Investigation Of The Equatorial Current Variability In Western Tropical Pacific During El Ni?o Events Based On Mooring Observations

The tropical western Pacific Ocean is a key sea area that drives the atmospheric circulation and the Asian monsoon.It is also an important area for the development of ENSO(El Nifio and Southern Oscillation).In order to study the sea-air characteristics of the sea area,especially the evolution of the El Ni?o phenomenon,a number of sea-air observation programs have been implemented by the international academic community since the 1980s and 1990s,which has greatly promoted the understanding and forecasting ability of the dynamic mechanism of ENSO.TAO/TRITON buoy arrays,such as those deployed by the United States and Japan,provide large,high-resolution continuous oceanographic observations of the tropical Pacific.However,TAO/TRITON buoy arrays are sparsely observed in the tropical western Pacific,lacking long-term,high-quality subsurface ocean observation data for the study purposes in the tropical western Pacific.The Tropical Western Pacific Scientific Observation Network,established by the Institute of Oceanography,Chinese Academy of Sciences(IOCAS),has deployed more than 20 deep-sea moorings in 130°E,140°E and 142°E sections of the western Pacific Ocean,as well as in the Yap-Mariana Trench connection area.The observation network provides the evolution of ocean temperature,salt and ocean currents associated with ENSO events in recent years,effectively complementing the short-board observa-tions of ENSO events.This work analyzes the upper ocean current observation data of the observation network from 2014 to 2016,focusing on the characteristics and re-lated dynamic processes of the equatorial circulation variation in the western Pacific during the 2015/2016 super El Ni?o event.In-depth research was carried out on the new variability found in the subsurface observations:(1)intraseasonal variation(ISV)of the upper ocean current during the El Ni?o developing phase in 2015;(2)The strong eastward current anomaly of the upper equatorial western Pacific in the decaying phase of the 2016 El Ni?o event.This paper mainly achieved the following results:(1)By analyzing the subsurface observation data and comparing the historical ob-servation data,it is found that there are significant differences in the upper ocean ISV between the 1997-1998 and 2015-2016 El Nifio events.Compared with the 1997-1998 event,the marine ISV intensity of the 2015-2016 event was significantly reduced by approximately 30%-50%in the equatorial Pacific basin,and the maximum ISV moved from 140°E in 1997 to near 160°E in 2015.In particular,the difference in intensity of the 142°E stations in the western Pacific was the most obvious,and the statistical results showed that it was weakened by 43%.The ocean current ISV in the western Pacific has a strong relationship with the forcing of the westerly wind event.The difference in marine ISV also affects the difference in sea surface temperature(SST)through zonal advection.In 1997,the advection of ocean currents was significantly stronger than in 2015,and it contributed significantly to several intraseasonal warmings.These differ-ences can be explained to a large extent by differences in the intensity and location of marine ISVs caused by the onset of intraseasonal westerly winds from different sources.In 2015,the sea surface zonal wind ISV was generally weaker than in 1997.In 1997,the westerly winds originated in the Indian Ocean and occurred near the western bound-ary(about 140°E).The westerly wind event caused by the turbulence in 2015 mainly came from the subtropical northeast Pacific Ocean and caused the maximum value of the upper ocean current ISV to appear near the equator 160°E.These ISV differences may be related to the decadal warming of the subtropical northeastern Pacific during the El Ni?o event during 2015-2016 and the weakening of intraseasonal oscillations in the Indian Ocean atmosphere.The different roles played by marine ISVs in the two El Ni?o events may be one of the factors affecting the diversity of ENSO.(2)By analyzing the subsurface observation data of IOCAS at the equator,142°E station,it is found that an abnormal eastward strong current occurred in the upper layer of the El Ni?o decaying phase in the summer of 2016,and the maximum current veloc-ity reached 67 cm/s.The vertical range extends up to the surface layer,and the surface westward current almost disappears.This phenomenon was also observed by JAM-STEC in the summer of 1998.Further analysis of three sets of subsurface observation data from the Tropical West Pacific Scientific Observation Network during 2014-2016 revealed that the 2°N,140°E stations north of the equator and the 1°S and 142°E sta-tions south of the equator also had significant eastward current enhancements in the summer of 2016,with maximum current speed of 90 cm/s and 63 cm/s,respectively.While the northerly 3°N,143.55°E station experienced a decrease of eastward velocity from 88 cm/s to about 30 cm/s,this indicates that the western Pacific equatorial circu-lation has a complex spatial structure.Further analysis of OSCAR surface current data,GODAS ocean reanalysis data and 1.5-layer nonlinear reduced gravity model results show that there is a cross-equatorial counterclockwise circulation anomaly in the upper ocean of the western boundary of the El Ni?o decaying phase.Therefore,the subsur-face moorings showed an increase of the eastward current velocity on the equator and at the two stations in the north and the south,while the eastern current was observed to be weakened in the north.The results of the 1.5-layer model driven by the wind stress of different El Ni?o phases and regions show that the easterly anomaly wind of the equatorial eastern Pacific during the mature phase of El Ni?o event is the specific time and key area for driving the variation of circulation.Its excited off-equatorial warming Rossby wave passes to the west boundary and then reflects into the equatorial warm Kelvin wave,resulting in two high sea surface height anomalies(SLA)in the equato-rial western Pacific.By changing the distribution of the pressure field,the SLA causes the anomalous westward flow on the equator and the eastward flow anomaly south of the equator,forming a cross-equatorial counterclockwise circulation anomaly.In addi-tion,experiments by idealized models show that the sea-land distribution of the western boundary plays an important role in this counterclockwise circulation.Therefore,the cross-equatorial counterclockwise circulation anomaly over the western boundary of the western Pacific in the El Ni?o decaying phase is the result of the interaction between the equatorial eastern Pacific wind stress of the El Ni?o mature period and the sea-land distribution of the western boundary.In summary,this paper finds two variations of the equatorial circulation in the western Pacific during the 2015-2016 El Ni?o event by analyzing the western Pacific subsurface mooring array,namely(1)During the El Ni?o developing phase in 2015,the intraseasonal variation of the upper ocean current was weaker than that of 1997;(2)The cross-equatorial counterclockwise anomalous circulation of the upper equatorial formation in the western Pacific Ocean during the decaying phase of the El Ni?o event.The phenomenon(1)is caused by the westerly wind events from different sources in two events.The phenomenon(2)is the result of the interaction between the equatorial eastern Pacific wind stress of the El Ni?o mature phase and the sea-land distribution of the western boundary.These results provide a new understanding of the study of ocean circulation variation and related dynamics during ENSO,laying the foundation for the subsequent study of the western Pacific ocean circulation structure and multi-scale vari-ation,highlighting the high-resolution upper ocean continuous subsurface observations in the circulation and structural variation and importance in the evolution of ENSO events.