While the warming of northern hemisphere oceans almost stalled, the southern hemisphere oceans are heating up

Observations from Argo floats and satellite shows that the rate of global ocean warming during 2005-2015 is largely consistent with the climate simulations under the increasing greenhouse gas concentration. However, during that period, the southern hemisphere oceans have absorbed up to 98% of the net global ocean heat gain, while the warming of northern hemisphere oceans almost stalled. Some studies suggested that this asymmetric warming of the global ocean is due to natural decadal variability. Others suggested that the higher concentrations of anthropogenic aerosols in the northern hemisphere caused the radiative cooling of the northern hemisphere. A study recently published in Nature Communications explored potential causes of the observed hemispheric asymmetry in global ocean warming. The study found a similar asymmetric variation in the pre-industrial control simulations from multiple climate models (i.e., forced with a fixed greenhouse gas concentration). The study also showed that while the mid-layer (700–2000 m) experienced steady anthropogenic warming, the upper-layer (0–700 m) experienced asymmetric warming, consistent with the pre-industrial control simulations. Therefore, they concluded that the observed hemispheric asymmetry in ocean heat gain can be explained by the Earth’s internal climate variability without invoking alternate hypotheses, such as asymmetric concentrations of anthropogenic aerosols. However, this study did not offer what types of internal climate variability are involved or if there is any corresponding signals in the atmosphere.

Table 1 from Rathore et al. (2020). Observed and simulated ocean heat content anomaly trend: Ocean heat content anomaly trend (10²² J decade⁻¹) from the observational mean and multi-model mean (MMM) as the average of RCP 4.5 and RCP 8.5 scenarios (in parentheses) in different depth layers during 2005–2015. Error bars are the 95% confidence interval.

Rathore, S., Bindoff, N.L., Phillips, H.E. et al. Recent hemispheric asymmetry in global ocean warming induced by climate change and internal variability. Nat Commun 11, 2008 (2020). https://doi.org/10.1038/s41467-020-15754-3.