Northwest monsoon upwelling within the Indonesian seas

ABSTRACT

The previous studies of upwelling within the Indonesian seas only focused on the Southeast Monsoon (SEM) season. The western coast of Sumatra Island; the southern coast of Java Island, Sunda Islands, and Sulawesi Island; the Banda Sea, the Maluku Sea, and the Arafura Sea are well-known SEM upwelling areas. However, the upwelling events during Northwest monsoon (NWM) have never been investigated. The investigation of NWM upwelling is challenging due to the broad cloud coverage, limiting the observation of infrared and visible sensors of the satellites. This study used the blended products of the satellite-based sea surface temperature (SST), chlorophyll-a (chl-a), sea level anomaly (SLA), surface wind, and rainfall to identify the NWM upwelling occurrences. Remote sensing reflectances 443 nm and 555 nm were also used to examine the influence of suspended sediment and organic matter which may bias the chl-a concentration. Along the northern coast of the Lesser Sunda Islands, the north coast of Papua Island and the Malacca Strait are the areas of NWM upwelling as denoted by the positive anomaly of chl-a, the negative anomaly of SST, and the negative anomaly of SLA. Focusing on the area along the northern coast of the Lesser Sunda Islands, the NWM upwelling is generated by offshore Ekman Mass Transport that reduces SST and increases chl-a. From 2010 onwards, El Niňo Southern Oscillation has a more consistent effect on the NWM upwelling than Indian Ocean Dipole. El Niňo (La Niňa) tends to weaken (strengthen) upwelling. The magnitude of NWM upwelling is weaker than SEM upwelling.

Acknowledgements

The first, fourth, and fifth authors thank the Program of Indonesia Collaborative Research held by Diponegoro University, Hasanuddin University, and Universitas Brawijaya with the contract no. 193.07/UN7.6.1/PP/2020, no. 1269/UN4.22/PT.01.03/2020, and no. 455.1/UN10.C10/PN/2020. Anindya Wirasatriya thanks to the World Class Professor programme managed by the Indonesian Ministry of Education and Culture contract no. 101.2/E4.3/KU/2020. R. Dwi Susanto is supported by NASA grants#80NSSC18K0777 and NNX17AE79A through the University of Maryland, College Park. GHRSST Lv. 4 is provided by https://podaac.jpl.nasa.gov/dataset/JPL_OUROCEAN-L4UHfnd-GLOB-G1SST?ids=Processing%20Levels:Keywords:Projects&values=4%20-%20Gridded%20Model%20Output::Oceans:Ocean%20Temperature::GHRSST. Meanwhile, OC-CCI 4.2 and GSMAP data can be downloaded from https://esa-oceancolour-cci.org/and https://sharaku.eorc.jaxa.jp/GSMaP/, respectively. ASCAT, SLA, and vertical profile of temperature and density are from https://resources.marine.copernicus.eu/?option=com_csw&task=results. ONI index was obtained from https://www.cpc.ncep.noaa.gov/data/indices/oni.ascii.txt. Meanwhile, the weekly DMI index is provided byhttps://stateoftheocean.osmc.noaa.gov/sur/ind/dmi.php.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Program of Indonesia Collaborative Research, Diponegoro University, Hasanuddin University, Brawijaya University [1269/UN4.22/PT.01.03/2020,193.07/UN7.6.1/PP/2020,455.1/UN10.C10/PN/2020]; NASA [80NSSC18K0777,NNX17AE79A]; World Class Professor programme managed by the Indonesian Ministry of Education and Culture [101.2/E4.3/KU/2020].