International Journal of Remote Sensing and Earth Sciences Vol. 11 No. 2 December 2014 :137 – 142

ANALYSIS OF SEA SURFACE HEIGHT ANOMALY CHARACTERISTICS BASED ON SATELLITE ALTIMETRY DATA (CASE STUDY: SEAS SURROUNDING JAVA ISLAND)

Sartono Marpaung* and Wawan K. Harsanugraha Remote Sensing Application Center - LAPAN * e-mail: [email protected] Received: 7 July 2014; Revised: 19 August 2014; Approved: 12 October 2014

Abstract. Sea surface height anomaly is a oceanographic parameter that has spatial and temporal variability. This paper aims to determine the characters of sea surface height anomaly in the south and north seas of Java Island. To find these characters, a descriptive analysis of monthly anomaly data is performed spatially, zonally and temporally. Based on satellite altimetry data from 1993 to 2010, the analysis shows that the average of sea surface height anomaly varies, ranging from -15 cm to 15 cm. Spatially and zonally, there are three patterns that can be concidered as sea surface height anomaly characteristics: anomaly is higher in coastal areas than in open seas, anomaly is lower in coastal areas than in open seas and anomaly in coastal area is almost the same as in open seas. The first and second patterns occur in the south and north seas of Java Island. The third pattern occurs simultaneously in south and north seas of Java Island. Characteristics of temporal anomaly have a sinusoidal pattern in south and north seas of Java Island.

Keywords: Altimetry, Zonal, Anomaly, Characteristics and Sea Surface Height

1 INTRODUCTION factors or parameters that are dominant Indonesia is a maritime continent, in the dynamics (Digby, 1999). about 70 percent of its territory consists of Nowadays, there are several satellite water and flanked by two great ; altimetry used to monitor or observe Indian and Pacific Ocean. The very vast global water/sea like Jason-2, Cryosat-2, has great potential and Saral and HY-2A (Hai Yang). Previous influence on various sectors of human satellite altimetry: GeoSat, Topex/ Poseidon, life. Those potentials and influences should GFO (GeoSat Follow On) and Jason- be studied and identified for public 1. Each satellite performs measurements welfare. An understanding of oceanic with different orbits and references and physical dynamic or circulation through establishes the track density or data analysis can be used to improve trajectory. Thus, the obtained satellite human welfare (Dwi, 2010). Adequate data can complement each other to availability of temporal and spatial data generate data with optimal spatial and from oceanographic parameters are required temporal coverage. Satellite altimetry in in research activities. monitoring produces The presence of satellite altimetry some oceanographic parameters: sea becomes the appropriate solution in surface height , chlorophyll-a, meeting both regional and global needs of sea surface height, , sea surface oceanographic data (Handoko, 2004). The height currents and waves. These analysed data generated from satellite parameters can be used as indicator of the altimetry shows pictures of the occurring various phenomena that occur at the processes of ocean dynamics and the sea. Sea surface height temperature is an

@National Institute of Aeronautics and Space of Indonesia (LAPAN) 137

Sartono Marpaung and Wawan K. Harsanugraha

indicator of weather/climate phenomena boundary of latitude: S 3.42 ⁰ to 12.01 ⁰. (La Nina, El Nino and Dipole Mode). The Four points are determined for temporal previously mentioned parameters of the analysis: location A (107.2 ⁰ E, 5.2 ⁰ S) ocean can also be used to determine and location B (112.5 ⁰ E, 5.9 ⁰ S) in north potential fishing area. In addition to these sea, and location C (106, 2 ⁰ E, 8.7 ⁰ S) functions, sea surface height has an and the location of D (112.5 ⁰ E, 9.2 ⁰ S) in important role as indicator of climate south sea. Selection of the study points is change phenomena in global and regional based on representative of closed waters scale (Susanto et al., 2001). As a result of in north and open waters in south (Figure global warming and polar ice melting, 2-1). ocean water volume increases so that the This study used descriptive data sea surface height rises. analysis method. To know the characteristics One of oceanographic parameters of sea surface height anomaly spatially, associated with physical dynamics that monthly mean of the same months during are discussed in this study are sea surface the observation period are calculated height anomaly. Sea surface height anomaly (Sudjana, 2005). Then, the average zonal is the magnitude of the deviation on the anomaly for each month are calculated. average sea surface height condition Mean zonal value is averaging values at a (Steward, 2008). Sea surface height is the latitude to the pixel values of all the distance between the sea surfaces to the longitude (Isawaki, 1998). Mean zonal reference ellipsoid. With the availability of value is analyzed to determine the satellite altimetry data from remote characteristics based on different sensing technology monitoring, analysis of latitude. Analysis of temporal sea surface sea surface height anomaly on seas height anomaly characteristics is done surrounding Java Island can be done. The based on the time series of the four purpose of this study is to compare the assessment places, two locations in the characteristics of sea surface height north (location A and B) and two locations anomalies spatially (different areas) and in the south (locations C and D). temporally (different time). Teh obtained characteristics of sea surface height anomaly can be used to determine the zone. Upwelling zones is a potential reservoir for fishing operations.

