Scientific study: The development of the La Niña phenomenon and its association with the collapse of the stratospheric polar vortex during the winter
Arab Weather - With the Northern Hemisphere entering the astronomical fall season, we at the "Arab Weather" Center shed light on the weather phenomena expected during the current winter season 2024/2025, the most important of which indicates the development of what is known as the tropical La Niña phenomenon in the Pacific Ocean. What are the possible effects during the current season with the development of this phenomenon?
Weather forecasters at Arabia Weather said that archival data indicates that the La Niña phenomenon has been active in many previous winter seasons over the past decades, and this allows us to analyze previous data to see how winter weather patterns typically develop under the La Niña phenomenon.
How does La Niña affect weather in the Northern Hemisphere?
La Niña is a tropical phenomenon that occurs in the tropical waters of the Pacific Ocean and is characterized by a cooling of the surface waters in that region. It greatly affects rainfall patterns and pressure in tropical regions, which leads to a change in the feedback system between the atmosphere and the ocean. The impact of La Niña appears globally through this system, which leads to a change in temperature patterns and snowfall in the winter in the Northern Hemisphere.
La Niña typically creates a high pressure system over the North Pacific Ocean, which promotes the development of a low pressure area over Alaska and western Canada and pushes the jet stream down between the two pressure systems, meaning more rain in the northern United States and warmer than usual weather in the southern states. Meanwhile, the northward push of the polar jet stream creates a high pressure system over Europe that acts as a barrier to low pressure systems coming from northern Europe, concentrating rain and snowfall on northern European countries.
The evolution of the La Niña phenomenon and its association with the collapse of the stratospheric polar vortex during the winter
A strong polar vortex usually means a strong polar circulation, which usually traps cold air in the polar regions, resulting in milder weather conditions across much of the United States and Europe. Conversely, a weak polar vortex means that the polar jet stream can break free and is less able to contain cold air, which can be pushed from the polar regions into the United States and Europe, and possibly into lower latitudes in some cases.
That's why we look at the wind speeds within the stratospheric polar vortex to determine its strength. The stronger the polar vortex, the stronger the winds it produces in the stratosphere. A weak polar vortex has weaker winds in the stratosphere. So by looking at the current forecasts of wind speeds in the stratosphere, we see the potential for temporary warming, which means a weakening of the polar vortex and its rotation.
What could a weakening polar vortex bring to weather patterns around the world?
Experts at Arab Weather say that the weakness of the polar vortex is linked to cold weather and snow across the eastern United States and Europe, but the weakness of the polar vortex does not mean the presence of stratospheric warming. Usually, if we want to see strong stratospheric warming, the temperature and pressure in the stratosphere must rise significantly, which leads to the collapse of the polar cycle above. Weather data indicates that the weakness of the polar vortex may often lead to the extension of the polar jet stream to large parts of the European continent, especially its center and west, which leads to significant cold weather and waves of snow at times.
How does La Niña and the weakening of the polar vortex affect the Arab region?
Usually, according to climate statistics, the development of the La Niña phenomenon and the weakness of the polar vortex are associated with more stable periods in the Arab region, especially the eastern Mediterranean and the Arabian Peninsula, represented by weak rainfall and colder weather with the possibility of extreme cases and occasional snowfall in high mountainous areas, while the situation is different in the central Mediterranean, where the collapse of the polar vortex affects the rush of polar winds towards the center and west of the European continent, extending to the countries of the central Mediterranean and the Maghreb, which means higher than average rainfall and perhaps snow over high elevations, but other factors cannot be ignored, represented by the behavior of the North Atlantic coefficient and the Arctic Oscillation coefficient, which play an important role in directing the polar jet stream, which is often considered rapidly changing.
What is the relationship between the development of the La Niña phenomenon and the collapse of the polar vortex?
Statistically, the probability of a stratospheric warming event (SSW) occurring in a La Niña winter is more than 60%, and studies have shown this in some winter seasons that were accompanied by the La Niña phenomenon. However, despite this, one cannot completely rely on the statistical factor and ignore other weather factors that have a significant impact on weather patterns around the world.
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