Are the changes in the Arctic messing with our weather? The Future of Blocking
In my blogs, I work hard to find words that are not overly burdened with jargon. I am struggling to do that with this blog, and say, keep it below 5000 words. Apologies in advance.
I ended the last entry by introducing the term blocking. Blocking describes a pattern of atmospheric flow, perhaps a particular configuration of the jet stream. Blocking slows or stops the normal movement of storms around the Earth, from west to east. Here is a link to 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 the 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 Northern Hemisphere, we logically look to changes in blocking.
There have been many papers during the past decade investigating blocking. In 2003, Pelly and Hoskins
open their paper “A New Perspective on Blocking,” with “Since the late 1940s it has been recognized that blocking is one of the most important aspects of the weather in middle latitudes: the usual mobile weather systems of the middle latitudes are diverted toward polar latitudes and the ambient westerly winds are replaced by easterlies.” They then investigate and interpret blocking in the context of developments in dynamical meteorology of the past 50 years. There are other studies looking at the challenges of predicting blocking in weather models and how well climate models represent blocking (for example, Scaife et al., 2010
There are number of studies that look at the representation of blocking in the CMIP5 models
that are used in climate-change assessments (for example, Dunn-Sigouin and Son, 2013
). These papers show that the climate models do reproduce the signature of blocking in their simulations; however, there are some differences between the observations and the simulations that suggest fundamental shortcomings in how blocking is represented in the models.
Recently Barnes et al. (2014)
searched for trends in blocking in the past 15 years using reanalysis data
. Barnes et al. investigate the sensitivity of the trends to the definition used to identify blocking and the choice of dataset. Though they do identify some significant trends, the sensitivity to choice of dataset and definition suggest high uncertainty in the conclusions. The difficulty of definition and identification of blocking combined with uncertainties in the observations and the short duration of the observational information are all challenges that will not be easily overcome, especially in the identification of trends. The CMIP5 models have also been used to make statements about the future of blocking and the Arctic Oscillation
. The suggestion from these models is that the Arctic Oscillation will trend towards its positive phase, which would suggest less blocking.
Part of my job these days is to explain to people planning adaptation strategies in a changing climate how to use the information from observations and models. In the case of blocking and the Arctic Oscillation, the recent weather and the work of Francis and Vavrus
suggesting a relation between changes in Arctic and the U.S. weather have opened important discussion and research. Understanding cause and effect is important because of implications for the future. If the recent behavior of the jet stream, the Arctic Oscillation and blocking is directly related to changes in the Arctic, then weather patterns are likely, quite abruptly in fact, to be far different in the next decades from what they were in the past decades. If the recent behavior is linked more strongly to global circulation (for example, Trenberth and Fasullo
), then we can, perhaps, expect a return to the weather patterns of the previous decades. However, if there is a return to conditions more like previous years, then the Arctic will be a fundamentally different place. It is and will be much warmer and much less frozen; these changes will alter the weather, though the spatial extent might be smaller than suggested by Francis and Vavrus
When I analyze the recent discussion of the Arctic Oscillation, blocking and the polar vortex, I am taken back to my early research. I have a long history of building models, and my research has often focused on the polar vortex, the wobbling of the jet stream and how mass is transported between the polar vortex and midlatitudes. In my experience, when building a global model, a common challenge is the representation of the polar vortex. It is easy to simulate a strong vortex encircling the pole, but difficult to simulate the wobbles. The wobbling is sensitive to many messy details, for example mountains, temperature contrast between land and water, mixing caused by waves in the atmosphere that are far smaller than represented in weather models and complex structure at the surface, like sea ice versus open ocean. All the evidence that I know suggests that the bias of too strong a polar vortex remains in climate models; therefore, there is a bias in the models for the Arctic Oscillation to be in its positive phase.
For many years, there has been a documented relation between blocking and edge of the polar vortex. When it comes to the simulation of blocking, this is a problem that requires exquisite fidelity of the numerical schemes used in the models. Specifically, and this is personal experience, how the numerical scheme dissipates momentum or energy at the smallest scales and how energy moves from large (weather) scales to small scales is important. This relationship is one between nonlinearity and dissipation, which is a neglected topic of attention.
In addition to these deficiencies in model formulation, another important issue in the models is where do you put the top of the model? Curious question. When a scientist builds a model, there is a choice of where to put the top. In order to represent the physics important for the polar vortex and the wobbles and wiggles in the jet stream at its edge, the top of the model needs to be far above the troposphere. The top needs to be above the stratosphere. The lowest model top that makes sense to me is 70-80 km, and some would argue that this is too low. Why does the top need to be so high? The energy in the waves that cause weather move, not only from west to east, but also by transporting momentum and energy upwards. The messy details that cause wobbles at the edge of vortex occur not just locally, but also at higher altitudes (High-top and low-top CMIP5 models
In terms of trends in blocking and potential changes in the Arctic Oscillation, there are fundamental elements of model formulation that need to be improved. There needs to be more attention to the numerical dissipation as well as better treatment of the upper atmosphere in CMIP models. With these improvements, then the recent results suggesting the importance of higher resolution and improved representation of topography become more convincing.
When I return to that part of my job explaining to people how to use the information from observations and models, here is how I frame this discussion. The observations of the past decade are intriguing. A number of researchers have pointed out the extreme meanders of the jet stream. My reading of the local versus global argument is that we are not likely to reach a yes-no answer. We are living in a time where the climate is changing quite rapidly. There is reason to expect that the changes in the sea ice will have large local effects – I would argue by analogy to look at the impacts on wintertime precipitation and storm development of whether Lake Superior is frozen or not. Further investigation will improve our knowledge of whether or not the effects of sea ice decline and broader changes in the Arctic have global effects. As for the information that models bring us about the Arctic Oscillation and blocking, I believe that this is a weak part of the discussion because there are underlying weaknesses in model construction and performance that limit how models inform these processes. At the start of this blog I stated that blocking is associated with the meanders in the jet stream, but large meanders do not always mean that the definition of “block” is fulfilled. Alternatively, large meanders in the jet will be directly associated with changes in the weather, and we do not require that “blocking” be realized to conclude that changes are real. That is why I include this body of research as part of the most important in the past year.
rLink to Rood Chapter on ReanalysisLink to Rood Chapter on Modeling Cold Weather in Denver: Climate Change and Arctic Oscillation (8)Climate Change and the Arctic Oscillation 2Climate Change and the Arctic Oscillation 1Wobbles in the BarriersBarriers in the AtmosphereBehaviorDefinitions and Some BackgroundAugust Arctic Oscillation presentationCPC Climate Glossary
“The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases.”