Spatio-temporal variability of surface chlorophyll-a in the Halmahera Sea and its relation to ENSO and the Indian Ocean Dipole

ABSTRACT

Long-term satellite data are used to investigate the variability of ocean surface chlorophyll-a (chl-a) concentration in the Halmahera Sea (HS) under influence of the Australian-Indonesian Monsoon (AIM), the El Niño-Southern Oscillation (ENSO), and the Indian Ocean Dipole (IOD). In this study, we first analysed the seasonal variability of chl-a, and then examine the relationship between surface chl-a, sea surface temperature (SST), and sea surface wind stress in the area. Our results suggest that prevailing southeasterly winds play a fundamental role in generating chl-a blooms in the HS. Particularly on a seasonal timescale, through the mechanism of Ekman mass transport, strengthening of southeasterly wind stress during the Southeast Monsoon season (June – August) produces enhanced chl-a concentrations associated with ocean surface cooling in the area of study. On the other hand, the chl-a bloom completely diminishes during the Northwest Monsoon season (December – February) due to weakening of wind stress and Ekman transport. On an interannual timescale, sea level pressure and wind stress are coherent with ENSO and IOD phases. During El Niño and positive IOD events (La Niña and negative IOD events), both sea level pressure and wind stress greatly increase (decrease) over the HS. These conditions cause an anomaly in southerly (northerly) wind stress, which is favourable to an enhancement (reduction) of the chl-a concentration in the region. This study demonstrates that sea level pressure and wind stress are the critical factors in determining the magnitude of chl-a bloom in the HS.

Acknowledgements

The authors thank NASA Goddard Space Flight Center, Ocean Ecology Laboratory, Ocean Biology Processing Group. QuikScat and ASCAT data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. JRA-55 data were provided by the Japan Meteorological Agency. The authors also wish to thank the anonymous reviewers for their careful reading of the manuscript and constructive suggestions for improvement. A. Wirasatriya thanks to Diponegoro University that supports the research fund under the scheme of International Publication Research 2016–2018 with contract number: 1052-24/UN7.5.1/PG/2016; 276/22/UN7.5.1/PG/2017; and 474-78/UN7.P4.3/PP/2018. The last author is supported by the Penelitian Unggulan Profesi Universitas Sriwijaya 2018. RYS thanks Nella Kharisma and Via Vallen for their inspiration lyrics.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary materials

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Additional information

Funding

R.Y. Setiawan acknowledges funding support from the Directorate of Research and Community Service of the Ministry of Research Technology and Higher Education, Republic of Indonesia through a scheme of ‘Penelitian Pasca Doktor (PPD)’ (1797/UN1/DITLIT/DIT-LIT/LT/2018).