Candid Admission By UCAR – “Blocking The Way – Predicting The Atmospheric Detours That Lead To Weather Troubles”

There was an interesting article in the Fall issue of the UCAR Magazine titled by Bob Henson titled

Blocking The Way – Predicting The Atmospheric Detours That Lead To Weather Troubles

Although embedded in the article are the usual claims by some that are quoted that future climate can be skillfully inferred from what the models already produce, this article includes text and quotes by several climate scientists who candidly discuss limitations in current climate models to skillfully predict a major aspect of the atmospheric circulation. The article focuses on what are called “blocks” and it is these restrictions to the east-west movement of weather systems that can persist for months that produces record weather such as the 2011 heat and drought in Texas.

Excerpts from text of the article read [highlight added]

The concept of atmospheric blocking might not be familiar to the general public, but millions have come face to face with the results of spectacular blocks over the last couple of years. Every so often, a dome of upper-level high pressure sits in place for a few days, sometimes as long as several weeks. A major block can produce seemingly endless stretches of blazing heat or bitter cold. It also blocks the typical eastward flow of the polar jet stream (thus the label “blocking”) and throws storm systems far from their usual tracks. Along those displaced paths, the storms can generate successive bouts of heavy rain or snow. By the time it dissipates, a major block may leave behind a whole stack of broken weather records and an array of disastrous consequences.

Relentless high pressure over Russia led to unprecedented summer heat in 2010, with estimates of more than 10,000 people killed either directly or indirectly. Toward the south side of the Russian block, unusually strong monsoonal flow sent vast amounts of moisture into Pakistan, leading to catastrophic flooding. And when huge bubbles of high pressure popped up in and near Greenland over the last two winters, cities from Washington, D.C., to London found themselves grappling with heavy snow even as parts of the Arctic experienced periods of record mildness.

The recent high-profile blocks have put questions of predictability on the front burner. There’s more than a touch of mystery in what makes a block form and dissipate. Phenomena such as El Niño tend to favor blocking in specific areas, but it’s still difficult for weather prediction models to peg the start and stop times of a particular block. And climate models tend to underestimate the frequency of blocking, which could have an influence on their seasonal-scale averaging of future climate.

“Blocking highs significantly influence climate events over large portions of the Northern Hemisphere,” notes James Hurrell, the new head of the NCAR Earth System Laboratory.

In the case of Russia, the team’s initial report pointed to a block that was unprecedentedly strong and unusually long-lasting, with a particularly intense stretch from early July into mid-August. On average, the region gets only about 10 blocking days during those two months. The strength and duration of the 2010 block allowed plants and soil to dry out, which helped send surface air temperatures into uncharted territory. Forest fires and long-burning fires in peat bogs poured smoke into the stagnant block, which degraded air quality and added to the heat wave’s deadly impact.

How much could global warming have contributed? Although the heat wave unfolded during one of the warmest years on record globally, the NOAA group found no evidence of a significant trend in blocking during July over western Russia in the last 60-plus years of upper-air records. And they noted that average July surface temperatures have not risen significantly over western Russia, unlike some other parts of the world and the world as a whole.

Climate models don’t yet have a firm enough handle on blocks to tell us exactly how they’ll change in coming decades. NCAR’s Neale is one of three co-chairs of the working group that oversees the atmospheric component of the NCAR Community Earth System Model (CESM) and its predecessor, the NCAR Community Climate System Model (CCSM4). By and large, he says, climate models do a good job depicting extratropical cyclones, the lows that regularly sweep across midlatitudes. Blocking is a different matter.

“The jury is still out on how blocks develop, how they become persistent, and how they break down,” says Neale.

According to NCAR’s Clara Deser, “The future state of the NAO is highly uncertain due to the large amount of internal variability, even on 50- to 100-year time scales, compared to the changes driven by increased levels of greenhouse gases.” Deser has carried out several model-based analyses of future ocean-atmosphere interactions in the North Atlantic and Pacific.

As climate models get a better handle on blocking and on Arctic sea ice, they may paint a more consistent and realistic picture of details in the Northern Hemisphere’s future climate. “There is not really a robust statement yet on the future of blocking,” says Scaife.

This article is an informative summary of this subject, and the entire article should be read.

source of image

Comments Off on Candid Admission By UCAR – “Blocking The Way – Predicting The Atmospheric Detours That Lead To Weather Troubles”

Filed under Climate Change Forcings & Feedbacks, Climate Models

Comments are closed.