Questions: Subtropical Jet Streams and Upper-Level Winds

Question 1

The subtropical jet stream weakens during summer compared to winter primarily because:

• A. The Hadley cell reverses direction in summer, reducing upper-level poleward flow
• B. The Coriolis effect weakens at subtropical latitudes during summer months
• C. The equator-to-pole temperature gradient is smaller in summer
• D. Upper-tropospheric humidity increases in summer, creating drag on the jet

Answer: (C)

Jet stream strength is governed by the thermal wind relationship: a stronger meridional temperature gradient produces stronger vertical wind shear and a faster jet. In winter, the poles cool dramatically while the tropics stay warm, maximizing the temperature contrast. In summer, the poles warm and the gradient relaxes, weakening the jet. The Hadley cell does not reverse direction, and the Coriolis effect does not vary seasonally.

Question 2

What is the primary mechanism that concentrates upper-level winds into the subtropical jet stream?

• A. Differential solar heating creating a direct pressure gradient at 30° latitude
• B. Coriolis deflection of poleward-moving Hadley cell air conserving angular momentum
• C. Convergence of trade winds at the ITCZ forcing air upward and outward
• D. Radiative cooling of the tropopause creating a sharp density inversion

Answer: (B)

As air rises near the equator and flows poleward in the upper troposphere, it conserves angular momentum. Moving to smaller-radius latitude circles, it accelerates relative to Earth's surface and is deflected strongly eastward by the Coriolis effect. By ~30° latitude, this deflection has concentrated the air into a fast zonal ribbon — the subtropical jet. This is angular momentum transport driven by the Hadley circulation, not direct heating.

Question 3

The subtropical jet stream and the polar front jet stream have different underlying origins: the subtropical jet arises from Hadley cell angular momentum transport, while the polar jet is driven by baroclinic instability along the polar front.

• T. True
• F. False

Answer: True

These are mechanistically distinct features that happen to look similar (both are upper-tropospheric wind maxima). The subtropical jet is a direct consequence of upper-tropospheric poleward flow being deflected by Coriolis as it exits the Hadley cell. The polar jet forms at ~50–60° latitude along the polar front, driven by baroclinic instability where cold polar air meets warmer mid-latitude air. Both are maintained by thermal wind balance, but their energy sources differ.

Question 4

Jet streams maintain a nearly fixed east-west path throughout the year, shifting primarily slightly with the seasons.

• T. True
• F. False

Answer: False

Jet streams meander substantially and continuously, forming large-amplitude ridges and troughs (Rossby wave patterns) that evolve on timescales of days to weeks. They also migrate poleward in summer and equatorward in winter. This variability in position and amplitude is precisely what drives mid-latitude weather changes — when the jet develops persistent large meanders, weather systems stagnate and extreme events (heat waves, floods, cold snaps) become more likely.

Question 5

Explain why the subtropical jet stream intensifies in winter. Connect the seasonal change to the physics governing jet stream strength.

Model answer: The subtropical jet's strength is governed by the thermal wind relationship: a stronger meridional (pole-to-equator) temperature gradient produces stronger vertical wind shear and thus a faster jet core. In winter, the poles cool dramatically while the tropics remain relatively warm, maximizing this temperature contrast. In summer, polar warming weakens the gradient, reducing vertical wind shear and producing a weaker, more variable jet.

The thermal wind equation quantifies this: ∂u/∂z ∝ −∂T/∂y. A large poleward temperature decrease (large |∂T/∂y|) forces large vertical wind shear, concentrating fast winds at jet level. The seasonal variation in polar versus tropical temperatures is the direct driver of seasonal jet variability — the Hadley cell itself persists year-round, but its associated jet strengthens and weakens with the temperature gradient.