Why Is Global Warming Causing An Increase In The Likelihood Of Extreme Weather Events?

in #meteorology3 years ago

The most obvious potential reason that comes to mind might be the fact that with more thermal energy being trapped in the atmosphere by the higher concentration of CO2 in the atmosphere there'd be more energy to drive dynamic phenomena such as winds in it. But what really drives all wind systems is temperature differences between locations. However, when it comes to extreme weather events in the temperate regions, there are some twists.


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One of the biggest drivers of prevailing winds is the temperature difference at surface level between the polar regions and lower latitudes. What happens in the tropics where solar radiation is strongest is that air heated by the sun rises to an altitude of up to 16-18 km where the lower and denser part of the atmosphere, the troposphere meets the stratosphere. Above the poles, the upper edge of the troposphere is usually at an altitude of about 8 km. The strongest convection zone is at the Intertropical Convergence Zone usually close to the equator from which the air flows away from the tropics to descend at around the 30th latitudes both north and south of the equator. Much weaker convection zones occur at around the 60th latitudes from which the some of the air flows toward the poles (in each of the hemispheres respectively) and back toward the 30th latitudes where air entering the upper troposphere tends to sink toward the surface. In the winter a large amount of sinking of air towards the ground occurs above that polar region which is undergoing winter at the moment. That happens because during the winter in the polar regions, there is no solar irradiation at all and that's when the temperature difference between that pole and the lower latitudes is at a maximum.


A polar vortex centered above the arctic parts of North America

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The sinking air in the upper parts of the troposphere forms a vortex (because of the Coriolis effect cause by the rotation of the planet). On the upper edges of the troposphere above the polar regions, there are bands of high velocity winds called the polar jet streams. The strength, speed and the directness of the trajectory a polar jet stream is dependent on how large the temperature difference between the polar region and the lower latitudes is. There are cyclical variations to it, some multi-decadal such as the North Atlantic Oscillation, but because it has been observed that the overall warming of the planet is causing the polar regions to warm faster than the rest of the planet, jet streams have a tendency to become weaker.


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Why are jet streams important? That's because they have been observed to control the trajectories of frontal low pressure systems at the surface level. When polar jet streams become weaker, they begin to meander. What that means is that surface-level frontal (meaning fronts between different air masses in the mid latitudes) low pressure systems as well as high pressure areas between them move slower. Blocking high pressure pressure systems have always had the tendency to be able to stay put for weeks. With weakening polar vortices and polar jet streams, the tendency of a type of weather prevailing longer at a certain region becomes more pronounced. What this means is that not only does the overall warming of the atmosphere cause heatwaves to become hotter but their duration will increase as well. In those areas where solar irradiation is sufficient with cloudless summer skies to cause the temperature of air at sea level to increase to levels uncomfortable or even dangerous to humans (such as warm temperate areas), such events will become more likely. Rainfall lasting for weeks will become more likely as well.


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