Sensitivity of equatorial Pacific Ocean circulation to solar radiation absorbed by phytoplankton

Sensitivity experiments conducted with the MIT ocean general circulation model reveal the potential influence of solar radiation absorbed by phytoplankton on the thermal structure and currents of the equatorial Pacific Ocean. In the model, vertical attenuation of solar radiation is parameterized as a function of chlorophyll pigment concentration, the major variable affecting turbidity in the euphotic zone. To isolate turbidity effects, the model is run from 1948 to 2001 with either a constant minimum pigment concentration of 0.02 mgm^-3 during the entire period or spatially and temporally varying pigment concentration from the Sea-viewing Wide Field-of-view Sensor during 1997-2001. The two model runs are compared for 2001, a relatively normal year following the strong 1997-1998 El Nino and subsequent La Nina. Due to phytoplankton-radiation forcing, equatorial sea surface temperature is decreased by 0.3K on average annually between 100W and 160W, but the negative temperature change is more pronounced in sub-surface layers, reaching -1.5K at 110W. In that region, heat trapping by phytoplankton causes the mixed layer to shallow and isotherms to shoal toward the equator, generating geostrophic currents that enhance the south equatorial current. These surface currents diverge north and south of the equator as they progress westward, creating equatorial divergence, convergence at the level of the equatorial undercurrent, and upwelling, explaining the change in thermal structure. The equatorial undercurrent is strengthened by as much as 4 cms-1 at its core. The findings support previous results obtained with the MHI Ocean isoPYCnal general circulation model and pigment concentration from the Coastal Zone Color Scanner. They indicate that biology-induced buoyancy my play a significant role in the equatorial Pacific Ocean circulation and suggest the existence of a biophysical feedback mechanism that contributes to maintaining the cold tongue in the eastern equatorial Pacific Ocean, with implications for inter-annual variability associated with El Nino.

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