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

Abstract:

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.

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