Impact of Eastern and Central Pacific El Niño on Lower Tropospheric Ozone in China

Tropospheric ozone is an essential atmospheric component as it plays a significant role in influencing radiation equilibrium and ecological health. It is affected not only by anthropogenic activities but also by natural climate variabilities. Here we examine the tropospheric ozone change in China associated with the Eastern Pacific (EP) and Central Pacific (CP) El Niño using satellite observations from 2007 to 2017 and GEOS-Chem simulations from 1980 to 2017. GEOS-Chem simulations reasonably reproduce the satellite-retrieved lower tropospheric ozone (LTO) changes despite a slight underestimation. Results show that El Niño generally exerts negative impacts on LTO concentration in China, except for southeastern China during the pre-CP El Niño autumn and post-EP El Niño summer. The budget analysis further indicates that for both events, LTO changes are dominated by the transport process controlled by circulation patterns and the chemical process influenced by local meteorological anomalies associated with El Niño, especially the solar radiation and relative humidity changes. The differences between EP and CP-induced LTO changes mostly lie in southern China. The different strengths, positions, and duration of western North Pacific anomalous anticyclone (WNPAC) induced by tropical warming are likely responsible for the different EP and CP LTO changes. During the post-EP El Niño summer, the Indian ocean capacitor also plays an important role in controlling LTO changes over southern China.

Change of East-Asian Summer Precipitation Associated With Strong El Niño Under the Future Emission Scenarios

Strong Eastern-Pacific type El Niño (EP-El Niño) events have significant impacts on the decaying-summer precipitation over East Asia (EA). It has been demonstrated that frequency of strong EP-El Niños will increase and associated precipitation will become more severe and complex under future high emission scenarios. In this study, using simulations of CMIP5 and CMIP6, changes of the summer precipitation pattern related to strong EP-El Niño during its decay phase and the possible mechanism as responding to high emission scenarios are examined. Precipitation anomaly patterns over EA of strong EP events show a large inter-model spread in historical simulations between the CMIP models where CMIP6 is not superior to CMIP5. Under high emission scenarios, changes of summer precipitation anomalies related to strong EP events tend to increase over the southern EA and decrease around the northern EA from CMIP5, while there is an overall increase in the whole EA from CMIP6. The common change is featured by the increase of precipitation over southeastern China under high emission scenarios. This could be mainly attributed to the anticyclonic circulation from the South China Sea to the western North Pacific as a delayed response to more frequent strong EP-El Niños, which favors an increase in water vapor fluxes converging into the southeastern China.

The North Equatorial Countercurrent East of the Dateline, Its Variations, and Its Relationship to the El Niño Event

Using forty years (1978–2017) of Ocean Reanalysis System 4 (ORAS4) dataset, the purpose of this study is to investigate the fluctuation of the North Equatorial Countercurrent (NECC) to the east of the dateline in relation to the presence of three kinds of El Niño events. From spring (MAM) through summer (JJA), we found that the NECC was stronger during the Eastern Pacific El Niño (EP El Niño) and the MIX El Niño than during the Central Pacific El Niño (CP El Niño). When it comes to winter (DJF), on the other hand, the NECC was stronger during the CP and MIX El Niño and weaker during the EP El Niño. This NECC variability was affected by the fluctuations of thermocline depth near the equatorial Pacific. Moreover, we also found that the seasonal southward shift of the NECC occurred between winter and spring, but the shift was absent during the CP and MIX El Niño events. This meridional shift was strongly affected by the local wind stress.

Water vapor anomaly over the tropical western Pacific in El Niño winters from radiosonde and satellite observations and ERA5 reanalysis data

