Upwelling

Upwelling: The Ocean’s Upward Surge

Upwelling, a rather dramatic oceanographic phenomenon, is essentially the ocean’s way of breathing. It’s the wind-driven motion of dense, cooler, and, crucially, nutrient-rich water from the abyssal depths to the sunlit ocean surface. Think of it as a deep, refreshing sigh from the sea, replacing the often depleted and warmer surface waters. This influx isn’t just about temperature; it’s a vital nutrient delivery service. The rich soup of nutrients, brought up from below, ignites the growth and reproduction of primary producers like phytoplankton . These microscopic powerhouses form the base of the marine food web, and their vibrant blooms are what give these upwelling zones their distinctive signature: cool sea surface temperatures and high concentrations of chlorophyll a , a tell-tale sign of life’s abundance.

The significance of upwelling extends far beyond the microscopic. These nutrient-rich waters are biological hotspots, driving incredibly high levels of primary production . This, in turn, fuels some of the world’s most productive fisheries . Astonishingly, a staggering 25% of the global marine fish catch originates from just five major upwelling zones, which collectively occupy a mere 5% of the ocean’s vast surface area. The most impactful upwellings, those driven by coastal currents or open ocean divergence, are directly responsible for this bounty, shaping global fishery yields.

Mechanisms of the Ascent

The choreography of upwelling is orchestrated by a trio of fundamental forces: wind , the Coriolis effect , and Ekman transport . While their interplay varies depending on the specific type of upwelling, their cumulative effect is consistent: bringing deep waters to the surface. The process begins with the wind, its force rippling across the sea’s surface, initiating a transfer of energy to the water. This wind-driven motion, however, doesn’t simply follow the wind’s direction. Due to the Earth’s rotation, the Coriolis effect , and the phenomenon known as Ekman transport , the surface layer of water is deflected. Specifically, Ekman transport causes the surface water to move at an angle of approximately 45 degrees to the wind direction. This initial movement then influences the layers beneath it through friction, creating a spiraling down-column effect known as the Ekman Spiral . The direction of this net transport is dictated by the Coriolis forces: to the right of the wind in the Northern Hemisphere and to the left in the Southern Hemisphere. When this net movement of surface water diverges, pulling away from a particular area, the void is filled by deeper water rising to replace it – this is upwelling.

Types of Upwelling: A Diverse Phenomenon

The ocean’s upwelling activity isn’t a monolithic event; it manifests in several distinct forms, each with its own unique characteristics and driving forces. The primary driver for most major upwellings is the divergence of currents, which then facilitates the upward movement of nutrient-laden deep waters. These types include coastal upwelling, large-scale wind-driven upwelling in the open ocean, upwelling associated with eddies, topographically-driven upwelling, and broad-diffusive upwelling in the ocean interior.

Coastal Upwelling: The Engine of Productivity

Coastal upwelling is perhaps the most recognized and economically significant form of this phenomenon. It’s the bedrock of some of the planet’s most vital fisheries . This type of upwelling occurs when winds blow parallel to the coastline, generating currents that are then deflected by the Coriolis effect . In the Northern Hemisphere, this deflection is to the right of the wind, and in the Southern Hemisphere, it’s to the left. The resultant Ekman transport moves surface water away from the coast, creating a vacuum that is filled by deeper, colder, and denser water rising to the surface. This process typically unfolds at a rate of 5 to 10 meters per day, though the intensity and proximity of the upwelling can fluctuate with wind strength and distance.

The deep waters brought to the surface are a treasure trove of essential nutrients – nitrate , phosphate , and silicic acid . These nutrients are remnants of the decomposition of organic matter , primarily dead plankton, that has sunk from the surface. Once at the surface, these nutrients, along with dissolved CO 2 (carbon dioxide ) and the sun’s light energy, become the raw materials for photosynthesis by phytoplankton . Consequently, upwelling regions exhibit exceptionally high levels of primary production , contributing approximately 50% of the global marine productivity. This high productivity cascades up the food chain , supporting a rich and diverse ecosystem.

The typical progression of this vibrant food web is as follows: • Phytoplankton → Zooplankton → Predatory zooplankton → Filter feeders → Predatory fish → Marine birds, marine mammals.

Coastal upwelling systems can be classified as either year-round, known as major coastal upwelling systems, or seasonal, occurring only during specific months. Many of these systems are recognized as Large Marine Ecosystems due to their high carbon productivity.

