ENSO-related variability in latitudinal distribution of annual mean atmospheric potential oxygen (APO) in the equatorial Western Pacific

Abstract

We examined temporal variations in the latitudinal distribution of annual mean atmospheric potential oxygen (APO=O2+1.1×CO2), a useful tracer for studying ocean biogeochemical processes. To compute APO, we used atmospheric CO2 and O2 concentrations from flask samples and in-situ measurements onboard commercial cargo ships sailing between Japan and Australia/New Zealand. Most of the observed latitudinal distributions of the annual mean APO for the years 2002–2012 showed equatorial bulges, indicating tropical APO outgassing fluxes. However, the equatorial bulge was noticeably absent during the 2009/2010 El Niño period, especially in the Southern Hemisphere. The temporal variation in the 25–0°S latitudinal APO gradient correlated significantly with the El Niño/Southern Oscillation (ENSO); the equatorward APO gradients decreased (increased) during the El Niño (La Niña) period with a variability of about ±0.1 per meg/degree. Simulated APO based on an atmospheric transport model driven by climatological/constant flux fields and reanalysis meteorological data reproduced the overall characteristic of the observed temporal variation in the APO gradients well, suggesting that the atmospheric transport contributed substantially to the observed interannual variation in the global APO distributions. However, the model simulation underestimated the variability in the APO gradients by about 25%, compared to the observations. These discrepancies suggest a possibility of the existence of additional APO flux variability in the tropical Pacific, enhancing the ENSO-related variability in the observed APO gradients.

Keywords

• air–sea gas exchange
• atmospheric O2/N2 ratio
• APO
• ENSO
• atmospheric transport
• shipboard observation