Желдің әсерінен теңіз суының көтерілуі және оның маңызы

Replacement by deep water moving upwards of surface water driven offshore by wind

Upwelling is an oceanographic phenomenon that involves wind driven motion of dense, cooler, and usually nutrient rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient depleted surface water. The nutrient rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. The biomass of phytoplankton and the presence of cool water in those regions allow upwelling zones to be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll a. The increased availability of nutrients in upwelling regions results in high levels of primary production and thus fishery production. Approximately 25% of the total global marine fish catches come from five upwellings, which occupy only 5% of the total ocean area. Upwellings that are driven by coastal currents or diverging open ocean have the greatest impact on nutrient enriched waters and global fishery yields.

Local and intermittent upwellings may occur when offshore islands, ridges, or seamounts cause a deflection of deep currents, providing a nutrient rich area in otherwise low productivity ocean areas. Examples include upwellings around the Galapagos Islands and the Seychelles Islands, which have major pelagic fisheries. Artificial upwelling is produced by devices that use ocean wave energy or ocean thermal energy conversion to pump water to the surface. Ocean wind turbines are also known to produce upwellings. Ocean wave devices have been shown to produce plankton blooms.

Upwelling intensity depends on wind strength and seasonal variability, as well as the vertical structure of the water, variations in the bottom bathymetry, and instabilities in the currents. In some areas, upwelling is a seasonal event leading to periodic bursts of productivity similar to spring blooms in coastal waters. Wind induced upwelling is generated by temperature differences between the warm, light air above the land and the cooler denser air over the sea. In temperate latitudes, the temperature contrast is greatly seasonably variable, creating periods of strong upwelling in the spring and summer, to weak or no upwelling in the winter. For example, off the coast of Oregon, there are four or five strong upwelling events separated by periods of little to no upwelling during the six month season of upwelling. In contrast, tropical latitudes have a more constant temperature contrast, creating constant upwelling throughout the year. The Peruvian upwelling, for instance, occurs throughout most of the year, resulting in one of the world's largest marine fisheries for sardines and anchovies.

Coastal upwelling has a major influence over the affected region's local climate. This effect is magnified if the ocean current is already cool. As the cold, nutrient rich water moves upwards and the sea surface temperature gets cooler, the air immediately above it also cools down and is likely to condensate, forming sea fog and stratus clouds. This also inhibits the formation of higher altitude clouds, showers and thunderstorms and results in rainfall over the ocean leaving the land dry. In year round upwelling systems (like that of the western coasts of Southern Africa and South America), temperatures are generally cooler and precipitation scarce. Seasonal upwelling systems are often paired with seasonal downwelling systems (like that of the western coasts of the United States and Iberian Peninsula), resulting in cooler, drier than average summers and milder, wetter than average winters. Permanent upwelling locations typically have semi arid/desert climates while seasonal upwelling locations usually have Mediterranean/semi arid climates, oceanic in some cases. Some worldwide cities affected by strong upwelling regimes include: San Francisco, Antofagasta, Sines, Essaouira, Walvis Bay, among others.