El Niño is a significant climate phenomenon originating in the Pacific Ocean, influencing weather patterns across the globe. This natural cycle involves variations in winds and sea surface temperatures, leading to widespread environmental and economic impacts. El Niño events usually last nine to twelve months, sometimes several years, occurring every two to seven years. These oceanic processes help explain how El Niño triggers extreme weather, such as droughts and heavy rainfall, globally.
What is Upwelling and Its Normal Location?
Upwelling is an oceanic process where deeper, cooler, and nutrient-rich water rises to the surface, replacing warmer, nutrient-depleted surface water. This upward movement is primarily driven by wind patterns and ocean currents. The nutrients brought to the surface act as a natural fertilizer for phytoplankton.
Phytoplankton, microscopic plants that perform photosynthesis, form the base of marine food webs, supporting zooplankton, fish, marine mammals, and seabirds. As a result, upwelling zones are among the most productive marine ecosystems globally, accounting for approximately 50% of the world’s fisheries landings despite occupying only about 1% of the ocean surface. In the Pacific Ocean, strong upwelling normally occurs along the equator in the eastern and central parts, extending from the Galapagos Islands westward towards the date line. This equatorial upwelling is driven by prevailing easterly trade winds, which push warm surface water westward, allowing cooler, deeper water to rise and cool the air above the sea surface.
How El Niño Suppresses Upwelling
During an El Niño event, normal atmospheric and oceanic conditions in the tropical Pacific change significantly, suppressing upwelling. Strong easterly trade winds usually blow across the equatorial Pacific, pushing warm surface water towards the western Pacific. This westward movement allows cooler, nutrient-rich water from the deep ocean to rise to the surface in the eastern and central equatorial Pacific.
During El Niño, these easterly trade winds weaken or can even reverse direction. This weakening is linked to a reversal in atmospheric pressure patterns. As the trade winds diminish, the warm surface water that normally accumulates in the western Pacific is no longer held there and instead surges eastward across the equator towards the coasts of the Americas.
This eastward movement of warm water causes the thermocline, the boundary between warm surface water and cold deep water, to deepen in the eastern Pacific. A deeper thermocline restricts the ability of cooler, nutrient-rich water from below to reach the surface, weakening or completely stopping the upwelling process in the eastern and central equatorial Pacific. The resulting accumulation of unusually warm surface water in these regions is a defining characteristic of El Niño.
Consequences of Suppressed Upwelling
The suppression of upwelling during an El Niño event leads to ecological and climatic impacts, particularly in the equatorial Pacific. With reduced or absent upwelling of nutrient-rich water, the primary productivity of marine ecosystems declines sharply. This lack of nutrients directly affects phytoplankton, which form the base of the marine food web, leading to a decrease in their populations.
The reduction in phytoplankton then cascades up the food chain, impacting zooplankton and commercially important fish populations, such as anchovies off the coast of Peru. During El Niño events, anchovies in Peruvian waters move deeper in search of cooler temperatures and food, leading to delays or cancellations of fishing seasons. A past El Niño event, for example, decimated Peru’s anchovy population, causing economic losses and impacting seabird populations that rely on them for food.
Beyond marine life, the warmer ocean surface in the central and eastern Pacific during El Niño influences atmospheric circulation patterns globally. This shift can lead to altered rainfall patterns, causing increased precipitation and flooding in some regions. Conversely, other areas may experience drier conditions and an increased risk of droughts and wildfires. These changes in ocean temperature and atmospheric circulation also contribute to a rise in global average temperatures.