Implications of the Near Interhemispheric Symmetry of the Planet's Water and Energy Balances

When one looks at the vastly different geographies of the two hemispheres and their very different cloud populations one might expect that their radiative and energy balances would be quite different. Two recent papers (Stephens et al. 2015, 2016) suggest otherwise! In summary, the two hemispheres, on an annual basis, reflect the same amount of solar radiation. That is, the albedo of the two hemispheres is essentially the same. However, the surface energy balance is slightly asymmetric between the hemispheres requiring a small oceanic northward flux of energy from the southern to the northern hemisphere.

Here we add one more component to the peculiar nature of near-interhemispheric symmetry. We examine total accumulated rainfall (m³/day) as a function of time and season. Global rainfall varies between 0.1-1% between seasons. The annual accumulated precipitation between hemispheres varies by about 2% with greater precipitation occurring in the Southern Hemisphere. Over the globe, there is roughly three times more precipitation over oceans than land while the ratio between accumulated rainfall over land and ocean has remained essentially constant over the last three decades. This constancy has occurred while the surface temperature of the planet has increased, the area of the warm pool has enlarged and while, it appears, that the northern hemisphere is warming at a greater rate than the southern.

We examine what these curious near-symmetry may tell us about the evolving climate. Does it propose a fundamental constraint on climate? Does it help us understand why ice-ages and warm periods in the past occur globally even though the changes in orbital forcing that are thought to produce them are distinctly hemispheric! Do these near-symmetries provide us with a measure to test how well our climate models are simulating climate?

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

Stephens G. L., Hakuba, M. Z., Hawcroft M., Hatwood J., Kay J. E., Webster PJ., 2016: The curious nature of the hemispheric symmetry of the Earth’s water and energy balances. Current Climate Change Reports (in press)