Recent hemispheric asymmetry in global ocean warming induced by climate change and internal variability

Recent research shows that 90% of the net global ocean heat gain during 2005–2015 was confined to the southern hemisphere with little corresponding heat gain in the northern hemisphere ocean. We propose that this heating pattern of the ocean is driven by anthropogenic climate change and an asymmetric climate variation between the two hemispheres. This asymmetric variation is found in the pre-industrial control simulations from 11 climate models. While both layers (0–700 m and 700–2000 m) experience steady anthropogenic warming, the 0–700 m layer experiences large internal variability, which primarily drives the observed hemispheric asymmetry of global ocean heat gain in 0–2000 m layer. We infer that the rate of global ocean warming is consistent with the climate simulations for this period. However, the observed hemispheric asymmetry in heat gain can be explained by the Earth’s internal climate variability without invoking alternate hypotheses, such as asymmetric aerosol loading. Observations of global ocean heat content during 2005–2015 have shown a strong hemispheric asymmetry, and the southern hemisphere accounts 92% of the total heat gain. Here, the authors show that the rate of observed global ocean warming is consistent with a forced symmetric climate change signal and an asymmetric climate variation for this period.