The ocean is a dynamic environment, characterized by constant movement of water in vast currents and localized flows. These motions redistribute heat, marine life, and chemical compounds throughout the global ocean. One fundamental oceanic process involves the vertical movement of water from deeper layers to the surface. This phenomenon plays a significant role in shaping marine ecosystems and influencing broader oceanic conditions.
Understanding Upwelling
Upwelling is an oceanographic phenomenon involving the upward movement of water from deeper parts of the ocean towards the surface. This rising water originates from depths where sunlight does not penetrate, making it cooler than the surface water it replaces. The upwelled water is also dense, a characteristic linked to its colder temperature and higher concentration of dissolved substances.
The Driving Forces Behind Upwelling
Upwelling is driven by a combination of factors, including wind patterns, the Earth’s rotation, and seafloor topography. Prevailing winds blowing across the ocean surface can push surface water away from coastlines, a process known as coastal upwelling. As surface water moves offshore, deeper water rises to fill the void created by the displaced surface water. The Coriolis effect, resulting from the Earth’s rotation, deflects these moving waters: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection, known as Ekman transport, causes a net movement of surface water at a 90-degree angle to the wind direction, which helps force surface waters away from a coast or create divergence in the open ocean.
Equatorial upwelling occurs where trade winds blow westward along the equator, causing surface waters to diverge both north and south of the equator, allowing deeper water to rise. Although the Coriolis effect is negligible directly at the equator, it causes water movement away from the equator just north and south of it, leading to this upwelling. The presence of underwater features like seamounts or ridges can also force deep ocean currents upward, contributing to localized upwelling.
The Vital Role of Upwelling
The rising deep water during upwelling brings with it a supply of dissolved inorganic nutrients, such as nitrates, phosphates, and silicic acid, which accumulate from the decomposition of organic matter that sinks from surface waters. These nutrients act as a natural fertilizer for the sunlit surface waters. The increased availability of these nutrients stimulates the growth of microscopic marine plants called phytoplankton. Phytoplankton form the base of the marine food web, converting sunlight and nutrients into organic matter through photosynthesis.
The abundant phytoplankton populations then support communities of zooplankton, which are tiny marine animals that feed on phytoplankton. This energy transfer continues up the food chain, sustaining larger organisms like small fish, predatory fish, marine mammals, and seabirds. Upwelling regions are productive ecosystems with significant biodiversity. For example, coastal upwelling regions, despite accounting for only about one percent of the ocean surface, contribute approximately 25% to 50% of the total global marine fish catches. Upwelling also contributes to the ocean’s carbon cycle by bringing carbon from deeper waters to the surface where it can be utilized by phytoplankton.