El Niño or Climate Change?

We might have just experienced one of the warmest Christmas in 50 years, accompanied by series of extreme global weather around the world, thanks to El Niño. From record-breaking temperatures over the Christmas holiday in the US (the New Yorkers were complaining that they didn’t get the White Christmas they used to have this year) and forest fires in Australia, to a lacking of snowfall in the Alps which made the skiing condition around the area less desirable, and some of the worst floods ever known in the UK. Study reveals a reciprocal relationship between El Niño effects and impacts of anthropogenic global warming - climate change is likely to intensify the effects and double the occurrences of El Niño in the future in response to global warming, which create more tumultuous weather events around the world that make the effects of climate change even worse. 

The natural phenomenon El Niño, also known as the El Niño Southern Oscillation (ENSO), is caused by warm ocean waters forming in the central and east-central Pacific, which then leads to a chain of climatic events that can cause significant changes to global weather patterns. During a moderate El Niño period (i.e. Modoki El Niño) (see Fig. 1), the eastern margin of the warm pool and the atmospheric convection zone migrate eastwards to just east of the Date Line, whereas the ITCZ locates north of the Equator, leading to a strong anomalous warming in the central tropical Pacific (fall in air and surface pressure) and cooling in the eastern and western tropical Pacific (rise in air and surface pressure). 

Fig.1 Anomalous SST during Modoki El Niño, 

Source: JAMSTEC

During an extreme El Niño (see Fig. 2), the warm pool expands eastward until it covers the entire eastern equatorial Pacific, weakening the equatorial east-west and meridional sea surface temperatures gradient. The oceanic convection then follows the highest SSTs and extends eastward while the ITCZ shifts equatorward, bringing atmospheric convection and anomalous rainfall patterns (> 5mm per day) in the normally dry eastern equatorial Pacific. As such, westerly winds dominate the easterlies and suppress the cool eastern equatorial upwelling (Anomalous El Niño, Nature), thus reinforcing the anomalous high SSTs in the region. 

Fig.2 Anomalous SST during extreme El Niño, 

Source: JAMSTEC

El Niño is certainly not a result of climate change but the latter has the impact to make its effect worse. Under anthropogenic global warming, the eastern equatorial Pacific experiences a faster warming rate and weaker zonal and meridional SST gradients than other parts of the world. Weaker changes in SST gradients assist shifts in convection zones more easily, thus increasing the chances of occurrence of extreme El Niño in the future (see Fig. 4).  

Fig. 3 Multi-model statistics associated with the increase in the frequency of extreme El Niño events.  Multi-model histograms of the meridional SST gradient in the eastern equatorial Pacific for all samples, and for extreme El Niño alone. Blue represents the moderate periods while red indicates climate change periods. 

Source: Wenju Cai et al. 

Fig. 4 Schematic depicting the mechanism for increased occurrences of extreme El Niño under greenhouse warming. Source: Wenju Cai et al. 

Extreme weathers are found more common around the world, mostly as a result of climate change, but also with the warmth reinforcement of the El Niño effect. If climate change is proved to have impact on increasing the occurrence and doubling the effect of El Niño, the world will certainly be expecting more extreme weather and stronger weather contrasts. 

Source: Inkcinct Cartoon

New climate adaptation measures like cooling measures, flood and drought defences, cyclones and typhoons defences should be constantly updated. According to Gail Whiteman, the chair of the Peatland centre for sustainability at Lancaster University, ‘[w]hat may appear to be sufficient to withstand a 1 i  100-year event can become quickly out of date as the incidence of extreme weather ramps up and becomes more unpredictable’.