Abstract
Abstract. 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. We use the Max Planck Institute Earth System Model (MPI-ESM), include light absorption by cyanobacteria, and find a considerable cooling effect on tropical sea surface temperature (SST) in the order of 0.5 K on a climatological timescale. This cooling is caused by local shading of subtropical subsurface water by cyanobacteria that is upwelled at the Equator and in eastern boundary upwelling systems. Implications for the climate system include a westward shift of the Walker circulation and a weakening of the Hadley 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 approx. 20 %. This study emphasizes the sensitivity of the tropical climate system to light absorption by cyanobacteria due to its regulative effect on tropical SST. Generally, including phytoplankton-dependent light attenuation instead of a globally uniform attenuation depth improves some of the major model temperature biases, indicating the relevance of taking this biophysical feedback into account in climate models.
Highlights
Phytoplankton pigments, predominantly chlorophyll a (Chl), absorb light and thereby modify the vertical distribution of radiative heating in the upper ocean (e.g., Lewis et al, 1990; Strutton and Chavez, 2004)
The chlorophyll content of cyanobacteria is in general rather low compared to that of other phytoplankton (e.g., Berman-Frank et al, 2001; Carpenter et al, 2004; Sathyendranath et al, 2009), the dense accumulations of biomass in cyanobacteria blooms result in high chlorophyll concentrations (e.g., Subramaniam et al, 2001; Westberry and Siegel, 2006) and strong light absorption and heat trapping at the ocean surface
If the reference turbidity within the subtropical gyres is lower than the prognostic value, including the feedback leads to a cooling. In this case the higher chlorophyll values shade and cool the subsurface water that is upwelled at the Equator
Summary
Phytoplankton pigments, predominantly chlorophyll a (Chl), absorb light and thereby modify the vertical distribution of radiative heating in the upper ocean (e.g., Lewis et al, 1990; Strutton and Chavez, 2004). We investigate the effects of light absorption by the phytoplankton group of positively buoyant marine cyanobacteria on the tropical climate system. Cyanobacteria are abundant throughout the tropical and subtropical ocean, where they often represent a dominant fraction of total phytoplankton biomass (e.g., Carpenter and Romans, 1991; Capone et al, 1997; Luo et al, 2012). The chlorophyll content of cyanobacteria is in general rather low compared to that of other phytoplankton (e.g., Berman-Frank et al, 2001; Carpenter et al, 2004; Sathyendranath et al, 2009), the dense accumulations of biomass in cyanobacteria blooms result in high chlorophyll concentrations (e.g., Subramaniam et al, 2001; Westberry and Siegel, 2006) and strong light absorption and heat trapping at the ocean surface
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