Origin and pathway of the Luzon Undercurrent identified by a simulated adjoint tracer

Abstract

The origin and pathway of Luzon Undercurrent (LUC) water are investigated using a simulated adjoint tracer, based on circulation estimates of a global general circulation model. The results demonstrate that most of the LUC water comes from the subtropical North Pacific, confirming the earlier hypothesis on its relation to the northward shift of the North Equatorial Current (NEC) bifurcation at subsurface. Of the total volume of initially tracer-tagged LUC water, approximately 41% is traced back to the winter mixed layer in the Kuroshio extension after 50 years of integration, coinciding with the formation of subtropical mode waters, while the rest is trapped in the northern subtropical gyre with its density >26.8 kg m-3, indicative of a significant South Pacific origin. As these waters move toward the western boundary, they get mixed and finally reach the density range of the LUC water. This result provides quantitative evidence for the dramatic impact of mixing on the route of subtropical water to becoming the LUC water. Copyright 2012 by the American Geophysical Union.