Horizontal and Vertical Distribution of Temperature

The diagram is a single line graph, and its job is to show you how ocean temperature behaves as you go deeper. The horizontal axis carries Increasing Temperature, marked from 0°C up to 24°C. The vertical axis carries Increasing Depth, but it is inverted — depth grows downward, so the surface sits at the top of the graph and the sea floor at the bottom. That inversion is the whole trick of the figure: you read it like a cross-section of the ocean, with the warm sunlit surface at the top and the cold abyss below.

The curve itself tells a three-part story. It begins at the surface near 22–24°C and holds almost steady through the first 300 m — that flat, warm stretch is the shallow surface layer, where sunlight keeps the water well mixed and uniformly warm. Then, between roughly 300 m and 1000 m, the line plunges almost vertically. This steep drop is the thermocline, and the diagram labels that band explicitly. Below 1000 m the curve flattens out again, easing gradually toward 0–4°C as it descends to 4500 m. That long, gentle tail is the deep cold water that fills most of the ocean's volume.

What the graph teaches is that temperature does not decrease uniformly with depth — it collapses in one narrow zone and is nearly constant everywhere else. The thermocline is that boundary region where the rapid decrease happens, and it acts as a lid separating warm surface water from the cold deep ocean. Notice the asymmetry: the warm layer is thin, the thermocline is a sharp transition, and the cold layer below is enormous. That is why the textbook says about 90 per cent of ocean water sits below the thermocline — the graph makes that visually obvious, because the curve spends most of its vertical travel in the cold, nearly flat zone. …

The map is a world map, with the ocean surface shown in place of the usual land-focused view. The key elements are the isotherms — lines joining points of equal surface temperature — drawn across the Atlantic, Pacific, and Indian Oceans. Each line is labelled with a temperature in degrees Celsius: 0, 5, 10, 15, 20, 25, and 28. These lines run roughly parallel to the latitude circles, which is the first thing the diagram teaches: ocean surface temperature is primarily a function of latitude, not of longitude or distance from land.

The warmest water, at 28°C, sits near the equator, but not exactly on it. The description places this maximum in the Indian and Pacific Oceans, slightly north of the equator — matching the text's point that the highest temperature is not at the equator itself. From this warm core, the isotherms step down symmetrically toward both poles: 25°C, then 20°C, 15°C, 10°C, 5°C, and finally 0°C near the Arctic and Antarctic circles. The pattern is a steady, poleward cooling, with no sudden jumps or closed loops of warm or cold water interrupting the general trend.

What the diagram teaches is the horizontal distribution of ocean surface temperature. It visualises the text's key figures: the average surface temperature of about 27°C, the gradual decrease from equator to poles, and the rate of roughly 0.5°C per degree of latitude. You can read the isotherms as a contour map of that gradient — closely spaced lines would mean a rapid temperature change, widely spaced lines a gentle one. The diagram also shows the asymmetry the text mentions: the northern hemisphere oceans run warmer than the southern at corresponding latitudes, a consequence of the unequal distribution of land and water between the two hemispheres. The 0°C isotherm reaches farther toward the equator in the south, reflecting the colder southern oceans. …