Observed Upper Deep Branch of the Pacific Meridional Overturning Circulation North of New Guinea

The deep channel north of New Guinea (NG) is the choke site for the upper deep branches of the Pacific meridional overturning circulation (U-PMOC). The U-PMOC is a crucial element of the ocean's climate and biogeochemi-cal systems. It carries the mixed water of the Upper Circumpolar Water and North Pacific Deep Water with a potential temperature over 1.2 degrees-2.2 degrees C. The pathway and volume transport of U-PMOC through the deep channel north of NG are revealed by mooring measurements from 2014 to 2019. Mean U-PMOC is located at 2000-3500 m with a velocity core at 2550 m and is directed eastward. The U-PMOC shows a strong seasonal variability with a direction reversal from June to September. The oceanic reanalysis product GLORYS12V1 well reproduces the observed U-PMOC and is thus used to es-timate the mean and standard deviation of U-PMOC's volume transport as 2.19 +/- 11.4 Sv (1 Sv = 106 m3 s-1) and to ex-plore the underlying dynamics of the U-PMOC. The seasonality of U-PMOC is induced by the vertical propagation of the Rossby energy through the upper ocean in the eastern Pacific to the deep ocean in the western Pacific. The mean eastward U-PMOC transport is forced by the zonal deep pressure gradient, which is mainly determined by the local upper-ocean processes above 500 m.