Cold in the Greenhouse
Christkind every day - a nightmare scenario for climatologists. Because what is meant is not Christmas, but the climate and ocean event El Niño, named after the Christ Child: If it occurs, weather chaos follows worldwide. Are rising temperatures now making the problematic Christkind a permanent condition?
The so-called Walker circulation is one of the most important components of the atmospheric climate process, which is almost unknown outside of the scientific community North to South but parallel to the equator in the Pacific Rim.
It is characterized by rising air masses in the western Pacific off the coasts of New Guinea and Indonesia, which - as a sort of compensatory movement - descend again in the east off the shores of Ecuador and Peru. For this reason, there is constant cloud formation and heavy rainfall in the Asian islands, while stable high pressure systems on the other side of the ocean produce deserts that are sometimes extremely dry and hostile to life.
The situation in the sea is completely different. Since the air that flows in at high altitude and then sinks at high altitude also has to flow out again, permanent winds arise at the lowest level of the Walker cell, which in turn blow from South America to Asia. Due to their constant blowing, they drive enormous masses of water in front of them, so that the Pacific gets into an imbalance: A "mountain" of warm water in the west is opposed to a supercooled "hole" in the east off Peru, since cold deep water is now slightly up to the surface there can rise.
Unlike on land, this temperature sink creates one of the most fertile marine regions on earth, as fresh nutrients are available in abundance. Every few years, however, this cycle breaks down, the trade winds off South America's shores literally die down and the warm mountain sloshes eastwards. As a result, the pressure conditions are also reversed in the atmosphere. Now highs and dry air reign over the islands of Asia, while lows bring copious rainfall to the arid regions of South America's Pacific coast – El Niño arrives.
Named after the Christ Child by local fishermen because it returns in a weaker form every year at Christmas, it appears in its extreme form every five to seven years. And it then disrupts weather patterns not just in the Pacific, but across the planet, causing droughts and famines in southern Africa, flood victims in Peru, or widespread forest fires in Australia and Borneo. El Niño in the steady state as a possible consequence of climate change is therefore considered one of the nightmare scenarios among atmospheric researchers.
How realistic this scenario actually is is a subject of constant controversy. Scientists led by Gabriel Vecchi from the American Agency for Ocean and Atmospheric Research (NOAA) are now introducing a new argument. They compared time series of air pressure and wind speed in different areas of the Walker circulation and then checked their possible influencing factors in the climate model.
Since the mid-19th century, the researchers found, the Walker circulation has actually weakened by about 3.5 percent. This means that the pressure differences between the low in the west and the high in the east of the Pacific have decreased and the winds are also blowing less strongly on average. This fits in with the steady temperature increase in the ocean between the archipelagos of Southeast Asia and Oceania on the one hand and the waters between the Galapagos and the South American continent, which has averaged 0.5 to 0.6 degrees over the past 150 years.
The two are closely related, because the increasing heating of the water and the air layers above it, which are also heating up, means that more moisture evaporates into the already more receptive, warm lower troposphere - for every degree the surface warms by seven percent. However, various physical processes in the upper troposphere prevent the precipitation that ultimately results from the water vapor from increasing to the same extent. They therefore only increase by two percent per degree of heating.
However, since there must be a balance between rising steam and falling rain, the rate at which moisture is transported from the lower to the upper layer of the atmosphere is reduced. Ultimately, this congestion causes the entire walker circulation to slow down and thus weaker trade winds at the sea surface.
But what triggers this braking maneuver? Greenhouse gases released by humans, such as carbon dioxide? Natural events like volcanic eruptions? Or extraterrestrial changes in solar radiation? Vecchi and his team checked all of these factors in combination or individually using various computer simulations. But only one explained the actually observed weakening of the Walker circulation to its current extent: the model that also took into account the full extent of the proven increase in greenhouse gases in the Earth's atmosphere.
Based on this result, the researchers continued their considerations into the future. Accordingly, the strength of the equator-parallel cell could decrease by another ten percent. And of course, because the winds will die down, the associated ocean currents will also decrease in strength - even by up to a fifth of their current value, since wind speed has a disproportionate influence on current intensity.
Nevertheless, this forecast does not necessarily mean a permanent Niño, since the predicted slowdown in the tropical easterly winds - i.e. those that originate off South America - should mainly be felt off Indonesia and Australia. In contrast, off South America the cold water influences remained largely normal and the world was spared weather chaos for most of the time. However, El Niño events are occurring at shorter intervals today than in the past: the discussions in climatological circles are likely to continue as.
