A Visibly Extreme Jet Stream in Advance of Irma

On Tuesday, I wrote this blog about how Jet Stream behavior and related severe weather during summer of 2017 jibed with the findings of recent climate science. About how human-forced polar warming appears to be impacting extreme summer weather patterns by altering the upper level winds — with a particular focus on impacts to North America.

Yesterday, I looked at the upper level wind patterns running over North America in advance of Irma’s approach and saw this:

(Classic ridge-trough pattern like that identified by Dr Jennifer Francis and Dr Michael Mann. One that, according to their related research, increases the likelihood of certain kinds of extreme weather patterns and events. One that these scientists associate with polar warming set off by human-caused climate change. Image capture from 1500 UTC on September 6. Image source: Earth Nullschool.)

It’s a classic high amplitude wave form in the Jet Stream. One that shows an extremely deep trough digging all the way down to the Gulf Coast in the east and arching back up into a pointed ridge north of Alaska and into the Arctic Ocean in the west. This kind of high amplitude wave pattern is not typical. Or if such a pattern did appear in the past, it tended not to stick around for so long. But during this summer, such intense high amplitude ridges have been forming again and again over the west and such deep troughs have been forming again and again in the east.

New Precipitation and Temperature Extremes

The most apparent visible effect of this ridge-west — trough-east pattern has been to produce record heat, drought, and wildfires in the west and record rainfall in conjunction with an extremely stormy weather pattern in the south and east. You can plainly see this dipolar relationship in the precipitation and temperature anomaly maps provided by NOAA below:

These maps cover precipitation and temperature observations for the last 30 days compared to climatological averages. In the west we find that precipitation for large regions has been less than 10 percent of normal (less than 1/10th normal). Meanwhile temperatures in the west have ranged between 1 and 4 C above average. In the south and east, large regions have seen between 200 and 800 percent of typical precipitation amounts (2 to 8 times the norm). Temperatures, meanwhile have ranged between 1 and 3 C below average.

This is the very definition of heightened extremes. Looking at the prevalent upper level air pattern over the U.S. for the summer of 2017, it’s clear that south to north upper level winds pulling air up from the Equatorial zone toward the pole are facilitating one side of the extreme and that a countervailing upper level wind originating near the pole and running south toward the tropics is driving the opposite extreme.

Slowing Upper Level Winds in a North-South Orientation Weakens the Steering Currents

Unfortunately, prevalent and long lasting heat or heavy rainfall isn’t the only apparent impact of this new pattern. Another aspect of this extreme dipole is a weakening of the west to east steering currents that typically begin to pick up in a region between 25 and 30 degrees North Latitude and to intensify further beyond the 30 N line. This effect is due to the fact that upper level wind patterns are oriented more in a north-south (west) or south-north (east) direction and due to the fact that under such large Jet Stream meanders the upper level steering winds tend to slow down.

(It’s not just Harvey and Irma. Weak upper level steering currents are contributing to a long range potential that Jose might loop back to strike South Florida.)

For Hurricanes like Harvey and Irma, stronger west to east steering winds have had two protective effects for the United States. First, they have helped storms to keep moving — working to generally prevent the kind of long duration stall we saw that helped to produce such catastrophic flooding during Harvey. Second, they have tended to deflect storms away from the U.S. East Coast. And for Irma, what this means is that this storm is more likely to strike the U.S. East Coast if the upper level steering winds that would typically turn it to the east are weak.

This is a dynamic upstream aspect of human-forced polar warming. One that produces added extreme weather risks on top of those already generated by warming ocean waters — which increase peak potential storm intensity — and rising atmospheric water vapor — which helps to add latent heat, lift and related convective available potential energy that increases top limits for storm intensity and heavy rainfall.

And as we sit here hoping and praying that Irma will re-curve away from the U.S. east coast, we should consider how polar warming may be helping to make such a terrible strike more likely — increasing risks to so many people and to so much that we all hold dear.



Dr Jennifer Francis

GFS Model Runs illustrated by Earth Nullschool

Extreme Weather Events Linked to Climate Change’s Impact on Jet Stream

This is the Pattern Climate Scientists Warned us About


Hat tip to Scott

Hat tip to Wharf Rat

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