Planet Earth/5f. La Nina and El Nino: the sloshing of the Pacific Ocean

El Niño Southern Oscillation
For centuries Peruvian fisherman would observe warm ocean waters as they set out their fishing nets during late December, although not every year produced such odd warming of the ocean. The fish caught during these strange warm ocean water events where different species than the fisherman normally found in their fishing nets. Such events became known as El Niño de Navidad, but later shortened to El Niño (the boy). The event was often followed by intense inland rain storms, where deserts flooded and rivers swelled. These strange weather patterns were observed for centuries by the Incas, and later by the Spanish. It was later discovered that the Pacific coastal waters also undergo periods of cooling, which became known as La Niña (the girl). People began observing an oscillating cycle along the equatorial region off the Pacific coast of Peru and Ecuador. The La Nina/El Niño cycle (often abbreviated as ENSO; El Niño Southern Oscillation) is an important cycle of warming and cooling surface ocean waters that occur within the Intertropical Convergence Zone (ITCZ) near the equator. This warming and cooling cycle has a profound effect on weather patterns elsewhere, particularly in the countries bordering the Pacific Ocean, such as the United States.

Until the 1980s, scientists did not have a clean understanding of ENSO (El Niño Southern Oscillation), and most evidence was based on sailors logs which recorded surface water temperatures while crossing the Pacific Ocean. But the doldrums near the equator were rarely visited by sailors, so a clear picture of variations of ocean water temperatures in these warm equatorial waters was lacking. In 1985, the Tropical Ocean Global Atmosphere program (TOGA) was funded by Pacific nations to study the temperatures of the ocean in the hopes that it could be used to aid weather forecasting. This included deploying buoys that would record ocean water temperatures at various depths in the ocean. The success of the recordings was followed up with today’s Tropical Atmospheric Ocean (TAO) deployment, which is a series of about 70 moored buoys spread across the equatorial Pacific Ocean which record ocean water temperatures at various depths in the ocean and send this information using satellite communication. This data gives a glimpse into the dynamic nature of equatorial tropical ocean water, and how it changes from month to month. As discussed earlier, the equatorial regions of the world’s ocean are affected by the near surface Equatorial Counter-Current which is an eastward flowing, westerly wind-driven current which extends to depths of 100–150m in the Atlantic, Indian, and Pacific Oceans. It is limited to the equatorial regions between 0° to 5° latitudes. It divides the two large gyres in the Pacific and Atlantic Oceans. This surface ocean current is a result of westerly winds, which are influenced by the ocean surface temperature oscillations resulting from Earth’s seasonal changes in its tilt. Because this ocean water does not travel north or south beyond this zone, the Coriolis effect is minimum, with the surface ocean current more strongly affected by wind. The Intertropical Convergence Zone is a low-pressure system caused by air moving into the Equator replacing air rising over the tropical warm ocean. The rising air results in intense rain-storms over these regions of Earth, but also the doldrums that slow wind speeds. Within the Intertropical Convergence Zone, these equatorial ocean water temperatures can vary between equatorial regions, east and west. If ocean water is relatively hot near the surface, the air above the ocean will rise, resulting in a strong low-pressure zone and will be relatively wetter, while if the ocean water is relatively cold near the surface, the air above the ocean will sink, resulting in a strong high-pressure zone within these tropical regions, and will be relatively drier.

During an El Niño event the hot surface ocean water off the coast of Ecuador and Peru, will result in a low-pressure zone, with air moving into the region and rising high into the atmosphere. This rising warm air will contain an abundance of moisture which will rain out over the South and North American continents, resulting in devastating flooding. During a La Nina event the cold surface ocean waters off the coast of Ecuador and Peru will result in a high-pressure zone, with air moving out into the Pacific Ocean, resulting in dry weather patterns in North and South America. It also flows toward low pressure zones above the Gulf of Mexico, which can increase the frequency of hurricanes. The westerly Equatorial Counter-Current pushes the equatorial Pacific Ocean waters east against the South American coastline, but are also being held back by easterly trade winds from the southern‑ and northernmost regions of the large Pacific gyre to the north and south, which act like a brake on this eastward motion. Every decade or more, the easterly trade winds will slow, resulting in the westerly Equatorial Counter-Current to push hot ocean water against the South American coastline (El Niño event), and every decade or more, the easterly trade winds will increase, resulting in the westerly Equatorial Counter-Current to push less of this hot ocean water against the South American coastline (La Nina event), resulting in cooler waters in these regions. This oscillation results in what are called Kelvin Waves. Kelvin Waves behave like the sloshing of water in a bathtub. In a bathtub, hot water comes from the faucet, and the water cools as it moves away from the faucet. The coldest water accumulates on the far end of the bathtub on the opposite side. If you were to slosh back and forth in the bathtub, the colder water on one side of the bathtub would mix with the warmer water from the other side, resulting in a better distribution of water temperature, and a more pleasant bathtub experience. Kelvin waves are a natural way that ocean water temperatures are spread out through sloshing back and forth resulting from the tug of the westerly equatorial counter-current and the easterly trade winds. However, the Pacific Ocean is 10,000 miles (16,000 kilometers) wide, and the amount of water sloshing back and forth is immense! Resulting in major weather patterns, and devastating flooding during these events, so scientists keep close observations on this cycle.

