The fact that the ocean is not perfectly clear results in shortwave radiation being trapped closer to the surface than would otherwise be the case. In a one-dimensional sense, this effect would be expect to cause warming at the ocean surface and cooling at depth. However, water that is shaded from solar heating is brought to the surface in upwelling zones, particularly along the equator, where it can trigger changes in atmospheric circulation. We examine how these changes can lead to significant changes in tropical variability which depend on where the shading occurs. Shading in the clear ocean gyres results in a shallower thermocline, which is then more able to produce subsurface temperature changes in response to changes in wind stress. Additionally, the weaker Hadley cell associated with off-equatorial warming allows these temperature changes to move further into the equatorial Pacific, producing a stronger wind stress response and thus a stronger El Nino. By contrast, shading in the mesotrophic regions overlying the oxygen minimum zones cools the deep ocean substantially, resulting in a stronger cold tongue and stronger Walker circulation, which traps the convection closer to Indonesia. This change in the atmospheric mean state is more important than the shallowing the of the thermocline and so the El Nino becomes weaker as a result of shading in this region. The regional distribution of chlorophyll and other absorbers can thus play an important role in how coupled climate models simulate variability.
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