El Niño, Watersheds, and Global Circulation
- Explain Hadley cells and the resulting global distribution of deserts and rainforests
- Describe El Niño and La Niña and their regional consequences
- Define a watershed and connect land use within it to downstream water quality
Why deserts sit at 30 degrees
Global circulation explains the map of biomes, and the mechanism is worth having. Intense equatorial heating drives air upward; rising air cools and loses moisture, producing the heavy rainfall of the tropical rainforest belt at the equator. That air then moves poleward at altitude, cools further, and descends at roughly 30° north and south — and descending air warms and absorbs moisture, producing the great desert belts. That single convection loop is a Hadley cell, and it is why the Sahara, the Arabian, the Sonoran, the Kalahari and the Australian deserts all sit at similar latitudes. Add the Coriolis effect deflecting winds — right in the Northern Hemisphere, left in the Southern — and you have the trade winds and the westerlies. A rain-shadow question is the same physics applied to a mountain: air rises and rains on the windward side, descends and dries on the leeward.
El Niño and La Niña
Normally, trade winds push warm surface water west across the Pacific, and cold, nutrient-rich water upwells off the coast of South America, supporting one of the world's great fisheries. During El Niño the trade winds weaken or reverse, warm water sloshes east, and upwelling is suppressed — so the fishery collapses, Peru and Ecuador get heavy rain and flooding, and Australia and Indonesia get drought and fire. La Niña is the intensified normal state: stronger trades, stronger upwelling, wetter Australasia, drier South America. The clearest way to hold it is by the upwelling: El Niño suppresses upwelling and the fishery; La Niña strengthens both. The effects reach far beyond the Pacific through atmospheric teleconnections, which is why these cycles appear in North American weather questions.
Watersheds connect land use to water quality
A watershed or drainage basin is all the land whose runoff drains to a common body of water, bounded by ridgelines. The concept matters because it makes water quality a land-use question: fertilizer applied anywhere in the basin, sediment from any construction site, and runoff from any parking lot all arrive at the same river. That is why non-point source pollution is so hard to regulate — there is no pipe to permit, only a whole landscape of contributors. Two consequences worth stating: impervious surface in a watershed raises the height and speed of storm runoff, because water that would have infiltrated instead flows overland, increasing flood peaks and erosion; and riparian buffers of vegetation along streams intercept sediment and nutrients before they reach the water, which is the cheapest single intervention available.
A Peruvian fishing community reports catch collapsing while Indonesia experiences severe drought and fires. Identify the phenomenon and explain the mechanism linking both effects.
- 1.Both symptoms point the same way: the eastern Pacific is unusually warm and the western Pacific unusually dry. That is El Niño.
- 2.Mechanism, eastern side: weakened trade winds allow warm surface water to move east, and warm surface water suppresses the upwelling of cold nutrient-rich water off Peru.
- 3.Without upwelling, phytoplankton lose their nutrient supply, primary productivity falls, and the fishery that depends on it collapses.
- 4.Mechanism, western side: the warm water and the convection that goes with it have moved east, so Indonesia and Australia lose the rainfall they normally receive, producing drought and conditions for fire.
For any climate-pattern question, ask which way is the air moving. Rising means cooling, condensation and rain; descending means warming, evaporation and dryness. That one question answers rainforest belts, desert belts, rain shadows and monsoons.
Major deserts cluster near 30° N and 30° S latitude because
Increasing impervious surface within a watershed most directly causes
Answer the 2 checkpoints as you read.
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