Hydraulics and Mixing of the Deep Overflow in the Lifamatola Passage of the Indonesian Seas

Abstract

Hydrographic measurements recently acquired along the thalweg of the Lifamatola Passage combined with historical moored velocity measurements immediately downstream of the sill are used to study the hydraulics, transport, mixing, and entrainment in the dense overflow. The observations suggest that the mean overflow is nearly critical at the mooring site, suggesting that a weir formula may be appropriate for estimating the overflow transport. Our assessment suggests that the weir formulae corresponding to a rectangular, triangular, or parabolic cross section all result in transports very close to the observation, suggesting their potential usage in long-term monitoring of the overflow transport or parameterizing the transport in numerical models. Analyses also suggest that deep signals within the overflow layer are blocked by the shear flow from propagating upstream, whereas the shallow wave modes of the full-depth continuously stratified flow are able to propagate upstream from the Banda Sea into the Maluku Sea. Strong mixing is found immediately downstream of the sill crest, with Thorpe-scale-based estimates of the mean dissipation rate within the overflow up to 1.1 × 10−7 W kg−1 and the region-averaged diapycnal diffusivity within the downstream overflow in the range of 2.3 × 10−3 m2 s−1 to 10.1 × 10−3 m2 s−1. Mixing in the Lifamatola Passage results in 0.6-1.2 Sv entrainment transport added to the overflow, enhancing the deep-water renewal in the Banda Sea. A bulk diffusivity coefficient estimated in the deep Banda Sea yields 1.6 × 10−3 ± 5 × 10−4 m2 s−1, with associated downward turbulent heat flux of 9 W m−2.

A Maluku Sea intermediate western boundary current connecting Pacific Ocean circulation to the Indonesian Throughflow

“…During the redeployment in October 2017, a downward looking ADCP was mounted at 1500 m to measure the bottom current profile (Table 1 ). The deep current that follows the topography is suggested to be sub-critical at the LF mooring site (slightly upstream of the sill), in contrast to the supercritical currents with a hydraulic jump downstream of the sill according to a recent analysis 21 .…”

“…2g ), with the salinity minimum largely eroded by the strong mixing inside the Maluku Sea. Considering that the southward flow through the Lifamatola Passage can be elevated above the 1000 m threshold of the ITF exits by the strong mixing inside the Indonesian seas 19 – 21 , this intermediate-depth throughflow is suggested to carry the intermediate waters from the South Pacific (Supplementary Figs. 2 and 3 ) and to the tropical southeastern Indian Ocean through the eastern Indonesian seas.…”

“…4 ), suggesting that the overflow is partially fed by the southward transport in the intermediate layer. In addition, the channel is wider downstream of the saddle point and flow reversals often occur above the bottom overflow experiencing strong turbulence and entrainment 19 – 21 , which suggest potential existence of horizontal recirculation associated with the western intensified currents and the reversal currents above the bottom overflow.…”

The spatial current structure in the Indonesian Seas in November 2014, during The Expedition of Widya Nusantara (EWIN)

“…It is likely triggered by the pressure gradient between the Maluku Sea and the Seram Sea. Since most of previous studies (see Tan et al, 2020;van Aken et al, 2009) has been focused on the deep overflow in this passage,therefore comparing those studies with the observations in this study is not possible. The Lifamatola throughflow plays an important role as it is responsible for enhancing the upwelling event in the Banda Sea via Ekman pumping mechanism (Zhu et al, 2019).…”

Percampuran vertikal di Perairan Laut Maluku dan Talaud pada bulan Februari 2021

“…Pada jalur barat Arlindo, sebanyak 11,6 Sv (1 Sv = 10 6 m 3 /s) massa air masuk ke perairan Indonesia melalui Selat Makassar (Gordon et al, 2008) dan 2,44 Sv mengalir melalui jalur timur, Selat Halmahera (Li et al, 2020). Pada beberapa lokasi ditemukan adanya pembalikan sirkulasi massa air, di antaranya di Perairan Sangihe, Perairan bagian utara Laut Maluku dan Perairan Talaud, di celah Lifamatola (Kashino et al, 1999;Tan et al, 2020;Yuan et al, 2018), atau dikenal dengan retrofleksi.…”