Impact of marine phytoplankton on tropical Pacific and Arctic climate using GFDL CM2.1
The presentation will give key results of recent two papers of Lim et al. (2018a; 2018b).
In the tropical Pacific, climate modeling groups nowadays develop earth system models (ESMs) by incorporating biogeochemical processes in their climate models. The ESMs, however, often show substantial bias in simulated marine biogeochemistry which can potentially introduce an undesirable bias in physical ocean fields through biogeophysical interactions. We examined how and how much the chlorophyll bias in GFDL CM2.1 affects the mean and seasonal cycle of tropical Pacific sea‑surface temperature (SST). The model used in the present study shows a sizeable positive bias in the simulated tropical chlorophyll. We found that the correction of the chlorophyll bias can reduce the ESM’s intrinsic cold SST mean bias in the equatorial Pacific. The biologically‑induced cold SST bias is strongly affected by seasonally-dependent air-sea coupling strength.
In the Arctic Ocean, chlorophyll substantially influences the Arctic climate via biogeophysical feedback, i.e., an increase in the mean chlorophyll concentration absorbs more shortwave radiation in the surface ocean layer, which leads to Arctic surface warming. Here, we identified that in addition to the effect of the mean chlorophyll change, an interannual chlorophyll variability substantially influences the Arctic mean climate state, even though the mean chlorophyll remains the same. We found that two nonlinear rectifications of chlorophyll variability induced Arctic cooling. One was due to the effect of a nonlinear shortwave heating term, which was induced by the positive ice–phytoplankton covariability in the boreal summer. The other was due to a cooling effect by rectification of a nonlinear function of the shortwave absorption rate, which reduced the shortwave absorption rate by interannually varying chlorophyll.