Midlatitude Surface Warming and the Arctic Free Troposphere

Observed warming of the Arctic free troposphere has been linked to intensified atmospheric energy transport within the storm tracks, but the mechanisms by which this occurs are poorly understood. Here, we show that aspects of the Arctic midtropospheric warming can be obtained by a simple theory in which midlatitude near-surface warming is propagated along climatological zonal mean isentropes that connect the midlatitude surface to the Arctic free troposphere. It is most clearly seen in the greenhouse warming response in the CMIP5 climate models but consistency of this theory with historical observed trends is also seen.

In most models, it is observed that the Arctic midtropospheric warming can be obtained from the propagation of midlatitude near-surface warming anomalies according to a dynamic that is intermediate between dry and moist adiabatic propagation, with most models tending towards moist adiabatic propagation. The analysis suggests that the Arctic midtropospheric warming can be partly described in terms of enhanced latent heat transport by synoptic eddies along slanted moist isentropes. Thus, as for the tropical atmosphere, theories of high latitude midtropospheric warming based on changes in heat transport should include a moist component. -2013-->