Although leap year isn’t until 2016, 2015 is shaping up to see an event that only takes place every four to six years: El Niño.
Despite affecting weather around the globe, El Niño is not a single weather system; it’s a climatic event. Ocean temperatures, currents and wind patterns are its building blocks, which always begin in the Pacific.
Towards the end of each year, shortly after Christmas, a warm ocean current flows along the coast of Peru and Ecuador. Every few years, it is stronger than normal and flows farther south, resulting in extra-heavy rains across that part of the world, which brings an abundance of crops. This led to the name El Niño, “The Child,” referring to the Christ Child, due to the increase in produce shortly after Christmas.
But El Niño is rarely so benevolent to the rest of the world, where it’s associated with severe flooding or drought, or a complete shift in typical rainfall.
Just off the western South American coast, there usually exists a large area of high pressure over the ocean. It’s from here that the southern trade winds blow westward towards a large area of low pressure positioned over Indonesia. The trade winds are typically strong and steady, bringing cool waters with them from near South America that warms up due to contact with the atmosphere and exposure to the sun.
The water level in the western Pacific can be as much as 16 inches higher than it is near South America, causing a thick layer of warm water over the western Pacific. Meanwhile, a deeper current of milder water, known as the “countercurrent,” flows back eastward, gently bringing warm water back with it, where it again cools off near South America, and the cycle begins anew. Much of the warm water evaporates into the sky, which contributes to rain systems, such as the monsoon.
Air that rises farther up, where it becomes drier, is carried from west to east by strong upper level winds. It cools and descends, adding itself to the high-pressure system off South America, where the entire cycle begins. That’s the “normal” climatic pattern.
During El Niño, the trade winds relax, allowing the warm water near Indonesia to surge eastward, strengthening the countercurrent and forming a warm layer of water much closer to the coast of South America.
This causes the air above to warm up, lowering the atmospheric pressure and weakening the trade winds even further. Ultimately, the normal direction of the trade winds is reversed for much of an El Niño year—sometimes even longer.
While crops thrive in western South America due to increased rain, fishing activities are negatively affected. Normally, the cooler water rising off the coast brings up with it rich nutrients that support some of the world’s most abundant fisheries.
During El Niño, however, this cooler water is trapped beneath a thick layer of warmer water, blocking the nutrients from reaching the fisheries. The surface becomes uninhabitable for the fish, which either leave or die. In our global world, this can lead to a major depletion of the world’s supply of fishmeal or animal feed.
About 30 degrees north of the equator, the subtropical jet stream—high-level winds that flow all around the Northern Hemisphere—pushes smaller weather systems beneath it. When El Niño arrives, the basic pattern of the jet stream, like the trade winds, is altered.
A large system of low pressure develops over the western Pacific, which drags the jet stream south, carrying thick clouds over the ocean eastward, where the coast of California and Mexico see more storms.
But this repositioned jet stream also shears off the tops of Atlantic storms, hindering the development of hurricanes later in the year, which can be of great benefit to many southern and eastern coastal cities.
The effects of El Niño are inconsistent. The El Niño of 1997-98 led to dramatically high rainfall in east Africa, while a severe drought occurred in Brazil. Central Asia and the northwest United States and Canada saw heat waves, while floods ravaged central Europe.
In the U.S., El Niño causes storm systems out west to dip south as they travel towards the Atlantic, then rise north as they move towards the east coast. Less precipitation is the typical result and, during colder months, we generally see an increase in air from the west that suggests a warmer winter. Meanwhile, spring months could see more severe weather in the southern prairie.
Our local forecast calls for some milder weather in late January. I’m not sure that can be attributed to El Niño, since overall temperatures this month will be about average due to a bitterly cold New Year’s. Our snowfall totals so far this winter haven’t been up to par, but look for some catching up this month as well.