2 METHOD This study used sea surface height anomaly data from combined (merged) monitoring of multiple satellite altimetry: Topex/Poseidon, Jason-1, Envisat, Jason- 2 and Cryosat-2. The data has a spatial resolution of 0.33 ° x 0.33 ° and monthly temporal. The data used in the analysis are from 1993 to 2010. The data sources can be found at ftp://aviso.oceanobs. com/pub/seadatanet/. The study area is the south and north sea of Java Island Figure 2-1: Area of research study and four with zonal boundary from longitude: E assessment points/locations (A, B, C 105.33 ⁰ to 114.67 ⁰ and meridional and D)

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3 RESULTS AND DISCUSSION push masses from open seas Based on the analysis conducted on towards the north coast so that the monthly data of sea surface heights anomaly becomes higher. In April and anomaly from 1993 to 2008, the monthly May, the anomaly distribution seems mean of sea surface height anomaly from different than the distribution characteristics January to December are presented in that have been described. The distribution Figure 3-1. Figure 3-1 shows the monthly of anomaly on the south seas between the mean scale of sea surface height anomaly coast area and the open sea is almost with anomalous values ranging from -15 equal. On the north seas, the anomalous cm to 15 cm. The results show that in value is almost spatially the same. This is January, February, March, November and presumably due to the influence of December, at south sea, the anomalies in irregular wind direction in the transitional the coastal areas is higher than in the period of March-April-May. open seas to the south. In coastal areas, To complete the results that have been the anomalous value is up to 15 cm, while obtained and reinforce the characteristics at open sea, the value is smaller. There of sea surface height anomaly in the south are two factors that cause it. The influence and north (analysis based on the of the dynamic circulation of the Indian difference in latitude), averaging zonal Ocean and the westerlies that occur in anomaly values is carried. The results in these months. These factors reinforce Figure 3-2 show that the mean zonal of each other because of both’s west to east sea surface height anomaly has high directions cause the mass of seawater variability based on latitude. Generally, pushed into the south coast. This causes latitude: 8⁰ to 9 ⁰S (the sea near the the sea surface height anomaly on the southern part of the mainland), the mean south coast becomes higher. The anomalies value is high in January, distribution of anomalies in the north sea February, March, November and is different from the south. Anomalies in December. By contrast, in the same the coastal areas are lower, and going position in June to October, mean north, the anomaly increases. This is due anomalies has low value. While in to the influence of westerlies that drive latitude: 5 ⁰ to 6 ⁰ S (the waters near the seawater masses from the north coast to northern part of the mainland) variability the east and lower the anomaly on the of sea surface height anomaly is lower north coast. In June and October, it than in the seas of the southern part. shows that the anomaly in the south, Bima et al., 2014, states that the especially in the coastal areas, is lower monsoon system in southern seas of than that on the open seas. This is caused Indonesia is characterized by seasonal by the easterlies occuring from June to reversal of wind direction that causes a October. The winds blow from east to different movement pattern of water west, pushing the mass of seawater from masses. Variability of sea surface height the south coast to the open sea. The anomaly indicates that the upwelling influence of the Indian Ocean dynamic and are stronger on the circulation becomes weakened or non- south sea than on the north one. This is existent because of the dominance of the presumably due to the influence of the easterlies. This incident resulted in the geographical position of the two seas. The lowering of sea surface height anomaly on south sea is an open sea with influence the south coast. Meanwhile, in the northern from the Indian Ocean circulation while part, the anomaly is higher on coastal the north sea is located in a closed sea. areas and lower on open seas. Easterlies

