009). In 2009, due to its potential marine resources, Sawu Sea was established as Marine National Park by the Ministry of Marine Affairs and Fisheries of the Republic of Indonesia. To the best of our knowledge, studies on upwelling variation in both space and time along the southern coast of Bali and Nusa Tenggara Waters have been limited. Sprintall et al. (2003) monitored temperature and salinity variability in the five major exit passages of Lesser Sunda Islands (Ombai Strait, Timor Passage, Sawu/Dao Strait, Sumba Strait, and Lombok Strait, as shown in Fig 1) based on 3.5 year time series (December 1995 until May 1999) of measured temperature and salinity data, which is insufficient for a comprehensive analysis. The temperature and salinity data show strong variability over all time scales related to the local regional and remote forcing mechanisms of heat, freshwater, and wind. Meanwhile, Moore and Marra (2002)

Upwelling Variability along the Southern Coast of Bali and in Nusa Tenggara Waters 51 concluded that there are persistent flow-induced upwelling and frontal features in the Strait of Ombai (Fig. 1), in which thermocline depth influenced by monsoons and ENSO is important to the development of upwelling features in the strait. In addition, 3-year velocity measurements (2004 2006) carried out by Sprintall et al. (2010) in the Ombai Strait confirm the eastward flowing surface South Java Current (SJC) and its deeper Undercurrent (SJUC) cross the Sawu Sea to reach the strait. The SJC and SJUC play an important role in distributing freshwater into and out of the southeast Indian Ocean. Based on surface temperature maps, they found the presence of a front during the SE monsoon that seems to trap the SJC in the northern boundary of the Ombai Strait. They suggested that the SJC and the frontal location are related to a complex interplay among local wind-driven Ekman dynamics, the strong Indonesian Throughflow (ITF), and topography. The motivation of this study is to further investigate the basic features and inter-annual variability of upwelling along the southern coast of Bali and in the Nusa Tenggara waters. It is important to obtain a good understanding of this upwelling phenomenon both for scientific reasons and marine resource conservation. The main aim of this paper is therefore to investigate the spatial and temporal variability of upwelling along the southern coast of Bali and in the Nusa Tenggara waters. The structure of the paper is as follows. Section 2 describes the materials and methods used in this study. Next, the existence of seasonal and inter-annual variability related to the upwelling in the study domain is discussed in Section 3. Lastly, we end with conclusions in Section 4. 2. Materials and Methods In this study, the variability of upwelling was investigated by using SST and chlorophyll-a data along the southern coast of Bali and in the Nusa Tenggara waters (8 S to 13 S and 114 E to 128 E, as shown in Fig. 1) from September 1997 to December 2008. The SST data from September 1997 to December 2002 were derived from NOAA - AVHRR satellite images, whereas the SST data from January 2003 to December 2008 were derived from the Terra Aqua/ MODIS satellite images. Meanwhile, chlorophyll-a data were derived from the SeaWiFS from September 1997 to December 2008. In addition, wind data obtained from NCEP (National Centers for Environmental Prediction) were also used to support the analysis in this study. The Oceanic Niño and Dipole Mode Indices (ONI and DMI, respectively) from 1997 to 2008 were used to identify climate conditions. The ONI is defined as the 3-month running mean of SST anomalies in the Niño 3.4 region (5 N-5 S and 120 W-170 W) (Lubbock, TX Weather Forecast Office, 2011). The DMI is defined as the difference in SST anomaly between the tropical western Indian Ocean (50 E-70 E, 10 S-10 N) and the tropical southeastern Indian Ocean (90 E-110 E, 10 S-0 S) (Saji et al. 1999). Monthly anomaly values of SST and chlorophyll-a data were used to analyze the spatial and temporal variability of upwelling. Following Susanto and Marra (2005) and based on the length of the data (1998-2008), in this study, the monthly anomalies are calculated as departures from the 1998-2008 mean for the considered month. 3. Results Seasonal variability Figure 2 shows average SST and chlorophyll-a concentration from January 1998 to December 2008 both during NW and SE monsoons. Upwelling regions along the south coast of Bali and in the Nusa Tenggara waters including the Sawu Sea, indicated by cold temperature and high chlorophyll-a concentration, correspond very well to the SE monsoon, as displayed in Fig. 2. During the SE monsoon, the SST (chlorophyll-a concentration) is colder (higher) than that during the NW monsoon (Fig. 2). The values of SST during the SE and NW monsoons range from 24.5 to 28.0 C and from 28.7 to 30.2 C, respectively. Meanwhile, ranges of the chlorophyll-a concentration are about 0.42-5.31 mg/m 3 for the SE monsoon and about 0.11-2.21 mg/m 3 for the NW monsoon. During the SE monsoon, in the southeastern part of Bali and in the Nusa Tenggara waters, high chlorophyll-a concentration spreads out over more than 200 km from the coasts (Fig. 2). In this period, along-shore southeasterly winds from Australia drive offshore Ekman transport that result in upwelling, bringing higher nutrient levels and cooler waters to the sea surface. Hence, the SST (chlorophylla concentration) is colder (higher) in comparison to that during the NW monsoon. The present results agree with what previous investigators found in the annual cycle of temperature in the studied area by using both measurement data (Sprintall et al. 2003) and satellite images (Moore and

