Why does the earth get so cold in winter even it gets hotter?
The Negative Phase of Arctic Oscillation
Polar Vortex
Polar Vortex is low pressure region of cold air rotating from west to east. It changes shape over time because of the shifts in atmospheric pressure and temperature.
source: sciencephoto
AO & its two modes
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What is Arctic Oscillation (AO)?
AO is a pattern that the polar vortex shifts between being stronger and weaker from days to months. It describes how pressure patterns are distributed over the Arctic region and the middle latitudes of the Northern Hemisphere.
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Two modes in AO
When the difference in pressure between the middle latitudes and a polar region is strong, the AO is in positive mode. And the negative mode is when the difference is less strong. The AO can persist in either phase.
Jet Stream, Polar Vortex, and AO
Jet stream has a quite important role in this mechanism. It is positioning in lower latitude than the Polar Vortex so it can prevent the polar vortex from moving toward south regions in general.
When the Arctic Oscillation is in its positive phase, the jet stream tends to shift to the north, so the polar vortex is generally a neat, tight circle. However, when the AO is in negative mode, the vortex is wavier and becomes more influential to the south.
Positive and negative phases of the Arctic Oscillation
source: Greendale Weather
Cold Wave Mechanism related to AO
source: straitstimes
To describe the cold wave mechanism in South Korea, therefore, we can say that because of the climate change, the jet stream gets weaker so the polar vortex moves to south with cold air of polar regions in the negative phase of AO. Then, we finally feel this cold wave in winter.
It’s true that AO is powerful, but scientists say that we should notice that it is not a single factor that affects the weather, usually affecting along with other large scale patterns such as El Nino and La Niña.
Appendix
*You can get AO data in this site:
https://www.ncdc.noaa.gov/teleconnections/ao/
The pictures below are the examples in the site.
AO data from 1960 to 2020
AO data from 1995 to 2020
AO data from 2008 to 2020
Source: NOAA