Climate Change Conundrum: Warmer World, Slower Circulation, but More Extremes

Abstract:

In the recent decade, the global mean surface temperature has risen half a degree in reference to that of the 1980’s. This means that the earth’s total (potential + internal) energy in the atmosphere has increased. In contrary, a slowdown of general circulation under the warming climate is likely in progress. Furthermore, weather extremes, including severe cold surges, occur as before.

This study attempts to explain these somewhat contradictory changes — warmer world, slower circulation, but more extremes — by expanding a pioneering work by Lorenz (1955) that explained the maintenance of general circulation from the energetics point of view. In a global scale, the mean available potential energy is generated by diabatic heating, and is converted to eddy energy through baroclinic energy conversion. Kinetic energy conversion either reinforces mean wind or drains energy from the mean flow through barotropic conversion depending on the regions.

It was found that all energy terms as well as global mean surface temperature show meaningful changes from the 1980’s to the 2000’s. The available potential energy decreases mostly due to polar amplification, i.e., colder regions warming faster. This supports the slowdown of the mean circulation. Likewise, global mean kinetic energy decreases, most notably at the northern hemisphere. Instead, the baroclinic conversion of the mean available potential energy significantly increases at the subtropics and mid-latitudes. As a result, the eddy energy that originates from the departures from the mean state considerably increases over the mid-latitudes. Although the eddy energy terms may not have direct correspondence to an individual extreme weather event, they are indicative of deviations from the mean flow. The increase in eddy energy terms may indicate more or stronger anomalous weather events.

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