The subtropical jet stream is strongest in winter and weakest in summer. What is the best explanation for this seasonal variation?
AJet streams only form during winter because the atmosphere is colder overall
BThe equator-to-pole temperature gradient steepens in winter, increasing thermal wind shear and concentrating the jet
CWinter storms generate stronger surface winds that merge to form the upper-level jet
DThe Hadley cell expands in summer, pushing the jet to higher latitudes where it weakens and disperses
Jet streams are driven by the thermal wind mechanism: horizontal temperature gradients cause vertical wind shear, and concentrated shear produces a jet maximum. In winter, the poles cool dramatically while the tropics remain relatively warm, steepening the equator-to-pole temperature gradient. The stronger gradient drives stronger thermal wind shear and produces a more intense, narrower jet. In summer, polar temperatures moderate and the gradient weakens, resulting in a weaker, more diffuse jet. The seasonal cycle in jet strength traces directly to the thermal wind relationship.
Question 2 Multiple Choice
The polar jet stream becomes highly meridional — making large northward ridges and southward troughs — over a continental region. What weather pattern is most likely to result?
ARapid succession of different weather systems, as storms move quickly across the region under the fast-moving jet
BPersistent weather patterns lasting weeks, such as prolonged heat waves, cold outbreaks, or drought
CNo significant surface weather impact, because the jet stream only affects aviation, not surface conditions
DIncreased precipitation everywhere, since meridional flow always draws moisture from the tropics
A zonal (west-to-east) polar jet steers weather systems briskly — no single pattern dominates for long. A highly meridional jet meanders in large Rossby waves that move slowly or become stationary (blocking). When a ridge of high pressure parks over a region, it can produce weeks of warm, dry conditions (heat waves, drought). An adjacent trough delivers persistent cold and precipitation. This blocking pattern is associated with some of the most extreme weather events on record — the 2003 European heat wave and 2010 Russian heat wave both involved blocking ridges maintained by a meridional polar jet.
Question 3 True / False
The polar jet stream is considerably more variable in position and strength than the subtropical jet stream.
TTrue
FFalse
Answer: True
True. The subtropical jet forms at the poleward edge of the Hadley cell, which is a thermally direct, persistent circulation driven by equatorial heating. The Hadley cell's position and strength change relatively little day-to-day, keeping the subtropical jet relatively steady near 30° latitude. The polar jet forms along the baroclinic zone between polar and midlatitude air masses, which is inherently unstable to growing Rossby waves and extratropical cyclones. The polar front shifts with individual weather systems, making the polar jet highly variable in location and intensity on daily to weekly timescales.
Question 4 True / False
Jet streams are easterly winds — they blow from east to west at upper levels of the atmosphere.
TTrue
FFalse
Answer: False
False. Jet streams are strongly *westerly* winds — they blow from west to east. This is a common geographic misconception. In the midlatitudes, the combination of Earth's rotation (Coriolis force) and the equator-to-pole temperature gradient drives upper-level winds from west to east. Easterly winds (blowing from east to west) are found in the tropics (trade winds near the surface) and at the poles, but midlatitude jet streams are definitively westerly. This is why transatlantic flights from North America to Europe are significantly faster than the return trip.
Question 5 Short Answer
Explain why the polar jet stream meanders in large Rossby waves rather than flowing in a smooth, uniform band around the globe.
Think about your answer, then reveal below.
Model answer: The polar jet is baroclinically unstable — small perturbations grow rather than decay. Rossby waves arise from conservation of potential vorticity on a rotating sphere: when air parcels are displaced meridionally, the latitude-dependent Coriolis parameter (β-effect) provides a restoring force that generates planetary-scale wave oscillations. These waves are also excited by orographic forcing (the Rocky Mountains and Himalayas deflect the flow) and by land-ocean heating contrasts. The jet responds to these perturbations by developing a quasi-stationary wavy structure of ridges and troughs superimposed on the mean westerly flow.
Rossby waves are the fundamental large-scale wave mode of rotating atmospheres and oceans. They propagate because conservation of potential vorticity (the sum of planetary and relative vorticity divided by layer thickness) requires air displaced poleward to develop anticyclonic relative vorticity (ridges) and air displaced equatorward to develop cyclonic vorticity (troughs). The amplitude and phase speed of these waves determine midlatitude weather patterns: low-amplitude, fast-moving waves produce active, changeable weather; high-amplitude, stationary (blocking) waves produce extreme persistent conditions.