Climate Denial Crock of the Week

The real debate in atmospheric science is, of course, not whether man-caused climate change is happening, but exactly how that change is playing out – affecting atmospheric circulation, and thereby, our weather.

I’ve covered this debate from early on by interviewing proponents of two seemingly opposing viewpoints, one, made famous by Dr. Jennifer Francis of Rutgers, is that changes in Arctic ice cover may be breaking down the temperature gradient between north and south, and thereby affecting the flow of the northern hemisphere jet stream – causing weather to get “stuck”, like this past winter’s “Ridiculously Resillient Ridge”, which brought warmth and rain to the arctic while pouring frigid arctic air onto eastern north America.

Conversely, Dr. Kevin Trenberth maintains the heat flux in polar regions is not large enough to drive changes on the scale which we are observing. The answer, he feels, is in the tropics.

Tim Palmer of Oxford University, writing in the new issue of Science, adds a new puzzle piece that might help square the circle.

Science (paywalled):

Given this overall decreasing tendency in cold winters, it seems impossible to argue that the record-breaking snowy winter in the Midwest could be connected to climate change. However, there is a plausible link. To understand this link we must consider the atmospheric circulation patterns that were associated with this winter and ask whether there is evidence that climate change might have increased the likelihood of these patterns. Large-scale atmospheric circulation patterns are controlled by the position of the jet stream. The Northern Hemisphere jet stream flows from west to east at mid- latitudes; it deviates from a line of latitude through a series of ripples called Rossby waves. Regions above which the jet stream is flowing from the north are likely to expe- rience cold weather. Conversely, in regions above which it flows from the south, the weather is likely to be relatively warm. The larger the amplitude of the Rossby waves, the more anomalous the weather is likely to be at the surface.

Why does the jet stream across the United States produce cold winters in the Midwest and on the east coast? A key factor is the strength of thunderstorm activity in the tropical West Pacific, associated with above- normal sea surface temperatures (SSTs) in this region (2). Latent heat release in this region, which occurs as warm moist air condenses into water droplets in these thunderstorms, acts as a source of energy to excite particularly large-amplitude Rossby waves in the Northern Hemisphere jet stream, with just the right phase to produce cold weather in the Midwest and East Coast. In the 2013– 2014 winter, SSTs have been particularly warm in the tropical West Pacific .

Since the late 1990s, global mean temperatures have been rising quite slowly. This pause or “hiatus” in global warming is linked to an increase in the westward trade winds across the tropical Pacific (4). The ocean currents, forced by these intensified trade winds, have been drawing down much of the excess heat associated with human climate change to the deep ocean. How- ever, one region that has not been cooling is the tropical West Pacific. Over the period of the hiatus, warm surface waters have been piled up in the tropical West Pacific by the intensified trade winds.

The surface waters in the tropical West Pacific will have been warmed further during this hiatus period through the local effects of man-made enhanced greenhouse gas forcing. Even though this enhanced warming may be small (it is not currently possible to estimate reliably its magnitude), its effect can be important in a region where SSTs are among the highest in the world. Any further warming in this region will lead to a relatively large increase in atmospheric water loading, leading to unusually strong latent heat release. Consistent with this, there was a very active typhoon season over the tropical West Pacific in 2013, including typhoon Haiyan, which devastated parts of the Philippines. These intense tropical weather systems continued into the 2013−14 winter season. Anomalous latent heat release in the tropical West Pacific can produce a particularly strong Rossby wave response in the Northern Hemisphere. The phase of this Rossby wave response is consistent with the cold and snowy season seen in the U.S. Midwest and East Coast. If this line of argument is correct, the extremely cold and snowy season in parts of the United States may indeed have been caused at least in part by increased greenhouse gas concentrations.