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| Map of average sea-surface temperatures. The Warm Pool in the West Pacific and India Ocean is an area of 30 °C (86 °F) water bigger than Russia! |
Straddling the Equator in the tropical Pacific and Indian Oceans lies the Great Warm Pool: an enormous expanse of seawater that remains perpetually 30 °C (86 °F). The Warm Pool is bigger than Russia, and we will spend the first month of our voyage bathing in its swelter.
The Equatorial Atlantic and East Pacific are nowhere near as warm as the West Pacific and Indian Ocean. Why is there a Warm Pool only in the Eastern Hemisphere?
The answer has to do with:
- the lack of a Coriolis Effect at the Equator, and
- the unique geography of the Pacific Ocean Basin
The Coriolis Effect is strongest at the poles where the axis of Earth's rotation is aligned with the local vertical. As latitude decreases, the axis of rotation tilts farther and farer from the vertical until it's horizontal at the Equator. Then it reverses sign and gets stronger as latitude increases toward the other pole. A moving object gets "pushed by the Dead Hand of Doctor Coriolis" toward the right in the north, toward the left in the Deep South, but not at all at the Equator!
The lack of Coriolis means that the east-to-west moving Trade Winds simply push the surface seawater westward across the Equatorial Pacific. Sun-warmed water flows thousands of miles to the west until it "runs out of room" along the shallower waters of the Maritime Continent (Indonesia and Malaysia) and then trickles over their sill into the Equatorial Indian Ocean.
Nowhere on Earth is the Equatorial Ocean as "wide" (unbroken by land) as the Equatorial Pacific, and this stretch is barely broken by the somewhat porous islands of the Maritime Continent. The enormous area of very warm seawater collected and transported westward by the Trade Winds is the source of heat for the Warm Pool.
The Warm Pool is unlike any other part of the world ocean. Its steady heat and humidity is the fuel for the great Indian Monsoon, the most powerful weather phenomenon on the planet. The Asian Summer Monsoon pushes the hot humid air above the Warm Pool up against South Asia and into the Himalayas. This results in torrential seasonal rainfall whose moisture feeds billions of people. In winter, the flow reverses and seasonal rains drench northern Australia.
Recall that warm water is buoyant and floats stably atop the cold dense water of Davy Jones Locker. The Warm Pool is a tremendous wedge of warm water that is forced downward by the steady inflow from the east. Around Indonesia, the warm water is 300 meters deep (about 1000 feet)! It's the only place in the world with such deep warm water, and the heat it contains is unlike any other form of heat storage in Earth's climate system.
The surface of the Warm Pool is elevated relative to the rest of the ocean, pushed upward by the Trade Winds and buoyed by the huge bulge of hot water it contains. Under the surface, the bulge is about 10 times as steep as the surface bulge. This bulge represents gravitational potential energy -- the water is held up by the pressure of the Trade Winds.
The atmosphere above the Equatorial Pacific responds to the huge heat reservoir of the Warm Pool. Evaporation from all that hot water is the source of Monsoon rains, which release latent heat and cause the atmosphere to rise and expand. This atmospheric heat source drives the Walker Circulation: westerly (eastward-moving) winds aloft and an intensified easterly return flow (Trade Winds) path the surface. The Walker Circulation and the Warm Pool form an interacting, self-reinforcing system that keeps the western ocean warm, maintains the Monsoon, and drives the strong Trades.
Every few years, the Trade Winds in this region weaken and the stored potential energy is released as a plenary-scale wave that propagates from west to east across the tropical Pacific. This phenomenon is called El Niño and it has profound implications for weather and climate all over the world. During El Niño events, the Warm Pool is noticeably cooler and the Indian Monsoon frequently fails. By contrast the waters of the tropical East Pacific off of Peru become unusually warm and rains drench the Atacama Desert.
