Nonlinear Response of Midlatitude Weather to the Changing Arctic

Are continuing changes in the Arctic influencing wind patterns and the occurrence of extreme weather events in northern midlatitudes? The chaotic nature of atmospheric circulation precludes easy answers. Yet the topic is a major science challenge, as continued Arctic temperature increases are an inevitable aspect of anthropogenic global change. The question is not whether the melting Arctic will influence midlatitude weather patterns over the next decades, but rather what is the nature and magnitude of this influence relative to non-Arctic factors, and is it limited to specific regions, seasons, or types of weather events? Arctic temperature increases impose thermodynamic influences through increase in geopotential thickness and changes in horizontal temperature gradients.  Complexity comes through coupling to atmospheric dynamics, that singular Arctic-midlatitude linkage pathways are unlikely as nonlinearities in the climate system are particularly important in the Arctic and subarctic. A linkage pathway that may appear to be responsible for one series of events may not exist in another scenario with similar forcing. This is potentially reflected in observationally based studies that have struggled to find robust linkages.  Further, multiple runs of the same model with similar but slightly different initial conditions, termed ensemble members, show linkages in some subsets of ensemble runs but not in others. Four properties (limitations) that contribute to the complexity of attribution of linkages are:  itinerancy [seemingly random variations from state to state], intermittency [apparently different atmospheric responses under conditions of similar external forcing, such as sea-ice loss], multiple influences [simultaneous forcing by various factors, such as sea-surface temperature anomalies in the tropics, midlatitudes and Arctic], and state dependence [a response dependent on the prior state of the atmospheric circulation, e.g., the phase of the Arctic Oscillation (AO) atmospheric circulation index or the strength of the stratospheric vortex]. We propose a system-level approach that recognizes multiple simultaneous processes, internal instabilities, and feedbacks. Progress requires the use of probabilistic model forecasts that are based on case studies and high-resolution, ensemble solutions to the equations of motion and thermodynamics. Community coordinated model experiments and diagnostic studies of atmospheric dynamics are essential to resolve controversy and benefit efforts to communicate the impacts of linkages and uncertainties with a broad public. Such activities have begun through national and international coordination such as the Polar Prediction Program and US CLIVAR.