The Role of Earth’s Major Cloud Systems in the Hemispheric Albedo Symmetry

Abstract We conduct an in-depth investigation of the role of major climatological cloud systems in the well-known near symmetry at multiannual scales between the Northern and Southern Hemisphere reflected solar radiation (RSR). Here, we quantify the degree of symmetry for the entirety of a recent 22-yr period while also analyzing how its interannual variability is modulated by cloud systems which we have previously termed “regimes of regimes” (RORs). By pairing RORs with monthly RSR values from the Clouds and the Earth’s Radiant Energy System (CERES) EBAF dataset, we elucidate how several large RSR asymmetries at the ROR level, mainly due to strong ROR hemispheric population contrasts, combine in a compensatory way to produce near symmetry at the hemispheric level. We also investigate how interannual variations of small RSR asymmetries relate to interannual asymmetry variations at the ROR level, which we also find to be mainly because of trends in ROR populations. Overall, we find statistically significant darkening in both hemispheres but have lower confidence in the darkening’s hemispheric contrast. Finally, we probe the role of the complementary symmetry in absorbed solar radiation (ASR), and specifically of its atmospheric component, in the broader context of hemispheric energetics. Our analysis sheds light on the role of major global cloud systems in modulating the mean state and variability of the equitable contribution of the two hemispheres to the global RSR. Significance Statement The nearly equal amounts of solar energy reflected by Earth’s two hemispheres on a multiannual basis are a well-observed aspect of the planet’s energetics, but its significance remains unexplained. While the role of clouds in this phenomenon has been previously investigated, this work goes into more detail in investigating it with the help of a new classification of cloud systems. We find that even if the prevailing systems differ between the two hemispheres, the uneven contributions to reflected solar radiation cancel each other almost exactly to produce symmetric reflected solar energy, while other components of the energy budget are asymmetric.