Changes in controlling factors of dissolved oxygen in the Eastern Equatorial Pacific from the past to the future

Abstract:

Variability of the volume of oceanic oxygen-deficient waters is critical for aerobic organisms. Several climate models consistently predict the decreasing oceanic oxygen inventory from the second half of the 20^th to the 21^st centuries. However, the projections about how the volume of oxygen-deficient waters in the future are not consistent with each other. In this study, we investigated the factors controlling the variability of volumes of oxygen-deficient waters in the Eastern Equatorial Pacific (EEP) where the largest oxygen-deficient zone exists in the ocean. Hindcast (from years 1850 to 2005) and forecast (from years 2006 to 2100) experiments with an offline global ocean biogeochemical model were performed by using outputs of physical field by the Earth System Model, MIROC-ESM, under the RCP4.5 and 8.5 scenarios. The model results illustrated that the volume of oxygen-deficient waters in EEP remains relatively constant from years 1850 to 1950, rapidly increases from years 1950 to 2000, and gradually declines from years 2000 to 2100. Available observations are consistent with the change of oxygen concentration from years 1950 to 2000. The budget analysis in EEP shows the following: (1) During 1850 to 1950, the horizontal advection via the Equatorial Undercurrent (EUC) is the major source of dissolved oxygen to the oxygen-deficient waters in EEP and that is almost consumed by the regeneration of organic matter. (2) During 1950 to 2000, the horizontal advection decreases and the declilne is not compensated by the reduction of regeneration of organic matter, which results in the expanding volume of oxygen-deficient waters. (3) In the 21^st century, the horizontal advection declines further but the decrease in the regeneration of organic matter and increase in vertical advective supply are in excess of the decrease, leading to the shrink of oxygen-deficient water volume. Our model suggests that the key mechanisms controlling the oxygen-deficient water volume in EEP in the future are the changes in EUC transport and primary production in EEP related to the change of EUC transport. Multi-model analysis focusing on these processes probably leads to estimating uncertainties of the oxygen-deficient water volume change to the future by the present climate models.