Given the odd weather of late, you may be aware that we are in the midst of what could be a record-setting El Niño. Rumors of a switch to La Niña later this year have also danced into the public’s ear, particularly those with an interest in commodity markets.
But comprehension of such a scenario, and what it may mean, is quite difficult without an understanding of what El Niño and La Niña are and why they exist.
To start, it’s important to note that both are naturally recurring climatic events. They are not storms. Chaotic weather is not the guaranteed outcome of either. They are not a product of climate change.
El Niño and La Niña are part of the El Niño-Southern Oscillation, a climate pattern affected by variations of sea surface temperatures and air pressure tendencies in and around the equatorial Pacific Ocean. The warm phase of ENSO is known as El Niño and the cool phase is La Niña.
The equatorial Pacific is split into four regions over which sea surface temperatures, or SSTs, are monitored. One region, Niño 3.4, is where scientists have found the best linkage between SSTs and climate patterns.
For a weather event or pattern to be classified as an El Niño or a La Niña, the SSTs in the 3.4 region must be at least 0.5 degrees Celsius — about one degree Fahrenheit — above or below the long-term average for a minimum of roughly six or seven consecutive months.
On average, ENSO undergoes a full cycle every two to seven years. El Niño and La Niña events occur every three to five years and reach peak strength between October and March, when SSTs across the equatorial Pacific are the warmest.
Basic ocean and atmospheric circulation set the scene. Cold waters deep in the equatorial Pacific move eastward toward the Americas, causing the upwelling, or surfacing, of these cool waters off the coast of Peru.
Easterly trade winds — that is, from the east — coming along the equator carry surface waters westward toward Asia. During the journey, the waters are warmed by the sun. Upon arrival, the waters sink down to the ocean floor, restarting the cycle.
One of the biggest signs that an El Niño or La Niña event may be in the works is when trade winds and pressure tendencies are disturbed from their normal state.
Air tends to flow from areas of high pressure to areas of low pressure. So when the pressure is higher near Tahiti than in Darwin, Australia, a strong easterly flow in the equatorial Pacific is created. Since the default state of trade winds near the equator is easterly, the trade winds are amplified, blowing to the west much more strongly than usual.
In this situation, the cold waters churned up off the west coast of South America are hurried westward faster than normal, displacing and cooling the solar-warmed waters in the Niño 3.4 region. This process is known as La Niña.
El Niño happens in the opposite way. If the Darwin-Tahiti pressure tendency is reversed and air is forced to move eastward, the trade winds will slow or even reverse direction.
They then push back on the cooler ocean waters trying to make their way westward from South America. Because the distribution of the cool waters is significantly lessened, equatorial waters become somewhat stagnant, and are essentially left to bake in the sun. This results in an El Niño pattern.
If a close eye is kept on trade winds and air pressure over the tropical Pacific, an El Niño or La Niña event is often highly detectable, sometimes months in advance.
Warmer waters favor low pressure and convection, or the development of thunderstorm activity. This allows for some ENSO-based seasonal weather predictability around the globe.
The wet pattern is focused where the ocean waters are relatively warmer. Therefore, La Niña often brings floods to Southeast Asia and Oceania and drought to the Americas, as eastern Pacific waters are cooler. El Niño frequently presents the reverse effects.
In North America, ENSO modifies the jet stream, which carries and distributes weather patterns from west to east. Warmer winters are generally expected during El Niño because the jet stream takes on a more southerly track, fighting off Arctic air domination.
But the high-amplitude, northerly track that the jet stream takes on during La Niña winters allows for frequent Arctic air intrusions in Canada and much of the United States, often resulting in colder winters. The loss of southerly flow removes the source of warm, moist air over the southern United States, sometimes inducing severe drought from California to Texas.
However, weather inputs are complicated. Several small- and large-scale climate patterns are always at play, and anomalies can always be found where the ENSO mold was broken and the weather effects were completely opposite of what was expected.