Satellite sea surface temperature: a powerful tool for interpreting in situ pCO2 measurements in the equatorial Pacific Ocean

Abstract

In order to determine the seasonal and interannual variability of the CO 2 released to the atmosphere from the equatorial Pacific, we have developed p CO 2 -temperature relationships based upon shipboard oceanic CO 2 partial pressure measurements, p CO 2 , and satellite sea surface temperature, SST, measurements. We interpret the spatial variability in p CO 2 with the help of the SST imagery. In the eastern equatorial Pacific, at 5°S, p CO 2 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 p CO 2 signature of the TIW is removed from the Alize II cruise measurements in January 1991, the equatorial p CO 2 data exhibit a diel cycle of about 10 matm with maximum values occurring at night. In the western equatorial Pacific, the variability in p CO 2 is primarily governed by the displacement of the boundary between warm pool waters, where air–sea CO 2 fluxes are weak, and equatorial upwelled waters which release high CO 2 fluxes to the atmosphere. We detect this boundary using satellite SST maps. East of the warm pool, Δ P is related to SST and SST anomalies. The 1985–97 CO 2 flux is computed in a 5° wide latitudinal band as a combination of Δ P and CO 2 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 Δ P to the flux variability is about half the contribution of K . The mean CO 2 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%. DOI: 10.1034/j.1600-0889.1999.00025.x