Ocean Dynamics

Wind friction moves the sea surface, but Earth's rotation applies the Coriolis effect so water flows at a twisted angle, not straight with the wind. Deeper layers turn farther and weaken in the Ekman spiral; summed over depth that becomes Ekman transport at right angles to the wind, and a sea-surface slope balanced by Coriolis yields geostrophic flow. Coastal wind direction also drives upwelling and downwelling. Here you can change wind speed and latitude to watch current direction by depth in the Ekman spiral.

The wind moves the sea

🌊 Wind on a coffee cup

①Blow on hot coffee — surface coffee is pushed away

②The ocean is similar — wind exerts friction on the sea surface

③But Earth rotates! → Coriolis effect kicks in

④Water doesn't move in the wind direction — it moves at a 'twisted' angle

⑤This is the core principle of Ekman transport

Ekman spiral and Ekman transport

Wind speed5

Latitude37

Ekman transport direction

N. Hemisphere: 90° to the right of wind / S. Hemisphere: 90° to the left

net water transport is at right angles to the wind due to Coriolis

📐 Ekman spiral principle

①Wind → surface current (about 45° deflected from wind direction)

②Surface current → friction transferred to next layer

③Each layer further deflected by Coriolis + speed decreases

④Result: deeper layers rotate clockwise (N. Hem.) and weaken

⑤Sum over all layers → 90° to wind direction = Ekman transport

Geostrophic flow — balance of sea-surface slope and Coriolis

Geostrophic balance

pressure gradient force = Coriolis → flow parallel to isobars/isobaric surface

core flow that maintains ocean circulation even without wind

🏔️ The sea surface has 'hills' and 'valleys' too

①When Ekman transport piles up water → sea surface bulges

②Pressure gradient force from high to low surface

③Coriolis cancels it → flow parallel to surface contours = geostrophic

④N. Hemisphere: high water level kept on the right of the flow

⑤Mechanism behind subtropical gyres (Kuroshio, Gulf Stream)

Western intensification

western boundary currents (Kuroshio, Gulf) ≫ eastern (California, Canary)

latitudinal variation of Coriolis (β-effect) makes the western side narrow and fast

Upwelling and downwelling

Comparison

ChartUpwelling vs Downwelling

Item	Upwelling	Downwelling
Cause	surface water moves away → deep water rises	surface water converges → sinks
Coastal	wind parallel to coast + Ekman transport offshore	opposite direction
Temperature	cold deep water rises → cools surface	warm surface water descends
Nutrients	nutrient-rich → forms fisheries!	nutrient-poor
Examples	Peru coast, California coast	convergence zones

🐟 Upwelling and fisheries

①Upwelling areas: nutrient-rich deep water rises

②Phytoplankton bloom → food chain becomes active

③Most of the world's top fisheries are near upwelling areas

④El Niño: weakened upwelling → catch plummets (Peru coast)

Worked Examples and Exam Practice

Example 1

In the open ocean of the Northern Hemisphere with a steady wind, in which direction relative to the wind does the whole surface layer (Ekman transport) move?

Because of the Coriolis effect, the surface current is deflected away from the wind direction.

Summing the currents over depth, the net transport in the Northern Hemisphere is 90° to the right of the wind (Ekman transport).

▸ 90° to the right of the wind (Northern Hemisphere)

In the Southern Hemisphere it is 90° to the left. Ekman transport piles up or removes water, causing upwelling or downwelling.

Example 2

When wind parallel to the coast pushes surface water out to sea, what rises to fill the gap, and how does it affect fisheries?

As Ekman transport carries surface water offshore, cold deep water rises to fill the gap (coastal upwelling).

Deep water is nutrient-rich, so phytoplankton flourish → productive fishing grounds form.

▸ Cold deep water upwells → nutrient-rich → productive fisheries

The rich fisheries off Peru and California owe to coastal upwelling. When El Niño weakens upwelling, catches plummet.

School-exam style

Which statement about ocean motion is correct?

①In the Northern Hemisphere, Ekman transport is 90° to the left of the wind

②Coastal upwelling enriches nutrients and forms productive fisheries

③Geostrophic flow occurs when the pressure-gradient force and Coriolis force are not balanced

④In the Ekman spiral, current speed increases with depth

⑤Downwelling regions are nutrient-rich

▸ ② Coastal upwelling enriches nutrients and forms productive fisheries

Upwelling lifts cold, nutrient-rich deep water to the surface, boosting plankton and forming fisheries.

Northern-Hemisphere Ekman transport is 90° to the right (①), geostrophic flow occurs when the two forces balance (③), the Ekman spiral slows with depth (④), and downwelling regions are nutrient-poor (⑤).

Summary

Ocean motion core

wind → Ekman transport (right angle) → sea-surface slope → geostrophic flow → ocean circulation

wind energy creates ocean circulation through Coriolis

🎯 Exam Points

①Ekman transport: at right angles to wind (right in N. Hem., left in S. Hem.)

②Ekman spiral: deeper layers rotate clockwise (N. Hem.) + decreasing speed

③Geostrophic flow = balance of pressure gradient and Coriolis (along isobaric contours)

④Western intensification: β-effect → narrow, fast western currents

⑤Coastal upwelling: coast-parallel wind → deep water rises → fishery forms

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