What would happen to the midlatitude jet stream if all transient eddy activity (cyclones and anticyclones) were somehow eliminated?
AThe jet stream would intensify because eddy drag no longer opposes it
BThe jet stream would become weaker and broader because eddies normally converge momentum into the jet's latitude band
CThe jet stream would shift poleward because eddy heat transport no longer reduces the equator-to-pole temperature gradient
DNothing would change — the jet stream is maintained by the Hadley cell alone
This is the counterintuitive core of eddy-mean flow interaction. Eddies (via Rossby wave breaking and momentum flux convergence) concentrate westerly momentum into the jet's latitude band, actually maintaining and sharpening it. Without eddies, the midlatitude westerlies would spread out and weaken — the Hadley cell alone cannot sustain the sharp, strong jet observed in the real atmosphere. The jet stream is not simply the cause of eddies; eddies and the jet co-produce each other through a two-way interaction.
Question 2 Multiple Choice
In climate projections, global warming causes the Arctic to warm faster than the tropics at the surface, but the tropical upper troposphere warms faster. For midlatitude eddy activity, these two effects are:
ABoth favor stronger eddies: reduced surface gradient and enhanced upper gradient both increase baroclinic instability
BBoth favor weaker eddies: both effects reduce the north-south temperature contrasts that drive storm development
COpposing: reduced surface gradient weakens baroclinic instability while enhanced upper-tropospheric gradient strengthens it, making the net effect uncertain
DIrrelevant to eddies, which are driven by land-sea contrasts rather than meridional temperature gradients
Baroclinic instability — the energy source for midlatitude eddies — depends on meridional temperature gradients. Rapid Arctic warming reduces the surface temperature gradient (less energy available for eddies), while enhanced tropical upper-tropospheric warming increases the temperature gradient aloft (more energy available). These effects compete, producing genuine uncertainty in projections of future storm track intensity and position. Climate models generally project a poleward shift of storm tracks, but the magnitude and regional details remain areas of active research.
Question 3 True / False
The relationship between midlatitude eddies and the temperature gradient is self-regulating: eddies transport heat poleward, weakening the very gradient that generated them, limiting how strong the temperature contrast can grow.
TTrue
FFalse
Answer: True
This describes a classic negative feedback loop. The equator-to-pole temperature gradient generates available potential energy, which baroclinic instability converts into kinetic energy of growing eddies. Those eddies then transport heat poleward, reducing the gradient that created them. The result is a self-regulating system: stronger gradients create more vigorous eddies, which transport more heat and weaken the gradient. This prevents the temperature contrast from growing without bound and is a key reason the midlatitude climate has a characteristic level of storminess.
Question 4 True / False
Eddies in the midlatitudes are passive features of the atmosphere — they form because of the jet stream, but they do not significantly influence the jet's position or strength.
TTrue
FFalse
Answer: False
This is the central misconception the topic corrects. Eddies actively maintain the jet through eddy momentum transport. As Rossby waves propagate equatorward and break, they deposit westward momentum at low latitudes and remove it from higher latitudes, creating a net convergence of westerly momentum in the storm track region that sustains the jet. This two-way coupling means the jet and storm tracks are co-determined — neither is simply the 'cause' of the other. Climate change is expected to shift both together, precisely because of this coupling.
Question 5 Short Answer
What is 'eddy momentum convergence,' and why does it matter for the strength and structure of the jet stream?
Think about your answer, then reveal below.
Model answer: Eddy momentum convergence refers to the net accumulation of westerly (eastward) momentum in a latitude band due to eddy activity. As Rossby waves generated in the storm tracks propagate equatorward and break, they deposit their momentum at lower latitudes and extract momentum from higher latitudes — the net effect is a flux of westerly momentum toward the jet's latitude band. This convergence effectively acts as a sustained source of westerly wind in the midlatitudes, keeping the jet stronger and narrower than the large-scale temperature gradient alone would produce. Without eddy-driven momentum convergence, the westerlies would be broader and weaker.
Eddy momentum flux convergence is the mechanism by which eddies 'build' the jet even as they extract energy from it. The jet is maintained not just by the thermal wind balance of the large-scale temperature gradient, but by the ongoing dynamical forcing of breaking Rossby waves — a feedback that only becomes apparent when analyzing the zonal mean momentum budget. This is why the storm tracks and the jet are so closely co-located: the region of eddy generation and breaking is precisely where the momentum convergence is largest.