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Xiang Li
,
Dongliang Yuan
,
Zheng Wang
,
Yao Li
,
Corry Corvianawatie
,
Dewi Surinati
,
Asep Sandra
,
Ahmad Bayhaqi
,
Praditya Avianto
,
Edi Kusmanto
,
Dirham Dirhamsyah
, and
Zainal Arifin

Abstract

The ocean currents in the Halmahera Sea are studied using a subsurface mooring deployed in the Jailolo Strait from November 2015 to October 2017. The subtidal currents of the mooring measurements are characterized by a two-layer system, with the current variability below about 200 m in opposite phases to that in the upper layer. The mean along-strait velocity (ASV) is toward the Indonesian seas in the whole water column, producing an estimated mean transport of 2.44 ± 0.42 Sv (1 Sv ≡ 106 m3 s−1). The errors of the transport calculation based on the single mooring measurements are estimated to be less than 15% using simulations of high-resolution ocean models. A weak current is observed to flow northward during 2017 at the bottom of the strait. The ASV variability is found to be dominated by an annual cycle both in the upper and lower layers. The total transport, however, is dominated by semiannual variability because of the cancelation of the annual transports in the upper and lower layers. The variability of the transport is suggested to be driven by the pressure difference between the Pacific Ocean and the Indonesian seas, as evidenced by the agreement between the satellite pressure gradient and the two-layer transports. The transport of the Jailolo Strait during the 2015/16 super El Niño is found to be nearly the same as that during the 2016 La Niña, suggesting that the interannual variability of the transport is much smaller than the seasonal cycle.

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Xiang Li
,
Dongliang Yuan
,
Yao Li
,
Zheng Wang
,
Jing Wang
,
Xiaoyue Hu
,
Ya Yang
,
Corry Corvianawatie
,
Dewi Surinati
,
Asep Sandra Budiman
,
Ahmad Bayhaqi
,
Praditya Avianto
,
Edi Kusmanto
,
Priyadi Dwi Santoso
,
Adi Purwandana
,
Mochamad Furqon Azis Ismail
,
Dirhamsyah
, and
Zainal Arifin

Abstract

The currents and water mass properties at the Pacific entrance of the Indonesian seas are studied using measurements of three subsurface moorings deployed between the Talaud and Halmahera Islands. The moored current meter data show northeastward mean currents toward the Pacific Ocean in the upper 400 m during the nearly 2-yr mooring period, with the maximum velocity in the northern part of the channel. The mean transport between 60- and 300-m depths is estimated to be 10.1–13.2 Sv (1 Sv ≡ 106 m3 s−1) during 2016–17, when all three moorings have measurements. The variability of the along-channel velocity is dominated by low-frequency signals (periods > 150 days), with northeastward variations in boreal winter and southwestward variations in summer in the superposition of the annual and semiannual harmonics. The current variations evidence the seasonal movement of the Mindanao Current retroflection, which is supported by satellite sea level and ocean color data, showing a cyclonic intrusion into the northern Maluku Sea in boreal winter whereas a leaping path occurs north of the Talaud Islands in summer. During Apri–July, the moored CTDs near 200 m show southwestward currents carrying the salty South Pacific Tropical Water into the Maluku Sea.

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Dongliang Yuan
,
Xiang Li
,
Zheng Wang
,
Yao Li
,
Jing Wang
,
Ya Yang
,
Xiaoyue Hu
,
Shuwen Tan
,
Hui Zhou
,
Adhitya Kusuma Wardana
,
Dewi Surinati
,
Adi Purwandana
,
Mochamad Furqon Azis Ismail
,
Praditya Avianto
,
Dirham Dirhamsyah
,
Zainal Arifin
, and
Jin-Song von Storch

Abstract

The Maluku Channel is a major opening of the eastern Indonesian Seas to the western Pacific Ocean, the upper-ocean currents of which have rarely been observed historically. During December 2012–November 2016, long time series of the upper Maluku Channel transport are measured successfully for the first time using subsurface oceanic moorings. The measurements show significant intraseasonal-to-interannual variability of over 14 Sv (1 Sv ≡ 106 m3 s−1) in the upper 300 m or so, with a mean transport of 1.04–1.31 Sv northward and a significant southward interannual change of over 3.5 Sv in the spring of 2014. Coincident with the interannual transport change is the Mindanao Current, choked at the entrance of the Indonesian Seas, which is significantly different from its climatological retroflection in fall–winter. A high-resolution numerical simulation suggests that the variations of the Maluku Channel currents are associated with the shifting of the Mindanao Current retroflection. It is suggested that the shifting of the Mindanao Current outside the Sulawesi Sea in the spring of 2014 elevates the sea level at the entrance of the Indonesian Seas, which drives the anomalous transport through the Maluku Channel. The results suggest the importance of the western boundary current nonlinearity in driving the transport variability of the Indonesian Throughflow.

Full access
Xueli Yin
,
Dongliang Yuan
,
Xiang Li
,
Zheng Wang
,
Yao Li
,
Corry Corvianawatie
,
Adhitya Kusuma Wardana
,
Dewi Surinati
,
Adi Purwandana
,
Mochamad Furqon Azis Ismail
,
Asep Sandra Budiman
,
Ahmad Bayhaqi
,
Praditya Avianto
,
Edi Kusmanto
,
Priyadi Dwi Santoso
,
Dirhamsyah
, and
Zainal Arifin

Abstract

The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multi-year current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60 m - 450 m is northward towards the Pacific Ocean with a mean transport of 2.07 Sv - 2.60 Sv. In the depth range of 450 m - 1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anti-clockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83 Sv - 2.25 Sv, which is balanced by three transports: (1) 0.62 Sv - 0.93 Sv southward transport into the Seram-Banda Seas through the Lifamatola Passage, (2) 0.97 Sv-1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and (3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semi-enclosed basin.

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