A Cabinet of (Climate) Curiosities

We are used to thinking of our climate system as one controlled simply by the annual cycle of asymmetric radiative forcing about the equator and the complex distribution of land and ocean between hemispheres. This model has been used to explain the seasonality of weather regimes and climatic features. Within this simple construct was thought to reside our climate, albeit perhaps subject to anthropogenic influences.

However, the results of Stephens et al. (2015) questioned the simplicity of the construct showing a remarkable and unexpected interhemispheric symmetry in the magnitude of the seasonal means of the top-of the-atmosphere (TOA) radiation balance despite vast differences in surface albedo between hemispheres. This curious result suggests that irrespective of the state of climate, there appears to be some overall constraint governing the collective hemispheres despite vastly differences cloud populations, ocean and atmospheric dynamical structures and surface albedo. Stephens et al. suggested that cloud fields, a major component of the TOA, somehow adjust to produce this symmetry. But what controls these cloud fields? Can these constraints also explain other climate curiosities such as the synchronous timing of ice ages between the hemispheres even though the orbital reduction of incoming radiation, precursor of the major ice ages, occurs in the northern hemisphere winter?

We argue that the Haynes-McIntyre Impermeability Theorem (Haines and McIntyre, 1990) offers insight in explaining the symmetry issue and perhaps other curiosities, too. The theorem states that between two isentropic surfaces the isentropic potential vorticity (IPV) is conserved. Ortega et al. (2018), was able to show that advective PV fluxes towards the equator by Rossby Wave dynamics are closely balanced by return PV fluxes towards the poles by divergent phenomena. We extend the analysis globally and consider the reaction of a system to an anomalous forcing in one hemisphere to help explain the Stephens et al. curiosity. Can the theory also be used to explain the changing character of near-equatorial divergent circulations noted by Hoyos and Webster (2012)?

Haynes, P.H. and McIntyre, M.E. (1990) On the conservation and impermeability theorems for potential vorticity. J. Atmos. Sci, 47, 2021–2031.

1. Ortega, Peter J. Webster, Violeta Toma, Hai-Ru Chang, (2018): The effect of potential vorticity fluxes on the circulation of the tropical upper troposphere. Q. J. Roy. Met Soc., 144, 848-860

Stephens, G. L., D. O'Brien, P. J. Webster, P. Pilewski, S. Kato, and J.-l. Li (2015): The albedo of Earth, Rev. Geophys., 53, doi:10.1002/2014RG000449

Hoyos, C. D., and Webster, P. J., 2012: Evolution of the tropical warm pool: Past, present and future. Clim. Dyn. 38, 1501–1519