El Niño, La Niña, and Volatility in the Earth System

Image courtesy of NOAA. La Niña causes the jet stream to move northward and to weaken over the eastern Pacific. During La Niña winters, the South sees warmer and drier conditions than usual. The North and Canada tend to be wetter and colder.

In the Pacific Northwest, where I live, it's been a canonical La Niña summer: cool nights, moderately warm days that might require light layers, all punctuated by regular bouts of clouds and rain. Despite being thousands of miles from the tropical Pacific Ocean, its conditions significantly influence our local weather. And it's not just the Pacific Northwest – California has been baking, super-charged hurricanes are spinning through the Gulf of Mexico, and the South Asian Summer Monsoon is torrential. All these unusual weather patterns around the world are linked to the Tropical Pacific Ocean and the La Niña conditions brewing there. While La Niña doesn't guarantee specific weather anomalies, it significantly increases their likelihood.

La Niña and its notorious sibling El Niño are key components of the El Niño - Southern Oscillation (ENSO), the most important mode of inter-annual variability in the Earth system. ENSO is the single most important factor driving year-to-year fluctuations in global weather patterns. Although ENSO's center of action is in the tropical Pacific Ocean, it affects environmental conditions worldwide through teleconnections – connections between distant regions due to giant atmospheric and oceanic waves that move energy across the planet. Year-to-year variations in temperature, precipitation, and winds, as well as the incidence of severe weather (including hurricanes, tornadoes, and intense rainfall) are all linked to ENSO. When it comes to understanding why some years are anomalous – different from the norm, even record-breaking – and some years are not, El Niño and La Niña are crucial factors.

ENSO is caused by interactions between the atmosphere and ocean in the equatorial Pacific. In this region, trade winds usually blow over the surface ocean from east to west, drawing up cool water from the deep ocean. This creates a cold, nutrient-rich 'tongue' of water that covers a huge area of the equatorial Pacific Ocean, from the coast of South America, extending a couple thousand miles westward to the center of the Pacific Ocean.

Image Credit: NOAA. NOAA Climate.gov animation, based on Coral Reef Watch Data and maps from NOAA View

During a La Niña event, trade winds intensify, causing the cold tongue to expand dramatically. It can stretch nearly halfway across the planet, from South America to the Solomon Islands near Papua New Guinea. This phenomenon represents an intensification of the typical equatorial Pacific cold tongue.

Conversely, when these trade winds slow down or even stop, cold water no longer rises from the deep, and this whole area can suddenly become very warm. This is an El Niño event. This sudden switch from cold to warm causes major disruptions: fisheries off of Peru collapse for the year since there's no more nutrient-rich water at the surface; rainfall shifts away from the equatorial West Pacific, which is usually a warm and wet area, to the Central Pacific, leading to droughts over SE Asia and Oceania; and huge atmospheric waves ripple from the equatorial West Pacific Ocean to far reaches of the planet, disturbing temperature and precipitation patterns around the globe, including the Brazilian Amazon, Indian subcontinent, and African Sahel.

ENSO significantly impacts industries vulnerable to environmental conditions, such as agriculture, energy, insurance, logistics, and commodities markets. The economic impact is substantial: each of the last two major El Niño events has been associated with approximately $5 trillion USD in global economic losses, primarily affecting environmentally sensitive sectors. On the other hand, knowing what ENSO state is on the way can provide the information needed to assess risk and develop strategies to mitigate, transfer, or hedge risk through capital markets. In other words, skilled predictions of ENSO are immensely valuable – just a 1% improvement in forecast accuracy can help prevent billions in losses.

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