Chapter 31: Ocean Currents

Wind-driven circulation and gyres

Wind stress plus the Coriolis effect drives Ekman transport about 45° to the right of the wind in the northern hemisphere (left in the southern), organising the ocean into subtropical gyres — clockwise in the north, counter-clockwise in the south. Western boundary currents are narrow, deep and fast: the Gulf Stream at 3–5 knots, the Kuroshio, Agulhas and Brazil Currents. Eastern boundary currents such as the California and Benguela are broad, slow and cool, with coastal upwelling and fog.

• Northern hemisphere gyres circulate clockwise
• Western boundary currents are strong and warm; eastern are weak and cool
• Fair current can save days; foul current plus opposing sea is dangerous

Thermohaline circulation and practical effects

Cooling and salinification in the North Atlantic and around Antarctica sink dense water that spreads along the abyss and upwells elsewhere, closing a global conveyor with a timescale of centuries. For the navigator the immediate concerns are current-induced set, and the steep dangerous seas where a strong current opposes wind or swell — the Agulhas against a southwesterly buster being the classic hazard. Current atlases, Pilot Charts and satellite-derived analyses support routing decisions.