Satellite sea surface temperature: a powerful tool for interpreting in situ pCO₂measurements in the equatorial Pacific Ocean

Abstract

In order to determine the seasonal and interannual variability of the CO₂ released to the atmosphere from the equatorial Pacific, we have developed pCO₂-temperature relationships based upon shipboard oceanic CO₂ partial pressure measurements, pCO₂, and satellite sea surface temperature, SST, measurements. We interpret the spatial variability in pCO₂ with the help of the SST imagery. In the eastern equatorial Pacific, at 5°S, pCO₂ variations of up to 100 μatm are caused by undulations in the southern boundary of the equatorial upwelled waters. These undulations appear to be periodic with a phase and a wavelength comparable to tropical instability waves, TIW, observed at the northern boundary of the equatorial upwelling. Once the pCO₂ signature of the TIW is removed from the Alize II cruise measurements in January 1991, the equatorial pCO₂ data exhibit a diel cycle of about 10 matm with maximum values occurring at night. In the western equatorial Pacific, the variability in pCO₂ is primarily governed by the displacement of the boundary between warm pool waters, where air’sea CO₂ fluxes are weak, and equatorial upwelled waters which release high CO₂ fluxes to the atmosphere. We detect this boundary using satellite SST maps. East of the warm pool, ΔP is related to SST and SST anomalies. The 1985–97 CO₂ flux is computed in a 5° wide latitudinal band as a combination of ΔP and CO₂ exchange coefficient, K, deduced from satellite wind speeds, U. It exhibits up to a factor 2 seasonal variation caused by K-seasonal variation and a large interannual variability, a factor 5 variation between 1987 and 1988. The interannual variability is primarily driven by displacements of the warm pool that makes the surface area of the outgassing region variable. The contribution of ΔP to the flux variability is about half the contribution of K. The mean CO₂ flux computed using either the Liss and Merlivat (1986) or the Wanninkhof (1992) K’U parametrization amounts to 0.11 GtC yr^-1 or to 0.18 GtC yr^-1, respectively. The error in the integrated flux, without taking into account the uncertainty on the K’U parametrization, is less than 31%.