Abstract
The lower deep branch of the Pacific Meridional Overturning Circulation (L-PMOC) is responsible for the deep-water transport from Antarctic to the North Pacific and is a key ingredient in the regulation of global climate through its influence on the storage and residence time of heat and carbon. At the Pacific Yap-Mariana Junction (YMJ), a major gateway for deep-water flowing into the Western Pacific Ocean, we deployed five moorings from 2018 to 2019 in the Eastern, Southern, and Northern Channels in order to explore the pathways and variability of L-PMOC. We have identified three main patterns for L-PMOC pathways. In Pattern 1, the L-PMOC intrudes into the YMJ from the East Mariana Basin (EMB) through the Eastern Channel and then flows northward into the West Mariana Basin (WMB) through the Northern Channel and southward into the West Caroline Basin (WCB) through the Southern Channel. In Pattern 2, the L-PMOC intrudes into the YMJ from both the WCB and the EMB and then flows into the WMB. In Pattern 3, the L-PMOC comes from the WCB and then flows into the EMB and WMB. The volume transports of L-PMOC through the Eastern, Southern, and Northern Channels all exhibit seasonality. During November–April (May–October), the flow pathway conforms to Pattern 1 (Patterns 2 and 3), and the mean and standard deviation of L-PMOC transports are −4.44 ± 1.26 (−0.30 ± 1.47), −0.96 ± 1.13 (1.75 ± 1.49), and 1.49 ± 1.31 (1.07 ± 1.10) Sv in the Eastern, Southern, and Northern Channels, respectively. Further analysis of numerical ocean modeling results demonstrates that L-PMOC transport at the YMJ is forced by a deep pressure gradient between two adjacent basins, which is mainly determined by the sea surface height (SSH) and water masses in the upper 2,000-m layer. The seasonal variability of L-PMOC transport is attributed to local Ekman pumping and westward-propagating Rossby waves. The L-PMOC transport greater than 3,500 m is closely linked to the wind forcing and the upper ocean processes.
Highlights
The deep and bottom waters of the North Pacific Ocean have their origins in Antarctic and are carried by the northward lower deep branch of the Pacific Meridional Overturning Circulation (LPMOC), referred to as the Pacific Deep Western Boundary Current
At the Pacific Yap-Mariana Junction (YMJ), a major gateway for L-PMOC flowing into the Western Pacific Ocean (WPO), five subsurface moorings were deployed in three deep channels to reveal the pathways, volume transport, and seasonal variability of L-PMOC
The pathways of L-PMOC at the YMJ can be cataloged into three patterns
Summary
The deep and bottom waters of the North Pacific Ocean have their origins in Antarctic and are carried by the northward lower deep branch of the Pacific Meridional Overturning Circulation (LPMOC), referred to as the Pacific Deep Western Boundary Current. In the Northern Channel, the MO model performs well during most of the measurement period but fails in reproducing the northward transport from mid-September to mid-November, leading to a low correlation Despite these discrepancies, the similarity between the MO model and the observations in terms of the seasonal variation of the L-PMOC encourages further examination of the MO results to identify the seasonal variation of the L-PMOC over the whole YMJ region and over a longer time scale, and to explore the underlying mechanism. The “climatological” seasonal signals are obtained by applying the annual harmonic fit to each volume transport time series, and the fitted annual cycle explains 93, 94, and 80% of the total variance in the Eastern, Southern, and Northern Channels, respectively This provides further evidence of the seasonal variability of the L-PMOC at the YMJ. The L-PMOC transport greater than 3,500 m is closely linked to the wind forcing and the upper ocean processes at depths less than 2,000 m
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