Midlatitude circulation trends and the efficiency of poleward heat transport in the upper troposphere

The jet streams are bands of intense zonal winds in the Earth's troposphere and are a dominant feature of climate in the midlatitudes. In each hemisphere, the jet's latitude is in thermal wind balance with
the meridional equator-to-pole temperature gradient, @T=@y. Climate models predict differently signed changes in the zonal-mean @T=@y at different altitudes in response to anthropogenic forcing. As the
climate warms, models suggest a \tug-of-war" [1] between rapid polar surface warming { which acts to reduce @T=@y at low levels and push the jet equatorward { and enhanced warming of the tropical upper
troposphere (UT), with an increased @T=@y at high levels associated with a poleward shift of the jet. State-of-the-art climate models differ in their projections of jet shifts under anthropogenic forcing,
reflecting uncertainty in the outcome of this tug-of-war. Most models predict a small poleward shift of the jets by the end of the century [2]. However, reanalysis data covering the last 40 years suggest larger
poleward trends in the jets than are found in model simulations of the same period [3]. Additionally, while the amplification of polar surface warming is well observed in reanalysis data [4], trends in the
tropical upper troposphere are more uncertain [5], with some observational datasets showing smaller warming trends than in models [6].
Recent work has drawn attention to anomalous trends in poleward eddy heat and momentum fluxes in the upper troposphere [3] and has proposed that, in the zonal mean, the anomalous eddy heat flux
[v*T*] is linked to the meridional temperature gradient by a simple flux-gradient relationship
[v*T*] = D @T/@y
Here, [.] denotes a zonal mean and D is some diffusivity coefficient. The hypothesis is that the real atmosphere exports heat and momentum out of the tropics more efficiently than in models, consistent
with both the stronger trends in the eddy-driven jets and weaker warming trends in the tropical UT. In this framework, a circulation that is more sensitive to tropical heating corresponds to a higher value
of the diffusivity D.