1 September 2018
CSD's Sean Davis is part of a team of researchers asked by Nature Climate Change to write a review on tropical widening. The review is now published.
Tropical widening refers to a change in the boundaries of Hadley Cells – large-scale convection cells where hot air rises from the tropics and then sinks over subtropical deserts at around 30 degrees latitude. With tropical widening, the boundary where cool air sinks is moving closer and closer to the poles, a shift that has implications for the global hydrological cycle, since as the boundary moves closer to the poles, these expanded boundary areas can become drier.
Dr. Davis and colleagues combined the results of many peer-reviewed journal articles on the topic to write a "State of the Science" report. Based on this analysis, they concluded that tropical widening is indeed a robust feature of the last 40 years. Addressing the question of whether this phenomenon is caused by human activities or natural variation, the scientists conclude that human activities do contribute, although a large fraction of the movement is decadal variability.
Specifically, the observed tropical widening patterns fall within the upper bounds of model prediction, and a significant portion of the variation can be explained by El Niño, which changes large-scale air movements in the tropics every few years, and the Pacific Decadal Oscillation, which operates on a larger time scale.
However, these natural variations do not diminish the importance of the human contributions: human activities also contribute to tropical widening. For example, it is well documented that increased release of carbon dioxide can lead to amplified warming in the tropics and a poleward movement of the Hadley cell edges. Stratospheric ozone depletion can also have an effect, especially on the Hadley cell in the Southern Hemisphere. Changes in aerosols and ozone in the troposphere are also important.
There are many drivers of tropical widening, some natural and some human-caused. Because this is a large-scale phenomenon, it is possible that all of these mechanisms occur and contribute to tropical widening.
Natural variations with El Niño and the Pacific Decadal Oscillation can push the boundary in either direction – resulting in either tropical widening or tropical shrinking. If our emission patterns continue, human contributions from increased carbon dioxide emissions, stratospheric ozone depletion, and increased tropospheric ozone and aerosols, will continue to expand the boundary of the tropics in the long-term.
Staten, P.W., J. Lu, K.M. Grise, S.M. Davis, and T. Birner, Re-examining tropical expansion, Nature Climate Change, doi:10.1038/s41558-018-0246-2, 2018.
Observations reveal a poleward expansion of the tropics in recent decades, implying a potential role of human activity. However, although theory and modelling suggest increasing GHG concentrations should widen the tropics, previous observational-based studies depict disparate rates of expansion, including many that are far higher than those simulated by climate models. Here, we review the rates and possible causes of observed and projected tropical widening. By accounting for methodological differences, the tropics are found to have widened about 0.5° of latitude per decade since 1979. However, it is too early to detect robust anthropogenically induced widening imprints due to large internal variability. Future work should target the seasonal and regional signatures of forced widening, as well as the associated dynamical mechanisms.