The La Nina/El Niño Cycle and Earth’s History of Global Climate


Earth’s La Nina/El Niño Cycle, or ENSO is controlled by the geographic arrangement of the continents on Earth. Today the North and South America continents are connected together and extend nearly from pole to pole preventing the flow of warm equatorial Pacific Ocean water flowing into the Atlantic Ocean. The formation of the Isthmus of Panama has blocked this flow of water resulting in the modern ENSO weather patterns. Prior to this, warm tropical water was able to flow across into the Atlantic Ocean, and even pass between Eurasia and Africa, through an ancient seaway called the Tethys Sea, that covered North Africa and the Middle East in ocean waters about 50 million years ago. This warm westerly Equatorial Counter-Current pushed ocean water between the various continents resulting in a complete Global Circum-Equatorial Counter-Current of warm ocean water rotating around the entire Earth at the equator. Trade winds would slow the rotation of this circum-equatorial counter-current, allowing the warm ocean temperatures to warm the entire planet through convection, and the rising of moist warm air from this trans-equatorial ocean resulted in some of the warmest climates in Earth’s history. This configuration of the continents occurred in the millions of years leading up to the Early Eocene Epoch, about 50 million years ago, and resulted in a profoundly different Earth than exists today. For example, in Utah and Wyoming in the American West crocodiles swam in swampy tropical lakes, palms grew, and lemur-like primates lived in vast forests that covered the region. In the high arctic of Canada, browsing tapirs and forests of tall Metasequoia trees covered the high arctic landscape instead of ice and tundra. Globally the climate was much warmer than today, and lacked extreme seasonal temperatures, and the Arctic Ocean was free of sea ice. During late Eocene Period, the Circum-Equatorial Counter-Current was blocked by the development of Anatolia and the collision of India with Asia. This caused warm water to slow, and kept Europe and Asia warm, while North and South America cooled during the late Eocene around 40 million years ago. However, the biggest change in climate occurred at the end of the Eocene as a result of the Drake Passageway opening, and the development of another major ocean current, the Circumpolar Current in the Southern Ocean, which led to the freezing of Antarctica. The Isthmus of Panama formed in the late Miocene, resulting in the recent Ice Ages of the Pliocene and Pleistocene Epochs, that resulted in the Northern Hemisphere ice sheets to form and sea ice in the Arctic Ocean, as the passageway of ocean water was fully blocked. The La Nina/El Niño Cycle is extremely important in the Earth’s long-term climate, as well as in yearly weather prediction today.

Another example of a continental configuration like today, existed around 230 million years ago, during the Triassic Period, when all the continents were connected into a single large landmass called Pangea. This Super Continent Pangea extended from pole to pole, and was imbalanced on a single side of the planet, with the other side composed of the largest ocean to have ever existed in Earth’s history. Geologists called this massively large ancient ocean the Panthalassic Ocean, from the Greek meaning “All Sea.” This ancient ocean was nearly twice the size of the Pacific Ocean, which resulted in a very dramatic La Nina/El Niño like cycle, as equatorial ocean water was blocked from flowing eastward along the equator, by the Super Continent Pangea (similar to today how the American Continents together block this current). The effects of this miss-balanced world had a profound effect on the climate at the time. Warm equatorial ocean waters were blocked by the super continent and pooled on the western coastline of Pangea, similar to how they do today off the coast of Peru and Ecuador, but on the eastern coastline of Pangea, cooler equatorial ocean waters existed. Driven by similar trade winds that exist at the time, this block equatorial counter-current would oscillate over a decade or longer scale. The warm ocean waters on the western coast would drive low pressure, resulting in rising moist air, which would result in intense rain over the interior of the continent, before sinking on the eastern coastline over the cooler ocean waters on the opposite coastline. This strange arrangement of the Earth’s land masses, resulted in a climate that underwent extremes in rainfall. Rainstorms could last years, and would be followed by years of drought, depending on the ocean oscillations. These intense rainy years and drought years, resulted in much of the Red Rock country in eastern Utah, best exposed around Moab and Canyonlands National Park. Iron minerals in the sediments exposed to the alternating wet/dry cycle oxidized or rusted with the intense rain, followed by drought, forming the bright red sandstones and siltstones that form much of Utah’s scenic country. From the tall canyons of Zion to labyrinthian region of Canyonlands, the red color in Utah’s canyon country is a result of this massive superior continent. Geologists call this period of intense rain, the Carnian Pluvial Event, which occurred 230 million years ago during the deposition of the dark red Chinle and Windgate Formations that surround the canyons around the town of Moab, Utah. Such red rock layers are found in similar aged terrestrially deposited rocks in Texas, Connecticut, Scotland and southern Germany.

The power of La Nina/El Niño cycles and the flow of warm tropical equatorial ocean waters around the Earth, is an important driver of Earth’s habitable climate. The oceans have such a strong influence of Earth’s climate due to the high heat capacity of water. The convection of warm equatorial water across the entire ocean determines the distribution of Earth’s global temperatures. When convection of this heat is restricted, the polar regions freeze and the equators cook, while an unrestricted flow of ocean waters are able to distribute the heat toward the poles better, leading to a warmer climate, and less geographic extremes across Earth’s surface. The oceans of Earth are extremely important in maintaining the habitability of the entire planet between climatic extremes, and have changed over its long history.