El Niño Explained

If you live in the Midwest you have probably heard that we are in for a mild winter thanks to something called El Niño. Well that’s great and after last winter I’m not sure anyone is complaining, but what exactly is El Niño?

The ENSO Cycle

First, we need to explain some key concepts concerning global weather. Weather, like climate, goes through natural cycles over various periods of time. Wind gusts and storms are examples of very localized short-term variabilities. Fluctuations can occur on a seasonal, yearly, or multi-decade scale. The longer-term variabilities are most often associated with changes in atmospheric circulation patterns that encompass the globe. Big changes in one area of the world can have a profound affect on another. One such fluctuation is called the El Niño-Southern Oscillation (ENSO) cycle. It refers to the changes in temperature in ocean and air pressure in the atmosphere in the East and West Pacific ocean around the equator. Let’s dive a bit deeper to get a better understanding.

The Southern Oscillation

The Southern Oscillation describes the variation in atmospheric pressure at sea level between the East and West Pacific Ocean. Measurements are taken at observation stations in Darwin, Australia and Tahiti. Under normal conditions, the air pressure over Darwin is lower than that over Tahiti. The pressure [tooltip title=”In science, a gradient means the measured increase or decrease in something between one point and another” trigger=”hover” placement=”top”]gradient[/tooltip] causes winds to blow from the East Pacific to the West Pacific. These surface winds are also known as trade winds. They are part of a circulation cell known as the Walker Circulation.

Walker Circulation NeutralWalker Circulation El Niño

In neutral conditions, the east-to-west winds bring warm air and warm surface water to the western Pacific. It creates heavy precipitation and storms across the basin. The deep water (cold) layer circulates up and the [tooltip title=”An area of water that marks the barrier between surface water and deep water. It also marks a significant change in water temperature. ” trigger=”hover” placement=”top”]thermocline[/tooltip] rises.

Normal versus El Niño conditions in Pacific Ocean.

When the pressure gradient weakens, the western Pacific falls into drought and heavy precipitation falls over the East Pacific. The thermocline no longer moves and the surface water stagnates in the East Pacific where the water then continues to warm, which in turn warms the air above it.

El Niño

Now that we have a better understanding of the Southern Oscillation we can take a stab at describing the ENSO cycle warm phase, known as El Niño. El Niño is a breakdown of the atmospheric circulation we mentioned earlier. It is characterized by positive temperature anomalies in the eastern-central Pacific equatorial waters.

The El Niño event and the Southern Oscillation are intricately tied together–hence the name, ENSO cycle. Increased surface water temperature influences the atmospheric winds, which in turn influence the upper water level and the thermocline which in turn leads to further increase in surface water temperature. In science, this is known as a positive feedback loop. When conditions are favorable, the feedback loop creates the El Niño event. Warmer water means more convection (rising warm air), which means bigger changes in the Walker circulation, which again weakens the surface winds or in cases of very strong El Niño events is able to reverse them entirely. Again, we have a positive feedback loop and an example of how intertwined atmospheric pressure is with surface ocean temperatures.El Niño and La Niña ENSO Cycle Surface Temperature Graphic

The El Niño event often changes the atmospheric currents which in turn has a profound effect on weather systems all over the region and even the globe. In North America, those effects include warmer-than-average temperatures over western and central Canada, and over the western and northern United States. Wetter-than-average conditions are likely over portions of the U.S. Gulf Coast and Florida, while drier-than-average conditions can be expected in the Ohio Valley and the Pacific Northwest.

We are said to be in an El Niño event when a specific sea surface area of the equatorial Pacific shows temperature anomalies of at least .9°F for at least 5 consecutive measurement periods–one period being a three month average ending with the current month.

According to NOAA, “El Niño and La Niña episodes typically last nine to 12 months, but some prolonged events may last for years. While their frequency can be quite irregular, El Niño and La Niña events occur on average every two to seven years. Typically, El Niño occurs more frequently than La Niña.”

Why do we call it El Niño?

Many of the early inhabitants of western South America near the equator were of the Christian faith. The warming half of the ENSO cycle manifests itself late in the year around the Christian holiday of Christmas. The local population equated the two occurrences and named the warming currents El Niño, which is Spanish for Jesus.

Sources:

MET Office
NOAA National Ocean Service
NOAA ENSO Blog