| The North Equatorial countercurrent(NECC)in the Pacific Ocean is an important current in the equatorial current system of the tropical Pacific.It transports water from the western Pacific warm pool eastward to the eastern Pacific and plays an important role in the occurrence and development of the El Ni(?)o Southern Oscillation(ENSO).Previous studies have paid more attention to the structural characteristics and variation of NECC in the inner region,but due to the limitation of observation,the understanding of its three-dimensional structure and its variation mechanism is still very limited.As the nascent NECC is close to the western boundary and is the confluence of water masses in the South and North Pacific Ocean,it is rich in fishery resources,and it is also the source of Indonesian Throughflow(ITF).Understanding its three-dimensional structure and multi-scale variation characteristics and mechanism is of great significance for understanding local ocean circulation and water mass variability,air-sea interaction,bio-ecological effects,and even global ocean circulation and climate change.Based on the field observation data obtained in the Western Pacific in recent years,combined with satellite remote sensing data,reanalysis data,and high-resolution model data,this paper makes a systematic analysis on the variation and mechanism of nascent NECC in different time scales,focuses on the variation characteristics and energy conversion mechanism of NECC during strong El Ni(?)o events,its seasonal variation,and the impact of nascent NECC on regional marine primary productivity on the scale of interannual variation.At first,a spatially varying non-stationary gravest empirical mode(GEM)field is constructed by incorporating the calendar month as an extra parameter to infer the subsurface thermohaline structure from the Pressure-sensor equipped Inverted Echo Sounder(PIES)observations in the western equatorial Pacific Ocean.This monthly GEM(MGEM)overcomes the application limitation of traditional GEM in the equatorial ocean due to the poor vertical coherence thermohaline structure there.The MGEM can capture 77%of the thermohaline variance for the upper 500 dbar,which is only 62%by the traditional GEM method.The most significant improvement of MGEM is for the deep ocean between 500-1900 dbar,where only 12%of the thermohaline variance can be captured by the GEM,but improved to 44%by the MGEM method.Based on the MGEM,a 25-month volume transport time series is estimated from observations by three PIESs deployed across the nascent NECC at 4°N,6°N,and 8°N along the 130°E transect during 2014-2016.Based on long-term PIES observation data and hydrological observations,combined with altimeter data,Argo data,and reanalysis data,we studied the structure and changes of nascent NECC of the Western Pacific from 2014 to 2016.The nascent NECC is strengthened during the developing phase of El Ni(?)os,and is weakened during La Ni(?)as.The NECC shifts～1 degree southward and intensifies significantly with its transport exceeding 40 Sv(1 Sv=10~6m~3/s),nearly double its climatology value,during the developing phase of the 2015/16 El Ni(?)o event.A comparative analysis with the historical observations of the 137°E section of the Western Pacific revealed that the2015/16 El Ni(?)o exerts a much stronger impact on the nascent NECC compared to that of the 1997/98 event,which even obscures the NECC’s seasonality.Based on a 2.5-layer reduced gravity model,we further analyzed and found that baroclinic instability was an important energy source for eddy kinetic energy(EKE)in the nascent NECC region during the 2015/16 El Ni(?)o event.The enhanced vertical shear and the reduced density jump between the NECC layer and the subsurface North Equatorial Subsurface Current(NESC)layer renders the NECC–NESC system to be baroclinically unstable in the western Pacific Ocean during El Ni(?)o developing phase.During the 2015/16 El Ni(?)o,the NECC is a baroclinically unstable jet at its birthplace,and the baroclinic instability provides the main energy source for the mesoscale eddy field,which differs from the traditional understanding of the energy source of EKE in low latitudes ocean.Next,this paper reconstructs the seasonal cycle of nascent NECC based on PIES observations combined with satellite altimeter data.The nascent NECC between 4°N-6°N was found to have quite strong seasonal variations with the strongest transport of23 Sv occurring in February and the weakest transport of 11 Sv occurring in September.Under the influence of Mindanao eddy(ME)and Hamahera eddy(HE),the north-south position and intensity of the NECC flow axis in the source region are not completely consistent.The half-yearly periodic variation of nascent NECC is caused by the inconsistency between the complex characteristics of eddy variation and the variation of thermocline depth and thermocline depth gradient.The change of the depth of the regional thermocline and its meridional gradient controls the change of NECC through the adjustment of geostrophic balance.Finally,on the basis of the above research,this paper makes a preliminary discussion on the impact of NECC on regional marine primary productivity on the scale of interannual variation.It is found that the change of advection transport in the tropical western Pacific during the El Ni(?)o events changes the regional water mass distribution,and the advection effect of NECC plays an important role in the interannual variation of sea surface chlorophyll. |
