Chlorophyll-a concentration effects on equatorial Atlantic Ocean mean-state and interannual variability

Abstract. Chlorophyll-a concentration is known to influence the mean-state and interannual sea surface temperature (SST) variability of the tropics. Here, we investigate this effect in the equatorial Atlantic Ocean using a suite of ocean model simulations. In these simulations, the prescribed monthly climatology of chlorophyll-a concentration is multiplied by a factor ranging from 0.01 to 2. We find that a 'clear-ocean' simulation, i.e an ocean simulation with the monthly climatology of chlorophyll-a concentration multiplied by 0.01, results in a significantly warmer (+0.15 °C) eastern equatorial Atlantic SST and in a reduced (14 %) amplitude of SST seasonal cycle when compared against a simulation with realistic chlorophyll-a levels. Additionally, the equatorial Atlantic vertical temperature gradient is weakened, the mixed-layer and thermocline are deepened, and equatorial upwelling is reduced. These changes in the mean-state of the 'clear-ocean' simulation lead to a significant reduction (12.9 %) in eastern equatorial Atlantic SST variability. We also show that when the prescribed monthly climatology of chlorophyll-a concentration is scaled by 0.01, 0.5, 1, 1.5, and 2, the eastern equatorial Atlantic SST variability responds non-linearly. Our results also suggest that the ongoing observed decrease in tropical Atlantic chlorophyll-a concentration may weaken the interannual variability of SST.