Using radiosonde observations at five stations in the tropical western Pacific and reanalysis data for the 15 years from 2005 to 2019, we report an extremely negative anomaly in atmospheric water vapor during the super El Niño winter of 2015/16 and compare the anomaly with that in the other three El Niño winters of the period. A strong specific humidity anomaly is concentrated below 8 km of the troposphere with a peak at 2.5–3.5 km, and a column-integrated water vapor mass anomaly over the five radiosonde sites has a large negative correlation coefficient of −0.63 with the oceanic Niño3.4 index but with a lag of about 2–3 months. In general, the tropical circulation anomaly in the El Niño winter is characterized by divergence (convergence) in the lower troposphere over the tropical western (eastern) Pacific; thus, the water vapor decreases over the tropical western Pacific as upward motion is suppressed. The variability of the Hadley circulation is quite small and has little influence on the observed water vapor anomaly. The anomaly of the Walker circulation makes a considerable contribution to the total anomaly in all four El Niño winters, especially in the 2006/07 and 2015/16 eastern Pacific (EP) El Niño events. The monsoon circulation shows a remarkable change from one event to another, and its anomaly is large in the 2009/10 and 2018/19 central Pacific (CP) El Niño winters and small in the two EP El Niño winters. The observed water vapor anomaly is caused mainly by the Walker circulation anomaly in the super EP event of 2015/16 but is caused by the monsoon circulation anomaly in the strong CP event of 2009/10. The roles of the Hadley, Walker, and monsoon circulations in the EP and CP events are confirmed by the composite EP and CP El Niños based on the reanalysis data for 41 years. Owing to the anomalous decrease in upward transport of water vapor during the El Niño winter, lower cloud amounts and more outgoing longwave radiation over the five stations are clearly presented in satellite observation. In addition, a detailed comparison of water vapor in the reanalysis, radiosonde, and satellite data shows a fine confidence level for the datasets; nevertheless, the reanalysis seems to slightly underestimate the water vapor over the five stations in the 2009/10 winter.

Impacts of CP- and EP-El Niño events on the Antarctic sea ice in austral spring

Based on observational data analyses and idealized modeling experiments, we investigated the distinctive impacts of central Pacific (CP-) El Niño and eastern Pacific (EP-) El Niño on the Antarctic sea ice concentration (SIC) in austral spring (September to November). The tropical heat sources associated with EP-El Niño and the co-occurred positive phase of Indian Ocean Dipole (IOD) excite two branches of Rossby wave trains that propagate southeastward, causing an anomalous anticyclone over the eastern Ross-Amundsen-Bellingshausen Seas. Anomalous northerly (southerly) wind west (east) of the anomalous anticyclone favor poleward (offshore) movements of sea ice, resulting in a sea ice loss (growth) in the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas). Meanwhile, the anomalous northerly (southerly) wind also advected warmer and wetter (colder and drier) air into the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas), causing surface warming (cooling) through the enhanced (reduced) surface heat fluxes and thus contributing to the sea ice melting (growth). CP-El Niño, however, forces a Rossby wave train that generates an anomalous anticyclone in the eastern Ross-Amundsen Seas, 20° west of that caused by EP-El Niño. Consequently, a positive SIC anomaly occurs in the Bellingshausen Sea. A dry version of the Princeton atmospheric general circulation model was applied to verify the roles of anomalous heating in the tropics. The result showed that EP-El Niño can remotely induce an anomalous anticyclone and associated dipole temperature pattern in the Antarctic region, whereas CP-El Niño generates a similar anticyclone pattern with its location shift westward by 20° in longitudes.

On the Varying Responses of East Asian Winter Monsoon to Three Types of El Niño: Observations and Model Hindcasts

Using observational data and model hindcasts produced by a coupled climate model, we examine the response of the East Asian winter monsoon (EAWM) to three types of El Niño: eastern Pacific (EP) and central Pacific I (CP-I) and II (CP-II) El Niños. The observational analysis shows that all three El Niño types weaken the EAWM with varying degrees of impact. The EP El Niño has the largest weakening effect, while the CP-II El Niño has the second largest, and the CP-I El Niño has the smallest. We find that diverse El Niño types impact the EAWM by altering the responses of two anomalous anticyclones during El Niño mature winter: the western North Pacific anticyclone (WNPAC) and Kuroshio anticyclone (KAC). The WNPAC responses are controlled by the Gill response and Indian Ocean warming processes that both respond to the eastern-to-central tropical Pacific precipitation anomalies. The KAC responses are controlled by a poleward wave propagation responding to the northwestern tropical Pacific precipitation anomalies. We find that the model hindcasts significantly underestimate the weakening effect during the EP and CP-II El Niños. These underestimations are related to a model deficiency in which it produces a too-weak WNPAC response during the EP El Niño and completely misses the KAC response during both types of El Niño. The too-weak WNPAC response is caused by the model deficiency of simulating too-weak eastern-to-central tropical Pacific precipitation anomalies. The lack of KAC response arises from the unrealistic response of the model’s extratropical atmosphere to the northwestern tropical Pacific precipitation anomalies.

