Atmospheric Circulation & El Niño
- Explain how uneven solar heating and the Coriolis effect create global wind belts
- Describe how the El Niño–Southern Oscillation alters normal Pacific circulation
- Predict the regional weather impacts of El Niño and La Niña conditions
Uneven heating drives the atmosphere
The Sun heats Earth unevenly: the equator receives the most direct, concentrated sunlight, while the poles receive it at a low angle spread over more area. This warms equatorial air, which rises, and the imbalance sets up giant convection loops called atmospheric circulation cells (Hadley, Ferrel, and Polar cells). Rising air near the equator cools and drops its moisture — creating the wet tropics — then sinks around 30° latitude, where the dry descending air creates the world’s great deserts. These cells explain the latitudinal bands of wet and dry climate.
The Coriolis effect and wind belts
Because Earth rotates, moving air is deflected — the Coriolis effect — curving to the right in the Northern Hemisphere and to the left in the Southern. This turns the north–south airflow of the circulation cells into steady prevailing winds: the trade winds that blow from east to west in the tropics, and the westerlies of the mid-latitudes. Over the tropical Pacific, the east-to-west trade winds normally push warm surface water toward Asia and Australia, allowing cold, nutrient-rich water to upwell along the coast of South America.
El Niño and La Niña
The El Niño–Southern Oscillation (ENSO) is a periodic disruption of that normal pattern. In an El Niño, the trade winds weaken or reverse, so warm water sloshes back east toward South America. This shuts down the upwelling of cold, nutrient-rich water — collapsing fisheries off Peru — and shifts rainfall: the normally dry west coast of the Americas gets floods while Australia and Indonesia suffer drought. A La Niña is the opposite extreme: unusually strong trade winds intensify upwelling and push even more warm water and rain toward Asia. Both cycle every few years and reshape weather worldwide.
During an El Niño year, fishers off the coast of Peru report that their normally rich anchovy catch has collapsed. Explain the chain of cause and effect.
- 1.Normally, strong east-to-west trade winds drive coastal upwelling that brings cold, nutrient-rich water to the surface off Peru.
- 2.Those nutrients fuel phytoplankton, the base of a food web that supports the huge anchovy fishery.
- 3.In an El Niño, the trade winds weaken, so warm surface water returns eastward and caps the cold water below — the upwelling shuts down.
- 4.With upwelling gone, nutrients no longer reach the surface, phytoplankton production crashes, and the anchovy population — and the catch — collapses.
Why does the descending, dry air around 30° latitude tend to create large deserts?
Keep El Niño and La Niña straight by the trade winds. El Niño = weak or reversed trade winds, warm water returns east, upwelling shuts down. La Niña = extra-strong trade winds, intensified upwelling. Mixing up which one strengthens the winds is the most common ENSO error.
The Coriolis effect is responsible for which of the following?
Two reliable exam anchors: sinking dry air at ~30° latitude → deserts, and El Niño → weakened trade winds → collapsed upwelling → failed fisheries plus flipped rainfall (wet Americas, dry Australia). Being able to trace those cause-and-effect chains earns the free-response points.
Answer the 2 checkpoints as you read.
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