UPSC Prelims 2021 Q58: Trade Winds, Westerlies & Ocean Temperature
Consider the following statements:
1. In the tropical zone, the western sections of the oceans are warmer than the eastern sections owing to the influence of trade winds.
2. In the temperate zone, westerlies make the eastern sections of oceans warmer than the western sections.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (c) Both 1 and 2
Statement-wise explanation
- 1: correct — Trade winds blow from the east in both hemispheres, from the northeast and the southeast. They drive the equatorial currents westward and pile warm surface water on the western side of tropical oceans. On the eastern side, cold deep water wells up to replace it, for example off Peru. NOAA records that under normal conditions the western equatorial Pacific is roughly 8°C warmer than the water off South America, and its sea surface is about half a metre higher.
- 2: correct — Between about 35° and 60° latitude the westerlies blow from west to east, so they are onshore winds on the eastern margins of the oceans. They push warm water, such as the North Atlantic Drift, towards the western coasts of Europe and North America. This keeps those coasts mild, while the western margins (eastern coasts of continents) cool in winter under offshore continental winds and cold currents such as the Labrador. The pattern is the reverse of the tropical one.
How to eliminate
This question cannot be solved by trick. Both statements are factual, and neither uses an absolute word like 'only' or 'always'. What helps is one rule of wind direction: winds pile warm surface water up on the side of the ocean they blow towards. Trade winds blow from the east, so warm water collects in the west. Westerlies blow from the west, so the warm side moves to the east. Once you see that the two statements are mirror images driven by opposite winds, you can see that they fit together. They do not contradict each other. A quick check from known examples settles it: the Western Pacific Warm Pool and the cold Peru coast confirm statement 1. Ice-free Norwegian ports at the latitude of a frozen Labrador coast confirm statement 2. Fact to remember: tropics are warm-west, temperate zone is warm-east.
Know more
Why the two sides of an ocean differ in temperature
Sea surface temperature (SST) falls from about 27°C near the equator towards the poles, because insolation decreases with latitude. Along any one latitude, though, the prevailing wind decides which side of the ocean is warmer.
- Onshore wind: pushes warm surface water against the coast and raises its temperature.
- Offshore wind: drags surface water away from the coast. Cold, nutrient-rich water rises to replace it (upwelling). This is why the cold-water coasts of Peru, Namibia and California are rich fishing grounds.
| Belt | Prevailing wind | Warmer side of ocean | Example |
|---|---|---|---|
| Tropics (0–30°) | Trade winds (easterlies) | Western (eastern coasts of continents) | Western Pacific Warm Pool near Indonesia; cold Peru coast |
| Temperate (35–60°) | Westerlies | Eastern (western coasts of continents) | North Atlantic Drift keeps Norway's ports ice-free; Labrador coast freezes |
Link to El Niño. In normal years the trade winds hold warm water in the west Pacific. This keeps the thermocline deep in the west and shallow in the east. When the trades weaken, the warm water slides east, upwelling off Peru weakens, and an El Niño sets in. It often brings a weaker Indian monsoon.
A nuance. Warm western-boundary currents such as the Gulf Stream and the Kuroshio do warm the western side of the ocean in the lower middle latitudes. In winter, though, offshore continental winds and cold currents (Labrador, Oyashio) chill the western margins further north. The standard textbook generalisation for the westerly belt, which UPSC followed, is therefore 'eastern margins warmer'.
Other factors in SST: latitude, unequal land–water distribution (the Northern Hemisphere oceans are warmer), ocean currents, salinity and depth.
NCERT Concepts to Revise
- Trade winds: Statement 1 turns on the direction trade winds blow and where they push tropical surface water.
- Westerlies: Statement 2 depends on the westerlies' west-to-east direction across the 35°–60° latitude belt.
- Sea surface temperature: Both statements compare sea surface temperature on opposite sides of an ocean basin.
- Distribution of ocean surface temperature: The basic latitude pattern of ocean surface temperature, before the effect of winds is added.
- Southeast trade winds: The Southern Hemisphere half of the trade winds, which shows the westward push works in both hemispheres.