Subduction of N orth P acific T ropical W ater and its equatorward pathways as shown by a simulated passive tracer

Abstract

The subduction and equatorward pathways of North Pacific Tropical Water (NPTW) are investigated using a simulated passive tracer of the consortium Estimating the Circulation and Climate of the Ocean (ECCO). The results demonstrate that the subduction of NPTW occurs in a large area that extends from about 150°E to 130°W between 20°N and 30°N, but the main subduction region lies in its eastern part. After subduction, the main body of NPTW first spreads westward in the North Equatorial Current. Then it splits into two branches. One flows northward in the Kuroshio upon reaching the western boundary, and the other enters the tropical Pacific either via its western boundary pathway (WBP) or interior pathway (IP). Less than half of the transport through the WBP can eventually reach the central and eastern Pacific by the Equatorial Undercurrent, while the rest is seen to flow into the Indian Ocean by the Indonesian Throughflow. The IP is found to play a significant role in equatorward transport of the NPTW. About 30% of the NPTW that reached the equatorial Pacific is transported through the IP.

Isotopic Evidence for the Evolution of Subsurface Nitrate in the Western Equatorial Pacific

“…North of the equator, NPTW, which feeds the upper thermocline, originates in the North Pacific subtropical gyre along ∼25°N between 150°E and 130°W and enters the WEP and our study site off Mindanao via the broad westward flowing NEC and MC (Fine et al, ; Katsura et al, ; Nie et al, ; Tsuchiya, ). Off Mindanao, upper thermocline waters exhibit a clear minimum in δ 15 N NO3 , with values as low as 4.4‰.…”

“…The water masses and currents that converge at the equator at the western boundary have been investigated in a number of studies (e.g., Bingham & Lukas, 1995;Fine et al, 1994;Kashino et al, 2005;Lindstrom et al, 1987;Nie et al, 2016;Toole et al, 1988;Tsuchiya, 1968). Nevertheless, the relative transports to the EUC from north and south of the equator remain subject to debate.…”

Origins and Pathways of the Pacific Equatorial Undercurrent Identified by a Simulated Adjoint Tracer

“…As the North Pacific counterparts, the NPTW and NPESTMW contribute as much as only about one third of that of the SPTW and SPESTMW. This may be partially due to the existence of the Indonesian Throughflow, through which a large volume of the North Pacific subtropical water is transported into the Indian Ocean (Nie et al, ). The contribution from the rest subtropical water masses is minor in comparison with these water masses, and they need a longer transit time to merge into the EUC.…”

“…Then, the adjoint tracer is found to appear in the mixed layer of eastern subtropical gyre in both hemispheres, with a larger portion returning to the South Pacific (Figures c–f). These regions are where the South Pacific Tropical Water (SPTW; O'Connor et al, ; Qu et al, ), South Pacific Eastern Subtropical Mode Waters (SPESTMW; Qu et al, ), the North Pacific Tropical Water (NPTW; Nie et al, ), and the North Pacific Eastern Subtropical Mode Waters (NPESTMW; Hautala & Roemmich, ) form. This result suggests that the SPTW, SPESTMW, NPTW, and NPESTMW are important source waters of the EUC.…”

“…There is also a weak transport through the Celebes Sea as indicated by a small concentration of the adjoint tracer in the western coast of Mindanao Island centered at 120°E. It can also be regarded as part of the North Pacific WBP transport since it is originated from the MC in the upstream region (Nie et al, ). The deep stretch of the tracer concentration on WBPs indicates that there is strong diapycnal mixing, as has been suggested by earlier studies (Grenier et al, ; Qu et al, ).…”