Surface currents move through the upper ocean and are driven mainly by prevailing winds.
The Coriolis effect deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Continents redirect these flows into large circular systems called gyres.
Warm currents carry tropical water towards higher latitudes, while cold currents return cooler water towards lower latitudes.
Example
North Atlantic Drift transfers heat towards north-west Europe and moderates its maritime climate.
Humboldt Current carries cold water northward along the west coast of South America.
Upwelling Transfers Nutrients
Upwelling occurs when winds move surface water away and deeper water rises to replace it.
The rising water is cold and rich in dissolved nutrients released by decomposition at depth.
Nutrients support phytoplankton, which increases marine productivity and can sustain large fisheries.
Along Peru, coastal upwelling supports the anchoveta fishery, but weaker upwelling during El Niño reduces nutrient supply.
Common Mistake
Upwelling brings deeper water towards the surface.
Downwelling carries surface water, dissolved oxygen and heat into deeper layers.
Density Drives Deep-Ocean Circulation
Definition
Thermohaline Circulation
Thermohaline circulation, also known as the global ocean conveyor belt, is a large-scale ocean current system driven by differences in water temperature (thermal) and salinity (haline).
Temperature And Salinity Control Density
Seawater becomes denser when it becomes colder or saltier.
At high latitudes, cooling and sea-ice formation can leave dense salty water that sinks into the deep ocean.
Sinking water draws in more surface water and helps connect surface and deep currents between ocean basins.
The Conveyor Belt Links Ocean Basins
The global ocean conveyor belt combines wind-driven surface currents with density-driven deep circulation.
Warm shallow currents transfer thermal energy poleward before heat loss makes some water dense enough to sink.
Cold deep currents transport oxygen, carbon and nutrients through the Atlantic, Southern, Indian and Pacific oceans.
Mixing and upwelling eventually return deep water to the surface, so the circulation forms a connected global system.
Note
The conveyor-belt diagram is a simplified model of several connected currents rather than one narrow current with a fixed route.
A full circuit takes many centuries, so changes develop more slowly than changes in surface currents.
Ocean and Atmosphere Form One Energy System
Currents exchange heat with the atmosphere, changing air temperature, evaporation and rainfall over nearby regions.
Changes in winds can alter currents, while changes in sea-surface temperature can shift pressure and wind patterns.
This two-way link explains why disturbances in the tropical Pacific can reorganise weather far beyond the ocean where they begin.
Active recall
What three controls shape the direction of surface ocean currents?
How does upwelling support productive fisheries?
How do temperature and salinity changes cause deep water to sink?
What two forms of circulation make up the global ocean conveyor belt?
