Grade 11-12 (age 16-18)

high school Ocean Dynamics

When wind blows over the sea for a long time, friction drags the surface water along, but because the Earth rotates, the water does not simply follow the wind; it veers to the right in the Northern Hemisphere and to the left in the Southern. The deeper you go, the more the flow turns and the weaker it gets (the Ekman spiral), and the net movement of all these layers, the Ekman transport, is at right angles to the wind. When Ekman transport piles water up on one side, the sea surface tilts and a geostrophic current forms in which the pressure gradient force balances the Coriolis force, while along coasts the wind direction can bring upwelling or downwelling. Changing the wind speed and latitude in the diagram shows the direction and size of the spiral seen from above, along with how quickly the current weakens with depth.

How wind moves the sea

🌊 Wind and the Coriolis force

①Just as blowing on a hot drink pushes its surface along, wind over the sea drags the surface water by friction

②Because the Earth rotates, a moving object feels the Coriolis force, which turns it to the right in the Northern Hemisphere and to the left in the Southern

③So the surface current flows at an angle to the wind, and the layers beneath it turn even further

④At the equator the Coriolis force is zero, so this turning hardly happens

The Ekman spiral and Ekman transport

Wind speed (m/s)5

Latitude (°, negative = Southern Hemisphere)37

Direction of Ekman transport

Northern Hemisphere: 90° to the right of the wind / Southern Hemisphere: 90° to the left

the net direction of movement when the flows of every layer, from the surface to the bottom of the Ekman layer, are added up

📐 What to read in the diagram

①The surface current flows about 45° off the wind, to the right in the Northern Hemisphere and to the left in the Southern

②Each layer down turns further the same way and slows, so the arrow tips trace a spiral

③At the bottom of the Ekman layer the flow runs almost opposite to the surface current at only about 4% of its speed

④The stronger the wind, the faster the surface current and the deeper the Ekman layer, and for the same wind the layer is deeper at low latitudes, where the Coriolis force is weaker

⑤The Ekman transport, the sum of all layers (green dashed arrow), is at right angles to the wind

Geostrophic currents: a sloping sea surface balanced by the Coriolis force

Geostrophic balance

pressure gradient force = Coriolis force → flow parallel to lines of equal sea-surface height

It keeps flowing as long as the sea-surface slope built by Ekman transport lasts, and it forms the main body of the surface ocean circulation.

🏔️ The sea surface has hills and valleys too

①In subtropical waters, Ekman transport gathers water toward the middle of the ocean, raising the sea surface there by about 1 m

②Water is pushed from high sea surface toward low by the pressure gradient force

③Once the water moves, the Coriolis force acts on it, and when the two forces balance, the water flows along lines of equal sea-surface height (a geostrophic current)

④In the Northern Hemisphere, a geostrophic current flows with the higher sea surface on its right

⑤Subtropical gyres with currents such as the Kuroshio and the Gulf Stream are kept going this way

Western intensification

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

Because the Coriolis force grows with latitude (the β effect), the center of the gyre shifts west, so the western current is narrow, deep, and fast.

Upwelling and downwelling

Comparison

Upwelling vs Downwelling

Item	Upwelling	Downwelling
Cause	surface water moves away and deep water rises to replace it	surface water piles up and sinks
Along coasts	when wind blows parallel to the coast and Ekman transport heads offshore	when Ekman transport heads toward the coast
Temperature	cold water rises and the surface cools	warm surface water sinks
Nutrients	rich in nutrients, so plankton thrive and fishing grounds form	poor in nutrients
Examples	coasts of Peru and California	center of subtropical gyres (Sargasso Sea)

🐟 Upwelling and fishing grounds

①In upwelling zones, deep water rich in nutrients rises to the surface

②Phytoplankton multiply in the sunlit surface layer, and the zooplankton and fish that feed on them gather

③Major fishing grounds off Peru, California, northwest Africa, and southwest Africa lie in coastal upwelling zones

④During El Niño, upwelling off Peru weakens and the anchovy catch drops sharply

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. Setting a negative latitude in the diagram flips the spiral and the green arrow upside down.

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 fishing grounds off Peru and California owe their wealth to coastal upwelling, and when El Niño weakens the upwelling, the catch drops sharply.

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 (at right angles) → sea-surface slope → geostrophic current → ocean circulation

The wind's energy becomes the great ocean circulation by way of the Coriolis force.

🎯 Exam Points

①Ekman transport is at right angles to the wind (right in the Northern Hemisphere, left in the Southern)

②In the Ekman spiral, the current turns clockwise with depth in the Northern Hemisphere and weakens

③A geostrophic current is a flow in which the pressure gradient force and the Coriolis force balance, running along lines of equal sea-surface height

④Western intensification makes currents on the western side of oceans narrow and fast

⑤Coastal upwelling happens when wind blows parallel to the coast and Ekman transport heads offshore, and it creates good fishing grounds

← Previous

Atmospheric Thermodynamics

Celestial Coordinates

Last updated 2026-10-17

Was this helpful? Support seegongsik