National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2020

Ocean acidification from below in the tropical Pacific

Ishii, M., K.B. Rodgers, H.Y. Inoue, K. Toyama, D. Sasano, N. Kosugi, H. Ono, K. Enyo, T. Nakano, D. Iudicone, B. Blanke, O. Aumont, and R.A. Feely

Global Biogeochem. Cycles, 34(8), e2019GB006368, doi: 10.1029/2019GB006368, View online (2020)

Identifying ocean acidification and its controlling mechanisms is an important priority within the broader question of understanding how sustained anthropogenic CO2 emissions are harming the health of the ocean. Through extensive analysis of observational data products for ocean inorganic carbon, here we quantify the rate at which acidification is proceeding in the western tropical Pacific Warm Pool, revealing −0.0013 ± 0.0001 year−1 for pH and −0.0083 ± 0.0007 year−1 for the saturation index of aragonite for the years 1985–2016. However, the mean rate of total dissolved inorganic carbon increase (+0.81 ± 0.06 μmol · kg−1 · year−1) sustaining acidification was ~20% slower than what would be expected if it were simply controlled by the rate of atmospheric CO2 increase and transmitted through local air‐sea CO2 equilibration. Joint Lagrangian and Eulerian model diagnostics indicate that the acidification of the Warm Pool occurs primarily through the anthropogenic CO2 that invades the ocean in the extra‐tropics is transported to the tropics through the thermocline shallow overturning circulation and then re‐emerges into surface waters within the tropics through the Equatorial Undercurrent from below. An interior residence time of several years to decades, acting in conjunction with the accelerating CO2 growth in the atmosphere, can be expected to contribute to modulating the rate of Warm Pool acidification.

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