It’s an El Niño year, and there’s already been a major snowfall in my region to prove it. I’m sure you all have heard the name and may even be aware of how this phenomenon affects the weather in your region. But it might not be something you know much about practically, just a word you hear on the news every few years. Well, it was for me too, which is why I researched and wrote this.
The Mechanism
The Pacific Ocean is big. Very big. Bigger than you probably realize. The Pacific Ocean covers more of Earth’s surface area than all of the planet’s land masses combined. It covers 46% of the planet’s water surface, making it only slightly smaller than the rest of the world’s oceans combined. There are parts of the Pacific (specifically off the coast of Peru and Vietnam) that contain pairs of antipodal points. This means there are places where one could dig a tunnel straight through the center of the Earth and have both entrances be in the Pacific Ocean. It’s a big ocean. And this size means that climate phenomena which occur in the Pacific can become very powerful and can significantly influence the rest of the global climate. All oceans have a pattern of oscillation caused by air and ocean currents interacting in a way that strengthens or weakens those currents. The size of the Pacific means that its oscillations have dramatic effects.
In the tropics, the trade winds blow east-to-west due to the effect of Earth’s spin. These trade winds blow on the ocean to create east-to-west currents in the topmost ocean layer. The Sun heats this topmost layer of the ocean, and the east-to-west currents move this warm water toward the ocean’s western side. The Pacific’s size means that it contains a massive volume of warm water, so concentrating this warm water on the western side can create a significant temperature difference. The western tropical Pacific averages about 8 to 10°C warmer than the eastern side, with the warmest part being centered just east of Indonesia and north of Australia. This warm spot causes enormous amounts of evaporation and thus rainfall, resulting in a warmer, wetter climate, while also creating an enormous zone of rising air. Meanwhile, these currents mean the eastern side of the Pacific tropics are losing heat and moisture. The fact that so much surface water is being pulled away from the region means that cold water from the deep ocean must rise up to replace it. This makes the surrounding region (the west coast of Central and South America) cooler and drier, but this upwelling also carries up nutrients from the deep ocean which result in fertile waters and a larger fish population. This region of cold water causes the air above it to sink, creating a zone of high air pressure. Wind moves from this high pressure area near Latin America toward the low pressure area near Indonesia, reinforcing the existing east-to-west trade winds. And the rising air in Indonesia combined with the sinking air in Latin America creates a wind current in the upper atmosphere going from Asia to the Americas. This conveyor belt of air, called a Walker cell, drives the ocean currents which creates the temperature difference which fuels the Walker cell. This feedback loop strengthens and reinforces the currents, making them so strong that the Pacific’s waters are about 0.5 meters (1.5 feet) higher on the western side than on the eastern side.
This is how the Pacific’s ocean and air currents normally behave. But the self-reinforcing nature of these currents mean that a small change can snowball into a big effect. What initially triggers El Niño events is still poorly understood, natural climate phenomena are too complex and multifaceted to be predicted like that, but we know that it causes the trade winds to become mildly weaker. The feedback loop I mentioned means that weakening the trade winds lessens the temperature difference which fuels the trade winds, causing the trade winds to weaken further. This vicious cycle continues until the trade winds can no longer drive the temperature gradient and warm water becomes more evenly distributed than normal. The eastern Pacific becomes warmer and wetter than normal while the western Pacific becomes colder and drier than normal. In El Niño years, Southeast Asia and Australia experience droughts and heatwaves while Latin America experiences heavy rains, flooding, hurricanes, and declines in fish stocks. Also, El Niño is associated with higher global temperatures because this more distributed warm water can release more of its heat into the atmosphere. The loss of this temperature difference also causes the jet stream to move southward (more on the jet stream here), causing changes in temperature and precipitation in North America, Africa, and India. This trend is temporary; the weakening of the trade winds means that the warm waters of the Pacific are able to diffuse out and release all their heat. The chaotic wind currents associated with El Niño stop without fuel, allowing the trade winds to start back up again and restore the ocean’s neutral state. El Niño events can last for nine to twelve months.
The opposite also occurs. La Niña occurs when the trade winds become mildly stronger, again for reasons that are poorly understood. The feedback loop means that strengthening of the trade winds increases the temperature difference which fuels Walker circulation, causing the trade winds to become even stronger. This causes the eastern Pacific to become colder and drier than normal while the western Pacific becomes warmer and wetter than normal. La Niña is associated with flooding and storms in Southeastern Asia and droughts and heatwaves in Latin America. It’s also associated with cooler global temperatures as more heat gets concentrated in the western Pacific and the jet stream moving northward which causes downstream effects for climate in other regions worldwide.
Effects for Humans
El Niño-Southern Oscillation doesn’t take place on a regular cycle, instead switching back and forth between El Niño and La Niña with two-to-seven years between each like-phase, with neutral conditions between these extremes. La Niña conditions can last longer (some lasting for over two years), but El Niño is associated with worse conditions. Both El Niño and La Niña episodes tend to peak in severity around late December (hence the name, after the Christ child by 19th century Peruvian fisherman), though when they start and end can vary.
There are several ways ENSO can impact human civilization. Excessive rain and drought can negatively impact agriculture while the decline of fish stocks hurts fisheries. Both of these factors can negatively impact the global economy and commodity prices, particularly in the directly-affected regions of the Pacific tropics. An increase in ocean temperature and evaporation can also feed storm systems, so El Niño is associated with worse hurricane seasons in the Americas while La Niña is associated with worse typhons in East Asia. ENSO events can also lead to outbreaks of insect-borne diseases as more rain leads to more standing water which leads to more breeding habitats for mosquitos. These outbreaks are worst in the directly-affected regions, but can spread to neighboring regions along with moving mosquito populations. Due to all these factors, rates of civil conflict and unrest double in affected regions during ENSO episodes, with some estimates finding that one-fifth of all major civil conflicts since 1950 were at least partially worsened by ENSO events. Developing nations, such as the majority of nations in these affected regions, are particularly vulnerable to outbreaks of civil unrest, and the regular strain to sociopolitical infrastructure caused by ENSO events almost certainly makes development more difficult. And of course there are the effects on climate in other regions, from La Niña causing worse hurricane seasons in the North Atlantic and both ENSO phases can cause more or less rain in Africa depending on region and time of year. See the maps below to see how various regions are affected.
The obvious question that comes from any discussion of global climate is ‘how is this affected by anthropogenic climate change?’ The good news is that there doesn’t appear to be any evidence that climate change significantly affects the frequency or severity of ENSO events. This could be because we know so little about how ENSO events start and thus don’t have enough data to see something coming, but there are a few hypotheses for how the balancing effect of the Southern Oscillation could remain unaffected. It is true that the individual events caused by ENSO events (storms, droughts, etc.) do appear to be getting worse, but that’s not the same as ENSO itself getting worse. The best connection I can draw here is that ENSO events are a good model for the kinds of crises that fall when the climate shifts from what a region expects, so that alone is worth studying.
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