Distribution And Variation Of Interior Turbulent Mixing Layer And Weak Shear Layer In The Equatorial Pacific Thermocline

In the ocean,turbulent mixing refers to the mixing process between fluids of different properties.Turbulent mixing has important impacts on water exchange,material and energy transport,and different scales of seawater movement.Therefore,the study of the spatial and temporal distribution of turbulent mixing has always been one of the important contents in physical oceanography research.The latest research shows that due to the coupling of equatorially trapped planetary waves of different dynamical types,temporal periods and baroclinic modes along the equator,some complex layer structures of vertical velocity shear and hence turbulent mixing should frequently occur in the eastern equatorial Pacific thermocline: Interior Turbulent Mixing Layer(ITML)and Weak Shear Layer(WSL).At present,there are still deficiencies in research on this complex vertical velocity shear and mixing structure.Based on this,this paper further studies their spatial and temporal distribution patterns and some characteristics in detail.Based on velocity,temperature and salinity data from Tropical Atmosphere Ocean(TAO)buoy array,this study mainly analyzes the occurrence characteristics of ITML and WSL above the Equatorial Undercurrent(EUC)core at three equatorial sites(140°W,170°W,and 165°E).Specifically,at these three sites,the occurrence probabilities of ITML are about 68%,53%,and 48%,respectively;while those for WSL are about 27%,15%,and 28%.The accuracy and reliability of calculations are verified using Bootstrap method to construct 95% confidence intervals.These results show that there are continuous bursts of turbulent mixing in the thermocline at all three sites.The overall occurrence probability of ITML(WSL)shows obvious and similar biannual(annual)variations at 140°W and 170°W,which is higher in boreal late summer to winter and lower in spring.This seasonal variation confirms that turbulence mixing within ITML is closely related to shear instability of EUC and equatorial waves.Vertically,the ITML are primarily located above the EUC core and prevail in deeper(shallower)layers from late summer to winter(spring).At 165°E,the vertical distribution of ITML occurrence probability is different from that at140°W and 170°W,as it does not show a preference for depths.In order to further understand the mixing intensity in ITML,this paper also studies its thicknesses and durations.At Eq140 W and Eq170 W,ITML thickness is concentrated between 15–55 m with strong vertical mixing;while at Eq165 E,ITML thickness is concentrated between 5–25 m with relatively weak vertical mixing.The power spectral density(PSD)analysis results based on the Welch method(weighted overlap averaging method)show that the intensity of ITML may be related to the number of interacting equatorial waves as well as the energy of equatorial waves at a certain frequency.In addition,the durations of ITML and WSL are short,because one or more equatorial waves that produce them have short periods ranging from a few days to a month.Specifically,most ITML(70%)last for several hours to three days with a few(15%)lasting more than seven days at Eq140 W.These properties,particularly the high occurrence probability,seasonal variability,and short durations demonstrate the persistence of thermocline mixing in the western to eastern equatorial Pacific thermocline and confirm the generation mechanism by persistent equatorial waves.