52 Ningsih, N. S. et al. Fig. 2. Average SST from January 1998 to December 2008: (a) during the NW monsoon (December - February, or DJF) and (b) during the SE monsoon (June - August, or JJA). Average chlorophyll-a concentration from January 1998 to December 2008: (c) during the NW monsoon and (d) during the SE monsoon Marra 2002; Sprintall et al. 2010): warmest during the NW monsoon and coolest during the SE monsoon. In this study, we found seasonal variability of upwelling events and corresponding frontal features along the coasts (Fig. 2) indicated with a cross-front gradient in sea surface temperature (SST) of the order of 2 C and a gradient in chlorophyll-a concentration of over 2 mg/m 3. The frontal features coincide with large phytoplankton blooms in the studied area. This is consistent with Moore and Marra (2002) who show that the variability in the phytoplankton blooms events in the Strait of Ombai (Fig. 1) can be tied to seasonal monsoons and ENSO. Moore and Marra (2002) further suggest that the presence and strength of surface expressions of cold, nutrient rich waters are directly tied to periods of shallow thermocline depth in which the upwelling or nutrient rich waters variability is affected by monsoons and ENSO. Based on the 3-year velocity time series (2004-2006) from two moorings in the Ombai Strait (Fig. 1), Sprintall et al. (2010) found that the seasonal pattern in the front associated with location of the SST gradient is largely consistent with the velocity data, which confirm the existence of the SJC and SJUC in the strait. During the NW monsoon, strong eastward flow of the surface SJC is evident at both moorings and there is little cross passage SST gradients. Sprintall et al. (2010) further suggest that the eastward flow in the surface SJC could be driven by both Ekman flow in response to the westerly winds during the NW monsoon and semi-annual Kelvin waves. In contrast, during the SE monsoon the SJC has a subsurface maximum eastward flow at 50-100 m depth in the northern part of the Ombai Strait. According to Sprintall et al. (2010), upwelling dynamics along the southern Nusa Tenggara coastal region may contribute to the subsurface (50-100 m) maximum in eastward flow observed in the northern part of the Ombai Strait. Upwelling dynamics along the Nusa Tenggara coast during the SE monsoon can cause water to rise from mid-depths supported by an alongshore pressure gradient, which when balanced by friction

