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Why so wet? Arctic warming is a factor

There are both meteorological (shorter term patterns) and climatological (mean warming climate) reasons for this past year having been the wettest year nationally on record. Marshall Shepherd, director of the University of Georgia Atmospheric Sciences program, has compiled a great deal of the evidence behind this extraordinary year in his latest Forbes magazine article. Here is the basic picture.

You’ll note even portions of our Southern Tier came in with record wettest totals. The good news here is the yearslong and extraordinary drought in much of the west and southwest has come to an end.

And the seasonal outlook through August shows no return of drought conditions across most of the nation.

(Sidebar: The return of luxuriant vegetative growth in parts of the west, especially California, will almost certainly mean a horrendous wildfire season by late summer and the fall. That part of the year is always the peak dry season, and this drying growth will provide a tremendous source of fuel for those fires at that time.)

Some of you are familiar with forecasts of increased and lengthier drought in arid regions as seen in the climate models going back to the 1980s. Those forecasts cover a span of many decades and extend into the next century. The mean over the next 100 years will see expanded and worsened drought in already arid regions.

But breaks in the trend will be showing up periodically due to increased blocking and stagnation in the jet stream. When the jet stream weakens/slows, it “buckles” more, sagging into blocked patterns which greatly inhibit west-to-east winds aloft. This weakening is tied to arctic warming.

As predicted, the loss of highly reflective ice on the Arctic Ocean, and the thinning of ice due to a shorter freeze season to be replaced by dark blue heat absorbing waters for more of the year has led to a rate of warming in the arctic that is twice that of the rest of the globe. What was not originally foreseen in the arctic warming was the frequent weakening in the polar jet stream that would result. The lessening of the temperature gradient between the high latitudes and mid-latitudes does now more often lead to blocking patterns. Much of this work was pioneered by former Rutgers researcher Jennifer Francis, now at Woods Hole.

When these blocks develop, lengthier periods of more extreme conditions can develop in parts of the world. As I’ve written before, blocking led to the stalling of Hurricanes Florence and Harvey, producing catastrophic flooding when they drifted inland. Even as I write this article on Monday, this morning’s upper air shows a high amplitude flow with partial blocking.

This particular pattern is expected to produce a potentially disastrous severe weather outbreak today, Monday, in parts of Texas and especially Oklahoma, along with devastating flash flooding.

Make no mistake, this severe weather potential is a weather event. But the evidence of a strengthening tie-in between the mean warming climate and individual extreme weather events has become more nearly conclusive in many cases, with much peer-reviewed evidence to support the link. The most glaring example of such a connection between climate and weather comes in the increased number of extreme rainfall events. The warming oceans and atmosphere have increased evaporation, putting more water vapor into the lower atmosphere. Look at the trend over decades:

The physics in this trend are relatively simple. Greater warmth = more water vapor. During periods of blocking with nearly stalled storm systems, this increased volume of what we call precipitable water leads to more devastating flooding more often. This next graphic is a bit complex but of particular interest to the northeast U.S. The upper left shows the percentage of annual precipitation falling in the heaviest 1% of events, from 1901-2016. The upper right panel shows how that percentage has accelerated in the northeast since 1958, instead of 1901. Basically, this documents more extreme rainfall events in the northeast are occurring more often. Much of this is due to increase evaporation from both the Atlantic and the Gulf of Mexico.

What has unfolded in the research of arctic warming are the numerous major impacts of that warming. There is now a sub-discipline tied to the research by Jennifer Francis and others called Arctic Amplification. Arctic Amplification involves focused research on both weather and climate effects tied to rapid arctic warming, some of which were not clearly foreseen back in the 1980s.

The tie-in between climate change and extreme weather events is brought to light in this tweet from Deke Arndt, one of our leading climatologists, in the Forbes article: “A way to think about the relationship between changing climate and Big Weather. Consider parent/child, teacher/student, coach/athlete, mentor/apprentice relationships. The parent (climate) rarely dictates day-to-day actions, but is hugely influential in long-term outcomes. ... Change the climate, and the things that dictate 'extreme behavior' are largely the same, but those ingredients are coming together in different ways. In other words, the drivers of his day-to-day behaviors are largely the same, but the trajectory of his life has changed.”

Not that many years ago, it was thought connecting the mean warming climate and individual extreme weather events was very difficult at best. Now, another sub-discipline called Attribution is showing many – not all – extreme weather events and their increased numbers have a visible link to climate change, most especially the accelerated warming in the arctic.

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