The ocean consists of greatly different water masses layered by depth. Wind-driven circulation dominates the surface, while thermohaline circulation dominates the deep ocean.
Basic Knowledge
By depth, the ocean is divided into the mixed layer (surface to approximately 100 m, uniform because of wind and solar radiation), the thermocline (approximately 100–1000 m, a layer in which temperature decreases rapidly), and the deep layer (below 1000 m, cold, dense, and uniform).
Wind-driven circulation is surface circulation formed by the westerlies and trade winds. In the North Pacific, a clockwise subtropical gyre (the North Pacific subtropical gyre) forms. The Kuroshio (a strong warm current, 100 km wide, with a speed of 2 m/s) flows along the western boundary, while the North Pacific Current and California Current flow along the eastern boundary.
Ekman transport is the phenomenon in which seawater is transported at a right angle to the wind direction (to the right in the Northern Hemisphere) because of the Coriolis force when wind stress acts on the sea surface. The convergence of surface water toward the center of the subtropical region, which raises sea level (the subtropical convergence zone), is one factor that strengthens the Kuroshio.
Mixed layer (surface to 100 m; warm-water layer made uniform by wind and waves)
Thermocline (layer in which water temperature changes rapidly with depth; barrier to material exchange)
Wind-driven circulation (large-scale surface circulation driven by the westerlies and trade winds)
Kuroshio (North Pacific western boundary current; warm, salty, and fast; greatly influences Japan's climate)
Ekman transport (wind stress + Coriolis force → seawater transport 90° to the right of the wind direction)
Western boundary intensification (phenomenon in which currents become especially strong along the western boundary of a subtropical gyre; the Kuroshio and Gulf Stream are representative examples)
Further Study (Background and Significance)
Western boundary intensification results from the latitudinal variation of the Coriolis force (the beta effect). The North Pacific Kuroshio (near Japan) and the North Atlantic Gulf Stream (along the eastern coast of North America) are both products of western boundary intensification, transporting heat toward higher latitudes and warming northern Europe and Japan.
Ocean acidification is also an important topic. Atmospheric CO₂ dissolves in seawater and changes into H₂CO₃→HCO₃⁻→CO₃²⁻, lowering pH. The present ocean pH is approximately 8.1 (8.2 before the Industrial Revolution), making the CaCO₃ skeletons of shellfish and coral easier to dissolve.
The heat capacity of the ocean is approximately 1000 times that of the atmosphere, and the ocean absorbs about 90% of the heat added by global warming. Rising sea-surface temperatures can strengthen typhoons and change precipitation patterns.
- Ocean layers: mixed layer → thermocline → deep layer
- The thermocline obstructs the exchange of materials and heat
- Wind-driven circulation: clockwise subtropical gyre in the Northern Hemisphere
- Kuroshio = North Pacific western boundary current (warm and fast)
- Ekman transport: 90° to the right of the wind direction (Northern Hemisphere)
- Western boundary intensification: strong western currents caused by the beta effect of the Coriolis force
- Ocean acidification: pH decreases as the ocean absorbs CO₂
Points of Caution (Often Confused)
① Distinguish the Kuroshio (warm current, western boundary) from the Oyashio (cold current, eastern boundary). ② Ekman transport is 90° to the right of the wind direction in the Northern Hemisphere. ③ The thermocline is a boundary between layers, not a water layer itself. ④ Western boundary intensification contrasts with eastern-boundary currents (eastward-flowing cold currents).
Practice
- What is the strong warm current that flows northward along the western North Pacific?
- In Ekman transport, in which direction relative to the wind is seawater transported in the Northern Hemisphere?
- What is the layer in which water temperature changes rapidly with depth called?