Indonesia asserts its prominence as the world’s largest archipelagic nation, strategically positioned at the confluence of Asia and Australia, bordered by the Indian Ocean and the Pacific Ocean. The Indonesian Archipelago (IA), comprising over 18,000 islands, traces its geological origins over 300 million years ago to intricate processes of subduction and collision during the Cenozoic era, resulting from the rearrangement of Gondwana fragments, extensively detailed in previous studies [1–3].
Within the field of climatology, the IA is commonly identified as the Maritime Continent (MC) [4], strategically located in the western Pacific region, specifically within the Indo-Pacific Warm Pool (IPWP) [5]. Recognized as a substantial source of latent heat release in the atmosphere, the MC plays a critical role in initiating deep convection processes that intricately govern both the Hadley and Walker circulations within tropical regions [6, 7]. Consequently, the region stands out for harboring the most robust monsoonal activity on Earth [8]. Additionally, the MC assumes a pivotal role in global atmosphere–ocean interactions, exerting influence over phenomena such as the El Niño Southern Oscillation (ENSO) [9], Indian Ocean Dipole mode (IOD) [10], and Madden–Julian Oscillation (MJO) [11].
Oceanographically, the IA plays an important role, serving as the sole conduit for the transit of water masses from the Pacific Ocean to the Indian Ocean [12–15]. This transport process is recognized as the Indonesian Throughflow (ITF), which also represents the only low-latitude current connecting major ocean basins in the presentday [16]. On an annual time scale, the ITF transfers approximately 0–30 Sv (1 Sv ≡ 106 m3/s) of water mass, as estimated by various numerical models and observations [e. g., 12, 17–33]. Heat transfer through the ITF, based on estimates from the Global Climate Model (GCM) conducted by Hirst and Godfrey [25], is approximately 0.63 PW. This value represents approximately one-third of the total heat input in the equatorial Pacific region.
The ITF facilitates the transfer of warm, low-salinity water masses from the west equatorial Pacific Ocean to the east equatorial Indian Ocean through a defined route encompassing the Makassar Strait, Maluku Sea, and Halmahera Sea [15, 33–36]. In this context, the Makassar Strait is recognized as the primary entry point for the ITF. The water mass originating from the North Pacific navigates through the Celebes Sea before proceeding towards the Makassar Strait and the IA. Upon entering the IA, the current from the Makassar Strait bifurcates into two branches, one traversing southwest Indonesia, and the other moving towards the Indian Ocean through the Lombok Strait. Simultaneously, another branch moves eastward in the Indonesian region, specifically into the Banda Sea via the Flores Sea.
Within the Banda Sea, a complex mixing process unfolds as water masses originating from the South Pacific, entering through the Halmahera Sea, Maluku Sea, and Seram Sea, interact. This amalgamation of water masses in the Banda Sea subsequently progresses towards the Indian Ocean through the Ombai Strait and Timor Gap. The Maluku Sea stands as the second pivotal gateway for the ITF. Pacific water masses traverse the Maluku Sea to reach the Seram Sea via the Lifamatola Strait. From the Seram Sea, they continue their trajectory through the Manipa Strait towards the Banda Sea.
The Halmahera Sea represents the third key entry point for the ITF, with South Pacific water masses transiting through this sea towards the Seram Sea and the Aru Basin. Following a mixing process with water masses originating from the Banda Sea, the combined water mass moves towards the Indian Ocean via the eastern part of the Timor Sea.
On intra-annual, annual, and inter-annual time scales, the ITF is influenced and, in turn, also influences tidal movements, monsoons, and atmosphere-ocean interactions such as ENSO and IOD. Additionally, the ITF may be affected by decadal variability in the Pacific [33, 34, 36]. However, on a centennial time scale, the commonly used Island Rule theory [15], employed to estimate the strength of the ITF, failed to indicate a weakening trend based on Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel simulations [37]. This limitation arises because the Island Rule solely calculates the strength of the ITF as a line integral of wind stress and Coriolis terms along a defined boundary (dominated by wind-driven forces) without accounting for the long-term circulation of heat flux and freshwater. This long-term circulation induces interior mixing of temperature and salinity, known as thermohaline circulation (THC). Therefore, to present a realistic centennial projection, an additional term accounting for the contribution of THC in the Pacific is necessary, as demonstrated in the Ocean Forecasting Australia Model version 3 (OFAM3) simulation [38].
Apart from the temporal regulation of the ITF by wind-driven circulation, the THC has long been acknowledged as a contributing factor to the ITF’s strength within the upper segment of the global overturning circulation scheme [13]. The trajectory of the warm surface water initiates from the North Pacific Deep Water (NPDW), extending through the IA into the Indian Ocean. Within the 10-20oS belt of the Indian Ocean, a complex mixing of Pacific and Indian Ocean water occurs. The southward journey then proceeds through the Mozambique Channel, where it divides into two primary flows. The dominant portion, comprising Agulhas Current system, enters the Agulhas Retroflection into the Southern Ocean, while the remaining stream finds its way into the Atlantic.
Meanwhile, at the western boundary of the Indian Ocean, the convergence of northward and southward boundary currents facilitates the closure of tropical and subtropical gyres, aligning with the observations of Hughes et al. [39]. These gyres actively contribute to pumping water into the Antarctic Circumpolar Current (ACC) and the South Atlantic Gyre, a pivotal process in the formation of North Atlantic Deep Water (NADW), as detailed by Toole and Warren [24]. Notably, the ITF enhances the meridional steric height gradient, thereby influencing the strength of these gyres. The complex interplay implies that the ITF, in addition to its role in balancing the thermal and saline conditions between the Pacific and the Indian Ocean, might also have a pronounced influence on the physical conditions of the distant Atlantic. This cyclical process aligns with the concept known as the great ocean conveyor belt hypothesis [40], which was deemed accountable for abrupt climate changes during glacial-interglacial periods [e. g., 41–45].
Given the observed weakening trend in the Atlantic Meridional Overturning Circulation (AMOC) [e. g., 46–49], the principal driver of the modern THC, and its potential correlation with a density-driven weakening of the ITF due to recent anthropogenic climate change [37, 38, 50, 51], a comprehensive understanding of the ITF’s role in this circulation becomes imperative. This study employs classical numerical experiments, with a focus on the blockage of the IA, aiming to unravel the geographic significance of both IA and ITF in the global meridional THC.
In contrast to prior numerical experiments [16, 52–54] that concentrated on the short-term analysis of IA closure’s impact on the surface ocean, influencing climate, and featured a limited simulation time of up to 105 years [16], our approach prioritizes a more extended physical realization of the ocean. Although this method adopts low spatial resolution, it extends over a more extended simulation time, facilitating the attainment of quasi-steady-state conditions (equilibrium numerical solutions). This extended duration enables the analysis of the global meridional THC on a centennial time scale, providing valuable insights into the long-term dynamics of the circulation system.