The hammam effect or how a warm ocean enhances large scale atmospheric predictability

Will climate change make weather forecasting harder than it currently is? Or, could a warmer planet make prediction easier? The answer isn’t obvious. Because atmospheric dynamics are chaotic, there are practical limits to predictability, regardless of data precision and available computing power. These limits also depend on space and time, and can be strongly affected by changes in the large-scale state of atmospheric flows. As one example, the term “large-scale zonal flow” refers to a strong eastward (zonal) jet with few meridional oscillations. Persistent flows of this kind are easier to predict than more unstable flows involving mid-latitude transitions from blocked to zonal atmospheric states.

As LML External Fellow Davide Faranda and colleagues argue in a new paper, climate change could well alter the relative frequencies with which these distinct large-scale atmospheric states occur. This would thereby influence the degree of weather predictability. To estimate the effect, they use historical data as well as atmospheric simulations to analyse two empirical metrics that should reflect the intrinsic predictability of the atmosphere in different states. Practical methods for computing such metrics have only recently become available through dynamical systems techniques allowing the detection of approximate recurrences of instantaneous weather patterns.

Faranda and colleagues focus on atmospheric circulation over the North Atlantic, and find that their results mostly suggest that climate change will increase the atmosphere’s intrinsic predictability. What is the essential cause? By using simulations from the Atmospheric Model Intercomparison Project (AMIP) with ocean temperatures raised by 4 K degrees and atmospheric CO2 concentrations four times higher, they conclude that warmer oceans will play the most prominent role in driving greater predictability. The authors refer to this as the “hammam effect,” hammam being the Turkish word for a warm bath.

The paper is available at https://www.nature.com/articles/s41467-019-09305-8

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