General Circulation of the Atmosphere Questions and Answers

Answer: A

The convergence zone is a band of deep convection near the thermal equator where the trades of the two hemispheres meet, and it forms the rising branch of both cells. It appears in satellite imagery as a nearly continuous band of cloud. The subtropical highs mark the descending branch and the trade wind inversion is a feature of the subsiding air on their equatorward flank. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

The zone migrates into the summer hemisphere, reaching well north of the equator over land in July and south in January, with a lag reflecting the thermal inertia of the surface. Its movement over land is much larger than over ocean, which is why the monsoons are continental phenomena. The eastward propagation described belongs to the Madden-Julian Oscillation. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

Where a large landmass heats far more strongly than the adjacent ocean, the convergence zone is drawn far poleward and the resulting seasonal reversal of the low level flow is a monsoon. The Asian monsoon is the largest example. The Walker circulation is zonal and driven by sea surface temperature contrast along the equator rather than by seasonal land heating. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

The defining feature is a reversal of the prevailing low level wind between summer and winter, accompanied by a marked seasonal cycle of rainfall. The Somali jet and the reversal of the Arabian Sea current are consequences rather than definitions. The land-sea temperature contrast drives the reversal, so it changes sign too, but the wind reversal is what the term names. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

The Tibetan Plateau acts as an elevated heat source in the middle troposphere, strengthening the meridional temperature gradient aloft and reversing the upper level flow. Simple land-sea contrast alone would give a much weaker circulation. Cross-equatorial flow does experience Coriolis deflection, producing the Somali jet, but that is a consequence of the circulation rather than a driver of it. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

The Somali or Findlater jet is a low level cross-equatorial flow that turns eastward along the Somali coast and carries moisture from the southern Indian Ocean into the monsoon circulation. Its strength correlates with monsoon rainfall. The subtropical and polar front jets lie near the tropopause and the African easterly jet is a mid-level feature over West Africa. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

Warm water in the western Pacific supports deep convection while the cold eastern Pacific is dominated by subsidence, and the resulting east-west overturning is the Walker circulation. It is a zonal cell rather than a meridional one. Its weakening is the atmospheric signature of El Nino, which is why the circulation is monitored so closely.

Answer: D

During El Nino the eastern Pacific warms and the temperature contrast that drives the circulation is reduced, so the Walker cell weakens and the convection shifts eastward. La Nina produces the opposite. The associated pressure seesaw between Darwin and Tahiti is the Southern Oscillation, and the two together are known as the coupled ENSO phenomenon. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

The Southern Oscillation Index is based on the normalised sea level pressure difference between Tahiti and Darwin, and it goes negative during El Nino. The Nino 3.4 index measures sea surface temperature in the central Pacific instead, giving the oceanic rather than the atmospheric side of the same coupled phenomenon. The other indices describe quite different regions and timescales. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

Weakened trade winds reduce upwelling and warm the eastern Pacific, which further weakens the trades, giving a positive feedback that Bjerknes identified. Equatorial Kelvin and Rossby waves in the ocean supply the delayed negative feedback that terminates each event. Radiation and evaporation contribute to the surface energy budget without providing the coupling. Most errors here come from carrying a result across from a neighbouring case where it does not hold.