Surface Currents, Deep Currents & Upwelling: Sixth Grade Science
Ocean water circulates continuously through wind-driven surface gyres and density-driven deep thermohaline conveyor belts, regulating planetary heat and sustaining marine food webs.
The Two Regimes of Ocean Circulation
The global ocean is in continuous, dynamic circulation, functioning as Earth's central thermal heating and air-conditioning system. Ocean currents are coherent streams of seawater moving through surrounding waters along predictable paths. Oceanographers divide ocean currents into two fundamental physical regimes:
- Surface Currents (Wind-Driven): Comprise roughly 10 percent of ocean water, circulating in the upper 400 meters of the water column, driven by global atmospheric winds and steered by the Coriolis effect.
- Deep Ocean Currents (Thermohaline): Comprise roughly 90 percent of ocean water, circulating through the deep abyss, driven by differences in water density caused by temperature and salinity variations.
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| SURFACE VERSUS DEEP CURRENTS |
+---------------------+-----------------------+-------------------------------+
| Circulation Regime | Primary Driving Force | Spatial Scale & Velocity |
+---------------------+-----------------------+-------------------------------+
| Surface Currents | Global wind belts & | Fast (up to 10 km/h); |
| | Coriolis effect | circulates within upper 400 m |
+---------------------+-----------------------+-------------------------------+
| Deep Thermohaline | Density differences | Very slow (centimeters / sec);|
| Currents | (Temperature & Salt) | circulates entire ocean abyss |
| | under gravity | over a 1,000-year cycle |
+---------------------+-----------------------+-------------------------------+
Surface Gyres and the Coriolis Effect
Surface ocean currents do not flow in straight lines across ocean basins. Their paths are deflected by Earth's rotation—a phenomenon known as the Coriolis effect:
- Northern Hemisphere: Moving fluids are deflected toward their RIGHT, causing surface currents to circulate in giant clockwise rotating loops called gyres.
- Southern Hemisphere: Moving fluids are deflected toward their LEFT, causing surface currents to circulate in counter-clockwise gyres.
Earth contains five major subtropical ocean gyres: North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres.
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| THE FIVE SUBTROPICAL OCEAN GYRES |
| |
| NORTHERN HEMISPHERE: Clockwise Rotation |
| -----> (Eastward Current) -----> |
| / \ |
| (Warm Gulf Stream) (Cold California Current) |
| ^ v |
| \ / |
| <----- (Trade Winds West) <------ |
| |
| SOUTHERN HEMISPHERE: Counter-Clockwise Rotation |
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Climate Moderation by Boundary Currents
Surface currents transport vast quantities of thermal energy from the equator toward the poles:
- The Gulf Stream: A powerful, warm western boundary current that originates in the tropical Caribbean, carrying warm water northeast across the Atlantic toward Europe. The Gulf Stream releases massive quantities of heat into the westerly winds, giving cities like London and Paris mild, ice-free winters, even though they share the same northern latitude as freezing, snow-covered sub-arctic Canada!
- The California Current: A cold eastern boundary current carrying cold sub-polar waters southward along the western coast of North America, keeping coastal California cool and producing famous coastal summer fogs.
Deep Thermohaline Circulation: The Global Ocean Conveyor
Deep ocean circulation is driven by gravity acting on density differences (thermohaline: therme = heat, halos = salt). The global conveyor belt originates in the North Atlantic near Greenland and Iceland:
- Warm, salty water carried north by the Gulf Stream cools in arctic air.
- In winter, sea ice forms, expelling salt into the liquid water beneath via brine rejection.
- The cold, hyper-saline water becomes ultra-dense and sinks to the ocean floor, forming North Atlantic Deep Water (NADW).
- This deep current crawls southward along the Atlantic seafloor, flows around Antarctica, and branches into the Indian and Pacific oceans.
- Centuries later, the water slowly warms, becomes less dense, and upwells back to the surface to complete the global 1,000-year loop.
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| THE GLOBAL CONVEYOR BELT LOOP |
| |
| [ Cold Salty Water Sinks in North Atlantic ] |
| | |
| v (Travels South along Abyssal Ocean Floor) |
| [ Deep Atlantic Current ] ===> [ Circles Antarctica ] |
| | |
| v (Branches to Pacific / Indian) |
| [ Warm Surface Flow Returns ] <=== [ Deep Water Upwells in Pacific ] |
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Coastal Upwelling: Fueling Marine Life
Upwelling is the vertical upward movement of cold, deep, nutrient-rich ocean water to the sunlit surface:
- Coastal Winds: Winds blow parallel to the coast (e.g., blowing southward along the coast of California or Peru).
- Ekman Transport: Due to the Coriolis effect, surface water is deflected 90 degrees offshore, pushing warm surface water out to sea.
- Upwelling Replacement: Cold water from depths of 200 to 400 meters rises vertically to replace the displaced surface water.
- Ecological Explosion: The upwelling water brings decomposed nitrates and phosphates from the dark ocean floor into the sunlit surface. Phytoplankton bloom in astronomical numbers, fueling rich marine food webs supporting anchovy fisheries, sea lions, dolphins, and whales.
Exercises and Solutions
Exercise 1: Contrasting Climate at Identical Latitudes. London, England, and Calgary, Canada, both lie at approximately 51 degrees North latitude. Calgary experiences brutal, sub-zero continental winters with average January temperatures of -9 degrees Celsius and heavy snow. London experiences mild, temperate marine winters with average January temperatures of +5 degrees Celsius and virtually zero snow. Explain how ocean currents account for this 14-degree temperature disparity between two cities at the exact same latitude.
Solution 1: The temperature difference is driven by ocean surface currents and prevailing atmospheric winds. London sits downwind of the warm North Atlantic Current (the extension of the Gulf Stream). The Gulf Stream transports trillions of Joules of tropical solar heat from the Caribbean northeast across the Atlantic Ocean. As westerly atmospheric winds blow across this warm ocean current, they absorb thermal energy and moisture, blanketing the British Isles in warm marine air that moderates winter temperatures and prevents freezing. In contrast, Calgary is situated deep within the interior of the North American landmass, far from any warm ocean currents, and is exposed to freezing arctic air masses descending from the polar tundra without any marine thermal buffering.
Exercise 2: Evaluating the Biological Impacts of El Niño on Upwelling. Under normal conditions along the coast of Peru, strong trade winds drive offshore Ekman transport, fueling intense coastal upwelling that supports the world's most productive anchovy fishery. Every few years, an El Niño event occurs: trade winds weaken, and a thick pool of warm tropical water surges eastward across the Pacific, capping the coastal waters off Peru. Explain the biological consequences of this event for local anchovy fisheries and seabird populations.
Solution 2: When the warm tropical water caps the coastal ocean off Peru during an El Niño, it forms a thick, buoyant thermal lid that shuts down coastal upwelling. The deep, cold, nutrient-rich water can no longer rise to the sunlit surface layer. Without an injection of nitrates and phosphates, photosynthetic phytoplankton populations collapse immediately. Anchovies, which feed directly on phytoplankton, either starve or migrate away into deeper, colder waters out of reach of fishing fleets and predators. Marine seabirds (such as pelicans and boobies) and sea lions that depend on anchovies for survival suffer catastrophic starvation and reproductive failure, devastating the coastal marine ecosystem and local commercial fishing industries.