Mechanisms of low-frequency O2 variability in the North Pacific Taka Ito1 In collaboration with Matt Long2, Curtis Deutsch3, Shoshiro Minobe4 and Daoxun Sun1 1Georgia Institute of Technology 2National Center for Atmospheric Research 3University of Washington 4Hokkaido University Ocean Deoxygenation Kiel 2018 Session 03 Ventilation and Oxygen Supply

DO2 = DO2,forced + DO2,natural + err Motivation Observed linear O2 trend at 200m; 1960-2000, mMyr-1 (Ito et al., 2017) R2 Observed 40-year trends can only explain about 10-30% of variance. What controls the natural variability of O2 (DO2,natural )? DO2 = DO2,forced + DO2,natural + err

Ocean hindcast simulation Ocean-ice CESM (Yeager et al., 2018) forced by observed atmospheric state (CORE2-IAV; Large and Yeager, 2009) for 1948-2015 1° resolution; ecosystem and BGC (Moore et al., 2013)

Ocean hindcast simulation Ocean-ice CESM (Yeager et al., 2018) forced by observed atmospheric state (CORE2-IAV; Large and Yeager, 2009) for 1948-2015 1° resolution; ecosystem and BGC (Moore et al., 2013)

Observational dataset World Ocean Database 2013 binned into monthly 5°x5° bins Test the model’s ability to reproduce IAV of O2 Monthly anomalies are aggregated into Warm (AMJJAS) and Cool (ONDJFM) season and annually. No interpolation  minimize interpolation error O2 data count per 5°x5° grid cell per year

Direct model-data comparison Correlate CESM O2 with World Ocean Database 2013 in the 5°x5° grid (dots indicates statistical significance) Data rich regions  Western Pacific, California Current Correlation between obs and model O2 (200m), warm season (AMJJAS) Correlation between obs and model O2 (200m), cool season (ONDJFM)  Overall positive and significant correlation in data rich regions

Modes of O2 variability Model O2 PC-1 explains about 24% of variance, and is correlated with the PDO index (r=0.86)

Ventilation / subduction Mechanisms Vertical movement of isopycnal (heave) Ventilation / subduction

DO2 = DO2,heave + DO2,res

Mid-latitude ventilation Stronger mid-latitude wind under positive PDO Deeper MLD under positive PDO DO2,res particularly strong in the Kuroshio Extension region.

Mid-latitude ventilation Stronger mid-latitude wind under positive PDO Deeper MLD under positive PDO DO2,res particularly strong in the Kuroshio Extension region.

The role of biology OUR regressed onto PDO index, mM/yr/SD Weakened productivity reinforces higher tropical O2 under +PDO O2 vs O2,heave regression indicates stronger response of O2 in tropics, consistent with changes in export production.

Conclusion +PDO -PDO Ventilation variability  Subtropical thermocline O2 Isopycnal heave + biology  Tropics and eastern boundary regions Subpolar region appears to be driven by both Preprint and model output are  http://shadow.eas.gatech.edu/~Ito/webdata

DO2 = DO2,forced + DO2,natural + err O2 trends Simulated linear O2 trend at 200m by CMIP5 Earth System Models Observed linear O2 trend at 200m; 1960-2000, mMyr-1 (Ito et al., 2017) R2 Trends from CMIP5 models show diverse patterns. DO2 = DO2,forced + DO2,natural + err  DO2,natural likely important Observed 40-year trends’ R2 is 0.1 - 0.3 range.

Understanding O2 climatology World Ocean Atlas 2013 (Garcia et al., 2014) 200-600m average annual O2 “hypoxic” waters Mid/high-latitude ventilation = physical O2 supply Weakly ventilated tropics = low-O2 due to cumulative respiration AOU : Apparent Oxygen Utilization