Two pathways switch of the North Pacific Eastern Subtropical Mode Water toward the equatorial region identified in sigma-pi distance metric

This study uses a new distance metric to define the boundaries of Eastern Subtropical Mode Water (ESTMW), Subtropical Mode Water (STMW), and Central Mode Water (CMW) in the North Pacific based on orthogonal potential density-potential spicity (sigma-pi, σ-π) space. The distance metrics well capture the core and more stable part of mode waters and provide an efficient approach to identifying subduction regions. Different from the other two, the ESTMW directly connects the subtropics and tropics in the North Pacific, affecting the ocean’s vertical thermal structure along its pathway. The distribution of small σ-π distance on 24.7 kg m^− 3 isopycnal surface suggests that the ESTMW transports from the subtropics to the equatorial region by a stable Central Pathway and an unstable Western Pathway. In addition, part of ESTMW moves eastward along the Western Pathway in the western tropics to join the Central Pathway and ultimately flows to the equatorial Eastern Pacific through the Central Pathway. The intensity of the North Equatorial Current and North Equatorial Countercurrent, especially the latter, determines the existence of the Western Pathway. Our findings suggest that the σ-π distance metric provides a new insight into water mass transport pathway study.