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OUT OF THIS WORLD | What's Up In Climate Change - a weekly glance at the most important news about our changing world

Tornadoes are clustering into more extreme outbreaks: study

Scott Sutherland
Meteorologist/Science Writer

Thursday, March 3, 2016, 2:10 PM - Tornadoes clustering into more extreme outbreaks, a February that shatters temperature records and picks away at the Arctic ice sheet, and the longest global coral bleaching event ever observed continues. It's What's Up In Climate Change!

Extreme tornado outbreaks are more common

On April 27, 2011, the United States was hit by the biggest tornado outbreak every recorded. A total of 363 tornadoes were recorded - the greatest number ever seen in one outbreak - and according to a new study out of Columbia University, these kinds of outbreaks have become more common over the past 60 years.

Map of all tornado (red), severe thunderstorm (yellow), and flood (green) warnings issued on April 27, 2011. Credit: NWS/Wikimedia Commons

According to Columbia's Earth Institute:

For this study, the authors calculated the mean number of tornadoes per outbreak for each year as well as the variance, or scatter, around this mean. They found that while the total number of tornadoes rated F/EF1 and higher each year hasn't increased, the average number per outbreak has, rising from about 10 to about 15 since the 1950s.

To do this, the authors used a method known as Taylor's law, which shows how the density (number of individuals per area) of a population - be it plants, animals, stars, or even tornadoes - is a function of the average density of the population raised to some exponential power. If the exponent is simply 1, the population is evenly spread throughout the area. Higher exponents reveal greater and greater clustering of the population.

When applied to tornadoes over the past 60 years, they found that the mean, as quoted above, had risen from 10 tornadoes per outbreak to 15 tornadoes per outbreak. What was truly remarkable here was that the variance they found - exactly how much these tornadoes were clustering into severe outbreaks - was extremely high.

"In most ecological applications, the Taylor exponent seldom exceeds 2. To have an exponent of 4 is truly exceptional," says study coauthor Joel Cohen, a researcher and professor at Rockefeller University's Laboratory of Populations and Columbia University's Earth Institute. "It means that when it rains, it really, really, really pours."

While the authors do not pin this down as having a direct link to climate change, it supports previous research which has stated that, with more energy in the atmosphere due to global warming, "there is a low probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes."

February shatters temperature records, at the surface and above

We're two weeks away from NOAA and NASA releasing the official numbers on February's global temperatures, but the data is already revealing just how extreme the month was, when compared to what the globe usually sees at that time of year.

A comparison of just the past three Februarys reveals just how hot Feb 2016 was. Credit: WxBell/S. Sutherland

While the global anomalies seen in the animation may not match the official numbers from NOAA and NASA, due to differences in calculations, the overall pattern is consistent enough for comparison.

This spike was not only recorded by surface measurements, but by satellites reading temperatures in the lower atmosphere as well:

Credit: Dr. Roy Spencer/University of Alabama

Although the temperature spike during the 1997/98 El Niño dominated the satellite records of the February global lower atmosphere temperature anomalies until now, Feb 2016 has surpassed that record by nearly a tenth of a degree Celsius.

What is all that heat in the Arctic doing?

For over three-quarters of the month of February - from Feb 2-6 and again from Feb 11-27 - Arctic sea ice was at its lowest extent on record, roughly 200,000 square kilometres smaller than any previous low. At a time when sea ice extents are still growing towards the winter maximum, for the second of those record lows, the extent was actually declining.

Credit: National Snow and Ice Data Center (NSIDC)

According to the latest report from the NSIDC: "February 2016 sea ice extent was the lowest in the satellite record at 14.22 million square kilometers (5.48 million square miles)."

The NSIDC notes that the Arctic was predisposed to low extents starting this year, due to a cyclonic (counter-clockwise) circulation around the region, which limited the amount of sea ice drift in January. This was further impacted by anomalies, such as "Storm Frank", which brought very warm temperatures across the Arctic and strong winds pushing sea ice north of Europe closer to the pole. 

A late February/early March increase in sea ice has lifted the extent out of the record lows, putting it at roughly the same level as years 2006, 2011 and 2015. This is apparently due to an anticyclonic (clockwise) circulation of winds around the Arctic, which, according to the NSIDC "favored enhanced ice export out of Fram Strait, helping to flush old, thick ice out of the Arctic Ocean, leaving behind thinner ice that is more apt to melt away in summer."

"Whether this circulation pattern will continue and set the stage for very low September sea ice extent remains to be seen," the report added.

Largest global coral bleaching event is not slowing down

The world's coral reefs are in trouble.

Increasing ocean temperatures are stressing these important ecological niches to their limit, and it is resulting in the largest global coral bleaching event ever recorded.

90 day animation of alert levels for coral reefs around the world. Credit: NOAA Coral Reef Watch

Corresponding 90 day animation of ocean temperature anomalies around the world. Credit: NOAA Coral Reef Watch

As a comparison of the two animations above shows, it is the regions of highest temperature anomaly that have the greatest impacts on coral reefs, not only in the central Pacific, where a strong El Niño currently persists, but throughout the southern hemisphere (where the majority of the world's coral reefs are located).

Is this getting any better? Not according to NOAA's Coral Reef Watch, as their March-June outlook map reveals below:

Credit: NOAA Coral Reef Watch

Although the service also releases a map which shows the alert levels which at least 60 per cent of their models are flagging for the coming months, this high-confidence map (at least 90 per cent of models indicating Watch or higher potential stress levels) reveals just how bad things are expected to get through the spring.

Sources: Columbia University | WxBell | Roy Spencer/U. of Alabama | NSIDC | NOAA Coral Reef Watch

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