Abstract
Abstract We quantify the mechanisms governing interannual variability in the global, upper-ocean inorganic carbon system using a hindcast simulation (1979–2004) of an ecosystem-biogeochemistry model forced with time-evolving atmospheric physics and dust deposition. We analyze the variability of three key, interrelated metrics—air–sea CO 2 flux, surface-water carbon dioxide partial pressure p CO 2 , and upper-ocean dissolved inorganic carbon (DIC) inventory—presenting for each metric global spatial maps of the root mean square (rms) of anomalies from a model monthly climatology. The contribution of specific driving factors is diagnosed using Taylor expansions and linear regression analysis. The major regions of variability occur in the Southern Ocean, tropical Indo-Pacific, and Northern Hemisphere temperate and subpolar latitudes. Ocean circulation is the dominant factor driving variability over most of the ocean, modulating surface dissolved inorganic carbon that in turn alters surface-water p CO 2 and air–sea CO 2 flux variability (global integrated anomaly rms of 0.34 Pg C yr −1 ). Biological export and thermal solubility effects partially damp circulation-driven p CO 2 variability in the tropics, while in the subtropics, thermal solubility contributes positively to surface-water p CO 2 and air–sea CO 2 flux variability. Gas transfer and net freshwater inputs induce variability in the air–sea CO 2 flux in some specific regions. A component of air–sea CO 2 flux variability (global integrated anomaly rms of 0.14 Pg C yr −1 ) arises from variations in biological export production induced by variations in atmospheric iron deposition downwind of dust source regions. Beginning in the mid-1990s, reduced global dust deposition generates increased air–sea CO 2 outgassing in the Southern Ocean, consistent with trends derived from atmospheric CO 2 inversions.
Highlights
The ocean exhibits variability in physical circulation on subannual to decadal and longer time-scales that in turn drives substantial changes in regional to basin-scale biogeochemistry and air–sea CO2 fluxes (Chavez et al, 1999; Le Quereet al., 2000, 2003; Gruber et al, 2003; Dore et al, 2003; Bates et al, 2003; Corbiere et al 2007)
The interannual air–sea CO2 flux variability in the CCSM-3 ocean Biogeochemical Elemental Cycle (BEC) model is at the upper range reported for other models for the tropical Pacific Ocean models (70.13 to 70.3 Pg C yrÀ1) and about twice that estimated for the Southern Ocean (70.1 Pg C yrÀ1) (Le Quereet al., 2000; Obata and Kitamura, 2003; McKinley et al, 2004; Wetzel et al, 2005)
The deseasonalized anomalies in air–sea CO2 flux, surface-water pCO2, and upper-ocean inorganic carbon inventory are decomposed into the underlying processes using a linear Taylor expansion and partial derivatives of the variable of interest to individual forcing terms
Summary
The ocean exhibits variability in physical circulation on subannual to decadal and longer time-scales that in turn drives substantial changes in regional to basin-scale biogeochemistry and air–sea CO2 fluxes (Chavez et al, 1999; Le Quereet al., 2000, 2003; Gruber et al, 2003; Dore et al, 2003; Bates et al, 2003; Corbiere et al 2007). Historical ocean carbon data are too sparse, except for a few regions, to fully resolve upper-ocean carbon system and air–sea CO2 flux variability on the required regional and monthly scales (Bender et al, 2002) This will likely remain true in the near-term on a global-scale, even with the recent growth in instrumented biogeochemical moorings and volunteer observing ship (VOS) pCO2 transects (e.g., Metzl et al, 2007; Doney et al, 2009b). We present a globally consistent analysis of upper-ocean biogeochemical interannual variability from a numerical hindcast (1979–2004) that exhibits good skill relative to observations The analysis includes both air–sea CO2 fluxes and surface-water chemistry. We quantify and partition the underlying forcing factors including atmospheric physical forcing, dust deposition, and ocean circulation and biology
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