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The analysis results of the mean same as in the open seas. When the first spatial and the mean zonal show three pattern occurres in the south sea, the patterns or characters of sea surface second pattern occures in the north sea height anomaly. First, the anomaly is and vice versa. The first and second higher in coastal areas than in open seas. patterns occur in pairs in the sea around Second, the anomaly is lower in coastal the Java Island. Meanwhile, the third areas than in open seas. Third, the pattern occurs simultaneously in south anomaly in the coastal areas is almost the and north sea.

Figure 3-1: Monthly mean of sea surface height anomaly from January to December

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Figure 3-2: Average zonal anomalies in sea surface heights from January to December

To determine the temporal assesment point on south sea and in characteristics of sea surface height August/October at assesment point on anomaly in the south and north sea of north sea. In October to December, the Java Island, time series of anomaly for anomaly increases. Judging from the form four assessment points (Figure 3-3) are of time series on the four assesment made. Figure 3-3 shows that the sea points, sea surface height anomaly has surface height anomaly declines slightly in the same trends or tendencies; sinusoidal January to March and increases in April pattern. It shows the presence of factors to May. It appears that the anomaly that strongly influences the dynamics of maximum peak occurs in May. In June to sea surface height anomaly periodically; September, the anomaly decreases seasonal westerlies and easterlies which significantly. The minimum condition of occur alternately in Indonesia. anomaly occurs in September at

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Figure 3-3: The time series of monthly sea surface height anomaly at four assesment locations (A, B, C and D)

4 CONCLUSION Dwi BD, (2010), Penilaian Dampak Kenaikan Monthly sea surface height anomaly Muka Air Laut Pada Wilayah Pantai : Studi ranges from -15 cm to 15 cm. Spatially Kasus Kabupaten Indramayu. Jurnal and zonally, there are three patterns or Hidrosfer Indonesia 5 (2): 43-53. characteristic of sea surface height Handoko EY, (2004), Satelit Altimetri dan anomaly: the anomaly is higher in coastal Aplikasinya Dalam Bidang Kelautan. areas than in open seas, the anomaly is Scientific Journal, Pertemuan Ilmiah lower in coastal areas than in open seas Tahunan I. Teknik Geodesi – ITS, Surabaya, and anomaly in coastal area is almost the Indonesia. same as in open seas. The first and Iwasaki T., (1998), A Set of Zonal Mean Equations in second patterns occur in the south and a Pressure–Isentrope Hybrid Vertical north seas of Java Island. The third Coordinate. Journal of The Atmospheric pattern occurs simultaneously in south Science 55: 3000–3002. and north seas of Java Island. Kunarso, Ningsih, Supangat, (2005), Karakteristik Characteristics of temporal anomaly have Upwelling di Sepanjang Perairan Selatan a sinusoidal pattern in south and north NTT Hingga Barat Sumatera. Jurnal Ilmu seas of Java Island. Kelautan 10(1): 17- 23. Mann KH, Lazier JRN, (2006), Dynamics of ACKNOWLEDGEMENT Marine Ecosystems: Biological Physical The authors thank to Remote Interactions in the Oceans. 3rd ed., Sensing Application Center that Blackwell Publishing, Malden, 444. facilitated this research activity Stewart RH, (2008), Introduction to Physical . Departement Of REFERENCES Oceanography. Texas A and M University, Bima YR, Setyono, Harsono, (2014), Dinamika Texas, 313. Upwelling dan Downwelling Berdasarkan Sudjana, (2005), Metode Statistika Edisi ke-6. Variabilitas Suhu Permukaan Laut dan Bandung, Tarsito. Klorofil-A di Perairan Selatan Jawa. Jurnal Susanto RD, Gordon, Zheng, (2001), Upwelling Oseanografi 3(1): 56-66. along the coasts of Java and Sumatra and Digby S., (1999), Use of Altimeter Data, Jet its relation to ENSO. Geophysical Research Propulsion Laboratory California Institute Letters 28(8):1599-1602. of Technology, Pasadena, California.

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