Let’s try to break down what this phenomenon actually is, why it matters and how it could affect our weather into the fall.
¿Qué es? (What is it?)
El Niño is one of the three phases of what’s known as ENSO, which stands for El Niño Southern Oscillation. In the simplest terms, ENSO describes whether sea surface temperatures in the central and eastern tropical Pacific Ocean are warmer than average, average or cooler than average.
During the El Niño phase of ENSO, the water is warmer than average. During the opposite phase, La Niña, it’s cooler than average. And then there is the neutral phase, when the water temperature is right around average.
Right now, ENSO is in its neutral phase but starting to trend toward El Niño.
¿Por qué importa? (Why does it matter?)
What happens in the tropical Pacific Ocean doesn’t stay in the Pacific Ocean. Changing the temperature of the sea surface has important implications on the flow of the atmosphere directly above the ocean, which affects weather across the globe.
Essentially, large pools of cold or warm water shift the normal areas of large-scale rising and sinking air. From a physical standpoint, pools of warmer sea surface water will in turn warm the air immediately above the water — and as we know, warm air rises. This is important because rising air is what makes turbulent weather, while sinking air is responsible for quiet and stagnant weather.
¿Qué significa para nosotros? (What does it mean for us?)
Recent observations and computer model forecasts are hinting that we are at the onset of above-normal sea surface temperatures in the eastern and central Pacific, which could be a harbinger of El Niño.
We should stress that while El Niño is favored to develop, it is not guaranteed. Last year, some models predicted its development, and it didn’t happen.
But even the hint of an emerging El Niño can give us insight into how the rest of our summer plays out, and here are two of the most significant potential implications:
1) Increased rainfall and mugginess
A developing El Niño would favor increased moisture into our region due to an enhanced flow of air from the subtropics. We’ve already seen this tendency since mid-May, the result being above-normal rainfall.
The increased cloud cover resulting from the added moisture may moderate afternoon temperatures, resulting in fewer extreme-heat days. However, this enhanced moisture feed would increase humidity, nighttime temperatures and the number of days with thunderstorms.
Since 2000, after four of the six summers leading up to an El Niño event, the fall and winter have had above-normal to far-above-normal rainfall. Temperatures have mostly been close to normal.
2) Less Atlantic hurricane activity
El Niño may well have the beneficial effect of slowing down hurricane season. Tropical storms and hurricanes love to form in environments with warm ocean temperatures and low values of upper-level wind shear, or changing winds with altitude. Tropical storms have that recognizable circular shape in part because there is little conflict in the direction or speed of the wind from the surface all the way up to 30,000 feet.
During El Niño, the shifted global atmospheric circulation causes stronger upper-level westerly winds and stronger low-level easterly winds over the tropical Atlantic Ocean. This configuration, where you have strong wind speeds blowing from opposite directions, tends to rip tropical storms apart or prevent them from even forming in the first place.
To a certain extent, we are seeing an Atlantic tropical basin that is already unfavorable for tropical storm development. Persistent low-level easterly winds have pushed a gigantic area of dust from the Saharan desert into the atmosphere over the Atlantic, which also inhibits storm formation.
Stay tuned for a post next week that will look at the potential effects of El Niño next winter.
The Capital Weather Gang’s Jason Samenow contributed to this story.