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Will Storminess Increase in a Warmer World: Models Can't Clear the Skies

Hernandez-Deckers, D. and von Storch, J.S. 2012. Impact of the Warming Pattern on Global Energetics. Journal of Climate 25: 5223-5240.
Anthropogenic global warming (AGW) supporters have argued for decades that an increase in CO2 will bring hotter temperatures, and stronger and more destructive storms. These include more hurricanes, tornadoes, and mid-latitude cyclones to name a few. Recently, they have argued that the warmer temperatures will strengthen the water cycle, which will make these storms even stronger. Models have not been helpful in settling this argument as some models imply storminess will decrease, while others imply the opposite.

The strength of the general circulation is controlled by two factors. One is the equator-to-pole temperature difference (gradient). This ultimately drives our general circulation and gives rise to the jet streams. In the mid-latitudes, where the gradient is strong, storms arise and are driven by these gradients. The ultimate effect of storms is to decrease the equator-to-pole gradient. The impact of the horizontal temperature gradient is well-known. A stronger gradient will generally lead to more storminess.

Hernandez-Deckers and von Storch (2012) (hereafter HDvS12) show the second factor; atmospheric stability, is often overlooked in the climate change debate. Stability is simply the atmosphere's resistance to overturning. In general, warm air rises and cold air sinks. To make the atmosphere less (more) stable, one could warm (cool) the surface relative to the upper air.

HDvS12 used the European Centre for Medium Range Forecasting general circulation model (ECHAM5) coupled with an ocean model from the Max Planck Institute. They calculated the generation, flow, and dissipation of energy in the atmosphere in order to compare the impacts of the two factors above in changing the strength of the general circulation.

The authors test the proposition that AGW will warm the poles faster than the tropics near the Earth's surface (SFC - first trial). This argues for a weakening of the general circulation as horizontal temperature gradients decrease. However, warming the SFC polar region decreased the stability of the model atmosphere. Others have argued that AGW will warm the upper troposphere relative to the surface, especially in the tropics (UP - second trial). This increases tropical atmospheric stability, but increases the upper air temperature gradients. They then added the two effects together in a third trial (UP+SFC).

HDvS12 found that the general circulation "weakens by almost 10% in the UP experiment whereas it strengthens by almost 4% in the SFC experiment. In the FULL experiment, it weakens by about 5%." The FULL and the UP+SFC experiments were similar in outcome. They also note that "the expected effects due to mean static [atmospheric] stability and meridional temperature gradient change are opposite of each other." The results demonstrate that the stability impact seems to be the dominant one (Fig 1).

Figure 1. Adapted from Fig. 5 in HDvS12. Diagrams of atmospheric energy production, reservoirs, conversions, and dissipation. The numbers given are differences from the control run of the model (today's climate). The FULL, UP, SFC, and UP+SFC, represent an experiment with double CO2, warmer upper troposphere only, warmer polar surface temperatures only, and the combined effects, respectively. Units are W m-2. Arrows indicate the direction of energy flow. Each square contains a simple two box and a more complex four box energy model.

Many skeptics argue that under AGW scenarios, storminess should decrease due to the weakening of the equator-to-pole temperature differences. The answer is generally correct, but overlooks the impact of static stability. Models have not sorted out the storminess issue because different models employ different physics for processes that impact the temperature gradient. Models also handle the heating effect of CO2 differently. The results yield temperature gradients and upper tropospheric warming of various strengths. Although it is noted here that there is no strong observational evidence the upper troposphere is warming as AGW scenarios suggest. HDvS12 demonstrates the utility of models in breaking down complicated climate problems. More importantly, however, it is known that model construction can make a huge difference in the interpretation of the outcome.

Archived 7 August 2012