ITF (Indonesia Passage) is the only ocean passage connecting tropical Pacific Ocean and Indian Ocean, and it is the key link of global ocean hot salt circulation conveyor belt. Its massive water transport from the Pacific Ocean to the Indian Ocean regulates the temperature and salinity distribution between the two oceans, and then affects the global ocean circulation structure and long-term climate change. Therefore, the Indonesian cross-current has an important influence on the circulation of the Pacific Ocean and the Indian Ocean and the global climate system.

NESC, which flows westward against the north equator of the Pacific Ocean, is the ocean undercurrent with the longest flow path and the largest flow in the world. It was discovered and named by Professor Yuan on 20 14 (Yuan et al. 20 14). Its formation mechanism is the superposition of low-order baroclinic modal responses forced by wind curl. In summer, the middle water masses (X. Li et al. 2020) in the tropical North and South Pacific Ocean mix in the process of flowing from east to west. During the extreme El Nino event of 20 15/20 16, the wind field in the equatorial central Pacific Ocean was abnormal, which triggered baroclinic Rossby waves to propagate vertically to the western Pacific Ocean (Yang et al. 2020), thus causing abnormal enhancement in the summer of 20 16.

In this study, the path of the northern equatorial undercurrent invading makassar strait is put forward, which is a new path for the mixed water mass of the north and south of the tropical Pacific Ocean to enter the Indonesian current through the thermocline (Figure 1). This path is the only passage for the sub-thermocline water masses in the central and eastern equatorial Pacific to enter the Indonesian Sea and the Indian Ocean. The above results provide a scientific basis for the connection of the Pacific-Indian Ocean sub-thermocline circulation, and are an important supplement to the equatorial circulation and the global ocean conveyor belt theory.

The research results were published in the Geophysical Research Express in April, 20021year, entitled "The introduction of strong sub-thermocline to the strain of Makassar in 20 16 ‐ 20 17". This paper was completed by Yuan, an outstanding scholar from the First Institute of Oceanography of the Ministry of Natural Resources, in cooperation with Dr. Li Mingting from Sun Yat-sen University and Dr. China Academy of Sciences.

This research is supported by the National Key R&D Plan, the National Natural Science Foundation and the International Cooperation Foundation, the Chinese Academy of Sciences Pilot Project B, and the Southern Ocean Laboratory Innovation Team Building Fund.