Light absorption by marine cyanobacteria affects tropical climate mean state and variability.

Observations indicate that positively buoyant marine cyanobacteria, which are abundant throughout the tropical and subtropical ocean, have a strong local heating effect due to light absorption at the ocean surface. How these local changes in radiative heating affect the climate system on the large scale is unclear as of yet. We use the Max Planck Institute Earth System Model (MPI-ESM) and find that - in contrast to the heating effect which was reported in previous studies - cyanobacteria have a considerable cooling effect on tropical climatological sea surface temperature (SST) in the order of 0.5 K. This cooling is caused by local shading of subtropical subsurface water that is upwelled at the equator and in eastern boundary upwelling systems. Implications for the climate system include an expansion of the Hadley cells and a westward shift of the Walker circulation. The amplitude of the seasonal cycle of SST is increased in large parts of the tropical ocean by up to 25%, and the tropical Pacific interannual variability is enhanced by ~20%. This study emphasizes the sensitivity of the tropical climate system to light absorption by the specific phytoplankton group of cyanobacteria due to its regulative effect on tropical SST. Generally, including the phytoplankton-dependent light attenuation instead of a globally uniform attenuation depth improves some of the major model temperature biases, indicating the relevance of taking into account this bio-physical feedback in climate models. [ABSTRACT FROM AUTHOR]