Timescales and Magnitude of Internal Variability in Surface Ocean pCO2: 1975-2036

Abstract:

Ocean carbon uptake reduces the accumulation of carbon in the atmosphere and thus damps anthropogenic climate change. Ocean carbon uptake is driven by the air-sea difference in pCO₂. Carbon uptake by the ocean increases pCO₂^SW; the rate of increase determines the evolution of the ocean sink and is modulated by ocean circulation and biogeochemistry. Many recent observational studies have used in situ data to infer changes in the ocean carbon sink; however, there is substantial uncertainty regarding the spatial and temporal structure of the long-term response to rising atmospheric CO₂ and patterns associated with changes in physical climate, making detection of trends a challenge. We use results from the CESM Large Ensemble experiment to quantify the timescales and magnitude of internal variability in pCO₂^SW in comparison to the forced trend. Over most of the ocean and for most timescales, the forced pCO₂^SW trend is weaker than the trend in atmospheric pCO₂ (indicating a strengthening sink), particularly in regions of upwelling or deep vertical mixing. On 10-year timeframes, the amplitude of the internal response is comparable to 10-30% of the forced response in the subtropics and 60-100% in the Equatorial Pacific and high-latitudes. This equates to deviations from the forced response of generally < 0.4 ppmv/yr in the subtropics but up to 2 ppmv/yr in the Equatorial Pacific and high-latitudes. On 20-year timeframes, the internal variability diminishes substantially to <10% of the forced response in the subtropics and <20% in the Equatorial Pacific and high-latitudes. This transition from a 10 to 20 year timeframe increases the signal-to-noise ratio in the Equatorial Pacific and high-latitudes from 1 to >5. On 30-year timeframes, the internal variability is <10% of the forced response globally with signal-to-noise ratios exceeding 10.