Water Sources Of The Pacific Equatorial Undercurrent And Their Main Pathways

Equatorial Undercurrent(EUC)is a strong eastward current that exit in the subsurface of the tropical Pacific.It constituts the main part of the thermocline alone the equator and it is also the main water source for the upwelling in cold tounge region.Therefore,EUC play an important role in modulating the variation in equatorial Pacific,no matter in the surface or the subsurface.Besides,EUC is the main branch of several meridional overturning circulations on the equator.These meridional overturning circulations include the Subtropical Cells(STCs)and Tropical Cells(TCs)in each hemisphere and the interocean circulation(IOC)connecting the Pacific and Indian ocean.Water masses that formed in high and mid-latitude regions could be transported into the tropical region by STCs and IOC,and eventually converge within the EUC.So it is of great significance to have a comprehensive understanding of the water sources of the EUC and their equatorward pathways which can help us to learn more about the interaction between the tropical region and extratropical region in Pacific.An adjoint passive tracer of the consortium Estimating the Circulation and Climate of the Ocean(ECCO)is used to determine the water sources of the Pacific Ocean Equatorial Undercurrent(EUC)in a global circulation model.The primary water sources of the EUC are identified,as well as the transit time and pathways of these water sources.The results shows that,70% of the EUC in central Pacific is constitute of water sources that originated from low latitude regions,including tropical region and eastern subtropical regions in both hemispheres.Particularly,water masses originated fromt eastern subtropical region contribute 50% of the EUC transport,and the amount of water from the south is more than 3 times larger than that from the north.The majority of tropical water has a transit time less than 1 year.60% of the extratropical water has a transit time less than 20 years,among which most is originated from eastern subtropical regions.Although the main part of the EUC is constitude by extratropical water,the percent of tropical water and extratropical water within the EUC changes slightly from west to east.In western Pacific,the percent of extratropical water is relatively high;from western Pacific to central Pacific,the percent of extratropical water slightly reduced;from central Pacific to eastern Pacific,the percent of extratropical water and tropical water in the EUC remain basically the same.The contribution of the western boundary pathways and the interior pathways to the EUC changes from west to east as well.In general,the western boundary pathways is dominate in the western part of the EUC,while the interior pathways is dominate in the eastern part of the EUC.In the transportation of extratropical water from the formation region to the EUC,western boundary pathways are more important than the interior pathways,while the relative importance is more significant in the western Pacific.The tropical water is converged into the EUC mainly through the interior pathways.As shown by the results,at least 60% of the tropical water is transported via the TCs instead of local vertical processes.And the tropical water transport via the TC in north side of the equator is more than that in the south.At last,the distribution of EUC water sources against density,temperature and salinity in different period shows that the mixing effect within the tropical region is no weak than that in the extratropical region.To enhance the understanding of the interaction between the North Pacific Subtropical region and the tropical region,one water source of the EUC that originated from the subtropical North Pacific are investigated in this study.NPTW is one of the water sources of the EUC that originated from the eastern subtropical region in North Pacific,and it is characterized as a subsurface salinity maximum.The subduction and equatorward pathways of NPTW are investigated using a simulated passive tracer of ECCO.The results demonstrate that the the main subduction region of NPTW lies in the eastern part of the sea surface salinity maximum region.After subduction,the main body of NPTW first spreads westward in the North Equatorial Current.Then it splits into two branches.One flows northward in the Kuroshio upon reaching the western boundary,and the other enters the tropical Pacific either via its western boundary pathway(WBP)or interior pathway(IP).Less than half of the transport through the western boundary pathway can eventually reach the central and eastern Pacific by the Equatorial Undercurrent,while the rest is seen to flow into the Indian Ocean by the Indonesian Throughflow.As a result,the interior pathway is found to play a significant role in equatorward transport of the NPTW.About 30% of the NPTW that reached the equatorial Pacific is transported through the interior pathway.However,the percent of NPTW that transport through the interior pathway is still weaker than its counterpart,the South Pacific Tropical Water(SPTW).Also due to the bypass of the ITF,the percent of NPTW(18%)that could finally upwell into the mixed layer of central and eastern tropical Pacific is also less than the SPTW(30%).These result clearly show the subduction and transport of the NPTW,clarify the strong influence of ITF on equatorward transport of the NPTW and the differences between the NPTW and SPTW,and lay a foundation for further studies about the influence of NPTW and SPTW to the equatorial region.