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

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 vertical temperature gradient around the main oceanic thermocline of the equatorial Atlantic is weakened, the mixed layer and main oceanic thermocline are deepened, and the equatorial upwelling across the mixed-layer base 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, decreasing more strongly under low chlorophyll a concentrations than it increases under comparable high chlorophyll a conditions. Our results also suggest that the ongoing observed decrease in tropical Atlantic chlorophyll a concentration may weaken the interannual variability of SST.