The Indo-Pacific Intersection (IPI), centered in the Indonesian Seas (IS) and South China Sea (SCS), is a convergence zone of both hydrological and atmospheric circulations and plays an important role in the water cycle and global climatic system. This study presents a unique dataset of both ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios to trace Plutonium (Pu) source terms and their transport in the IPI, to examine constituent water masses, notably freshwater fluxes. The ²⁴⁰Pu/²³⁹Pu atom ratios in surface IPI seawater ranged from 0.194 to 0.258 with an average of 0.223 ± 0.021, values that were consistently higher than global fallout (~0.18). ^239+240Pu activities ranged widely from 0.55 to 2.8 mBqm⁻³ with a generally accumulating trend from the western Pacific Ocean (WP) to the IPI as previously observed. We further reveal, using a two end-member mixing model between global fallout and Pacific Proving Grounds (PPG), that the PPG source contributes 34 ± 8% of the Pu to the IS. In the IS, both ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios further show values between those in the WP and SCS, indicating a two-endmember mixing scheme between the SCS (characteristically lower-salinity but higher PPG Pu-fraction) and the WP (higher-salinity and higher PPG Pu-fraction). We estimate the fractional contribution of the SCS water mass as 35 ± 16% in the surface mixed layer and 49 ± 14% in the IS 50–235 m depth interval. This water mass transport is equivalent to a freshwater influx of 80 ± 47 mSv from the SCS to the IS (known as the South China Sea Throughflow (SCSTF), a branch of the Indonesian Throughflow (ITF)). It accounts for over half (53 ± 31%) of the total freshwater flux transported by the ITF into the Indian Ocean and supplies ~24% the freshwater net evaporation in the Indian Ocean over the 8°N – 32°S latitudinal range, and it is thus crucial in maintaining the fresh water balance therein. This freshwater influx to the IS from the SCS also leads to a ~ 0.2 kg m⁻³ decrease in the mean density of the upper 235 m water layer, which serves to inhibit the transport of upper layer warm water from the western tropical Pacific warm pool into the Indian Ocean. Therefore, the SCS, together with the SCSTF, play a critical role in regulating freshwater and heat balance in the IPI and Indian Ocean. These findings have strong implications for the integrative role of the SCS and SCSTF on the Indo-Pacific climate and water cycle.

以印度尼西亚海（IS）和南海（SCS）为中心的印太交汇处（IPI）是水文和大气环流的汇合带，在水循环和全球气候系统中发挥着重要作用。本研究提供了^239+240 Pu 活性和²⁴⁰ Pu/ ²³⁹ Pu 原子比的独特数据集，以追踪钚 (Pu) 源项及其在 IPI 中的传输，以检查组成水团，尤其是淡水通量。表层 IPI 海水中的²⁴⁰ Pu/ ²³⁹ Pu 原子比介于 0.194 至 0.258 之间，平均值为 0.223 ± 0.021，该值始终高于全球沉降 (~0.18)。^239+240Pu 活动范围广泛，从 0.55 到 2.8 mBqm ^-3具有从西太平洋 (WP) 到 IPI 的普遍累积趋势，如先前观察到的。我们进一步揭示，使用全球辐射和太平洋试验场 (PPG) 之间的两个端元混合模型，PPG 源对 IS 贡献了 34 ± 8% 的 Pu。在 IS 中，^239+240 Pu 活动和²⁴⁰ Pu/ ²³⁹Pu 原子比进一步显示了 WP 和 SCS 中的值之间的值，表明 SCS（特征为较低盐度但 PPG Pu 分数较高）和 WP（较高盐度和较高 PPG Pu分数）之间的双端元混合方案. 我们估计 SCS 水质量在地表混合层中的部分贡献为 35±16%，在 IS 50-235 m 深度间隔中为 49±14%。这种水团输送相当于 80 ± 47 毫希沃特的淡水从南海流入 IS（称为南海通流 (SCSTF)，印度尼西亚通流 (ITF) 的一个分支）。它占 ITF 输送到印度洋的淡水总通量的一半以上 (53 ± 31%)，并提供了印度洋 8°N – 32°S 纬度范围内约 24% 的淡水净蒸发量，因此，保持淡水平衡至关重要。这种从 SCS 流入 IS 的淡水也导致约 0.2 kg m⁻³上层235 m水层平均密度降低，抑制了上层暖水从热带西太平洋暖池向印度洋的输送。因此，SCS 与 SCSTF 在调节 IPI 和印度洋的淡水和热平衡方面发挥着关键作用。这些发现对 SCS 和 SCSTF 在印度-太平洋气候和水循环中的综合作用具有重要意义。