Oral

IAPSO

JMP09 - El Nino/Southern Oscillation and its Global and Regional Impacts

How important is the negative feedback associated with phytoplankton to ENSO?

The El Ni?o-Southern Oscillation (ENSO) is a primary driver of interannual global climate variability. While its impact on phytoplankton concentrations in the eastern equatorial Pacific via nutrient supply changes is well established, the potential for phytoplankton to feedback onto the ENSO system remains elusive. Chlorophyll in phytoplankton warms the upper ocean by absorbing solar radiation, and this effect is diminished (enhanced) by decreases (increases) in surface chlorophyll concentrations during El Ni?o (La Ni?a) events, potentially leading to a reduction in ENSO amplitude. Despite this proposed feedback, its precise impact on ENSO has been unclear. Here it is shown that phytoplankton concentration anomalies significantly dampen ENSO by cooling (warming) sea surface temperature by 0.71�� (1.51��) during their development phase of El Ni?o (La Ni?a) with the mean amplitude of 1.71�� (1.42��) and contribute to the amplitude asymmetry between El Ni?o and La Ni?a. This damping effect accounts for about 15% (20%) of the total negative feedback processes related to the shortwave radiation. The present approach is unique in that it employs an offline model excluding feedbacks from phytoplankton to physical variables is used, thereby isolating the effect of this negative feedback. These findings have important implications for understanding ENSO as well as improving seasonal prediction and future projections of ENSO.