The World Ahead |
Triple dip
The weather system that influences the world
The southern Pacific Ocean has been locked in its La Niña phase for three winters running. That is unusual, and dangerous
The trade winds trace eternal circles, east to west around the planet, driven by the sun’s heat and Earth’s rotation. In the south Pacific, they combine with ocean currents to create a climate system called the El Niño-Southern Oscillation (ENSO).
ENSO has three equilibrium points, states in which it can stay balanced for some time: neutral, El Niño and La Niña. In its neutral phase, shown here, winds are neither too strong nor too weak, and the ocean surface over which they blow oscillates gently between warm (red) and cold (blue).
But when trade winds strengthen or weaken just a little, they can tip ENSO into one of two other equilibrium states, El Niño or La Niña. Shown here is the formation of the El Niño phase of 1998. A great river of warm water stretches west from the coast of Peru towards Indonesia.
Weaker winds allow warm water from the western Pacific to spread east. Warmer seas heat the air above them, lowering its pressure and further damping east-west winds (as wind is just air that flows from areas of high pressure to low). This creates a feedback loop which locks ENSO in the El Niño phase.
Eventually stronger winds or ocean currents nudge the system out of El Niño and back into the neutral phase. If stronger winds persist they can lock the system into an opposite phase: La Niña. This is what ENSO looked like in 2022.
Stronger winds drive water west over the surface of the ocean. Cold water then wells up from the depths to replace it, cooling the air above. This raises air pressure over the south-eastern Pacific and creates stronger westerly winds, which drive more water before them, setting up the opposite feedback loop to El Niño, and keeping ENSO in the La Niña state.
For La Niña to span three Northern-hemisphere winters is unusual. It has been recorded just twice before, once in the mid-1970s and again at the turn of the millennium. Its long duration is a problem. The large, persistent mass of cold air and high pressure influences wind patterns known as jet streams across the Pacific and the Indian Ocean. Where jet streams that blow over land from the ocean are diverted, that land loses moisture and can suffer from drought. The land to which that jet stream is diverted gets more moisture, and can be flooded.
This, coupled with a warmer planet, has created a series of extreme weather events. Places that need the rain, like the Horn of Africa, have been deprived of it. Regions that have more water than they can handle like Pakistan, have been inundated. But as 2022 turns to 2023, La Niña is weakening. Its end will bring relief to many. But the climate will not stand still. The heat that humans have added to the earth’s atmosphere and oceans through decades of CO2 emissions is making its weather less predictable and more volatile than ever.
Soil moisture, difference from long-term average, Jul-Sep 2022
← Drier
Wetter →
No data
Floods in Pakistan
Some 1,700 people have died
and 33m have been displaced
Drought in the central Plains
The area is experiencing extreme
drought according to the US government
Variable jet streams →
AtLANTIC
OCean
Warmer
Equator
Cooler
← Strong trade winds
Pacific
OCEAN
Missed monsoon in Somalia
An extended drought has pushed
7m people into hunger
Drought in the Pantanal
Years of dry conditions have caused
wildfires in Brazil’s wetlands
An end to drought in America’s breadbasket
The southwestern United States has been in a state of drought for three winters running. The jet stream which blows in from the Pacific, carrying moisture with it, has been forced north by La Niña’s high pressure zone in the southern Pacific. That means more rain falls further north too, and less falls in the south. The National Integrated Drought Information System (NIDIS) shows that 70% of Oklahoma is in a state of exceptional or extreme drought, the most serious levels that NIDIS reports. September was the driest since 1956, according to Gary McManus, the state’s climatologist.
This La Niña-induced drought is stimulating wildfires in Kansas. The state is now so dusty that it is becoming a safety concern for drivers and cattle. The wheat crop is weaker than usual and the state is concerned that it will be too dry for much of it to survive the winter. Losses to cotton crops due to drought in Texas are estimated to be worth some $2bn. The end of La Niña will bring relief. America’s National Weather Service’s latest prediction gives a 57% likelihood of ENSO returning to its neutral phase between February and April 2023.
Image: AP
The Indus River breaks its banks
Flooding in Pakistan killed at least 1,700 people during the summer of 2022, and left 7.6m homeless, according to the UN monitoring organisation, ReliefWeb. The country’s climate minister, Sherry Rehman, said that the worst-affected provinces received between five and seven times their average rainfall in August. The Indus river, which runs the length of Pakistan, burst its banks to swamp thousands of square kilometres of land. Shehbaz Sharif, the prime minister, said they were the worst floods in his country’s history.
La Niña is partially responsible. Just as the colder southern Pacific equilibrium pushes moisture away from the southern United States, it happens to push moisture right on top of Pakistan. But La Niña is not acting alone. A warmer climate, thanks to humanity’s carbon dioxide emissions, also means a more flood-prone Pakistan. In a paper published in September, a group of climate scientists estimated that rainfall over the worst-impacted provinces was 75% more intense than it would have been without the 1.2C degrees of warming to which the planet has already been subjected.
Image: Getty
As the winds ease, the south-eastern Pacific will warm and ENSO will finally break out of La Niña. It may switch into its opposite state, El Niño, pulling jet streams round the world in the opposite direction, driving different regions into drought or flood. The greatest forces on the planet remain outside human control.
Sources: NOAA; Planet Labs PBC; The Economist