Upwelling Variability along the Southern Coast of Bali and in Nusa Tenggara Waters 53 along the shelf slope, may result in a coastal current down the direction of the pressure gradient and opposing the direction of the wind (Sprintall et al. 2010). During the SE monsoon, such a coastal current of the SJC, associated with the presence of a front of strong SST gradient in the studied area, would be directed eastward toward the Ombai Strait and become trapped along the northern boundary of the strait. The seasonal movement of the front in the Ombai Strait is largely consistent with the surface flow of SJC, suggesting that the front probably indicates the southward boundary of the SJC in the strait during the SE monsoon. Meanwhile, although it has been believed that the presence of SJUC in the Ombai Strait could have a significant influence on the distribution and exchange of freshwater within the studied area (Sprintall et al. 2010), it is still unclear what is the role of the deeper SJUC forced by intraseasonal and semi-annual winds in the equatorial Indian Ocean on upwelling dynamics in the studied area. This issue remains for the future study. Interannual variability In order to understand the influences of the monsoons and interannual forcing associated with ENSO and IOD on both spatial and temporal variations of upwelling in the study domain, we carried out time-longitude profiles of SST and chlorophyll-a concentration anomalies along the coasts of Bali - Alor (Transect of C - C in Fig. 1), as shown in Fig. 3. The time-longitude profiles of SST anomaly (Fig. 3(a)) and chlorophyll-a anomaly (Fig. 3(b)) clearly reveal annual upwelling from June to October, associated with the SE monsoon cycle, with colder SST and higher chlorophyll-a concentration. From Fig. 3, it is apparent that upwelling strength is remotely controlled by atmosphere-ocean circulation associated with both ENSO and IOD. During the SE monsoon (June to October) in normal ENSO and IOD years (2000 and 2001), upwelling appeared only with SST anomaly of about -1.1 to -2.5 C (Fig. 3(a)). Meanwhile, chlorophyll-a anomaly ranged mainly from 0.08 to 1.1 mg/m 3 except along the coast of Bali (denoted by B in Fig. 1) in the year of 2001, in which the negative anomalous feature that was dominant was about -1.03 mg/ m 3 (Fig. 3(b)). Furthermore, it can be seen from the figure that the upwelling strength along the southern coasts of Sumbawa, Flores, and Alor (denoted by Sw, F, and A in Fig. 1, respectively), which is indicated by the negative SST anomaly, is stronger than that along the coasts of Bali and Lombok (denoted by B and L in Fig. 1, respectively). This can be understood as a consequence of the stronger SE monsoon along the Sumbawa - Alor compared with that along the Bali - Lombok (represented by the wind speed of July 2000 and 2001 in Figs. 4(c) and (d), respectively). Meanwhile, during El Niño events that coincided with positive IOD (1997/1998, 2002/2003, 2003/2004, and 2006/ 2007) there were significant departures from the elevenyear (1998-2008) monthly mean SST and chlorophyll-a in magnitude, area, and timing of the seasonal response to the SE monsoon (Fig. 3). In these events, the SST and chlorophyll-a anomalies range from -1.1 to -3.8 C and from 0.08 to more than 4.5 mg/m 3, respectively. As reported by Susanto and Marra (2005), the present study also showed significant high chlorophyll-a concentrations along the southern coast of Bali during the peak of the 1997/1998 El Niño event that coincided with positive IOD (October - November 1997, Fig. 3(b)). We even found that the chlorophyll-a anomaly along the southern part of Bali (B) during this peak of the 1997/1998 El Niño/the positive IOD is highest during the whole period of observed data (1997 to 2008) (Fig. 3(b)), which is associated with the strong El Niño event that coincided with the strong positive IOD (Fig. 3(c)). In this event, the chlorophyll-a concentrations along the southern part of Bali (B), which is associated with the stronger SE monsoon (represented by the wind speed of September 1997 in Fig. 4(a)), were highest along the region compared with that along the south coasts of Sumbawa (Sw), Flores (F), Alor (A), and Sawu Sea. On the other hand, during the SE monsoon in the La Niña event that coincided with negative IOD (1998), upwelling strength was reduced significantly (Fig. 3). There was no significant chlorophylla concentration in this region and SST was warmer than during normal years. Influences of inter-annual forcing associated with ENSO and IOD on upwelling strength were still observed although the ENSO events did not coincide with the IOD (e.g. l999 for weak La Niña and normal IOD; 2004 for El Niño and normal IOD; 2005 for normal ENSO and negative IOD; and 2007 and 2008 for normal ENSO and Positive IOD), as shown in Fig. 3. During the SE monson in the El Niño (2004) year and in the positive IOD events (2007 and 2008), upwelling strength is stronger than the normal years (2000 and 2001), which is indicated by more negative (more positive) anomaly values of the SST (clorophyll-a), as displayed

54 Ningsih, N. S. et al. Fig. 3. Time-longitude profiles of: (a) SST anomaly and (b) Chlorophyll-a anomaly, along the Track C-C (in Fig. 1); and (c) the ONI and DMI. Note that white colours in the Figs. 3(a) and (b) indicate unavailable data. The alphabet letters of B, L, Sw, F, and A denote Bali, Lombok, Sumbawa, Flores, and Alor in Fig. 1, respectively

