Factors Affecting Temperature Distribution

The temperature of ocean surface water is not uniform. It changes from place to place, and the factors that cause this variation are the same ones that shape climate on land — latitude, the arrangement of land and water, winds, and ocean currents. Each of these acts on the water in a distinct way, and together they explain why one coast may be warm while another at a similar latitude is cold.

Latitude is the most fundamental control. Surface water temperature decreases steadily from the equator towards the poles, because the amount of insolation — incoming solar radiation — also decreases poleward. The equator receives the sun's rays most directly, so its waters are the warmest; as we move towards higher latitudes, the sun's energy arrives at a slant and spreads over a larger area, warming the water less.

The unequal distribution of land and water creates a difference between the two hemispheres. The oceans of the northern hemisphere come into contact with a much larger extent of land than those of the southern hemisphere. Land heats up and cools down faster than water, and it also warms the air above it. Because the northern oceans are surrounded by and in contact with this extensive landmass, they receive more heat overall than the southern oceans, which are dominated by vast open water.

Prevailing winds affect temperature through their direction relative to the coast. When winds blow from the land towards the ocean — offshore winds — they push the warm surface water away from the coast. This forces cold water from below to rise up and replace it, a process called upwelling. Upwelling lowers the temperature near the coast and is responsible for the longitudinal variation in temperature, that is, differences along the same latitude from one coast to another. The opposite happens with onshore winds, which blow from the sea towards the land. These winds pile up warm water near the coast, and this raises the temperature of that coastal water.

Ocean currents redistribute heat across the oceans. Warm currents raise the temperature of the cold areas they flow into, while cold currents lower the temperature of the warm areas they enter. The textbook gives a classic pair of examples: