What are AO, MJO and NAO? Large fluctuations that you can hear during the weather forecast
You’ve probably heard of El Niño and La Niña, which are part of the El Niño-Southern Oscillation, but many other oscillations can affect weather patterns.
The North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), and the Madden-Julian Oscillation (MJO) are three that can have significant impacts on the climate in North America.
What are the climate patterns of El Niño and La Niña?
An oscillation is a recurring climate cycle usually affected by sea surface temperatures, atmospheric pressures, and other daily weather variables.
When one oscillation goes into a negative phase, neighboring oscillations can go into a positive phase, and vice versa. Each can affect everything from air temperatures to precipitation patterns.
These relationships are called teleconnections, and according to scientists, some variables are still not well understood. But the more oscillations that indicate a pattern, the more confident forecasters will be in warning of potential extreme weather events.
Arctic Oscillation (AO)
The state of the Arctic Oscillation is determined by the atmospheric pressure above the Earth’s North Pole.
When pressures around the Arctic Circle are low, the AO is considered to be in a positive state, and when pressures are higher, the oscillation is said to be in a negative state.
During the positive phase, the polar current helps keep cold air confined around the pole.
“Cold air is being sucked back toward the poles, and that means leaving moderate air in the U.S.,” says FOX meteorologist Britta Merwin.
The combination of low pressure and air circulating around the area creates what is commonly referred to as a polar vortex.
During this phase, Arctic air masses are typically confined to Alaska, Canada, and areas above 50 degrees north latitude.
But higher-than-normal pressures over the polar region could make the AO negative, which could mean serious consequences.
Relaxing upper-level winds could allow cold air to escape from the polar region and across most of North America, potentially resulting in historically low temperatures.
For example, the AO index turned negative in January, and when it reached its lowest value in the new year, the United States experienced its most significant cold snap of the season.
What is a “polar vortex”?
North Atlantic Oscillation (NAO)
This cycle revolves around low pressure centered around Iceland and high pressure centered near the Azores Islands in the North Atlantic Ocean.
If the Icelandic decline and the Azores rise are stronger than usual, the NAO is considered to be in positive territory.
This positive value results in a stronger jet stream that is positioned at northern latitudes.
During the North American winter, this condition can lead to less stormy weather in the eastern half of the country and milder conditions.
No truer example can be found than in 2023, when the United States set a new record for warmest December.
According to NOAA, the average temperature for the month was 7.3 degrees Fahrenheit above normal, ranking it as the warmest December in 129 years of records.
On the other side of the seasonal spectrum, tropical cyclones are more likely to interact with the North American coast during a positive North Atlantic than when it is negative.
During the negative phase during the winter, the United States experiences more stormy weather, with declines in the jet stream and outbreaks of cold air.
The NAO index was in negative territory in the first half of January 2024, and this period became known for winter storms and severe weather.
Madden-Julian Oscillation (MJO)
The MJO is an eastward-moving pulse of waves of enhanced moisture and dry air that affects regions along the equator and subtropics.
It is estimated that any given point within the MJO takes 30 to 60 days to orbit the globe.
During the summer and fall, the MJO can lead to an increase in hurricane activity in areas considered to be in areas of enhanced rainfall and a decrease in tropical cyclone activity in suppressed areas.
Why does Tornado Alley turn east?
During the winter, the wet phase of the MJO can lead to increased precipitation along the West Coast, activating the Pineapple Express Train and a major cold invasion on the East Coast.
Take the case of the MJO on January 9, 2024, for example. An anomalous divergence zone — an area favorable for precipitation — exists over much of North America, and has likely contributed to extreme weather from snowstorms in the Midwest to severe weather in the Southeast.
Forecasters can monitor the phases of the MJO as it orbits the planet to get a general idea of which direction the forecast might lean — whether it will be stormy or calmer.