Walker Circulation Quiz: Trade Winds, Pressure, the Pacific

The Walker Circulation is a large atmospheric loop that operates over the tropical Pacific Ocean. It is driven by the temperature difference between the warm western Pacific and the cooler eastern Pacific. Air rises over the warm western Pacific, moves eastward at high altitude, sinks over the cooler eastern Pacific, and returns westward at the surface as trade winds. This loop plays a key role in regulating tropical climate and rainfall.

Under normal conditions, the trade winds blow from east to west across the tropical Pacific, driven by pressure differences between high pressure in the eastern Pacific and low pressure over the warm western Pacific. These winds push warm surface water westward, causing it to pile up near Australia and Indonesia and allowing cooler upwelled water to rise along the coast of South America.

When trade winds weaken during El Nino, the warm water that normally piles up in the western Pacific spreads eastward. This reduces the temperature contrast between east and west, which is the engine of the Walker Circulation. The loop weakens or even partially reverses, leading to reduced rainfall over Australia and Indonesia and increased rainfall over the central and eastern Pacific.

Trade winds are driven by the pressure gradient between the relatively high pressure over the cooler eastern Pacific and the low pressure over the warm western Pacific, where warm air rises. Surface air moves from high pressure toward low pressure, and the Coriolis effect deflects these winds, producing the characteristic easterly trade winds that blow westward across the tropical Pacific under normal conditions.

The western Pacific warm pool is maintained by the persistent easterly trade winds that push warm surface water westward. Over time this accumulation of warm water raises sea levels in the western Pacific by several centimeters compared to the east. The warm pool sits above the thermocline and is the primary source of heat and moisture that fuels the rising branch of the Walker Circulation.

During normal conditions, easterly trade winds push warm surface water westward, building the western Pacific warm pool. Cold water upwells along the South American coast in the eastern Pacific, keeping sea surface temperatures there much cooler. This east-west temperature difference drives the Walker Circulation. Equal temperatures east and west would weaken the Walker Circulation, which only occurs during El Nino.

The Walker Circulation is fundamentally a pressure-driven system. Warm ocean surfaces heat the overlying air, causing it to rise and creating areas of low atmospheric pressure. Cooler ocean surfaces maintain higher pressure. This east-west pressure gradient across the tropical Pacific drives the surface trade winds from the high-pressure eastern Pacific toward the low-pressure western Pacific, completing the circulation loop at the surface.

Coastal upwelling along South America is driven by southerly and southeasterly winds pushing surface water offshore, allowing cold nutrient-rich deep water to rise and replace it. During El Nino, when trade winds weaken, this offshore pushing mechanism is reduced. Less surface water is displaced, so less cold water upwells, allowing the sea surface along the South American coast to warm significantly compared to normal conditions.

El Nino is a naturally occurring phenomenon that has been part of Earth's climate system for thousands of years, long before industrialization. It arises from natural interactions between the tropical Pacific Ocean and the atmosphere. While climate change may be influencing the frequency and intensity of El Nino events, the phenomenon itself is natural and occurs irregularly every two to seven years regardless of human influence.

When trade winds weaken during El Nino, warm water spreads eastward rather than remaining in the western Pacific, raising sea surface temperatures in the central and eastern Pacific. Reduced trade winds also diminish the coastal upwelling that normally keeps the eastern Pacific cool. The Walker Circulation weakens rather than strengthens, and rainfall decreases over Australia and Indonesia rather than increasing.

During normal conditions, the rising branch of the Walker Circulation sits over the western Pacific near Australia and Indonesia, bringing heavy rainfall to those regions. When El Nino weakens the Walker Circulation, the rising motion shifts eastward toward the central Pacific and sinking air replaces the normal rising branch over Australia and Indonesia, suppressing rainfall and causing drought conditions across these normally wet tropical regions.

Sir Gilbert Walker was a British mathematician and meteorologist who studied the Indian Ocean and Pacific atmospheric pressure variations in the early 1900s. He identified a large-scale seesaw pattern of pressure between the eastern and western tropical Pacific, which he called the Southern Oscillation. The Walker Circulation, the atmospheric loop driven by this pressure pattern, was later named in his honor for his foundational contributions to understanding tropical climate variability.

The Southern Oscillation is the atmospheric component of the El Nino-Southern Oscillation system. It describes the seesaw pattern of sea-level pressure between the eastern Pacific near Tahiti and the western Pacific near Darwin, Australia. When pressure is high in the east and low in the west, trade winds are strong and Walker Circulation is active. When this pattern reverses, trade winds weaken and El Nino conditions develop.

Early indicators of El Nino development include rising sea surface temperatures in the central and eastern tropical Pacific as warm water spreads eastward, the migration of the western Pacific warm pool toward the central Pacific, and changes in the Southern Oscillation pressure pattern with pressure rising in the west and falling in the east. Increased rainfall over Australia and Indonesia would indicate strengthening of the Walker Circulation, the opposite of El Nino conditions.

El Nino events typically develop over several months, peak during Northern Hemisphere winter, and last a total of approximately nine to twelve months before conditions return to neutral or La Nina. They occur irregularly, with intervals of approximately two to seven years between events. No two El Nino events are identical in timing, intensity, or duration, making them challenging to predict more than several months in advance.