If you have never heard the term “El Nino,” you might have been living under a rock.
The common informed person is probably somewhat aware that we are amidst a record-setting El Nino, and rumors of a possible switch to La Nina 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 Nino and La Nina actually are and why they exist.
These topics always spark interest and curiosity among people worldwide, but actual clarity is often missing. This is a time when a return to basics becomes admittedly refreshing.
One very important thing to remember from the start is that both El Nino and La Nina are naturally recurring climatic events. They are not “storms.” Chaotic weather is not the guaranteed outcome. They are not a product of climate change. And there is certainly no conspiracy involved.
Both El Nino and its counterpart, La Nina, are part of a larger-scale climate pattern known as El Nino-Southern Oscillation, or ENSO. ENSO is based on the variation of both sea surface temperatures, or SSTs, and air pressure tendencies in and around the equatorial Pacific Ocean.
The equatorial Pacific is split into 4 regions, named “Nino” regions, over which SSTs are monitored. One region, Nino 3.4, is the location in which scientists have found the best linkage between SSTs and climate patterns. Therefore, the SST anomaly in the Nino 3.4 region ultimately defines which phase of ENSO is present.
The requirement to classify an El Nino or La Nina event is that SSTs in the Nino 3.4 region must be at least 0.5 degree C above or below the long-term average temperature for roughly six or seven consecutive months, at the minimum.
The warm phase of ENSO is known as El Nino and the cool phase is La Nina, so therefore by definition, the two events could never occur simultaneously. ENSO-Neutral conditions are said to be present when the SSTs are neither significantly warmer nor cooler than average.
ENSO’s existence owes to the ever-dynamic global circulation patterns. Because of the cyclic nature of the oceans and the atmosphere, ENSO operates on a semi-regular scale as well. And the cycle is always in progress.
On average, ENSO undergoes a full cycle every two to seven years. La Nina and El Nino events occur every three to five years and reach peak strength between October and March when SSTs across the equatorial Pacific are the warmest.
There has been roughly the same amount of El Nino and La Nina events since modern records began in 1950, but La Nina can span over three years, while El Nino typically does not last more than one year. There is no long term bias for either in recent decades.
How It Happens
Basic ocean and atmospheric circulation set the scene. Cold waters deep in the equatorial Pacific move eastward towards the Americas, causing the upwelling, or surfacing, of these cool waters off the coast of Peru.
Once reaching the surface, easterly – from the east – trade winds along the equator carry the surface waters westward towards Asia. During the journey, the waters are warmed by the sun’s direct rays. Upon arrival at the western basin, the waters sink back down to the ocean floor, restarting the cycle.
One of the biggest signs that an El Nino or La Nina event may be in the works is when trade winds and pressure tendencies over the Pacific Ocean are disturbed from their normal state. Air pressure and winds directly impact one another, but to ask which comes first presents a “chicken or egg” dilemma.
Air tends to flow from high to low pressure. So when the pressure is higher near Tahiti relative to that in Darwin, Australia, this creates a strong easterly flow in the equatorial Ocean. Since the default state of trade winds near the equator is already easterly, the trade winds are amplified, blowing towards 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 along the equator faster than normal, displacing and cooling down the solar-warmed waters in the Nino 3.4 region. This process is known as La Nina, or the cool phase of ENSO.
El Nino happens in the opposite way. If the Darwin-Tahiti pressure tendency is reversed, and air is forced to move eastward and oppose the easterly trade winds, the trade winds will slow or even reverse in direction if the force is strong enough.
When the trade winds slow or reverse, they 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 the ENSO warm cycle, or El Nino.
Therefore, if a close eye is kept on the trend in trade winds and air pressure over the tropical Pacific Ocean, an El Nino or La Nina event is often highly detectable, sometimes more than several months in advance.
Warmer waters favor the presence of 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 Nina often brings floods to Southeast Asia and Oceania and drought to the Americas, as eastern Pacific waters are cooler. El Nino 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 Nino since 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 Nina 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, this is a highly simplified view of global weather impacts, and should only be used as a general guideline. Several more small- and large-scale climate patterns are always at play and at least one or two years can always be found in history where the ENSO mould was broken, and the weather effects were completely opposite of what was expected.
(Karen Braun is a Reuters market analyst. Views expressed are her own.)
(Editing by David Evans)