El Niño is a climate phenomenon involving a warming of the central and eastern tropical Pacific Ocean, disrupting typical weather patterns globally. This natural climate cycle influences ocean processes, including upwelling, an important component of marine ecosystems. Understanding this interaction helps explain its environmental and biological consequences.
Understanding Ocean Upwelling
Ocean upwelling is a process where deeper, colder, and nutrient-rich water rises to the ocean surface. This vertical movement is driven by wind patterns that push surface waters away from coastlines or diverge them in open ocean areas. As surface water moves away, colder water from below moves upward to replace it.
The water brought to the surface during upwelling is abundant in nutrients like nitrates and phosphates, accumulated from deep-ocean organic matter. When these nutrients reach the sunlit surface layer, they fuel the growth of phytoplankton, microscopic marine plants that form the base of the marine food web. This high primary productivity supports a diverse array of marine life, including zooplankton, fish, seabirds, and marine mammals.
Upwelling zones, though making up less than 10% of the ocean’s surface, contribute to more than 90% of the world’s fish catch. An example is the coastal upwelling along the west coast of the Americas, where prevailing winds push warmer surface waters offshore. This allows cold, nutrient-rich water from the depths to rise, creating highly productive marine environments that sustain important fisheries.
The El Niño Climate Pattern
El Niño is a recurring climate pattern of warmer-than-average sea surface temperatures in the central and eastern tropical Pacific Ocean. This phenomenon is part of a larger cycle known as the El Niño-Southern Oscillation (ENSO), involving coupled changes in both the ocean and atmosphere. El Niño events typically occur every two to seven years and can last for nine to twelve months, though some persist for several years.
During normal conditions, strong easterly trade winds blow across the tropical Pacific, pushing warm surface water towards the western Pacific. This creates a higher sea level in the west, with the average sea-level height about 1.5 feet higher at Indonesia than at Peru. This westward movement of warm water also causes the thermocline—the boundary layer separating warmer surface waters from colder deep waters—to be deeper in the western Pacific, often around 450 feet (150 meters), and shallower in the east, around 90 feet (30 meters).
During an El Niño event, these trade winds weaken, sometimes even reversing direction. This weakening reduces the force that normally pushes warm water westward, allowing warm surface water to move eastward across the Pacific towards the coast of South America. As this warm water shifts, the thermocline in the eastern Pacific deepens, sometimes becoming less distinct.
El Niño’s Influence on Upwelling
The weakening of the trade winds during an El Niño event impacts the ocean’s upwelling process. Normally, these winds drive the upward movement of cold, nutrient-rich water by pushing surface water away from coastal areas and along the equator. With weaker winds, this driving force is diminished.
As warm surface water spreads eastward and the thermocline deepens in the eastern Pacific, the cold, nutrient-rich water that normally rises to the surface becomes harder to access. The deeper thermocline acts as a barrier, making it more difficult for the colder, denser, and nutrient-laden water from the deep ocean to reach the sunlit upper layers.
This results in a reduction or cessation of upwelling in affected regions, particularly along the west coast of South America and along the equator. The absence of this regular influx of cold, nutrient-rich water alters the ocean’s chemical and physical properties at the surface. Consequently, the characteristic cool, productive waters found in these upwelling zones become warmer and less fertile during an El Niño.
Consequences for Marine Ecosystems and Beyond
The reduction or cessation of upwelling during an El Niño event has effects throughout marine ecosystems. With less nutrient-rich water reaching the surface, the growth of phytoplankton, the primary producers at the base of the marine food web, declines. This reduction in primary productivity means there is less food available for zooplankton, which in turn impacts the fish that feed on them.
Fish populations, particularly species like anchovies and sardines that thrive in nutrient-rich upwelling zones, experience declines or shift their distributions to cooler, more productive waters. For example, the Peruvian anchovy fishery has historically faced collapses during El Niño events, such as in 1972. This disruption extends up the food chain, leading to reduced food availability for larger marine predators, including seabirds, seals, and other marine mammals, resulting in increased mortality rates.
The economic hardship faced by fishing communities is substantial due to reduced catches and altered fish distributions. Fisheries that rely on species affected by upwelling, such as those off the coasts of Peru and Chile, experience economic losses. Beyond marine life and fisheries, the warming of ocean temperatures and changes in ocean currents caused by El Niño can influence global weather patterns, contributing to altered rainfall, temperatures, and extreme weather events in various parts of the world.