Globally, five major coastal currents are intrinsically linked with upwelling areas: the Canary Current off Northwest Africa , the Benguela Current off southern Africa , the California Current off California and Oregon , the Humboldt Current off Peru and Chile , and the Somali Current off Somalia and Oman . All of these currents are pillars of significant fisheries. The Canary Current , Benguela Current , California Current , and Humboldt Current are the primary eastern boundary currents where coastal upwelling predominantly occurs. The Benguela Current , marking the eastern edge of the South Atlantic subtropical gyre , comprises northern and southern subsystems, both experiencing upwelling. Between them lies an area of permanent upwelling near Luderitz , recognized as the most intense upwelling zone globally. The California Current System , an eastern boundary current of the North Pacific, also exhibits a north-south division, with stronger upwelling in the north, north of Point Conception , California, and weaker upwelling in the south. The Canary Current , part of the North Atlantic Gyre , is similarly segmented by the presence of the Canary Islands . Lastly, the Humboldt Current , also known as the Peru Current, flows westward along the coast of South America from Peru to Chile , extending up to a thousand kilometers offshore. These four eastern boundary currents are the major sites of coastal upwelling across the world’s oceans.

Equatorial Upwelling: A Divergent Zone

Upwelling also occurs at the equator , a phenomenon intricately linked to the Intertropical Convergence Zone (ITCZ). While the ITCZ itself shifts, often residing just north or south of the equator, the easterly (westward) trade winds blowing from the Northeast and Southeast converge along the equator. Despite the absence of Coriolis forces directly at the equator, a divergence of surface waters occurs just north and south of it. This divergence compels denser, nutrient-rich water to well up from below. This process is so significant that the equatorial region in the Pacific is visibly identifiable from space as a broad band of high phytoplankton concentration.

Southern Ocean Upwelling: A Global Conveyor

The Southern Ocean is another significant site of large-scale upwelling. Here, powerful westerly (eastward) winds encircle Antarctica , driving a substantial flow of water northward. This can be considered a form of coastal upwelling, albeit on a massive scale. In the absence of continents across a wide latitudinal band between South America and the Antarctic Peninsula, some of this water originates from profound depths. In numerous numerical models and observational studies, Southern Ocean upwelling is identified as the primary mechanism for bringing deep, dense water to the surface. In certain Antarctic regions, wind-driven upwelling near the coast draws relatively warm Circumpolar deep water onto the continental shelf, which can accelerate ice shelf melt and influence the stability of the ice sheet. Shallower, wind-driven upwelling is also observed off the west coasts of the Americas, northwest and southwest Africa, and southwest and south Australia , all associated with oceanic subtropical high-pressure circulations, mirroring the dynamics of coastal upwelling.

Some ocean circulation models propose that broad-scale upwelling occurs in the tropics. This occurs as pressure-driven flows converge water towards lower latitudes, where it is then warmed diffusively from above. However, the diffusion coefficients required for such a process appear to be larger than what is typically observed in the real ocean, suggesting that while some diffusive upwelling likely takes place, it may not be as dominant as these models indicate.

Other Sources of Upwelling: Localized and Artificial

Beyond these major categories, upwelling can arise from various other sources:

• Topographically-Associated Upwelling: Localized and intermittent upwellings can occur when offshore islands, ridges , or seamounts disrupt deep currents, deflecting them upwards. These features can create nutrient-rich oases in otherwise less productive ocean areas. Notable examples include upwellings around the Galapagos Islands and the Seychelles Islands , both of which support significant pelagic fisheries.
• Wind Shear Driven Upwelling: Upwelling can be triggered anywhere with sufficient shear in the horizontal wind field. A prime example is the influence of a tropical cyclone passing over an area, particularly when moving at slower speeds (less than 8 km/h). The cyclonic winds induce a divergence in the surface water within the Ekman layer , necessitating the upwelling of deeper water to maintain continuity.
• Artificial Upwelling: Human ingenuity has also devised methods to induce upwelling. Devices that harness ocean wave energy or utilize ocean thermal energy conversion can pump water to the surface. Even large offshore wind turbines have been observed to influence and create localized upwellings. Research has also demonstrated that wave-powered devices can indeed stimulate plankton blooms.

Variations in Upwelling: A Dynamic System

The intensity and behavior of upwelling are far from static. They are influenced by a complex interplay of factors including wind strength, seasonal cycles, the vertical structure of the water column, variations in the underwater bathymetry , and instabilities within currents .

In many regions, upwelling is a distinctly seasonal event, leading to periodic surges of biological productivity that mirror spring blooms in coastal waters. The driving force behind this seasonality is often the temperature difference between warm, light air over land and cooler, denser air over the sea. In temperate latitudes , this temperature contrast fluctuates significantly throughout the year, resulting in strong upwelling during spring and summer and diminished or absent upwelling in winter. For instance, off the coast of Oregon, there can be four or five distinct upwelling events, interspersed with periods of calm, during the six-month upwelling season. In contrast, tropical latitudes experience a more consistent temperature contrast, leading to year-round upwelling. The Peruvian upwelling system, for example, operates for most of the year, supporting some of the world’s largest marine fisheries for sardines and anchovies .

