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When you're finished with this page, you should be able to:
- describe the positive and negative phases of the Arctic Oscillation (AO) and their impacts on temperature, precipitation, and winter weather patterns in the Northern Hemisphere.
- explain how various factors, such as Arctic sea ice, snow cover, and volcanic eruptions, contribute to shifts in the Arctic Oscillation (AO) phases.
- describe the positive and negative phases of the Antarctic Oscillation (AAO) and their influence on temperature, westerly winds, and rainfall patterns across the Southern Hemisphere.
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The Arctic Oscillation (AO)
Let’s discuss the Arctic Oscillation (AO), a key climate pattern that involves shifts in pressure between the Arctic and the mid-latitudes, particularly over the North Pacific and North Atlantic. These pressure differences influence how Arctic air moves across the Northern Hemisphere. The AO has two main phases—positive and negative—and each brings its own kind of weather.
When the AO is in its positive phase, surface pressure in the Arctic drops lower than usual, while pressure in the mid-latitudes rises. This setup strengthens the polar vortex, a large, persistent area of low-pressure and cold air that circulates around the polar regions, acting as a barrier that keeps colder air trapped near the poles. With a strengthened vortex, a positive AO effectively locks in that Arctic air. The result? The jet stream, which is like a fast-moving river of air high above us, stays further to the north, keeping colder temperatures confined to the Arctic. For those of us living in the mid-latitudes, this means milder winters, as the cold air doesn’t push as far south.
On the other hand, when the AO switches to its negative phase, things flip around. The pressure differences weaken the polar vortex, allowing it to wobble and send that cold Arctic air further south. As the jet stream dips down into the mid-latitudes, areas that might normally enjoy milder winters can suddenly get hit with frigid Arctic air, leading to much colder and stormier conditions. This phase is often associated with harsher winters in places like the eastern United States and Europe, thanks to those southward surges of cold air.
“Consequently, locations in the mid-latitudes are more likely to experience outbreaks of frigid, polar air during winters when the AO is negative.”
The Arctic Oscillation (AO) doesn't just influence temperature—it also affects where and how much it rains or snows. When the AO is in its positive phase, Northern Europe typically experiences wetter conditions, while Southern Europe and areas around the Mediterranean tend to dry out. But when the AO flips to its negative phase, these patterns can reverse, bringing more rain to Southern Europe and drier weather up north.
What drives the AO's shifts? A mix of internal and external factors plays a role. Changes in Arctic sea ice and snow cover, variations in solar radiation, and even volcanic eruptions can all influence how the AO behaves. These factors can interact with the atmosphere, nudging the AO into its positive or negative phase and shaping weather patterns across the Northern Hemisphere.
Antarctic Oscillation (AAO)
In the Southern Hemisphere, there's a similar climate pattern to the Arctic Oscillation (AO), called the Antarctic Oscillation (AAO). Scientists also refer to this as the Southern Annular Mode (SAM) -- it's fine if you want to use them interchangeably. Maybe researchers were just tired of having all the “AO”s and wanted an easier acronym to remember!
Unlike its northern counterpart, the AAO is less interrupted by landmasses, making it more zonal, or west-to-east, in nature. Because of this, the AAO has a stronger influence on the position of the westerly winds that circulate around Antarctica. These winds shift north and south depending on whether the AAO is in its positive or negative phase, representing a change in atmospheric mass between the mid-latitudes and the Antarctic region.
The AAO also impacts weather patterns across the Southern Hemisphere. In its positive phase, the westerly winds strengthen and move closer to Antarctica, keeping the mid-latitudes cooler and making conditions warmer over Antarctica. In the negative phase, the winds weaken and shift northward, leading to warmer temperatures in the mid-latitudes and cooler ones in Antarctica. These shifts in the AAO also influence rainfall patterns. A positive phase typically brings drier weather to parts of Australia, New Zealand, and South America, while the negative phase tends to increase rainfall in those regions.
The AAO is primarily driven by differences in atmospheric pressure between the mid-latitudes and Antarctica. The strength of this oscillation is closely tied to variations in the temperature gradient across the Southern Ocean, particularly changes in sea surface temperatures (SST). As sea ice around Antarctica expands or contracts, it can shift the dynamics of the AAO. Additionally, the state of the ozone hole—its formation and recovery—also plays a role in influencing the strength and behavior of this pattern, further affecting the weather and climate across the Southern Hemisphere.