By

Saya Goddard

Guest Contribution

The El Nino Southern Oscillation (ENSO) is a recurring climate phenomenon that plays a crucial role in climate variability patterns (Wang et al.). It is concentrated in the central and eastern equatorial Pacific (Met Office) but has widespread global effects. By the changes in sea temperatures and the corresponding atmospheric conditions, it can be characterised by a ‘cooling period’ called ‘La Niña’ or a ‘warming period’ called ‘El Niño’ (L’Heureux). This leads to globally colder or hotter years respectively. Understanding the mechanics of ENSO and the reasons behind El Niño-induced warming is crucial for grasping broader climate dynamics.

The ENSO cycle is as a result of ocean and atmospheric interactions. These interactions consist of the exchange and distribution of solar energy (National Oceanic and Atmospheric Administration) which play a huge role in shaping the planet’s climate. A large portion of the sun’s radiation is absorbed by the ocean and heats the surface. This energy can be transferred by sensible heat influx through the direct conduction of heat (Misra) from the warmer water molecules to the cooler air molecules at the interface. Since hot air is less dense in comparison to the surrounding cold air, it rises. To maintain equilibrium, cooler and denser air from the surroundings sinks to replace the rising warm air. This pattern of air movement is the fueling of convection cells which regulate wind systems, cloud formation and precipitation patterns (Grotjahn). The energy can also be transferred through latent heat flux whereby the sun’s radiation causes the water on the surface to evaporate into water vapour (Stewart). As the vapour rises into the atmosphere, where the air is colder, it condenses into clouds, again influencing precipitation patterns (Stewart). It should be noted that they both contribute to storm formation creating intense rainfall (Colbert) from increased moisture content and the created pressure gradient by the different temperatures of the air; warm air exerts less pressure and cold air exerts more (Read). The force of the pressure gradient acts like a ‘windward push’, causing air to flow from high to low-pressure zones (Read), where the steeper the gradient, the stronger the force (Met Office). These processes are key to understanding the mechanisms behind how the ENSO works and the grandeur of its impact.

The ENSO is a cyclical variability of the ocean-atmospheric interactions in the Pacific Ocean. The phenomena take place at 2-7-year intervals (Generoso et al.). While the cause of the ENSO remains unknown, the main component fueling the phenomena is the trade winds that are formed by intense heating of the equatorial regions and the produced equatorial low-pressure zone (Smithson et al. 118–19) . Under normal conditions, the tropical Pacific operates under the Walker Circulation (Misra). In the tropical west Pacific, trade winds blow towards the west (Dawson and O’Hare). Warm surface water is as a result pushed to the west which heats the atmosphere and a low pressure area develops to form convectional uplift and rainfall along the west including countries like Australia and Indonesia. In normal conditions in the tropical east pacific, the trade winds pull up cold water from below. The air sinks off the west coast of lower North America and South America and an area of high pressure develops to give dry stable conditions (Dawson and O’Hare). However, during an El Niño event the trade winds weaken (Met Office) and this causes the warm water in the west to accumulate eastwards. In the west the absence of warm water to heat the atmosphere will lead to dry stable weather conditions. (Burton et al.). In the east the warmer water will lead to convectional uplift and unstable wet weather conditions such as floods and intense storms (Burton et al.). As the influence of an El Nino event decreases and conditions return to normal occasionally a third weather condition develops – La Nina. This is an exaggerated version of the normal conditions where trade winds strengthen resulting in more warm water travelling west (Met Office), causing intensive rainfall creates flooding in the west and upwelling more colder water to the surface  intensive droughts in the east (Burton et al.). These are the key impacts of the ENSO and how it functions.

The effects of the ENSO have a global influence on climate variability. As mentioned, the ENSO has far-reaching impacts, and evidently the Pacific covering a third of the Earth (Bardach et al.) is in itself a reason for this. Although beyond this, through teleconnections which are long-lasting, connected climate anomalies (National Geographic) that take place on a global scale. The ENSO’s influence on warmer or cooler ocean temperatures and resulting change in pressure gradients has proven to alter the course of the jet stream which is a narrow zone of strong winds circling the globe. It carries atmospheric waves that guide weather patterns (Zavadoff and Arcodia) like storms, fronts and precipitation. This means large-scale change to distant regions such as affecting precipitation in Africa or temperature differences in Europe. To observe these effects on climate, the most generalised (considering there are varying impacts depending on region) way is to observe global mean surface temperature which takes into account the average temperature over the sea surface and land surface. Figure 1 is shown, where it compares temperature change from the 20th-century average temperature (Allan et al.) to sea surface temperature in the Niño 3-4 region which is in the Pacific Ocean that is particularly sensitive to ocean-atmospheric interactions. The coloured peaks in the bottom graph represent ENSO events that have taken place. It can be seen that in many of the ENSO events, it corresponds to a more extreme fluctuation in the temperature difference. This is true around 2015-2016, indicating a strong El Niño event, linked to more than a 1°C increase, or the 1988 la Niña event corresponding to the 0.25°C drop in temperature. Moreover, the values aren’t constant which presents the unpredictability of the intensity of impact as well as occurrence. It is also important to note that other factors play into temperature anomalies that may insinuate less of a correlation, but other bodies of research suggest similar conclusions to global temperature changes by the ENSO. This includes the Annual Report by the National Centres for Environmental Information (2023) that produced a particular section on ENSO cycles having a significant contribution to climate anomalies. This indicates the role ENSO plays in climate variability by recorded fluctuations in temperature as well as short-term differing weather conditions globally.

Examining El Niño in particular and how it leads to globally unusually hot years shows the complexity of the ENSO. In the previous explanation, one of the reasons for hotter temperatures was identified as the weakening of the trade winds which results in less upwelling of colder water and therefore more heat transfer to the atmosphere. There was also a reference to the distortion of the jet stream’s path, meaning some countries receive less of the ‘cool, moist air masses’ the jet stream usually carries (McCabe) and get hotter temperatures. In addition to this, with increased ocean surface temperatures, more evaporation occurs which can lead to increased cloud formation as seen by NASA’s Aqua Satellite (NASA Earth Observatory). Although clouds may reflect incoming sunlight, they also have the potential to “trap the heat from the surface at night” (NASA Earth Observatory). Furthermore, another focus of research to reason the hotter climate is the effect on phytoplankton. As there is less upwelling, there is less nutrient-rich water being cycled to the surface. This reduces the nutrients available to phytoplankton and hinders their ability to photosynthesise which removes CO2 from the atmosphere (Racault et al.). Though it may not have as much of an immediate impact, the Research By Recault et al. (2017) shows decreases in Chlorophyll in phytoplankton which may have persisting and long-term impacts in terms of CO2 fixing. Lastly, hotter recorded years during El Nino events can be from more indirect causes. For example, El Niño creates dryer conditions in regions which can lead to wildfires. Research by Lerato Shikwambana et al. (2022) shows that El Nino in Southern Africa intensifies the already dry conditions and droughts, which increases the number and severity of wildfires in the region. With this, in theory, with multiple regions facing a similar effect, large-scale emissions from wildfires can contribute to greenhouse gases that can cause hotter temperatures by trapping incoming solar radiation. Overall, the multitude of factors all contribute to unusually hotter years globally and show the intricate links the ENSO phenomenon has with other systems.

The ENSO phenomenon plays a pivotal role in shaping global climate variability. El Niño’s influence on global weather conditions, particularly unusually hot years, underscores the complex interplay of the ocean and atmosphere as well as its association with other processes. Understanding these dynamics is essential for forecasting and mitigating its contribution to meteorological extremes.