Anomalous years, characterized by weakened or reversed easterly trade winds , can drastically alter upwelling dynamics. During such periods, the upwelled water is warmer and nutrient-poor, leading to a sharp decline in biomass and phytoplankton productivity. This phenomenon is famously known as the El Niño-Southern Oscillation (ENSO) event. The Peruvian upwelling system is particularly susceptible to ENSO’s influence, experiencing extreme interannual variability in its productivity.

Changes in underwater topography, or bathymetry , can also significantly affect upwelling intensity. A submarine ridge extending from the coast, for example, can create more favorable upwelling conditions than surrounding areas. Upwelling often initiates at such ridges and remains strongest there, even as it develops in adjacent regions.

High Productivity: The Engine of Marine Life

Upwelling regions stand out as the most biologically productive and fertile areas within the ocean. They act as magnets for hundreds of species across all trophic levels , making them a focal point for marine research . Studies of these ecosystems have revealed a distinctive “wasp-waist” pattern of species richness. In this pattern, species diversity is high at the lower and upper trophic levels, but the intermediate trophic level is sparsely populated, often represented by only one or two dominant species.

This crucial intermediate layer typically consists of small, pelagic fish , which, despite their low species count (making up only 3-4% of the total fish diversity), play a vital role. They feed on the abundant phytoplankton and, in turn, form the primary food source for a multitude of predators. In most upwelling systems, these intermediate species are anchovies or sardines, and often, only one of these dominates, though sometimes two or three species may be present. These small pelagic fish are the linchpin of the entire marine ecosystem in these zones, connecting the phytoplankton base to the higher predators, including larger pelagic fish, marine mammals, and marine birds.

The typical food chain in these highly productive zones unfolds as follows: • Phytoplankton → Zooplankton → Predatory zooplankton → Filter feeders → Predatory fish → Marine birds, marine mammals.

Threats to Upwelling Ecosystems: A Delicate Balance

The very richness and productivity of upwelling ecosystems also make them vulnerable to a variety of threats, the most significant of which is commercial fishing . While these fisheries provide livelihoods and food sources, the intense fishing pressure can have devastating consequences.

In the intricate web of upwelling ecosystems, each species plays a critical role. The overexploitation of a single species, particularly the vital intermediate pelagic fish, can trigger cascading effects throughout the entire food chain. For instance, if fisheries target a popular prey species, harvesting hundreds of thousands of individuals, the food source for its predators dwindles. This can lead to a decline in predator populations, which in turn impacts the predators above them, potentially leading to a collapse of the entire ecosystem. While ecosystems can eventually recover, this process can be slow, and some species may struggle to re-establish themselves.

The targeting of intermediate pelagic fish by fisheries poses a particularly acute threat. These fish, forming the core of the upwelling trophic process, are a primary target for commercial fisheries, accounting for approximately 64% of their catch. The depletion of these species not only removes a food source but can also reduce the reproductive viability of predator populations due to decreased food availability. This can lead to declining populations, especially in species with slower reproductive rates or late maturity. Furthermore, the reduction in the population of a species can lead to a decrease in its genetic diversity, potentially hindering its ability to adapt to environmental changes and increasing the risk of population or ecosystem collapse.

Another significant threat is the El Niño-Southern Oscillation (ENSO) system, specifically the El Niño events. During normal and La Niña periods, strong easterly trade winds maintain robust upwelling. However, during El Niño events, these winds weaken, diminishing the divergence of water north and south of the equator and consequently reducing upwelling. Coastal upwelling zones also shrink as their wind-driven engine falters. This global reduction in upwelling leads to a sharp decline in productivity, as nutrient-rich waters are no longer brought to the surface. Without this essential nutrient supply, the entire trophic pyramid is destabilized, leading to the potential collapse of these rich upwelling ecosystems.

Effect on Climate: A Local Influence

Coastal upwelling exerts a considerable influence on the local climate of the affected regions. This effect is amplified when the upwelling current is already cool. As cold, nutrient-rich water ascends, it cools the overlying air. This cooling can lead to condensation, forming sea fog and stratus clouds . Simultaneously, it inhibits the formation of higher altitude clouds, thereby reducing the likelihood of showers and thunderstorms and often leaving coastal land areas dry.

In regions with year-round upwelling, such as the western coasts of Southern Africa and South America, temperatures tend to be cooler, and rainfall is scarce. Seasonal upwelling systems, often paired with seasonal downwelling (as seen on the western coasts of the United States and the Iberian Peninsula ), typically result in cooler, drier summers and milder, wetter winters. Permanent upwelling locations frequently exhibit semi-arid or desert conditions, while seasonal upwelling zones often present Mediterranean or semi-arid climates, and occasionally oceanic climates. Cities around the world significantly impacted by strong upwelling regimes include San Francisco , Antofagasta , Sines , Essaouira , Walvis Bay , and Curaçao , among others.