New insights into &amp;lt;i&amp;gt;f&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; variability in the tropical eastern Pacific Ocean using SMOS SSS

C Walker Brown,J Boutin,L Merlivat

doi:10.5194/bg-12-7315-2015

Abstract

Abstract. Complex oceanic circulation and air–sea interaction make the eastern tropical Pacific Ocean (ETPO) a highly variable source of CO2 to the atmosphere. Although the scientific community have amassed 70 000 surface fugacities of carbon dioxide (fCO2) data points within the ETPO region over the past 25 years, the spatial and temporal resolution of this data set is insufficient to fully quantify the seasonal to interannual variability of the region, a region where fCO2 has been observed to fluctuate by > 300 μatm. Upwelling and rainfall events dominate the surface physical and chemical characteristics of the ETPO, with both yielding unique signatures in sea surface temperature and salinity. Thus, we explore the potential of using a statistical description of fCO2 within sea-surface salinity–temperature space. These SSS/SST relationships are based on in situ surface ocean CO2 atlas (SOCAT) data collected within the ETPO. This statistical description is then applied to high-resolution (0.25°) Soil Moisture and Ocean Salinity (SMOS) sea surface salinity (SSS) and Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) sea surface temperature (SST) in order to compute regional fCO2. As a result, we are able to resolve fCO2 at sufficiently high resolution to elucidate the influence that various physical processes have on the fCO2 of the surface ETPO. Normalised (to 2014) oceanic fCO2 between July 2010 and June 2014 within the entire ETPO was 39 (±10.7) μatm supersaturated with respect to 2014 atmospheric partial pressures, and featured a CO2 outgassing of 1.51 (±0.41) mmol m−2 d−1. Values of fCO2 within the ETPO were found to be broadly split between the Gulf of Panama region and the rest of the tropical eastern Pacific Ocean. The northwest, central and offshore regions were supersaturated, with wintertime wind-jet-driven upwelling found to constitute the first-order control on fCO2 values. This contrasts with the southeastern/Gulf of Panama region, where heavy rainfall combined with rapid stratification of the upper water column act to dilute dissolved inorganic carbon, and yield fCO2 values undersaturated with respect to atmospheric fugacities of CO2.

Highlights

Perturbations to the global carbon cycle caused by anthropogenically driven increases in atmospheric partial pressures of CO2 produce an acute requirement to understand inter-reservoir carbon fluxes (Le Quéré et al, 2015)
We have demonstrated a technique using surface ocean CO2 atlas (SOCAT) data to identify the fugacity of CO2 (f CO2) signatures of water masses within the eastern tropical Pacific Ocean (ETPO) region, namely, the high f CO2 deep water, the near-equilibrium ETPO surface water and the undersaturated rainfall diluted surface waters
We used a look-up table (LUT) technique to produce a description of the f CO2 content of ETPO surface water using satellite sea surface temperature (SST) and sea surface salinity (SSS) data

Summary

Introduction

Perturbations to the global carbon cycle caused by anthropogenically driven increases in atmospheric partial pressures of CO2 (pCO2) produce an acute requirement to understand inter-reservoir carbon fluxes (Le Quéré et al, 2015). Considerable effort has been made in recording oceanic pCO2 (or fugacity – f CO2) over the past 50 years, with roughly 10 million individual measurements of global surface ocean pCO2 taken, processed, flagged and assembled into two large data sets: the surface ocean CO2 atlas (SOCAT) and Lamont-Doherty Earth Observatory (LDEO) carbon dioxide database (Bakker et al, 2014; Takahashi et al, 2014). Both databases make heavy use of “vessel-of-opportunity”-derived pCO2 data, resulting in a heterogeneous data set with the majority of measurements collected within the tight confines of commercial shipping lanes. The quality of these extrapolation methods has been refined over the past few years, in part due to the increasing number of in situ measurements, the interannual variability of the global air–sea CO2 flux obtained using different datadriven methods still substantially differs, and further work is required to unify our estimates and improve understanding of this air–sea flux

Results

Discussion

Conclusion