Water vapor anomaly over the tropical western Pacific in El Niño winters from radiosonde and satellite observations

Using radiosonde observations at five stations in the tropical western Pacific and reanalysis data for 15 years from 2005 to 2019, we report an extremely negative anomaly in atmospheric water vapor during the super El Niño winter of 2015/16, and compare the anomaly with that in the other three El Niño winters. Strong specific humidity anomaly is concentrated below 8 km of the troposphere with a peak at 2.5–3.5 km, and column integrated water vapor mass anomaly over the five radiosonde sites has a large negative correlation coefficient of −0.63 with oceanic Niño3.4 index, but with a lag of about 2–3 months. In general, the tropical circulation anomaly in the El Niño winter is characterized by divergence (convergence) in the lower troposphere over the tropical western (eastern) Pacific, thus the water vapor decreases over the tropical western Pacific as upward motion is suppressed. The variability of the Hadley circulation is quite small and has little influence on the observed water vapor anomaly. The anomaly of the Walker circulation makes a considerable contribution to the total anomaly in all the four El Niño winters, especially in the 2006/07 and 2015/16 eastern-Pacific (EP) El Niño events. The monsoon circulation shows a remarkable change from one to the other event, and its anomaly is large in the 2009/10 and 2018/19 central-Pacific (CP) El Niño winters and small in the two EP El Niño winters. The observed water vapor anomaly is caused mainly by the Walker circulation anomaly in the supper EP event of 2015/16 but by the monsoon circulation anomaly in the strong CP event of 2009/10. Owing to the anomalous decrease in upward transport of water vapor during the El Niño winter, less cloud amount and more outgoing longwave radiation over the five stations are clearly presented in satellite observation.

Examining the sensitivity of the terrestrial carbon cycle to the expression of El Niño

The El Niño‐-Southern Oscillation (ENSO) influences the global climate and the variability in the terrestrial carbon cycle on interannual timescales. Two different expressions of El Niño have recently been identified: (i) central Pacific (CP) and (ii) eastern Pacific (EP). Both types of El Niño are characterised by above-average sea surface temperature anomalies at the respective locations. Studies exploring the impact of these expressions of El Niño on the carbon cycle have identified changes in the amplitude of the concentration of interannual atmospheric carbon dioxide (CO2) variability following increased tropical near-surface air temperature and decreased precipitation. We employ the dynamic global vegetation model LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator) within a synthetic experimental framework to examine the sensitivity and potential long-term impacts of these two expressions of El Niño on the terrestrial carbon cycle. We manipulated the occurrence of CP and EP events in two climate reanalysis datasets during the latter half of the 20th and early 21st century by replacing all EP with CP and separately all CP with EP El Niño events. We found that the different expressions of El Niño affect interannual variability in the terrestrial carbon cycle. However, the effect on longer timescales was small for both climate reanalysis datasets. We conclude that capturing any future trends in the relative frequency of CP and EP El Niño events may not be critical for robust simulations of the terrestrial carbon cycle.

Monthly Variations of the Winter Precipitation over the Philippines During the Mature Phase of Eastern Pacific El Niño

The influence of ENSO events on winter precipitation anomalies in the Philippines has been well known since decades, but whether this effect is different between months needs further exploration. In this study, the monthly variations of precipitation over the Philippines in winter during the mature phases of ENSO events are investigated with datasets of reanalysis and observations from 1979 to 2019. Results indicate that only the eastern Pacific (EP) El Niño shows different influences on the Philippines winter precipitation among different months. In December during mature EP El Niño events, precipitation deficiency is not significant over the whole Philippines, whereas in January and February, precipitation decreases significantly over the southern Philippines as well as the areas to the southeast of the Philippines. Besides, the correlation between consecutive dry days over the southeast Philippines and ENSO is significantly positive in January and February but not in December. The eastward propagation of EP El Niño–related anomalous anticyclone over the western North Pacific (WNPAC) from December to February is proved responsible for the changed relationship between EP El Niño and precipitation. In December, the center of the WNPAC is located to the southeast of the Indo-China Peninsula, inducing weak lower-level wind anomalies and, consequently, weak vertical movement and water vapor transport anomalies over the Philippines, which exerts limited influence on the local precipitation. In January and February, by contrast, the center of WNPAC is located to the southeast of the Philippines, and therefore the southern Philippines is occupied by anticyclonic moisture transports and downward vertical motions, favoring less precipitations and larger than normal consecutive dry days over there.