Ocean Circulation


Thermohaline Circulation


Upwelling schematic

The ocean is a major driver of global climate. It redistributes large amounts of heat around the planet via global ocean currents – through regional scale upwelling and downwelling, and via a process called thermohaline circulation.1,2 Marine and coastal ecosystems have adapted over time to the ocean circulation patterns as we know them today. Global climate change alters environmental forcing mechanisms – or factors that impact ocean circulation – such as wind, precipitation, temperature, and salinity patterns. These changes in forcing mechanisms may lead to a change in ocean circulation, as well as an increase in storm activity.

Thermohaline circulation behaves like a conveyor belt. Originating in the Northern Atlantic Ocean, cold, dense water sinks to the deep ocean. These waters travel across ocean basins to the tropics where they warm and upwell to the surface. The warmer, less dense, tropical waters are then drawn to polar latitudes to replace the cold sinking water. Here, heat is transferred to the atmosphere, causing the water to become cold and dense and thus renewing the conveyor cycle. Melting of polar ice could reduce the salinity and thus density of polar waters, which could weaken the rate at which this water sinks. Such melting would thus alter movement of heat around the globe.

Atmospheric pressure systems create strong winds along the eastern Pacific. These northwesterly winds lead to upwelling along the western coast of North America.1 Upwelling, where cold, nutrient-rich waters are brought to the surface, can drive ocean productivity, supporting most productive fisheries worldwide. Such wind patterns are expected to change considerably due to global climate change.2-4 Changes in global air temperatures over land and the ocean, as well as increased temperature variation, will alter atmospheric pressure gradients that drive the strength of winds over the ocean. Stronger winds induce a rapid, intense upwelling that provides a large influx of nutrients in a short amount of time.  This influx can increase the frequency and distribution of hypoxic events (low oxygen zones).5,6  Increased variability of winds due to the changing global climate is also expected to cause stronger and longer El Nino-Southern Oscillation regimes,7 which are periods of time when the waters in the tropical Pacific are warmer (or cooler) than usual, causing weather and climate consequences throughout the globe. Such increases in adverse ocean conditions may not be tolerable for many marine animals.8

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