Weakening of subduction in the Subtropical Mode Water formation region observed during 2003-2013

Abstract

Subduction plays an important role in the formation of the Subtropical Mode Water (STMW). Variation of subduction in the western North Pacific was studied using gridded monthly data from Argo float profiles. The results revealed that there exists a weakening trend of subduction in the STMW formation region (28°N-35°N, 142°E-175°W) due to decreasing winter mixed layer depth (MLD) during 2003-2013. In the STMW formation region, the mean subduction rate was about 64 m yr-1 and showed a decreasing trend at -3.44 ± 2.47 m yr-2 during 2003-2013. Meanwhile, the late winter (March) MLD showed a decreasing trend at -4.02 ± 2.41 m yr-1. Associated with the weakening subduction, the STMW volume had a similar decreasing trend in late winter. Diagnostic calculation indicated that change of the mixed layer temperature (MLT) is the key factor in determining the MLD variations in the STMW formation region. It is demonstrated that the increasing MLT tends to decrease oceanic density and stabilize the upper ocean. This oceanic processes act to weaken the vertical mixing and decrease the MLD, resulting in the weakening of subduction.