Persistence and variability of Earth's inter-hemispheric albedo symmetry

Earth's albedo is observed to be symmetric about the equator on long time scales despite having an asymmetric distribution of land and aerosol sources between the northern and southern hemispheres. This is made possible by the distribution of clouds, which compensates the clear-sky albedo asymmetry almost exactly. We investigate the variability of the inter-hemispheric difference in reflected solar radiation (asymmetry) on the monthly time scale using decomposed reflected radiative fluxes in the CERES EBAF satellite data record. We find that the variations in the degree of symmetry on shorter timescales is strongly controlled by tropical and subtropical processes affecting cloud distributions. States of high asymmetry coincide with opposing phases of the El Niño-Southern Oscillation (ENSO); during El Niño (La Niña) conditions, the southern (northern) hemisphere is reflecting anomalously more than the other, perturbing the inter-hemispheric albedo symmetry. This perturbation also impacts the inter-hemispheric difference in net radiative fluxes, i.e. during states of asymmetry, the hemisphere that is reflecting less solar radiation also absorbs more energy in the net radiation balance.

We also compare the variability of the asymmetry in simulations from coupled models in Phase 6 of the Coupled Model Intercomparison Project with observations, and find that model mean asymmetry bias is primarily determined by biases in reflected radiation in the midlatitudes. Models that overestimate the variability of the asymmetry also have larger biases in reflected radiation over the tropics. Both bias and variability are generally improved in atmospheric model simulations driven with historical sea surface temperatures.