Two Engines of Ocean Currents: Wind & the Coriolis Effect

Ocean currents are rivers within the sea. Two forces write their global map:

1. Wind — The Primary Motor

• Trade winds (NE/SE) push equatorial water westward → Equatorial Currents
• Westerlies (mid-latitudes) drive eastward flow → North Atlantic Drift, Antarctic Circumpolar Current
• Ekman transport: Surface layer moves 45° to wind (right NH, left SH); net transport 90° → creates subtropical gyres
• Stress magnitude: τ = ρ_airC_D|U|²; typical 0.1–0.3 N/m²

2. Coriolis Effect — The Steering Wheel

• Earth's rotation deflects moving mass: right in NH, left in SH
• Magnitude: f = 2Ω sin φ (Ω = 7.29×10⁻⁵ rad/s; φ = latitude)
• Zero at equator → max at poles
• Turns wind-driven flow into geostrophic balance (pressure gradient = Coriolis) → clockwise gyres NH, counterclockwise SH

Supporting Cast

Why Just Two?

Wind supplies energy (~1 TW to surface currents). Coriolis supplies geometry (turns energy into organized gyres). Everything else modulates. Together, they explain the first-order map of surface circulation.