Two decades of Pacific anthropogenic carbon storage and ocean acidification along Global Ocean Ship-lebased Hydrographic Investigations Program sections P16 and P02

A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (C-anth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30 degrees N, and P16 is a meridional section crossing the North and South Pacific at similar to 150 degrees W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to similar to 1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s-2000s) and recent (2000s-2010s) decades along P02, at rates of 0.53 +/- 0.11 and 0.46 +/- 0.11 mol Cm-2 a(-1), respectively. By contrast, Canth storage accelerated from 0.29 +/- 0.10 to 0.45 +/- 0.11 mol Cm-2 a(-1) along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60 degrees S and 60 degrees N of 6.1 +/- 1.5 PgC decade(-1) in the earlier decade and 8.8 +/- 2.2 PgC decade(-1) in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by -0.08 to -0.14 and aragonite saturation state by -0.57 to -0.82.