While it is normally understood that marine biogeochemical processes strongly depend on the physical drivers, but less so in the converse, here the authors demonstrate that nonlinear responses are present in a series of experiments that modify the absorption rate of light based on chlorophyll concentrations. While the leading order mechanism is clear (more absorption, less radiative forcing on the ocean), the more detailed governing mechanisms for the oceanic response are less obvious, and the authors provide some investigations towards that end with a focus on tropical regions. They also suggest that the ongoing observed decrease in tropical Atlantic chlorophyll-a concentration may weaken the interannual variability of sea surface temperature.

Climate projections should be able to reproduce long-term and low-frequency variability of the system. A method is proposed to estimate the low-frequency component of the ocean variability from observations using statistics from a prior long-term simulation. The paper presents a significant step forward in combining numerical ensembles together with satellite data to provide relevant reconstructions of a given ocean variable (in this case the low frequency variability of the sea surface height, SSH) in a way that reduces systematic errors of the prior pdf. It is likely that the application will lead to future improvement in the context of data assimilation. The paper includes significant theoretical advances but at the same time provides a concrete application for SSH estimation from observation and model results.