1. 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

2. DO2 = DO2,forced + DO2,natural + err
Motivation
Observed linear O2 trend at 200m; , 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

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

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

5. 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

6. 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

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

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

9. DO = DO2,heave DO2,res

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

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

12. 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.

13. 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 

14. 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; , 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 range.

15. Understanding O2 climatology
World Ocean Atlas 2013
(Garcia et al., 2014)
m 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