I'm a professor at U Michigan and lead a course on climate change problem solving. These articles often come from and contribute to the course.
By: Dr. Ricky Rood , 9:20 PM GMT on January 26, 2014
Are the changes in the Arctic messing with our weather? Analysis
In the last blog, I promised an analysis of why I conclude that what is happening in the Arctic makes it to my list of the big-ticket items of the past year.
I want to start with the work of Jennifer Francis and her collaborators. Professor Francis gave an excellent seminar in my department last week, which can be viewed here. This seminar uses as a foundation the paper Francis and Vavrus (2012), Evidence linking Arctic amplification to extreme weather in mid-latitudes. There is a whole set of coherent and convergent evidence that documents the changes in the Arctic. There is an increase in temperature, which is much greater in the Arctic than at lower latitudes and in the tropics (Polar or Arctic amplification). This has led to large changes in Arctic sea ice and springtime snow cover. There has been a lengthening of the growing season and an increase in activity in the northern forests – the greening of the Arctic (200 blogs ago, Getting Ready for Spring 5).
In the past, roughly, 15 years, there has been an observed change in the of the Arctic sea-level atmospheric pressure (see previous blog). The pressure is slightly higher, which leads to a weakening of the stream of air that flows around the North Pole. I wrote a tutorial about this in Wobbles in the Barrier. Also in the past decade there have been a number of researchers, for example, Liu et al. (2012) who in Impact of declining Arctic sea ice on winter snowfall – noted circulation patterns that have “ … some resemblance to the negative phase of the winter Arctic oscillation. However, the atmospheric circulation change linked to the reduction of sea ice shows much broader meridional meanders in midlatitudes and clearly different interannual variability than the classical Arctic oscillation.”
These papers lead to a few questions. Are the changes in the Arctic sea-level pressure a direct consequence of local changes in the Arctic, or are they more closely related to changes in global circulation patterns? Are changes in the Arctic sea-level pressure causing changes in weather in the middle latitudes? Are the differences we have seen in the past 15 years indicative of a climate-change related differences in weather patterns? Is what we have traditionally called the Arctic Oscillation changing?
Trenberth and Fasullo are following the heat of the warming earth, with the primary goal of understanding of how much heat is contributing to warming the Earth’s surface air temperature versus how much is going to heating the ocean and melting ice and snow. Their focus is on approximately the past 15 years. Therefore, they pay attention to known ways that the atmosphere and ocean vary (Some previous tutorials: Still Following the Heat and Ocean, Atmosphere, Ice and Land). Trenberth and Fasullo document the strong influence of the 1997-1998 El Nino. El Nino typically has a large effect on global temperature. The 1997-1998 El Nino was especially large. Trenberth and Fasullo show that the temperature in the atmosphere and oceans still remembers the 1997-1998 El Nino. They also examine the Pacific Decadal Oscillation, which is characterized by sea surface temperature differences being above (or below) average in the north-central Pacific while they are below (or above) in the north and east Pacific near the Aleutian Islands and the Gulf of Alaska. The Pacific Decadal Oscillation has been in a pattern of being cooler than average in the north and east Pacific since the 1997-1998 El Nino. Trenberth and Fasullo document a pattern that spans the globe, and the changes in the Arctic are part of that pattern. Conversely, their analysis would suggest that the global aspects of circulation pattern are too large to be caused by changes in the Arctic – it just takes too much energy.
What might be a scientifically based difference between whether changes in the Arctic are part of a global pattern or caused by the loss of sea ice changing the absorption and reflection of solar energy is to some extent not relevant to the question about weather patterns over the U.S. My experience in scientific controversies of this nature is that there are usually both global and local pieces to the puzzle. Further, changes in the U.S. weather could be directly linked to changes in the Arctic as well as to global patterns. In both the Trenberth and Fasullo and the Francis and Vavrus (2012) analysis there are consequential changes in jet stream pattern which is strongly influential to weather in the U.S. and, in fact, all of the middle latitudes of the Northern Hemisphere.
It’s not surprising that changes in the polar jet stream, the river of air that meanders around the North Pole, would have a profound effect on weather in the U.S. The waves that make up the weather systems of winter storms, for example, draw their energy from the environment that forms the jet stream. The jet stream steers these storms. In classes on dynamical meteorology, students learn that what is going on at the jet stream is often better information for forecasting weather than what is going on at the surface. Though there is a direct link between the jet stream and weather systems, the path of cause and effect in the changes in the Arctic, changes in the jet stream and changes to extreme events in the U.S. is not easy to map.
We have seen observations from Francis and Vavrus and Liu et al. (2012) that suggest large meanders in the jet stream. Both of these papers suggest that the scale of these meanders is unprecedented and does not fit easily into the framework we have used historically to describe the Arctic Oscillation - the primary way we describe correlated variability between the Arctic and the middle latitudes. In addition to the Arctic Oscillation, another characteristic we use to describe mid-latitude weather is blocking. Blocking describes a pattern of atmospheric flow, perhaps a particular configuration of the jet stream. Blocking slows or stops the normal west-to-east movement of storms around the Earth. Here is a nice description of blocking. Blocking is most common with high pressure, and high pressure is associated with the northern meanders of the jet stream. Note, blocking is associated with the meanders in the jet stream, but large meanders do not always mean that our definition of “block” is fulfilled. Blocking patterns are difficult to predict on a case-by-case basis. Blocking patterns are known to be associated with droughts, floods, heat waves and cold snaps. Therefore, when we look to a way that changes in the jet stream might change the weather over the U.S. we logically look a changes in blocking, which will discussed more fully in next blog.
Cold Weather in Denver: Climate Change and Arctic Oscillation (8)
Climate Change and the Arctic Oscillation 2
Climate Change and the Arctic Oscillation 1
Wobbles in the Barriers
Barriers in the Atmosphere
Definitions and Some Background
August Arctic Oscillation presentation
CPC Climate Glossary “The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases.”
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