Eff ect of the 1997 / 98 El Niño on Chlorophyll a Variability Along the Southern Coasts of Java and Sumatra

The 1997/1998 El Niño Southern Oscillation (ENSO) was detected through observation of satellite-derived ocean color data (expressed as chlorophyll a) in the Indonesian seas. In addition to ENSO, other forcings may infl uence chlorophyll a concentrations in the Indonesian seas, such as tides (Ffi eld and Gordon, 1996; Susanto et al., 2000), the Madden-Julian Oscillation (MJO) (Madden and Julian, 1994), Kelvin and Rossby waves (Arief and Murray, 1996; Sprintall et al., 2000), monsoons (Asanuma et al., 2003; Moore et al., 2003; Susanto et al., submitted), and the Indian Ocean Dipole (IOD) (Saji et al., 1999; Webster et al., 1999). Because of its longitudinal extent (90° to 141°E), the Indonesian seas contain a signifi cant part of the tropical ocean productivity. The majority of the ocean’s productivity occurs within the tropics along the equatorial band of 10°N to 10°S (Longhurst, 1993). The Indonesian seas are also a center of biological diversity (Veron, 1995). They are one of the regions in the world that exhibits high variability in ocean color (Yoder and Kennelly, 2003). And, the Indonesian seas are the sites of important fi sheries, such as off the Halmahera coast (western Pacifi c warm pool region; Lehodey et al., 1997) and in the upwelling region in the eastern tropical Indian Ocean along the southern coasts of Java and Sumatra.


T H E I N D O N E S I A N S E A S
The 1997/1998 El Niño Southern Oscillation (ENSO) was detected through observation of satellite-derived ocean color data (expressed as chlorophyll a) in the Indonesian seas.In addition to ENSO, other forcings may infl uence chlorophyll a concentrations in the Indonesian seas, such as tides (Ffi eld and Gordon, 1996;Susanto et al., 2000), the Madden-Julian Oscillation (MJO) (Madden and Julian, 1994), Kelvin and Rossby waves (Arief and Murray, 1996;Sprintall et al., 2000), monsoons (Asanuma et al., 2003;Moore et al., 2003;Susanto et al., submitted), and the Indian Ocean Dipole (IOD) (Saji et al., 1999;Webster et al., 1999).Because of its longitudinal extent (90° to 141°E), the Indonesian seas contain a signifi cant part of the tropical ocean productivity.
The majority of the ocean's productivity occurs within the tropics along the equatorial band of 10°N to 10°S (Longhurst, 1993).The Indonesian seas are also a center of biological diversity (Veron, 1995).They are one of the regions in the world that exhibits high variability in ocean color (Yoder and Kennelly, 2003).

UPWELLING REGION ALONG THE SOUTHERN COASTS OF JAVA AND SUMATR A
During the southeast monsoon (April to October), southeasterly wind from Australia generates upwelling, bringing cooler waters and increased nutrients to the surface along the southern coasts of Java and Sumatra.Conditions are reversed during the northwest monsoon (October to April).Coastally trapped Kelvin waves, generated along the equatorial Indian Ocean during the monsoon transitions (April and October), also affect upwelling and downwelling processes (Arief and Murray, 1996;Clarke and Liu, 1993;Sprintall et al., 2000).Understanding Higher chlorophyll a concentrations are matched very well with the upwelling during the southeast monsoon cycle.Interannual variability associated with the 1997/1998 El Niño, which coincided with Indian Ocean Dipole, produced higher chlorophyll a concentrations that extended further northwestward along the Sumatra coast.Note that the increase in ocean color between 2000 and 2003 may signal an increase in the monsoonal winds (Goes et al., 2005).Th e Niño3.4 index defi nes an average sea-surface-temperature anomaly within the region 120°W-170°W and 5°N-5°S.Th e Indian Ocean Dipole index is defi ned as the diff erence in sea-surface-temperature 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-Equator) (Saji et al., 1999).
Indonesian seas are the sites of important fi sheries, such as off the Halmahera coast (western Pacifi c warm pool region; Lehodey et al., 1997) and in the upwelling region in the eastern tropical Indian Ocean along the southern coasts of Java and Sumatra.

Figure 2 .
Figure 1.(a) Average chlorophyll a concentration derived from SeaWiFS from January 1998 to December 2003 during the northwest monsoon (December-February, or DJF).(b)Average chlorophyll a concentration during the southeast monsoon (June-August, or JJA).Southeasterly winds from Australia induce upwelling, bringing cooler and highernutrient waters to the surface along the south coasts of Java and Sumatra.Hence, chlorophyll a concentrations are higher in comparison to those during the northwest monsoon.

Figure 3 .
Figure 3. Satellite-derived chlorophyll a anomaly during the peak of the 1997/98 El Niño from October to December 1997.Monthly anomalies were computed as departures from the 1998-2003 mean for that month.Anomalous easterly winds during the 1997/98 El Niño coincided with Indian Ocean Dipole (IOD), which generated anomalous upwelling and produced a signifi cant departure of chlorophyll a concentrations along the southern coasts of Java and Sumatra.

Figure 4 .
Figure 4. Temporal variability of SeaWiFS-derived chlorophyll a in the Sunda Strait (blue) and Bali Strait (red), overlain with interannual indices, Niño3.4 index (shaded in magenta and cyan) and Indian Ocean Dipole index (black).Th e seasonal variability associated with monsoon and interannual variability associated with 1997/1998 El Niño and Indian Ocean Dipole as indicated by the Niño3.4 and IOD indices are clearly seen.

ure 4 .
Again, clearly seen is the seasonal variability associated with monsoon and interannual variability associated with 1997/1998 El Niño and Indian Ocean Dipole as indicated by the Niño3.4 and IOD indices.The impact of the ENSO cycle on pelagic fi sh remains to be determined.Catches of small pelagics (e.g., anchovy, sardine) likely follow the temporal variability in chlorophyll a.In summary, the strong 1997/1998 El Niño followed by a La Niña period, which also coincided with an Indian Ocean Dipole event, was associated with higher chlorophyll a values in the upwelling region along the southern coasts of Java and Sumatra.Higher chlorophyll a values were observed during the southeast monsoon cycle (April-October) than during the northwest monsoon cycle.