FY 2021

Characterising mean and extreme diurnal variability of ocean CO₂ system variables across marine environments

Torres, O., L. Kwiatkowski, A.J. Sutton, N. Dorey, and J.C. Orr

Geophys. Res. Lett., 48(5), e2020GL090228, doi: 10.1029/2020GL090228, View online (open access) (2021)

Diurnal variability of ocean CO₂ system variables is poorly constrained. Here, this variability and its drivers are assessed using 3‐h observations collected over 8–140 months at 37 stations located in diverse marine environments. Extreme diurnal variability, that is, when the daily amplitude exceeds the 99th percentile of diurnal variability, is comparable in magnitude to the seasonal amplitude and can surpass projected changes in mean states of pCO₂ and [H⁺] over the twenty‐first century. At coastal sites and near coral reefs, extremes in diurnal amplitudes reach 187 ± 85 and 149 ± 106 μatm forpCO₂, 0.21 ± 0.08 and 0.11 ± 0.07 for pH, and 1.2 ± 0.5 and 0.8 ± 0.4 for Ω_arag, respectively. Extreme diurnal variability is weaker in the open ocean, but still reaches 47 ± 18 μatm for pCO₂, 0.04 ± 0.01 for pH, and 0.25 ± 0.11 for Ω_arag. Diurnal variability of the ocean CO₂ system is considerable and likely to respond to increasing CO₂. Therefore, it should be represented in Earth system models.

Plain Language Summary. Our understanding of how ocean pH and related chemical variables vary during the day (known as diurnal variability) is not well established. Here, we use a recent data set of such observations collected every 3 h during 8–140 months from 37 buoys located across the oceans to assess these diurnal variations and what drives them. In extreme cases, observed changes over 24 h were found to be greater than those observed between seasons. Diurnal variations in these chemical variables are particularly large in coastal waters and near coral reefs and are not negligible further offshore. Along with the more gradual, long‐term acidification of the ocean from atmospheric CO₂ increases year after year, diurnal and seasonal variability of ocean chemistry is also expected to change dramatically. Understanding how this diurnal variability will change in the future is important because it modulates the levels of acidification experienced by marine organisms from long‐term yearly changes.