Fifteen Eighty Four

Since the 1950s, it has been known that atmosphere undergoes a drastic change in behaviour at around one week. In modern terms, for shorter periods, successive fluctuations tend to reinforce each other whereas over longer periods, they tend to cancel each other out. The result is familiar from daily experience: the short term “weather” is unstable whereas at longer term – our usual idea of climate – it is stable: “the climate is what you expect, the weather is what you get”.

Fundamental transitions demand fundamental explanations: here, they should involve solar energy fluxes – after all without the sun, everything would grind to a halt. Surprisingly, it was not until recently that that the transition was explained as the lifetime of planetary sized structures, with the latter being determined solely and directly by the size of the earth and by the solar power input: the “energy rate density” [Lovejoy and Schertzer, 2010]. This approach was later extended to the ocean although the corresponding power per mass was one hundred thousand times smaller, with the transition roughly at a year [Stolle et al., 2012].

Although seductive, this theory contradicts a belief held among certain theorists that the large scales are essentially flat – two dimensional. In this view the energy rate density is only relevant at small scales. The new theory can therefore only be plausible if the atmosphere is never completely flat. Indeed, since 1980s an alternative theory has increasingly gained support: that the atmosphere is increasingly stratified at larger scales it is “in between” 3D and 2D; it is 2.55 dimensional [Schertzer and Lovejoy, 1985]!

During a discussion last year, Maartan Ambaum (University of Reading) pointed out that if the theory was correct that, it should apply to other planets (and possible Titan). This is where Mars comes in: it has the best extraterrestrial data with which to make a test: if the theory was right, there should be an analogous Martian transition. By taking into account the Martian solar heating and atmospheric thickness, we predicted that the Martian temperature and wind would undergo a transition analogous to the Earth’s but at 1.5 sols (≈1.5 Earth days) rather than a week. Viking Lander data and Martian reanalyses (based on Martian orbiter data) confirmed this prediction quite accurately [Lovejoy et al., 2014].  Now we have a third example of such a transition.

But why expect macroweather and not climate?

The problem is the although fluctuations tend to cancel for periods longer than a week, a year, 1.5 sols (Earth atmosphere, ocean, Mars) –  averages tend to converge – at longer scales (the earth 30 – 100 years), they no longer cancel, rather they tend to reinforce each other again, they are again unstable.  Since the shorter periods undeniably correspond to our idea of the weather, and the longer periods to the climate, we called the intermediate regime “macroweather” [Lovejoy and Schertzer, 2013]: don’t expect the climate, expect macroweather!

References

Lovejoy, S., and D. Schertzer (2010), Towards a new synthesis for atmospheric dynamics: space-time cascades, Atmos. Res., 96, pp. 1-52 doi: doi: 10.1016/j.atmosres.2010.01.004.

Lovejoy, S., and D. Schertzer (2013), The Weather and  Climate: Emergent Laws and Multifractal Cascades, 496 pp., Cambridge University Press, Cambridge.

Lovejoy, S., J. P. Muller, and J. P. Boisvert (2014), On Mars too, expect macroweather, Geophys. Res. Lett., in press.

Schertzer, D., and S. Lovejoy (1985), The dimension and intermittency of atmospheric dynamics, in Turbulent Shear Flow, edited by L. J. S. B. e. al., pp. 7-33, Springer-Verlag.

Stolle, J., S. Lovejoy, and D. Schertzer (2012), The temporal cascade structure and space-time relations for reanalyses and Global Circulation models, Quart. J. of the Royal Meteor. Soc., in press.