A Multiparametric Nonlinear Regression Approach for the Estimation of Global Surface Ocean pCO2Using Satellite Oceanographic Data

Abstract

Estimation of the partial pressure of carbon dioxide (pCO2) and its space-time variability in global surface ocean waters is essential for understanding the carbon cycle and predicting the future atmospheric CO2 concentration. Until recently, only basin-scale distribution of pCO2 has been reported by using satellite-derived climatological data due to the lack of models for global-scale applications. In the present work, a multiparametric nonlinear regression (MPNR) for the estimation of global-scale distribution of pCO2 on the ocean surface is developed using continuous in-situ measurements of pCO2, chlorophyll-a (Chla) concentration, sea surface temperature (SST), and sea surface salinity (SSS) obtained on a number of cruise programs in various regional oceanic waters. Analysis of these measurement data showed strong relationships of pCO2 with Chla, SST, and SSS, because these three parameters are governed by the complex interactions of oceanographic (physical, biological, and chemical) and meteorological processes and thus influence pCO2 levels over different spatial and temporal scales. In order to account for regional differences in the influences of these processes on pCO2, model parameterizations are derived as a function of Chla, SST, and SSS data with different boundary conditions. Because the strength of each influencing parameters on pCO2 differed at different Chla, SST, and SSS ranges, measurement data were grouped with reference to the Chla, SST, and SSS ranges and significant correlations of the pCO2 with dominant processes were established: for example, an inverse correlation of the pCO2 with Chla, SST, and SSS in polar and subpolar regions, a positive correlation of the pCO2 with SST and SSS and an inverse correlation of the pCO2 with Chla in tropical and subtropical regions, and an inverse correlation of the pCO2 with SST and a positive correlation of the pCO2 with Chla and SSS in equatorial regions. This indicates that the relationship of pCO2 versus biological and physical parameters is more complex and an individual parameter alone would not serve as an accurate estimator of basin- and global-scale pCO2 trends. Thus, changes in Chla, SST, and SSS were systematically analyzed as they account for biological and physical effects on pCO2 and best constrained based upon their strong relationships with pCO2 using the MPNR regression approach. The accuracy of the MPNR was assessed using independent in-situ data and satellite pCO2 data derived from global Level-3 Chla, SST, and SSS data. Validation results showed that satellite-derived pCO2 data agreed with direct in-situ pCO2 measurements with an RMSE 6.68-7.5 μatm and a relative error less than 5%, which is significantly small as compared to the errors associated with earlier satellite pCO2 computations. The distribution and magnitude of spatial and temporal (monthly and seasonal) amplitude of satellite-derived pCO2 in climatic zones and ocean basins were further examined and agreed well with the shipboard pCO2 observations and climatological surface ocean pCO2 data.