Explaining Two Centers of pCO2 and CO2 Flux Variability in the Equatorial Pacific Induced by ENSO

Analysis of sea surface pCO2 reconstructions from 1982-2011 reveals two main centers of activity along the equatorial Pacific. The underlying dynamics of this variability and its connection to the El Niño-Southern Oscillation (ENSO) phenomenon will be elucidated. Using a multi-century pre-industrial simulation of the Community Earth System Model (CESM), coupled to the Biogeochemistry-Ecosystem-Circulation model (BEC) we show that ENSO-driven changes of upper ocean currents are responsible for the natural variability in the tropical Pacific carbonate system. Based on a Taylor expansion of dissolved inorganic carbon (DIC), pCO2 and CO2 flux we find that during El Niño events, the anomalous meridional advection of low salinity water towards the equator leads to a dilution of Alkalinity (TA) and DIC in the western Pacific (155◦E to 165◦W), and a subsequent net reduction of surface pCO2. Combined with the anomalous westerly winds this causes an anomalous flux of CO2 into the ocean.

In the eastern equatorial Pacific (90◦W to 125◦W), surface DIC decreases during El Niño events due a reduction of upwelling. This in turn diminishes surface ocean pCO2. Its effect on eastern Pacific CO2 flux is overcompensated by the effect of El Niño-driven changes in mean wind speed, thus leading to an increased flux of CO2 out of the eastern Pacific ocean.

The presentation will also discuss the phase relationship between the western and eastern tropical Pacific anomalies.