Influence of El Niño on atmospheric CO2 over the tropical Pacific Ocean: Findings from NASA’s OCO-2 mission

INTRODUCTION The Orbiting Carbon Observatory-2 (OCO-2) is NASA’s first satellite designed to measure atmospheric carbon dioxide (CO 2 ) with the precision, resolution, and coverage necessary to quantify regional carbon sources and sinks. OCO-2 launched on 2 July 2014, and during the first 2 years of its operation, a major El Nino occurred: the 2015–2016 El Nino, which was one of the strongest events ever recorded. El Nino and its cold counterpart La Nina (collectively known as the El Nino–Southern Oscillation or ENSO) are the dominant modes of tropical climate variability. ENSO originates in the tropical Pacific Ocean but spurs a variety of anomalous weather patterns around the globe. Not surprisingly, it also leaves an imprint on the global carbon cycle. Understanding the magnitude and phasing of the ENSO-CO 2 relationship has important implications for improving the predictability of carbon-climate feedbacks. The high-density observations from NASA’s OCO-2 mission, coupled with surface ocean CO 2 measurements from NOAA buoys, have provided us with a unique data set to track the atmospheric CO 2 concentrations and unravel the timing of the response of the ocean and the terrestrial carbon cycle during the 2015–2016 El Nino. RATIONALE During strong El Nino events, there is an overall increase in global atmospheric CO 2 concentrations. This increase is predominantly due to the response of the terrestrial carbon cycle to El Nino–induced changes in weather patterns. But along with the terrestrial component, the tropical Pacific Ocean also plays an important role. Typically, the tropical Pacific Ocean is a source of CO 2 to the atmosphere due to equatorial upwelling that brings CO 2 -rich water from the interior ocean to the surface. During El Nino, this equatorial upwelling is suppressed in the eastern and the central Pacific Ocean, reducing the supply of CO 2 to the surface. If CO 2 fluxes were to remain constant elsewhere, this reduction in ocean-to-atmosphere CO 2 fluxes should contribute to a slowdown in the growth of atmospheric CO 2 . This hypothesis cannot be verified, however, without large-scale CO 2 observations over the tropical Pacific Ocean. RESULTS OCO-2 observations confirm that the tropical Pacific Ocean played an early and important role in the response of atmospheric CO 2 concentrations to the 2015–2016 El Nino. By analyzing trends in the time series of atmospheric CO 2 , we see clear evidence of an initial decrease in atmospheric CO 2 concentrations over the tropical Pacific Ocean, specifically during the early stages of the El Nino event (March through July 2015). Atmospheric CO 2 concentration anomalies suggest a flux reduction of 26 to 54% that is validated by the NOAA Tropical Atmosphere Ocean (TAO) mooring CO 2 data. Both the OCO-2 and TAO data further show that the reduction in ocean-to-atmosphere fluxes is spatially variable and has strong gradients across the tropical Pacific Ocean. During the later stages of the El Nino (August 2015 and later), the OCO-2 observations register a rise in atmospheric CO 2 concentrations. We attribute this increase to the response from the terrestrial component of the carbon cycle—a combination of reduction in biospheric uptake of CO 2 over pan-tropical regions and an enhancement in biomass burning emissions over Southeast Asia and Indonesia. The net impact of the 2015–2016 El Nino event on the global carbon cycle is an increase in atmospheric CO 2 concentrations, which would likely be larger if it were not for the reduction in outgassing from the ocean. CONCLUSION The strong El Nino event of 2015–2016 provided us with an opportunity to study how the global carbon cycle responds to a change in the physical climate system. Space-based observations of atmospheric CO 2 , such as from OCO-2, allow us to observe and monitor the temporal sequence of El Nino–induced changes in CO 2 concentrations. Disentangling the timing of the ocean and terrestrial responses is the first step toward interpreting their relative contribution to the global atmospheric CO 2 growth rate, and thereby understanding the sensitivity of the carbon cycle to climate forcing on interannual to decadal time scales.