There are probably many answers, here are a couple that I am familiar with to get started.
Cold currents are generally from deep water that has sunk at the poles and travelled deep below the surface. When this cold, nutrient rich water surfaces at an upwelling the nutrients become available to phytoplankton that photosynthesise them powered by the sun. This explosion of growth fuels the food chain and is associated with some of the major fisheries in the World; eg the Peruvian Sardine fishery, which support local populations.
During periods of El Nino the trade winds in the Pacific lessen interfering with the driving of warm surface waters West and eliminating the upwelling cool currents resulting in reduced fisheries productivity.
El Nino years are also associated with warmer surface waters which cause coral reef bleaching. When the sea surface temperature exceeds a threshold temperature for the coral over a period of time the organism becomes stressed and releases the zooxanthellae essential for photosynthesis within the coral tissue. In time the coral will die and be replaced by more vigorously pioneering algal species that prevent the recolonisation of the reef by coral polyps. Once this happens the fish leave and it is difficult for the reef to recover. This is one of the greatest challenges being faced by countries that rely on reefs for food and tourism.
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The currents that keep coastal regions cooler than inland climates year-round are called maritime currents. These currents bring cool oceanic air to coastal areas, moderating temperatures and creating milder climates compared to the more extreme temperatures found inland.
Warm water currents significantly influence coastal areas by moderating temperatures, leading to milder climates and promoting diverse marine ecosystems. These currents can enhance nutrient availability, supporting fisheries and marine life. Additionally, they can impact weather patterns, potentially increasing precipitation and altering storm trajectories. Overall, warm currents contribute to the ecological and economic vitality of coastal regions.
They create currents and micro climates for different sea life.
Currents in the coastal water masses
The type of current that creates warmer climates along a coastal area is a warm ocean current. Warm ocean currents bring warm water from the tropics to higher latitudes, resulting in milder temperatures along the coast.
Warm Ocean currents warms the air above it and some move from the tropic towards both poles ---------------------------------------------------------AND----------------------------------------------------------------------------- Cold Ocean currents also affect weather patterns by bringing cold water from polar to the equator
Ocean currents that originate from high latitudes and flow towards the equator create cooler climates in coastal areas. These currents bring cold water from polar regions, which in turn cools the surrounding air and moderates temperatures along the coast. An example of this is the California Current along the west coast of the United States.
Currents in the coastal water masses
Ocean currents significantly influence the climate of coastal regions by redistributing heat across the planet. Warm currents, such as the Gulf Stream, raise temperatures in nearby coastal areas, leading to milder winters, while cold currents can have a cooling effect. For example, the California Current brings cooler water down the western U.S. coast, resulting in a more temperate climate and foggy conditions in coastal California. This interaction between ocean currents and coastal climates plays a crucial role in shaping local weather patterns and ecosystems.
Surface currents flowing away from the equator typically carry warm water. These currents are influenced by wind patterns and the Earth's rotation, transporting warm, less dense water towards higher latitudes. As warm water moves poleward, it can affect local climates and marine ecosystems.