Roles Of Equatorial Pacific Ocean Processes In ENSO And Its Variability

El Ni?o-Southern Oscillation(ENSO)is the most prominent interannual signal in the global climate system,which has severely impacts on Pacific and even global climate system.Although lots of efforts have been made to ENSO study for half a century,its low-frequency variability and changes under climate changes keep challenging our current understanding.Understanding of ENSO dynamics is a basis of correct prediction of ENSO,but many important issues remain in ENSO dynamics to date.In this study,we constructed a novel temperature variance equation,andapplied it into ENSO dynamics study for the first time.Through heat budget analysis based on temperature equation and temperature variance equations in key Ni?o regions,we investigated quantificationally the role of oceanic feedbacks in ENSO sea surface temperature(SST)variability shift at the beginning of 21 st century,ENSO asymmetry and 2014–2016 ENSO warm events.Besides,our multi-model product based approach provides a robust assessment of dominant mechanisms that account for these ENSO variabilities in key Ni?o regions.Main results are shown below.(1)SST variability associated with ENSO shows zonal shift at the beginning of 21 st century.ENSO-related SST variability during 2000s(period P2)weakened significantly in the equatorial eastern Pacific but slightly increased in the central Pacific Ocean relative to 1980–1999(period P1).Meanwhile,greater prominence of central Pacific(CP)El Ni?o events is clear during P2 relative to P1 when eastern Pacific(EP)events are dominant.Temperature and temperature variance budgets are examined in the mixed layer of the Ni?o3(5°S–5°N,150°W–90°W)and Ni?o4(5°S–5°N,160°E–150°W)regions from seven reanalysis datasets and model products.Results show that,the thermocline feedback(TCF)and zonal advective feedback(ZAF)are the most important positive feedbacks for generating ENSO SST variance,while the thermodynamic damping(TD)from sea surface heat flux is the largest negative feedback for damping ENSO variance.As more CP El Ni?os after 2000,TCF experienced a substantial reduction while ZAF were less affected.Negative feedbacks likewise weakened after 2000,particularly TD in the Ni?o3 region and the nonlinear sink of variance in both Ni?o3 and Ni?o4 regions.Compared to P1,the decreased ENSO SST variability in Ni?o3 in the 2000 s,is attributed to the significant decease in TCF,while the decease of negative NL contributes to the slight increase of ENSO SST variability in the Ni?o4 region.The role of thermocline feedbacks in the recharge oscillator theory is also examined using the temperature variance equation.TCF is divided into two parts: one related to the zonal-gradient thermocline variability which contributes to the ENSO SST growth as a part of Bjerknes positive feedback;The other related to zonal-mean thermocline variability which reveals the recharge/ discharge oscillator and contributes to ENSO phase transitions.The decrease in the latter in the 2000 s reveals the weakening of recharge/ discharge processes.(2)Based on temperature variance budget,we diagnosed oceanic roles in these ENSO asymmetric features from three aspects: the amplitude of anomalous temperature during the mature phase of El Ni?o events is larger than La Ni?a in the Ni?o3 region;In the Ni?o4 region,the amplitude in mature La Ni?a is larger than El Ni?o;La Ni?a events basically last longer than El Ni?os.In the Ni?o3 region,the larger temperature amplitude of El Ni?o than La Ni?a is attributed to a greater growth rate during El Ni?o development phase than La Ni?a events.Stronger positive oceanic feedbacks are found during El Ni?o development phase with the most contribution from ZAF,following by Ekman feedback(EKF)and TCF.Difference of the negative feedbacks between the El Ni?o and La Ni?acevents is mostly from the damping effects of TD.However,in Ni?o4,La Ni?a has greater growth rate than El Ni?o,which is induced by larger effects of positive TCF in its developing phase while ZAF contributes equally to developing these two events.ENSO asymmetry in duriation in the Ni?o3 region,is because the decay rate in La Ni?a events is smaller than that of El Ni?o events by 0.2 °C2 mon-1.Strong positive TCF still persists during the decaying of La Ni?a,which slows down the fade of the cold events.It is shown that the difference of TCF between El Ni?o and La Ni?a decaying phases is induced by the asymmetry of discharge/ recharge processes between the equator and off-equator.(3)Why did the predicted “super El Ni?o” fade out in the summer of 2014 and the following 2015 developed into one of the three strongest El Ni?o on record? Though there are already some hypothesis and studies,a systematic analysis from the perspective of quantitative analysis of oceanic feedbacks' contribution is still absent yet.Using ECCO2 simulation outputs and satellite-based observations,we diagnosed oceanic processes in the evolution of 2014–2016 events,by executing temperature budgets in the Ni?o3,Ni?o3.4 and Ni?o4 regions.Driven by the westley wind busts(WWBs)in the spring of 2014,largest temperature anomalies appeared in the Ni?o3 region,close to 1°C in May,but suddently declined in June–July.From the perspective of temperature budget,the mixed layer temperature(MLT)anomaly shows positive tendency during March–May with a large peak of about 0.8 °C mon-1,and tends negative in mid-May.EKF induced by anomalous downwelling is the dominant process in leading this warming,with a peak of 0.4°C mon-1.ZAF by the anomalous eastward current at the west boundary contributes secondly(about 0.25°C mon-1).TCF is relatively smaller and lags behind the MLT tendency.EKF also plays a significant role in its decay,which is caused by anomalous upwelling beginning in April.This upwelling brings relatively colder water into the mixed layer and thus contributes to the decay of warm MLT.ZAF also helps kill the warm MLT as a cold advection induced by westward current anomalies in June.Strong WWBs appeared almost throughout the whole year 2015,in the west Pacific during the first half year and moved eastward to the dateline afterward.Under intense air-sea interactions,warm SSTs show up in the mid-east Pacific with a peak about 4°C in November while cold SSTs appear in the west.Based on temperature budget in the Ni?o3 region,there is a positive MLT tendency in boreal spring peaking at 0.45°C mon-1 ZAF and TCF play significant roles in this warming tendency as positive feedbacks.In the west Ni?o3.4 and Ni?o4 regions,the role of TCF is weakened and thus ZAF becomes the dominant positive feedback.