In 2011, a historic drought severely impacted Texas. Shortly after, record floods hit the central US, pushing the Mississippi and its tributaries to record high levels while ending the Texas drought. One year later, a 55 year drought hit the heartland of the US pushing the Mississippi back to record low levels in many regions.
Enter April 2013 and the weather has once more swung back to the flood extreme of what appears to be a new Drought, Flood, Drought paradigm. This winter, unusually heavy precipitation created a much larger than normal snowpack for the north-central US. Then, in April, warmer weather and a series of heavy rain events combined to push the Mississippi River and its tributaries from record low levels to record and near record high flood stage in many locations.
Now, along the Mississippi and its tributaries, flood gauges are recording moderate to severe flooding at over 60 stations. But this selective dump of large volumes of precipitation over the Mississippi River valley has still left much of the western US in the grips of drought. In fact, 47% of the contiguous US is still suffering from drought even as many places in the Heartland flood.
How can this happen? How can historic and opposite weather extremes such as severe drought and flood repeatedly affect the same region year after year? The answer lies in a re-currence of powerful blocking patterns that keep the polar jet stream in a fixed position for longer periods of time. The result is that weather in a given region tends to persist for longer and longer periods. So if weather gets stuck in a hot and dry pattern, as it did from April to December of 2012, then severe heat waves and drought conditions are most likely to follow. And if the jet stream switches back to a position where it plunges down from the Arctic, expect a long period of cooler and much wetter, stormier conditions, as the Central US experienced January to April of 2013 and which has resulted in the current major flood events.
Looking at the ECMWF weather model forecast for Friday, May 3rd, we can see a persistence of the cool, stormy, wet pattern continuing for the Central US.
Notice that long tongue of colder air plunging down all the way from northern Canada, through the central US and down into Texas? Sweeping along the trough is a low which will likely bring even more rain to flood-stricken areas later this week. This is the basic pattern that has persisted for the central US all throughout 2013. And the result is more cool air, more storms, and more precipitation for that region persisting for weeks and months on end.
If we look to the west of this cold, wet trough zone, we can find its culprit. A blocking high pressure system that has parked itself just west of the US and Canadian Pacific coasts since this past January. This high is pulling warm, drier air up from the south. It is responsible for persisting drought conditions for the western US. And it is responsible for a big northward bulge in the polar jet stream running up over west-central Canada before an equally exaggerated southward swoop plunges down into the central US.
Classic blocking pattern.
Now, if we rewind to last year, we find an opposite jet stream configuration emerging as a result of a powerful blocking high pressure system forming directly over the central US and bringing record hot temperatures over a broad region. It is the increasingly frequent emergence of these powerful blocking systems that are keeping the weather in a drought-flood bipolarization for the central US.
A growing number of climate scientists led by Jennifer Francis are attributing the greater frequency of blocking patterns and associated extreme weather events to the massive loss of Arctic sea ice since 1979. Overall, 80% of Arctic sea ice volume has been lost at end of Summer over the last 33 years. These climate scientists make a compelling observation that this ice which once trapped cold air to the north and kept warmer air confined to the south, has lost its insulating properties. Now, more warm air invades the Arctic even as more of the Arctic’s colder air tends to seep out into the mid-latitudes. The result is that the polar jet stream, which is powered by north-south temperature differences, both moves slower and forms the large, persistent, blocking wave patterns.
Such a climate regime of more persistent patterns of either extreme wet, cold, stormy weather or extreme warm, dry, drought conditions is likely to amplify as sea ice continues to recede and melt out. These conditions will probably worsen until glacial melt from Greenland reaches a tipping point. Once this tipping point is reached, cold ice bergs will invade the North Atlantic, pushing the cold air pole south and concentrating much colder air around the region of Greenland and the North Atlantic. At the same time that the North Atlantic becomes colder, the tropics become warmer. The result is a rapid acceleration of the polar jet in the region of the Atlantic Ocean. The large temperature differentials caused by this new climate state are likely to drive very powerful storms. It was the potential for such conditions to emerge by or after the mid 21rst Century that inspired Dr. James Hansen to write his ground breaking book “The Storms of My Grandchildren.”
After the current blocking pattern regime switches to the rapid Greenland ice melt regime, Dr. Hansen warns of the potential for ‘continent size frontal storms that pack the strength of hurricanes.’ Such storms would make Sandy seem like kitten’s play.
How do we avoid a continued worsening and more extreme climate? Simple. Stop emitting CO2 into the atmosphere. The sooner CO2 emission reduction and elimination policies are put into place, the less likely the very worst weather changes will emerge. But, until we make the wise, rational choice of CO2 reduction and elimination, we consign ourselves to what is most likely to be a decadal period of worsening and more extreme weather.