Upwelling Variability along the Southern Coast of Bali and in Nusa Tenggara Waters 55 Fig. 4. Monthly mean wind speed along the southern coast of Bali and in the Nusa Tenggara waters: (a) in September 1997 (strong El Niño and strong positive IOD), (b) in July 1999 (weak La Niña and normal IOD), (c) in July 2000 (normal ENSO and normal IOD), and (d) in July 2001 (normal ENSO and normal IOD) in Fig. 3. However, the upwelling strength in the years of El Niño (2004) and positive IOD (2007 and 2008) is not as strong as that during the El Niño events that coincided with positive IOD (1997/1998 and 2006/2007). It is associated with the strength of the ONI and DMI in Fig. 3(c). Again, the impact of inter-annual forcing on upwelling strength during the SE monsoon can be clearly seen in Fig. 3, namely for the case of the normal ENSO event that coincided with negative IOD (2005). In this event, the upwelling strength is weaker than in the normal years (2000 and 2001). It can be understood as a result of deepening thermocline depths during the negative IOD (Susanto et al. 2001). However, during the SE monsoon in the weak La Niña year that coincided with normal IOD (l999), it can be assumed that the upwelling strength would be weaker than during the normal years (2000 and 2001). In fact, the upwelling strength was not as weak as expected. This might be a consequence of the stronger SE monsoon in the year of 1999 compared with that of 2000 and 2001 (represented by the wind speed of July 1999, 2000, and 2001 in Figs. 4(b)(d), respectively). In addition, in Fig. 5 we present temporal variability of chlorophyll-a concentration along the southern coasts of Bali (B), Lombok (L), Sumbawa (Sw), Flores (F), Sumba (S), and Alor (A), overlaid with both ONI and DMI. It can be clearly seen from the Fig. 5 the existence of both seasonal and interannual variability of the chlorophyll-a concentration associated with the monsoon and both ENSO and IOD. In general, the variability in temperature and chlorophyll-a concentration on the inter-annual time scales (ENSO and IOD) in the studied area agrees with that of previous studies (Moore and Marra 2002; Sprintall et al. 2003; Susanto and Marra 2005): during El Niño/positive IOD, SST (chlorophyll-a concentration) is colder (higher). During La Niña/negative IOD periods, SST (chlorophyll-a concentration) is warmer (lower).

56 Ningsih, N. S. et al. Fig. 5. Temporal variability of chlorophyll-a concentration along the southern coasts of Bali (B), Lombok (L), Sumbawa (S), Flores (F), Sumba (S), and Alor (A), overlaid with interannual indices (the ONI and DMI) 4. Concluding Remarks In this paper, spatial and temporal variability of upwelling along the southern coast of Bali and in the Nusa Tenggara waters has been investigated by using sea surface temperature and chlorophyll-a data from September 1997 to December 2008. The present study found the existence of annual upwelling in June to October, associated with the SE monsoon cycle, with colder SST and higher chlorophyll-a concentration in comparison to those during the NW monsoon. Our results also show that the upwelling is not only forced locally by the SE monsoon but also forced remotely by atmosphereocean circulation associated with ENSO and IOD. In general, the present study found that during El Niño/positive IOD (La Niña/negative IOD) periods, the Bali - Nusa Tenggara upwelling strength increases (reduces). It is believed that this occurs due to shallowing (deepening) thermocline depths during El Niño/positive IOD (La Niña/negative IOD) periods, as reported by Susanto et al. (2001). Understanding the spatial and temporal variability of upwelling in this region, which is improved by the present study, is important for coastal fisheries. However, in reality the relationship between environmental factors and fish stock abundance are likely to be complex and variable. In this study, we only studied horizontally spatial variability of the upwelling, whereas in reality it would be essential to investigate vertical variability in a water column, such as stratification or mixing, which is important for placing longline fishing in the context of fish catching techniques. Therefore, a more complete study of upwelling dynamics in this region is needs to be carried out, one that would include temperature stratification or mixing of the water column by using a three-dimensional hydrodynamic model. This kind of study is currently in progress as an extension of this research. Acknowledgements We would like to thank the support given by the Research Group of Remote Sensing and Geographical Information Sciences, Faculty of Earth Sciences and Technology, ITB, Indonesia for carrying out the satellite images data processing. We also gratefully acknowledge the support given by the Graduate School for International Development and Cooperation (IDEC) at Hiroshima University, Japan, for making the writing of this paper possible. References Kahn B, Subijanto J (2009) Managing whale hot spots while protecting fisheries. http://www2.thejakartapost.com/news/

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