The Equatorial Undercurrent and Its Origin in the Region Between Mindanao and New Guinea

Herein, the spatial characteristics of the Equatorial Undercurrent (EUC) in the region between Mindanao and New Guinea and the origins of the EUC and the North Equatorial Countercurrent (NECC) were investigated using a combination of high‐resolution Argo hydrographic data and numerical modeling. The Mindanao Current (MC) results in the formation of a strong EUC‐depth eastward current that splits into two branches near the Halmahera Eddy west of 132°E, with one branch directly flowing eastward without experiencing the Halmahera Eddy rotation effect, and the other branch flowing southwestward and then turning northeastward. The EUC and NECC were difficult to distinguish between 132°E and 136°E, since the former combined with the latter to form a continuous equator‐tilted current. At ~137°E, the EUC axis started turning toward the equator, and an isolated EUC core was observed at 143°E and ~200‐m depth. The Pacific EUC was shown to originate at ~130°E, where it featured an isolated current core with a maximum velocity of 5–10 cm/s at ~1°N and 200‐ to 400‐m depth. The EUC and NECC in the western tropical Pacific were found to have the same water sources, namely, the MC and the New Guinea Coastal Undercurrent, with the shallow parts (centered at ~100‐m depth) of MC and New Guinea Coastal Undercurrent finally reaching the NECC and their deeper parts (centered at ~200‐m depth) finally reaching the EUC. Key Points The Pacific EUC starts at ~130°E with an isolated core at ~1°N and 200‐ to 400‐m depth A schematic map of ocean circulation in the western tropical Pacific at the depth of the EUC is presented The EUC and NECC in the far‐western tropical Pacific have same source waters, the MC and NGCUC