Study On The Seasonal And Interannual Variability Of Currents Off The Papua New Guinea Coast

The South Pacific low-latitude western boundary current mainly includes New Guinea Coastal Current(NGCC)and New Guinea Coastal Undercurrent(NGCUC),which play an important role in regulating the inter-hemispheric water masses redistribution and the interannual regulation of El Ni?o/Southern Oscillation(ENSO).Previous ADCP(Acoustic Doppler Current Profilers)observation data only included velocity above 250 m and the observation period was short,which was insufficient to cover the depth range of NGCUC.The vertical structure and multiscale variability of the current along the Papua New Guinea(PNG)coast are still unclear.In this study,the vertical structure and seasonal-interannual variations of coastal current system along the PNG coast were investigated using mooring observations deployed by the Institute of Oceanology,Chinese Academy of Sciences(IOCAS)during 2015-2019,and combined with eddy-resolving simulations of the Ocean General Circulation Model for the Earth Simulator(OFES)during 1980–2019.ADCP observation well captures the flow structure above 800 m along the PNG coast.In addition to the surface seasonal reversal NGCC and the subsurface stable westward NGCUC,a seasonally reversing current with speed of 10 cm/s is detected below the NGCUC,and was named as the New Guinea Coastal Intermediate Current(NGCIC)in this study.This also suggests that the effects of monsoon wind forcing are not just trapped in the upper ocean,but could reach the depth of 800 m,which is far greater than our previous understanding.Linear continuously stratified model successfully simulated the three-layer structure and seasonal variation of currents along the PNG coast,and found that the combination of the first and second baroclinic modes dominate the vertical structure and seasonal cycle of the currents.This also reflects the response of ocean circulation to low order modes of large-scale wind stress.In addition to significant seasonal variations,ADCP observation also show significant interannual variations of NGCUC.The velocity core of NGCUC shoals and extends to the sea surface during 2015-2016 El Nino,and it deepens to 220 m during La Nina.As the velocity core of NGCUC becomes shallower or deeper,the velocity above 400 m also show corresponding interannual variation,which is closely related to the fluctuation of the equatorial Pacific thermocline during ENSO.In general,OFES outputs reproduce this interannual variation reasonably well,albeit with slight differences in the current velocity and amplitude of interannual variation.In addition,multi-year velocity composite from OFES outputs also confirm that this interannual variation exists throughout El Nino or La Nina events,rather than just the decay period of the El Nino event.The interannual variations of currents along the PNG coast also exihibit significant spatial distribution differences.The mean transport of the currents above 400 m in Solomon Sea through the Vitiaz Strait(9.2 Sv)is relatively larger than that through the Solomon Strait(8.2 Sv),and the transport through George’s Channel(0.1 Sv)is weaker.However,the interannual signal through the Vitiaz Strait is 0.84 Sv,which is weaker than the signal of 1.1 Sv through the Solomon Strait,confirming the constraint of narrow topography in Vitiaz Strait on the interannual signals.After passing through the Vitiaz Strait,NGCUC still carries strong ENSO signals(0.84 Sv),which is about 28%less than the interannual signal of the NGCUC transport before passing through the strait.As NGCUC flows westward,this interannual variation gradually weakens and finally reaches down to 0.65 Sv at the 142°E section The results of transport budget show that the southern branch of SEC merges into NGCUC west of offshore section as the northwestward NGCUC gradually approaches the equator,which enhances the total volume transport but weakens the interannual fluctuationsIn addition,currents on different water mass levels along the PNG coast show different interannual variations during ENSO.The results f the OEFS indicates that the shoaling(deepening)of the NGCUC velocity core is not only related to the thermocline or pycnocline fluctuations associated with ENSO,but also influenced by the shift of velocity core between different isopycnals.During ENSO events,accompanied by the vertical movement of the velocity core of NGCUC on isopycnals,current above the thermocline(SW and UTW)is positively correlated with ENSO,while current below the thermocline is negatively correlated with ENSO.This opposite interannual variations of the currents above and below the thermocline are not just local features around 142°E section,but a part of the large-scale ocean circulation anomalies in the tropical Pacific.The anomalous clockwise and counterclockwise circulation anomalies straddling the equator occur above and below the thermocline during El Ni?o events,and the opposite circulation anomalies appear during La Ni?a events.The results indicates that the interannual signals from North Vanuatu Jet(NVJ)dominates the interannual variation of the total transport into the Solomon Sea,while the contribution of Gulf of Papua Current(GPC)is relatively small.The easterly anomalies in the eastern and western Pacific during the El Nino drive the transport into the Solomon Sea.Meanwhile,while the wind stress near the equator weakens the SEC,thus forming this cross-equatorial clockwise circulation.In addition,we find that the interannual variation of sea surface height(SSH)between the East Sea Basin and the West Sea basin in the Solomon Sea is quite different,and the interannual variation of SSHA in East Sea basin is larger than that in West Sea basin.Specifically,the lower SSH in the East China Sea Basin and the higher SSH in the West Basin produced a westward geostrophic current during El Nino,which enhanced the transport into the Solomon Sea.The opposite variation occurs during La Nina events,and eastward geostrophic current weakens its westward transport.Both the scale wind field from South Pacific and the local wind field in the Solomon Sea affect its interannual variation of transport in Solomon Sea.