On Asymmetric Internal Hemispheric Variability in the Ocean in 2005-2015

Abstract Hemispheric asymmetry in global ocean warming was observed during 2005–2015. Previously, we showed that 92% of net global ocean heat gain during this period was in the southern hemisphere and that this period represented one extreme of an asymmetric mode of internal ocean variability that is independent of anthropogenic warming. We now identify for the first time, the asymmetric mode’s consistent spatial pattern between its phase of southern hemisphere dominance in observations and CMIP5 pre-industrial simulations. The southern hemisphere dominance of this intensity (≥ 2 × 10 22 J decade − 1 ) observed in the first decade of the global Argo profiling array occurs 1% of the time in 10,000 CMIP5 years. The opposite phase, the northern hemisphere dominating heat uptake, occurs with similar frequency. We show that the asymmetric mode in 2005–2015 was associated with changes in the global overturning circulation, causing anomalous warming of 0.2×10 15 W south of 40 o S and convergence of heat in southern mid-latitudes. The asymmetric mode is uncorrelated with common climate modes.