The Variability And Mechanism Of Subsurface And Intermediate Currents In The Western Tropical Pacific Ocean

The Western Tropical Pacific Ocean(WTPO)is one of the most abundant areas with warming water and has a significant impact on both of local and global climate systems.Besides the current system in the upper layer,the subsurface and intermediate currents also play important roles in the entire circulation system,which are closely related to large-scale phenomena such as El Nino-Southern Oscillation(ENSO)and Pacific Decadal Oscillation(PDO).Moreover,the subsurface and intermediate currents can drive the east-west transport process of nutrients below the surface layer of the Pacific Ocean,and promote the carbon cycle and biological-ecological environment interaction between the east and west Pacific Ocean.However,at present,the observation measurements of ocean currents at depths below the surface layer are limited and impossible to make long-term and high-quality continuous observation.The Scientific Observing Network of the Chinese Academy of Sciences(CASSON)in the WTPO,established and maintained by the Institute of Oceanology,Chinese Academy of Sciences(IOCAS),makes up for this shortcoming and provides a good observation basis for studies of the subsurface and intermediate currents.In this paper,based on the mooring observations,the variability and mechanism of subsurface and intermediate currents in the WTPO are studied.The specific results are as follows:1)Intermediate-depth(below 1000 m)intraseasonal variability(ISV)at 20-90-day period in the western tropical Pacific can be interpreted in terms of equatorial short Rossby waves modified by the tilted southern boundary.Seven moorings were deployed between 0° and 7.5°N along 142°E from September 2014 to October 2015.The strongest ISV energy at 1200 m occurs at 4.5°N.Similar pattern is also seen in an eddy-resolving global circulation model.An analysis of the model output identifies that the ISV propagates as equatorial short Rossby waves with westward phase speed and southeastward and downward group velocity.Additionally,it is shown that a superposition of first three baroclinic modes is required to represent the ISV energy propagation.Further analysis using a 1.5-layer shallow water model suggests that the first meridional mode Rossby wave accounts for the specific meridional distribution of ISV in the western Pacific.The same model suggests that the tilted coastlines of Irian Jaya and Papua New Guinea,which lie to the south of the moorings,shift the location of the northern peak of ISV from 3°N to near 4.5°N.The tilt of this boundary with respect to a purely zonal alignment therefore needs to be taken into account to explain this meridional shift of the peak.Calculation of the barotropic conversion rate indicates that the intraseasonal kinetic energy below 1000 m can be transferred into the mean flows,suggesting a possible forcing mechanism for intermediate-depth zonal jets.2)Based on the analysis of the continuous mooring observation data at 4.7°N and140°E for six years,the multi time scale variability of North Equatorial Subsurface Current(NESC)and its response to ESNO event was studied.It was found that the intensity of the semiannual periodic variability of the NESC after 2016 ENSO event was significantly weakened(reduced by 67%).The ISV energy increased with the presence of strong eddy,and remained intensity thereafter.Based on the reanalysis data and the results of Mercator-Ocean model and Linearly Continuously Stratified(LCS)model,it is preliminarily speculated that the semiannual variability of NESC is dominated by the second meridional mode equatorial long Rossby wave,propagating westward and downward.Moreover,based on the surface wind field data,it is further verified that the energy of semiannual variability of wind stress in the sensitive area near 160°E also shows significant weakening after 2016 ENSO event.This phenomenon is related to the westerly wind burst event with ENSO.The intensity of westerly wind in strong ENSO event is stronger,and the range of westerly wind extends farther to the east.Therefore,the westerly wind in strong ENSO event is more likely to affect the wind field in sensitive area.The results of correlation analysis of ISV between NESC and seasurface currents suggest similar speculation to the previous chapter that ISV energy comes from the upper layer in the northwest.3)A new topographic rossby wave(TRW)covering ridge is derived theoretically.Although there have been many previous studies on the TRW,they all have been limited to the situation of single-sided sloping topography.Considering the topography of Eauripik rise between west and east Caroline Basin,utilizing a simplified homogenous,f-plane,shallow water model,we got a new form of TRW through the conservation of potential vorticity equation.The zonal distributions of amplitude are different between two sides of rise.The exponential distribution is on the right side of direction of phase velocity and oscillation form is on the left side.In view of the parameters of Eauripik rise,it is found that the period of this new TRW is generally greater than 69 days.