Features Of The Deep Circulation At The Yap-Mariana Junction

The meridional overtuning circulation plays an important role in global climate system as vast amounts of heat are redistributed meridionally and vertically.In contrast to upper ocean circluations,measurements in deep ocean are still sparse and synoptic,and hence there remains large gaps in our knowledge of deep ocean circulations.Recently,Chinese Academy of Sciences has built a scientific observing network in the western Pacific,comprising several subsurface moorings to continuously measure the full-depth ocean parameters.These data provide us a good opportunity to study the deep ocean circulations in this region.In this study,using the mooring observation data,observed conductive-temperature-depth(CTD)-lowered acoustic Doppler current profiler(LADCP)data,satellite data,and multiple sets of global ocean model outputs,we aim to study the structure and variability of Pacific Meridional Overturning Circulation(PMOC).The PMOC is less observed compared with the main limb of the global meridional overturning circulation in the Atlantic.However,the North Pacific possesses a long residence time and enormous volume as storage of heat and carbon,attracting more attentions than before from oceanography communite.We first study the characteristics of deep ocean circulations in the tropical western Pacific based on six ocean model outputs.The deep currents between 1000and 3000 m are dominated by alternating westward and eastward zonal jets.At depth below 3000 m,deep currents become discontinuous in the zonal direction due to topograohic barriers,and the watermass can be exchanged through the choke channels.Both the transport across these deep channels and current patter within deep basins show seasonal variation.Above analyses on the model outputs provide a reference for the following observational studies.The deep channel at the Yap-Mariana Junction(YMJ)is the major gateway for the PMOC flowing into the western Pacific,and thus we have made extentive observations in this region.We first present the spatial and seasonal variations of the lower and upper branches of PMOC(L-PMOC and U-PMOC)at the YMJ channel.On the western side of YMJ channel,mooring observations in 2017 and 1997 both revealed seasonal phase of L-PMOC at depths of 3800-4400 m:strong northward flow with speed exceeding 45 cm s^-1 at 4200 m and lasting 6 months from December to next May,and weak flow during the following 6 months.On the eastern side of the channel,mooring observation during 2014-2017 revealed two southward deep flows with broadly seasonal phases:one is the return flow of L-PMOC at deeper layer below?4000 m with the same phase of L-PMOC but reduced magnitude,and the other is the U-PMOC at shallower depth of 3000-3800 m with opposite phase of L-PMOC.Seasonal variations of the L-PMOC and U-PMOC are accompanied by the seasonal intrusions of the LCPW and UCPW in lower and upper deep layers,respectively.With the aid of the CTD/LADCP profiles and the model outputs,we find the seasonal intrusions of the LCPW and UCPW can change the isopycnal structure in the channel,and further influence the deep currents through geostrophic adjustment.Besides the seasonality,the intraseasonal variability of the PMOC is also present in the mooring observations in the YMJ channel.This intraseasonal variability is characterized as the intensified fluctuations with depth at 30-90-day period.Observed isotherm displacement can reach?600 m at 4200 m.Vertical profiles of observed currents reasonably conform to topographic Rossby wave(TRW)features of hyperbolic intensification with depth and highly vertical coherence in phase.A reanalysis product well reproduces observed TRW and is used to study their energy source.Besides the stong surface eddies which can induce the TRW in the deep layer,the energy transferred from the mean flow through barotropic and baroclinic instabilities are deemed to contribute to the generation of the TRW.Considering the favorable conditions of deep channel for TRW,and the relatively rapid teleconnection between Southern Ocean and bottom water warming in the North Pacific through TRW,we expect that TRW may have an important role in the inter-ocean and/or inter-basin exchanges.Two separate mooring observations in 1990s and 2010s show remarkably similar features of PMOC on seasonal and intraseaonal timescales.Using the two separate mooring observations,we further discussed the interdecadal variation of the L-PMOC.The time-mean of depth-integrated L-PMOC on the western side was reduced by 20%compared to the measurement two decades ago,which is slightly smaller but generally comparable to 27%on the western side of the Samoan Passage.No significant warming trend was found in the deep ocean of the western Pacific as compared to the 1990s,which is different from the situation in the Samoan Passage with significant warming.Based on the CTD and LADCP profiles data around the Magellan seamounts,which are located northeast of the YMJ channel,we discussed the path of the PMOC before it flows into the YMJ channel.Two kinds of deep watermass,including UCPW and LCPW,were observed in this region with separate depth at 3800 m.The deep flow around the Magellan seamounts is mainly southward.At the depth deeper than3800 m,the value of the dissolve oxygen around the seamounts is larger than that in YMJ channel.These suggest that the Antarctic origined LCPW first arrive at the Magellan seamounts region and then returns southwestward into the YMJ channel.The mixing features around the Magellan semounts are also studied using the CTD profiles.The value of eddy difusivity enhances below 1500 m,and can reach 10^-4 m²s^-1 in the layer of 500-1000 m above the bottom.The difusivity is relatively larger near the seamount than that on the farther flank of the seamount.