| Due to the unique geographical location of the western tropical Pacific Ocean,it has a complex three-dimensional ocean currents structure.Surface and subsurface current systems are an important element of the three-dimensional structure,and they play an important role in maintaining the mass conservation,heat balance,and climate prediction of the entire ocean.In the past,observation data was limited,and the study of the western tropical Pacific currents was mainly based on hydrological data(Temperature,salinity,density)to study its seasonal to interannual variability.It is impossible to know the intraseasonal variability of the western Pacific currents.In recent years,direct observation data have found that intraseasonal variability have contributed greatly to variability in the western tropical Pacific currents.Therefore,it is of great significance to study whether the source of energy for intraseasonal variability comes from atmospheric forcing or the ocean's own variability.The first part of the work in this article is aimed at the scientific question:Is there a spatial and temporal difference in the meridional direction of the intraseasonal variability of sea surface height at the same latitude?Utilized the sea level anomaly(SLA)dataset from 1993 to 2017,the overall distribution characteristics of intraseasonal variability(periods:20–120 days)in the sea surface height of the tropical western Pacific Ocean were first analyzed.The intraseasonal signal is the strongest in the region around 20°N(16°–24°N),and there is a seasonal scale change in the intraseasonal variability,which reaches the strongest of the year in June–August,and the weakest of the year corresponds to the December–February.The subsequent analysis of the intraseasonal variability in the sea surface height in the 20°N latitude region revealed that the intraseasonal signal period and propagation speed in the east side of the Luzon Strait are greater than those in the west side of the Luzon Strait,specifically about 70 days on the east side,the propagation speed is about 10.7–12.7cm/s;the corresponding western period is 60 days,and the propagation speed is 6.5–7.8 cm/s.Furthermore,the difference between the intraseasonal variability of the sea surface height near the Luzon Strait and the open ocean is analyzed.There is a quasi-90-day periodic signal in the open ocean,and the calculated propagation speed is about 10.3 cm/s.The intraseasonal signal of sea surface height propagates westward along the latitude from the ocean to the east of the Luzon Strait.The propagation path turns northwestward near the 123°E due to the influence of the Kuroshio.Finally,the wave velocity and theoretical period of the first baroclinic Rossby wave are calculated.It is believed that the intraseasonal variability period and propagation velocity of the sea surface height in the open ocean can be well explained by the theoretical period and wave velocity of the first baroclinic Rossby wave.However,the intraseasonal variability of sea surface height near the Luzon Strait is more related to the role of local background flow field.The other part of this article is aimed at the scientific question that the North Equatorial Current(NEC)transport was mostly estimated based on model data and hydrological data combines geostrophic equations.For the directly calculating the NEC transport volume using the mooring observation data,previous scholars have not dealt with it,and there is little work to further analyze the intraseasonal variability characteristics of the transport of NEC.In addition,few scholars have mentioned the research work on the intraseasonal variation of the North Equatorial Undercurrent(NEUC)transport using the mooring observation data.This part of the work is based on the one-year observation data from five moorings at the 130°E section to expand the intraseasonal variability transport by NEC and NEUC,and combines the two sets of climatology temperature-salinity data of RG-Argo and WOA18,and OFES model data.Firstly,the mean structures of NEC and NEUC observed by the mooring observation were analyzed,and it was verified that the NEUC proposed by previous scholars has a structure of three jets.The three cores are located at 8.5°N(?500 m),12.5°N(?700m)and 17.5°N(?900 m),with the middle core(12.5°N)being the strongest.It is obvious that as the latitude increases,the depth of the three cores gradually deepens.Subsequently,the volume transport of NEC estimated from the mooring observations was 52(±14)Sv(1Sv?106m3s-1),with a maximum of 83 Sv,and the direction is westward;the corresponding NEUC volume transport was 18(±13)Sv,with a maximum of 61 Sv,and the direction is eastward.The power spectrum analysis shows that the NEC and NEUC transport have a common intraseasonal variability signal of quasi-40-day.Furthermore,EOF analysis shows that the variability of the two has a similar vertical spatial distribution,and the transport of the two has the consistent phase change.That is,when the transport of NEC westward weakens,the transport of the NEUC eastward increases,and vice versa.The variation of th NEC is vertically coherent with NEUC.In the study,two sets of climatology temperature-salinity data were used to calculate the zonal geostrophic flow,and the calculated results based on the RG-Argo data were more consistent with the mooring observation.In addition,QSCAT-OFES model data is used to simulate the observation section.The zonal velocity structure is basically consistent with the results of mooring observations.This paper also calculated the transport of the NEC and NEUC by model data,and the results shown that there is a quasi-40-day intraseasonal variability signal.In this paper,we also calculated the 130°E section observed by the altimeter and transported the sea surface westward between the latitudinal bands of 8.5°–17.5°N.The results also have a quasi-40-day intraseasonal variability signal.Finally,considering the intraseasonal variation of the two currents and the intraseasonal variation of sea surface height as a whole,through composite analysis with the signal of MJO,it is believed that the quasi-40-day variability of NEC and NEUC transport may be related to the conversion between different phases of MJO.The specific process needs further research and analysis in future work. |
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