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Eddies (transient storms and waves) interact with the mean flow through eddy heat and momentum transport, exerting a net drag that drives or opposes the mean circulation. In the midlatitudes, eddy momentum convergence in the storm tracks opposes poleward expansion while eddy heat transport helps balance the mean flow. Changes in eddy activity with climate forcing can reshape the mean circulation, affecting regional climate patterns and extremes.
From Hadley cell dynamics, you know that the tropical atmosphere is organized into a relatively steady, thermally driven overturning circulation. But the midlatitudes are fundamentally different: the circulation there is dominated by transient eddies — the migrating high- and low-pressure systems (cyclones and anticyclones) that drive day-to-day weather. From Rossby waves, you know these eddies are not random turbulence but organized wave-like disturbances that propagate along the jet stream. The key insight of eddy-mean flow interaction is that these eddies are not passive riders on the background flow — they actively shape it, transport heat and momentum, and determine the position and strength of the jet streams.
Think of the mean flow (the time-averaged winds) and the eddies as engaged in a two-way conversation. The mean flow sets the stage: the temperature gradient between the tropics and poles creates available potential energy, and baroclinic instability (which you may know from prerequisites) converts this into kinetic energy of growing storm systems. The eddies then feed back on the mean flow by transporting heat poleward, which reduces the very temperature gradient that created them. This is a self-regulating system: stronger temperature gradients produce more vigorous eddies, which transport more heat poleward, which weakens the gradient.
Eddy momentum transport is equally important but less intuitive. As Rossby waves propagate equatorward and break (much like ocean waves breaking on a beach), they deposit their westward momentum at lower latitudes and extract momentum from higher latitudes. The net effect is a convergence of eastward (westerly) momentum into the latitude band of the jet stream, which actually maintains and sharpens the jet. Without eddies, the midlatitude westerlies would be much weaker and broader. The storm tracks — the preferred paths of eddies across the ocean basins — are therefore not just consequences of the jet stream but active participants in sustaining it.
This two-way coupling has profound implications for climate change. If global warming reduces the equator-to-pole temperature gradient (as it does at the surface, since the Arctic warms fastest), eddy activity might weaken, potentially shifting storm tracks and jet streams. But warming also increases the upper-tropospheric temperature gradient (the tropical upper troposphere warms fastest), which could strengthen eddies aloft. Climate models show that these competing effects lead to a poleward shift of the jet streams and storm tracks in most projections — a change that would alter precipitation patterns, drought risk, and extreme weather across the midlatitudes. Understanding eddy-mean flow interaction is therefore essential for predicting how regional climates will respond to global forcing.