| The western equatorial Pacific is a crossroads where water masses in the northern and southern hemispheres meet.It has the complex structure of three-dimensional circulation and contains multiscale processes of different dynamic motions.Changes in the upper ocean heat content and ocean circulation in the western equatorial Pacific are closely related to the zonal displacement of the Western Pacific warm pool and related air-sea interactions,and thus to the global climate change,especially the El Ni（?）o and Southern Oscillation（ENSO）.Diapycnal mixing plays a key role in the above processes by redistributing heat,salinity,and momentum.Therefore,understanding the spatial and temporal variability of diapycnal mixing in the western equatorial Pacific and its driving mechanisms is an important goal of ocean and climate research.However,due to the limited direct observation of turbulence in this region,the characteristics and mechanisms of its occurrence and variation are still not fully understood.Therefore,more observations are needed to define the features and changes of mixing in the equatorial western Pacific.Based on direct turbulence observational data from a vertical microstructure profiler（VMP）,as well as mooring data from the TAO/TRITON and the Scientific Observation Network of the Chinese Academy of Sciences in the Western Pacific Ocean,this paper confirms the existence of strong turbulence and mixing in the equatorial western Pacific,and analyzes its mechanism and similarities and differences with similar phenomena in the eastern equatorial Pacific.The spatial difference and interannual variation of the ratio of shear instability in the thermocline are obtained,which enriches the understanding of the upper ocean turbulent mixing process in the western equatorial Pacific.The vertical gradient of thermohaline in the thermocline is large,and if accompanied by relatively strong mixing process,a large vertical thermohaline flux will be generated,which plays an important potential role in the thermohaline redistribution of the upper ocean and related climate change processes.Based on a comprehensive observation data set of turbulent mixing in the equatorial western Pacific,this article provides direct evidence of strong turbulence and mixing processes in the thermocline and reveals their characteristics and dynamical mechanisms.In the mixed layer,diapycnal mixing is mainly related to the wind forcing.With increasing westerly wind speed,the squared buoyancy frequency weakened and the shear instability with Ri<0.25 occurred,resulting in the strong turbulence and diapycnal mixing,and the deepening mixed layer thickness from 20 m to 60 m.The“Deep Cycle Turbulence（DCT）”occurs 20-30 m below the mixed layer,withε～O(10-8-10-7)W kg-1 and Kρ～O(10-4)m2 s-1,respectively.This strong turbulence resembles the DCT in the eastern equatorial Pacific in their occurring depth and their corresponding values of Ri andε.Their difference is embodied in the different contributors to wind-related shear.In the eastern equatorial Pacific,the shear comes from the combined sources of zonal velocity differences within the Equatorial Undercurrent（EUC）and between the South Equatorial Current（SEC）and the EUC.The EUC core in the western part is located at a depth of～200 m,deeper than that in the eastern part.Thus,the shear within the EUC and at the interface layer between EUC and SEC contributes little to the DCT.The shear comes from the vertical difference of zonal velocity between the SEC and the Westerly Wind Event induced eastward flow above.Below the DCT,turbulence and mixing are generally weak,ε～O(10-10-10-9)W kg-1 and Kρ～O(10-7-10-6)m2 s-1,a prototype of the weak mixing nature of the low latitude western Pacific.In addition,the intrusion of South Pacific Tropical water（SPTW）is an important feature of the western equatorial Pacific water mass.Previous studies have shown that SPTW intrusion weakens stratification and enhances mixing.In this paper,the analysis shows that the shear environment of SPTW is relatively weak,which fails to trigger shear instability and enhance mixing.The real cause of strong mixing may be due to enhanced turbulence generation through thermohaline-shear instabilities.The observed turbulence below the deep cycle turbulence layer can be satisfactorily scaled by either the Mac Kinnon-Gregg model or the Richardson number-based model with tuned model parameters.By analyzing velocity,temperature,and salinity data of three representative stations from the TAO/TRITON array and the Scientific Observation Network of the Chinese Academy of Sciences in the Western Pacific Ocean with relatively long time scales,We found that strong shear variance and shear instability exist in the thermocline of the western,central and eastern part of the western equatorial Pacific,but with spatial variations in their occurrence ratio and intensity.The strong shear in the thermocline is mainly distributed at the interface between the surface eastward flow and the SEC（upper interface）and the interface between the SEC and the EUC（lower interface）.Under weaker stratification conditions,the occurrence ratio of shear instability at the upper interface is higher than that at the lower interface.The occurrence ratio of shear instability at the upper and lower interface is significantly different in different regions of the western equatorial Pacific.We found that occurrence ratio of shear instability are more frequent in the eastern region than the western at the upper interface.The shear variance is the weakest and stratification is strongest in the western region.And the central and eastern regions have similar shear variance,but the stratification is weaker in the eastern region,which facilitates shear instability occurring.The occurrence ratio of shear instability are least frequent in the western region and comparable in the central and eastern regions at the lower interface.The stratification are similar at lower interface across the western,central,and eastern region,but the shear variance is the smallest in the western region,resulting in the lowest shear instability occurrence ratio.We also investigated the relationship between the occurrence ratio of shear instability and ENSO.We discovered that the occurrence ratio of shear instability are negatively correlated with Ni（?）o Index at the upper interface in the western region,the upper and lower interfaces in the central region,and the lower interface in the eastern region.The occurrence ratio of shear instability are positively correlated with Ni（?）o Index at the upper interface in the eastern region.There is no correlation between occurrence ratio of shear instability and Ni（?）o Index at the lower interface in the western region.For the negatively correlated interfaces,during La Ni（?）a periods,the upper interface and lower interface have high occurrence ratio of shear instability due to reduced stratification and enhanced shear variance produced by SEC and EUC,respectively.For the upper interface in the eastern region,during El Ni（?）o periods,the surface eastward flow driven by westerly wind generates strong shear with westward SEC,which is four times that of La Ni（?）a periods,causing shear instability. |
