Ancient hurricanes may have caused and sustained warmer climate conditions
Although scientists know that the early Pliocene had carbon dioxide concentrations that are similar to those of today, it has remained a mystery what caused the high levels of greenhouse gas and how the Pliocene’s warm conditions, including an extensive warm pool in the Pacific Ocean and temperatures that were roughly 4°C warmer than today’s, were maintained.
In a paper published Feb. 25 in Nature, Kerry Emanuel, the Breene M. Kerr Professor of Atmospheric Science in the Department of Earth, Atmospheric and Planetary Science, and two colleagues from Yale University’s Department of Geology and Geophysics suggest that a positive feedback between tropical cyclones — commonly called hurricanes and typhoons — and the circulation in the Pacific could have been the mechanism that enabled the Pliocene’s warm climate.
How they did it: By combining a hurricane model and coupled ocean-atmosphere general circulation model to investigate the early Pliocene, Emanuel, Brierley and co-author Alexey Fedorov observed how vertical ocean mixing by hurricanes near the equator caused shallow parcels of water to heat up and later resurface in the eastern equatorial Pacific as part of the ocean wind-driven circulation. The researchers conclude from this pattern that frequent hurricanes in the central Pacific likely strengthened the warm pool in the eastern equatorial Pacific, which in turn increased hurricane frequency — an interaction described by Emanuel as a “two-way feedback process.”
Their observations included nearly twice the number of tropical cyclones than occur in our current climate system, including storms with life spans that averaged two to three days longer than our current system. The hurricanes appeared in places, such as Hawaii, that differ from where they typically occur today, and also occurred throughout the seasons.
The researchers believe that the intense hurricane activity likely created a permanent El Niño-like state in which very warm water in the eastern Pacific near the equator extended to higher latitudes. The El Niño weather pattern, which is caused when warm water replaces cold water in the Pacific, can impact the global climate by intermittently altering atmospheric circulation, temperature and precipitation patterns.
The research suggests that Earth’s climate system may have at least two states — the one we currently live in that has relatively few tropical cyclones and relatively cold water, including in the eastern part of the Pacific, and the one during the Pliocene that featured warm sea surface temperatures, permanent El Niño conditions and high tropical cyclone activity.
Although the paper does not suggest a direct link with current climate models, Fedorov said it is possible that future global warming could cause Earth to transition into a different equilibrium state that has more hurricanes and permanent El Niño conditions. “So far, there is no evidence in our simulations that this transition is going to occur at least in the next century. However, it’s still possible that the condition can occur in the future.”
Next steps: Additional research will focus on why the Pliocene was so warm at higher latitudes, including an iceless North Pole, and whether this resulted from moisture produced by the tropical cyclones, Fedorov said.
Brierley hopes to develop an interactive model to strengthen the group’s theory. Rather than examining individual components, such as sea surface temperatures, and then imposing that data onto a model to figure out potential ocean mixing and hurricane activity, the researchers would like to include everything in the same interactive model.
Resolving other issues, such as how to more precisely estimate the contribution of tropical cyclones to ocean mixing, will not only help improve the early Pliocene climate model, but also help predict future climate change for which the feedback between hurricanes and the ocean circulation could be crucial.
Source: “Tropical cyclones and permanent El Niño in the Early Pliocene,” by A. V. Fedorov, C. M. Brierley and K. Emanuel, in Nature, published online Feb. 25, 2010.
Funding: This work was supported by the National Science Foundation, the Department of Energy Office of Science and the David and Lucile Packard Foundation.
Written by: Morgan Bettex, MIT News Office