Upwelling and Downwelling in the ocean
Standing on the beach on a windy day you might be unhappy about the sand blowing in your eyes, but take a closer look, you may be witnessing waters of the ocean moving.
Upwelling and downwelling are important processes that describe mass movements of the ocean, which affect both surface and deep currents. These movements are essential to stirring the ocean, delivering oxygen to depth, distributing heat, and bringing nutrients to the surface. Stratification occurs when surface waters and deep waters are separated into layers by distinct differences in temperature and salinity. Think of swimming in the sea and how warm the top few centimetres feel compared with deeper waters. The top layer of ocean is called the “surface mixed layer”; it is often warm and without many nutrients.
Downwelling occurs when surface waters converge (come together), pushing the surface water downwards. Regions of downwelling have low productivity because the nutrients get used up and are not continuously resupplied by the cold, nutrient-rich water from below the surface. Upwelling is the movement of cold, deep, often nutrient-rich water to the surface mixed layer; and downwelling is the movement of surface water to deeper depths.
The relationship between downwelling, convergence zones and upwelling (Image: AE Nieblas, CSIRO).
Upwelling occurs when surface waters diverge (move apart), enabling upward movement of water. Upwelling brings water to the surface that is enriched with nutrients important for primary productivity (algal growth) that in turn supports richly productive marine ecosystems.
Some of the most important upwelling regions are along the coasts of continents. In these coastal upwelling regions, surface winds push water away from the shore and create a divergence at the coast, which is replaced by water from depth. For coastal upwelling to occur, the wind must be parallel to the coast because water is deflected to the left of the wind in the southern hemisphere, and the right of the wind in the northern hemisphere. This deflection is due to the Coriolis force which causes objects travelling in a straight line appear to curve or deflect due to the rotation of the earth.
Upwelling regions are often measured by their productivity due to the influx of nutrients to the surface mixed layer and euphotic zone (sunlit layer) by upwelling currents. This drives photosynthesis of phytoplankton (tiny alga), which form the base of the ocean food web. Upwelling regions are less than 1 per cent of the world’s ocean by area, but account for greater than 20 percent of the global fish catch.
Upwelling in the current climate off a southern hemisphere coast. Wind is parallel to the coast, and water is deflected to the left of the wind by the Coriolis force. Surface water is pushed offshore and is replaced by cool, nutrient-rich water from depth (Image: AE Nieblas, CSIRO).
Most major upwelling regions are found along the west coasts of continents, such as off California, Peru, Namibia and South Africa. Large-scale upwelling off the west coast of Australia is suppressed due to the poleward-flowing Leeuwin Current. However, smaller-scale regional upwelling is found around the Australian coastline, including the largest and most predictable upwelling off the Bonney Coast in southeastern Australia.
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(left) Global distribution of major upwelling regions are mostly found off the west coast of continents (image: CSIRO) (right) Australia does not have a large-scale upwelling area because it is suppressed by the poleward-flowing Leeuwin Current. However, regional-scale upwelling occurs around Australia at the Gascoyne region in northwestern Western Australia, in southwestern Western Australia, near the separation point of the East Australia Current from the east coast at Sugar Loaf Point, and in southeastern Australia, including the Bonney Coast (image: CSIRO).
Meet Anne-Elise Nieblas. She is studying upwelling regions as part of her work at CSIRO in Hobart. Anne-Elise uses computer models to help her understand how changes in environmental conditions due to climate change may affect Australian upwelling regions and their productivity. Her results indicate that upwelling winds are expected to increase in the 21st century, which will in turn increase phytoplankton productivity. However, complex interactions amongst marine organisms make impacts of climate change on fish difficult to predict.
Anne-Elise with a treasure found during a
snorkeling trip (Image: AE Nieblas, CSIRO).
Links and further information
You too can conduct an experiment like Anne Elise and determine how thermal currents work by conducting a small experiment enabling you to view a miniature version of downwelling and ocean stratification. Just follow this link to the experiment instructions and worksheets.
Want to know more about ocean upwelling?Click here for further information about the relationship between wind and upwelling.
Want to know more about the Coriolis force? Check out the National Oceanic and Atmospheric Administration Website.
Source and acknowledgements
A special thankyou to Anne-Elise for her detailed contribution to the Upwelling and Downwelling page.
CSIRO website
Nieblas AE, Sloyan BM, Hobday AJ, Coleman R, Richardson AJ (2009) Variability of biological production in low wind-forced regional upwelling systems: a case study off southeastern Australia Limnology and Oceanography 54:1548-1558
Pauly D, Christensen V (1995) Primary production required to sustain global fisheries. Nature 374:255 - 257
Nieblas AE, Sloyan BM, Coleman R, Richardson AJ (2009) Variability of biological production in low wind-forced regional upwelling systems: a case study off southeastern Australia Limnology and Oceanography 54:1548-1558
Thurman H, Trujillo A (1999) Essentials of Oceanography. Macmillan Publishing Company, New Jersey, USA. pp 527.
