Upwelling is a process in which deep, cold water rises toward the surface. Typically, water that rises to the surface as a result of upwelling is colder and rich in nutrients. This is the reason why coastal upwelling ecosystems are some of the most productive ecosystems in the world and support many of the world’s most important fisheries.
For example, the Eastern Boundary Upwelling Systems (EBUS), such as the California Current System (CalCS), the Canary Current System (CanCS), the Humboldt Current System (HCS), and the Benguela Current System (BenCS), are among the most productive marine ecosystems, supplying up to 20% of the global fish catches, although they only cover approximately 1% of the total ocean. Surface alongshore winds, force the offshore water transport and the divergence of the surface flow, thereby lifting nutrient-rich deep waters into the euphotic layer. The nutrient-rich upwelled water, in addition to the sunlight, sustains the blooms of phytoplankton that are the foundation of the aquatic food web.
Understanding the drivers and monitoring changes across EBUS is becoming increasingly important: many studies have in fact documented trends and changes at decadal scale in the EBUS ecosystem structure. Coastal warming increases the water stratification and it might limit the effectiveness of upwelling to bring nutrient-rich deep waters up to the surface. Increasing or decreasing of the upwelling-favourable winds might also mitigate or amplify the effect of coastal warming. Coastal waves may also influence the water column stratification modulating coastal biogeochemical conditions and triggering vertical displacements of the thermocline, which controls subsurface anomalies (e.g., salinity), and thus the impact on EBUS productivity.
Moreover, we have to mention the influence of the main large-scale ocean-atmosphere processes: the El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the North Pacific Gyre Oscillation (NPGO), the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) seem to play a role in controlling the upwelling variability.
Read more at Euro-Mediterranean Center on Climate Change
Image by Sebastian Ganso from Pixabay