1. Erin McCabe Alison Gray, Keith Rodgers, Ping Zhai
The Sensitivity of Ocean Biogeography to Nonlinearities in the Ocean Density Field
Erin McCabe
Alison Gray, Keith Rodgers, Ping Zhai

2. Introduction Jackett et al. 2006
Equation of state - density of seawater as a function of salinity, temperature, and pressure
25 term equation
Jackett et al. 2006

3. Nonlinearities in the Equation of State aquarius.nasa.gov
Cabbeling
mixing two waters of equal density, but different temperature and salinity, results in a denser water mass
Thermobaricity
compressibility of seawater - a cold water parcel at the surface will become more dense at depth than a warmer water parcel would
aquarius.nasa.gov

4. Potential Temperature at 23W Previous Studies θ(℃) θ(℃)
Nycander et al., 2015
Forced ocean models, 1 degree resolution
Showed cabbeling and thermobaricity have impact on layering and formation of water masses in the Southern Ocean
Cabbeling significant in formation of AAIW
Thermobaricity significant in layering of NADW & AABW
θ(℃)
Emphasize that they have only focused on physical aspects
NEMO
Consistent w/ observations

5. Question
How do nonlinearities in the equation of state affect the distribution of nutrients in the ocean and ultimately ocean biogeography?

6. Methods MOM5(dynamics)-TOPAZ(biogeochemistry) 1 degree resolution
Surface forcing from CORE II normal year
2 runs: both start off with nonlinear EOS spun up for ~1500 years
One run continues with nonlinear EOS for 300 years
The other continues with a linearized EOS for 300 years
ρ =1035-(0.25)T
*salinity not included in density dependence (β=0)

7. Nycander et al. MOM5 Nonlinear EOS - θ Exact EOS - θ θ(℃) θ(℃)
Linear EOS - Temperature
Linear EOS - θ
θ(℃)
θ(℃)
Relatively consistent w/ nycander, except deep linear ocean overall colder/more uniform in ours
Difference probably due to beta as 0

8. Nycander et al. MOM5 Nonlinear EOS - Salinity Exact EOS - Salinity
More freshwater reaching bottom 40-60
Again relatively consistent w/ nycander
Low salinity area around -50 supports nycander finding that AAIW is more prominent in the nonlinear case → cabbeling plays role in formation of AAIW

9. Nonlinear EOS - Nitrate Linear EOS - Nitrate Nonlinear EOS - Oxygen
Oxygen and Nitrate Concentrations - 32S
Nonlinear EOS - Nitrate
Linear EOS - Nitrate
Nonlinear EOS - Oxygen
Linear EOS - Oxygen
Pacific
Atlantic
Indian
Pacific
Atlantic
Indian

10. Closer Look at Oxygen Nonlinear EOS - Oxygen Linear EOS - Oxygen
Deep ocean is clearly perturbed
substantially higher oxygen concentration
Repositioning of oxygen minimum zones
Surface ocean response is less clear
Concentrations seem similar, but layer thickness different
→ look at surface productivity as well as nutrients in different water masses
Pacific
Atlantic
Indian
Pacific
Atlantic
Indian
Oxygen increase 10% per century for linear - gas exchange
Nitrate changes much smaller

11. Net Primary Productivity (upper 100m) Nonlinear EOS - NPP
10.7% increase in NPP for area-weighted average over 15S-15N
8.3% increase in NPP for area-weighted average over 30S-30N
Percent increase is for linear relative to nonlinear

12. Nutrient Transport in Water Masses
Watermass Boundaries for each case
Calculated using the method of Ivy Frenger based on Sallée et al. 2013
AAIW bounds based on salinity minimum
SAMW bounds based on potential vorticity minimum
Nutrient Transport across 32S
Nitrate transport calculated online
Integrated over density range of SAMW and AAIW

13. Salinity and AAIW - 32S Nonlinear Linear
Pacific
Atlantic
Indian
Pacific
Atlantic
Indian
Code does a good job -- boundaries are around the low salinity area
Linear substantially deeper in pacific -- almost 3000m (~1500 in nonlinear)
Subtract 1000 from density if possible
Make plots more legibleeeee

14. Potential Vorticity and SAMW - 32S Nonlinear Linear
Pacific
Atlantic
Indian
Pacific
Atlantic
Indian
Again code does good job -- outlines low pv area
*** try to get atlantic linear SAMW
Unable to identify pv in atlantic linear
Change color scale to show minimum
Make density more readable somehow???

15. Linear has overall greater magnitudes of transport
pacific aaiw opposite in sign
Explain calculation
Percent difference
Point out differences in mass vs nitrate

16. Summary Significant perturbations observed using a linear EOS
Varies by basin and water mass
Salinity, temperature, nitrate, oxygen
~10% changes in NPP over low latitudes
Changes to nitrate transport across 32S much smaller than changes to mass transport

17. Future Studies Current runs should reach steady state
Not quite there yet in this study, but expected that the current observed perturbations will only become greater in time
New runs with a linear EOS including salinity dependence
Compare with observed nutrient transports
Look at different latitudes

18. Acknowledgments Rick Slater Steve Griffies
Alison Gray, Keith Rodgers, Ping Zhai
PEI
Thank you/questions
Rick slater, steve griffies (built eos/mom5)