Jun 25 2019
For several years, researchers investigating a key climate occurrence have been contending with contradictory data that has threatened to destabilize confidence in the dependability of climate models on the whole. A new study, published recently in Nature Geoscience, resolves that debate concerning the tropical atmospheric circulation.
Clouds from deep convection over the tropical Pacific Ocean, photographed by the space shuttle. Such convective activity drives the Hadley circulation of the atmosphere. (Image credit: NASA)
The Hadley circulation, or Hadley cell—a universal tropical atmospheric circulation pattern that happens because of uneven solar heating at various latitudes surrounding the equator—causes air around the equator to increase to about 10-15 km, flow poleward (toward the North Pole above the equator, the South Pole below the equator), descend in the subtropics, and then return back to the equator along the surface of the Earth.
This circulation is extensively explored by climate researchers because it regulates precipitation in the subtropics and also creates a region known as the intertropical convergence zone, creating a band of major, extremely-precipitative storms.
The study, led by Rei Chemke, a Columbia Engineering postdoctoral research fellow, together with climate scientist Lorenzo Polvani, looks at a major inconsistency between climate models and re-analyses concerning possible strengthening or weakening of the Hadley circulation in the Northern Hemisphere as a consequence of anthropogenic emissions.
Historically, climate models have exhibited a progressive waning of the Hadley cell in the Northern Hemisphere. In the last 40 years re-analyses, which integrate models with observational and satellite data, have revealed just the opposite—a strengthening of the Hadley circulation in the Northern Hemisphere. Re-analyses offer the ideal approximation for the state of the atmosphere for researchers and are broadly used to confirm that model simulations are functioning well.
The difference in trends between models and re-analyses gives rise to a problem that goes way beyond whether the Hadley cell is going to deteriorate or toughen; the discrepancy itself is a major problem for researchers. Re-analyses are used to authenticate the dependability of climate models—if the two disagree, that means that either the re-analyses or models are imperfect.
Lead author Chemke, a NOAA Climate and Global Change postdoctoral fellow, explains the risk of this condition, “It’s a big problem if the models are wrong because we use them to project our climate and send our results to the IPCC (Intergovernmental Panel on Climate Change) and policy makers and so on.”
To learn the cause of this inconsistency, the researchers studied the different processes that affect circulation closely, establishing that latent heating is the cause of the discrepancy. To understand which data was precise—the models or the re-analyses—they had to compare the systems using a purely observational metric, unblemished by any model or simulation. Here, precipitation acted as an observational alternative for latent heating since it is equivalent to the net latent heating in the atmospheric column. This observational data exposed that the artifact, or fault, is in the re-analyses—settling that the model estimations for the future climate are, truly, correct.
The paper’s findings back earlier conclusions derived from a range of models—the Hadley circulation is weakening. That is important to understand, says Polvani, a professor of applied physics and applied mathematics and of earth and environmental sciences who studies the climate system at the Lamont-Doherty Earth Observatory. “One of the largest climatic signals associated with global warming is the drying of the subtropics, a region that already receives little rainfall,” he explained. “The Hadley cell is an important control on subtropical precipitation. Hence, any changes in the strength of the Hadley cell will result in a change in precipitation in that region. This is why it is important to determine if, as a consequence of anthropogenic emission, the Hadley cell will speed up or slow down in the coming decades.”
But these conclusions resonate way beyond the research in question. Resolving inconsistent results in scientific research is crucial to preserving integrity and accuracy in the scientific community. Owing to this new study, researchers now have added assurance that models are dependable tools for climate predictions.