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Grade 10-11 (age 15-17)

high school Ocean Currents & Properties

Water at the sea surface is pushed along by the winds blowing over it, and following the trade winds and the westerlies, the North Pacific forms a huge loop that turns clockwise. Warm currents carrying warm water from low to high latitudes and cold currents bringing cold water back toward the equator make up this loop and move heat, while deep in the ocean a circulation driven by differences in temperature and salinity flows very slowly. In the equatorial Pacific, the strength of the trade winds decides whether warm water piles up in the west or spreads east; when the trades weaken and the eastern Pacific warms, that is El Niño, and the opposite is La Niña. In the first diagram, changing the wind strength changes how thick the currents are, and the second compares the ocean and air in normal years, El Niño, and La Niña side by side.

What drives ocean currents

🌊 Why do currents flow?
①Surface water is pushed along by winds that blow steadily in one direction for long periods, and this flow is a surface current
②At low latitudes the trade winds push water from east to west, and at middle latitudes the westerlies push it from west to east
③Currents blocked by continents turn north or south, so in the North Pacific a subtropical gyre forms that turns clockwise
④Deep in the ocean, cold, salty, heavy water sinks and drives a deep circulation that slowly links all the world's oceans

Warm and cold currents

60%
💡 What to look for in the diagram
①Pushed by the trade winds, the North Equatorial Current flows west until Asia blocks it and turns it north, where it becomes the warm Kuroshio Current
②The Kuroshio continues east with the westerlies as the North Pacific Current, and when North America turns it south, it becomes the cold California Current
③A current carrying warm water from low to high latitudes is a warm current, and one carrying cold water from high to low latitudes is a cold current
④Weaken the wind and the currents thin out, which shows that wind drives surface currents
⑤In Korea's East Sea, the East Korea Warm Current, a branch of the Kuroshio, meets the North Korea Cold Current from the north, forming a current boundary zone that makes a rich fishing ground

Surface currents and deep circulation

Surface currents vs. deep circulation

Two kinds of ocean flow
FeatureSurface currentsDeep circulation
Driving forcewind (the atmosphere's general circulation)density differences from temperature and salinity (thermohaline circulation)
Depthsurface to a few hundred metersa few hundred to several thousand meters
Speedfairly fastvery slow (about 1000 years for one loop)
ExamplesKuroshio Current, Gulf StreamNorth Atlantic Deep Water, Antarctic Bottom Water

El Niño and La Niña

Normal year
El Niño
trade winds weaken → warm water spreads east → eastern Pacific warms and upwelling weakens → rain clouds shift east
Surface temperatures in the eastern equatorial Pacific stay at least 0.5 °C above normal for several months. Peru's coast sees smaller catches and heavy rain, while Indonesia and Australia tend toward drought.
La Niña
trade winds strengthen → more warm water piles up in the west → eastern Pacific cools and upwelling strengthens
The opposite of El Niño: surface temperatures in the eastern Pacific drop below normal, the western Pacific gets more rain, and the eastern Pacific gets drier.
🌡️ Properties of seawater
①By depth, seawater divides into the mixed layer, the thermocline, and the deep layer, and in the thermocline the temperature falls quickly with depth
②Salinity averages about 35 psu; it is higher where evaporation is strong and lower where there is heavy rain or river inflow
③The colder and saltier the water, the denser it is and the more it sinks, and this drives the deep circulation
④Upwelling is cold deep water rising to replace surface water pushed away by the wind, and because it is rich in nutrients, it makes good fishing grounds

Worked Examples and Exam Practice

Example 1
What is a surface current called that, like the Kuroshio, carries warm water from low to high latitudes?
1
A current that carries warm water from low to high latitudes is a warm current.
2
A current that carries cold water from high to low latitudes, such as the California Current, is a cold current.
▸ a warm current
In the North Pacific gyre in the diagram, the Kuroshio flowing north along the western edge is the warm current, and the California Current flowing south along the eastern edge is the cold current.
Example 2
How does the density of seawater change when its temperature drops or its salinity rises, and what flow results?
1
The colder and saltier seawater is, the denser it becomes.
2
Denser water sinks, and the slow flow of that sunken water along the deep ocean is the deep circulation (thermohaline circulation).
▸ It becomes denser and sinks, producing the deep circulation (thermohaline circulation).
Deep water forms in places such as the North Atlantic near Greenland, where seawater cools and sea ice forms, leaving the remaining water saltier.
CSAT-style problem
Which statement about ocean currents and the properties of seawater is correct?
①The main driving force of surface currents is differences in temperature and salinity
②During El Niño, surface temperatures in the eastern equatorial Pacific rise above normal
③Upwelling reduces nutrients and harms fishing grounds
④Cold currents flow from low to high latitudes
⑤The deep circulation is faster than surface currents
▸ ② During El Niño, surface temperatures in the eastern equatorial Pacific rise above normal
1
In El Niño, the trade winds weaken and warm water that had piled up in the west spreads east, so surface temperatures in the eastern equatorial Pacific rise above normal.
2
Wind drives surface currents (①), upwelling brings up nutrients and makes good fishing grounds (③), cold currents flow from high to low latitudes (④), and the deep circulation is very slow (⑤).

Summary

Ocean currents in brief
wind → surface currents / density differences from temperature and salinity → deep circulation → heat carried around the globe
Together, the atmosphere and the ocean carry heat from low to high latitudes and regulate the climate.
🎯 Exam Points
①Wind drives surface currents, and density differences from temperature and salinity drive the deep circulation
②A warm current flows from low to high latitudes, and a cold current flows from high to low latitudes
③Upwelling brings up cold, nutrient-rich water and makes good fishing grounds
④During El Niño the trade winds weaken, the eastern Pacific warms, and upwelling weakens; La Niña is the opposite
⑤In Korea's East Sea, the East Korea Warm Current and the North Korea Cold Current meet to form a current boundary zone
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