Sunday, November 9, 2014

On The Origin of Tornadoes - 5

Bell Curve Projection (not to scale)
The U.S. has experienced about 75% of the world's tornadoes in recent decades (US Tornadoes) and for some portion of time before that (On The Origin of Tornadoes - 3).

The other ~25% of the world's tornadoes take place in a handful of countries in general: Canada, Mexico, Europe, Bangladesh, Australia, Argentina, and South Africa (ibid, "US Tornadoes").

We have seen abrupt climate change in action over the past several years in the U.S. with regard to the number of tornadoes:
The data (number of tornadoes each year per decade):

1950-1959: 206, 261, 246, 443, 570, 606, 518, 868, 576, 614
1960-1969: 619, 721, 667, 476, 719, 918, 593, 939, 668, 617
1970-1979: 671, 904, 747, 1115, 974, 924, 848, 861, 802, 865
1980-1989: 876, 786, 1066, 939, 925, 704, 779, 662, 704, 872
1990-1999: 1151, 1144, 1312, 1186, 1094, 1253, 1181, 1154, 1440, 1364
2000-2009: 1079, 1232, 957, 1415, 1842, 1267, 1142, 1117, 1739, 1182
[2010-2014: 1315, 1777, 957, ~940, ~500]

The averages for each decade above total to:
1950-1959: 491
1960-1969: 694
1970-1979: 871
1980-1989: 831
1990-1999: 1228
2000-2009: 1297
[2010-2014: 1098]
(Source: NOAA database). The total tornadoes recorded in the NOAA tornado database is 58,169 during 1950-2012, and when divided by 62 years (1950-2012) it calculates out to an annual average of 938 tornadoes per year during that span of time.
(On The Origin of Tornadoes - 3, [updated]). The data show that a peak of 1,777 tornadoes was reached in 2011, then an abrupt drop in numbers took place.

The count went down to ~957 tornadoes in 2012, ~940 in 2013, and on target for about 500 confirmed in 2014.

This series is about the origin of tornadoes, not just spinning winds inside thunder storms.

We contemplate the larger picture, the climate dynamics and geological features that cause ~75% of the world's tornadoes to happen in the U.S. as well as the fact that only about ~25% of the Earth's tornadoes happen in other countries, a handful of other countries at that.

The abrupt climate change event we address today has had an abrupt impact on the number of tornadoes.

That is because, in the larger picture, it has had and is having an impact upon the Arctic polar vortex:
A mechanism to explain how the behavior of the stratosphere may affect tropospheric weather patterns has been proposed by scientists at the
Normal polar vortex
University of Illinois. If correct, the idea could be included in models to better understand the climate system and predict the weather.

“Recent observations have suggested that the strength of the stratospheric polar vortex influences circulation in the troposphere,” said Walter Robinson, a UI professor of atmospheric sciences. “We believe there is a weak forcing in the stratosphere, directed downward, that is pinging the lower atmosphere, stimulating modes of variability that are already there.”

The polar vortex is a wintertime feature of the stratosphere. Consisting of winds spinning counterclockwise above the pole, the vortex varies
At (d) a piece breaks away
in strength
on long time scales because of interactions with planetary waves global-scale disturbances that rise from the troposphere. “The polar vortex acts like a big flywheel,” Robinson said. “When it weakens, it tends to stay weakened for a while.”

Other researchers have noted a statistical correlation between periods when the polar vortex is weak and outbreaks of severe cold in many Northern Hemisphere cities.

“When the vortex is strong, the westerlies descend all the way to Earth's surface,” Robinson said. “This carries more air warmed by the ocean onto the land. When the vortex is weak, that's when the really deep cold occurs."
(Polar Vortex, Wayback Machine, emphasis added; cf. Science of Polar Vortex; Polar vortex 1853: The Living Age, Vol. 39 p. 430; 1971 Paper: Polar Vortex; Polar Vortex; Polar Vortex Can Break Up).

These surges of cold pieces of the vortex breaking off and flowing south as a large circulation --a large cold air mass --is forced by ongoing polar sea and air warming and the consequent polar ice melt.

The oceans and air at the poles are warming at a faster rate than they are at the lower latitudes of the globe.

Warming which eventually forces some of the cold away from the poles to the south, and thus destabilizes the polar vortex.

That warming melts ice and weakens the polar vortex which is spinning like a flywheel, and parts of the flywheel can fly off and flow south when conditions converge to make it so.

The usual U.S. tornado wind generating patterns are disrupted by the cooler or colder air masses, so fewer situations occur that typically generate tornadoes:
Under normal climate conditions, cold air is confined to the Arctic by the polar vortex winds, which circle counter-clockwise around the North Pole. As sea ice coverage decreases, the Arctic warms, high pressure builds, and the polar vortex weakens, sending cold air spilling southward into the mid-latitudes, bringing record cold and fierce snowstorms. At the same time, warm air will flow into the Arctic to replace the cold air spilling south, which drives more sea ice loss.
(Wunderground, emphasis added). A feedback loop develops which then accelerates the process of the ongoing disintegration of the polar vortex.

This cold air impact removes certain conditions in the unique U.S. birthplace, the tornado-favorable birthing environment:
"The reason the heart of the North American continent bears the brunt of these most powerful of twisters lies with its unique topography. Only in North America does a solid land mass stretch from the sub-tropics to the arctic with no mountain barriers to inhibit the mixture of air masses originating from these two regions."
(On The Origin of Tornadoes - 2, quoting Wunderground). Hence, lately we see lower tornado counts as the vortex degenerates and its pieces flow south to chill out the unique tornado birthing areas.

Add to that, on today's occasion, a typhoon (Nuri) remnant that has travelled past Japan on up into Alaska, and is now having an impact on jet streams and the polar vortex.

That is also forcing sections of the vortex to break away as a cold air mass that moves to the south (How the Polar Vortex is Affecting the American South).

Soon enough, when all the Arctic ice melts, and the Arctic Ocean warms enough, the polar vortex will eventually totally disintegrate.

The tornado count is likely to rebound significantly and severely then, as the impediments to tornado formation eventually subside (cf. IPCC Latest Report).

The next post in this series is here, the previous post in this series is here.

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