Seasonal Variation of the Deep Limb of the Pacific Meridional Overturning Circulation at Yap-Mariana Junction

This study reveals the seasonal variability of the lower and upper deep branches of the Pacific Meridional Overturning Circulation (L-PMOC and U-PMOC) in the Yap-Mariana Junction (YMJ) channel, a major gateway for deep flow into the western Pacific. On the western side of the YMJ channel, mooring observations in 2017 and in 1997 show the seasonal phase of the L-PMOC at depths of 3,800-4,400 m: strong northward flow with speed exceeding 20 cm s(-1)and lasting from December to next May and weak flow during the following 6 months. On the eastern side of the channel, mooring observations during 2014-2017 show two southward deep flows with broadly seasonal phases, one being the return flow of L-PMOC below similar to 4,000 m and with the same phase of L-PMOC but reduced magnitude. The second, shallower, southward deep flow corresponds to the U-PMOC observed within 3,000-3,800 m and with opposite phase of L-PMOC, that is, strong (weak) southward flow appearing during June-November (December-May). Seasonal variations of the L-PMOC and U-PMOC are accompanied by the seasonal intrusions of the Lower and Upper Circumpolar Waters (LCPW and UCPW) in lower and upper deep layers, which change the isopycnal structure and the deep currents in a way consistent with geostrophic balance.