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

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

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

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

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

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)

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

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

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

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

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

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

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

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???

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

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

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

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