Lauren Santi and Alison Gray Understanding oxygen in the Southern Ocean from models and observations Lauren Santi and Alison Gray

Motivation High variability of oxygen levels seasonal, interannual, regional Limited knowledge of the Southern Ocean historical observations are sparse, biased towards summer Importance of Southern Ocean in global oxygen cycle Annual mean zonally averaged oceanic uptake of oxygen for period 1990-2000 (Matear et al. 2000) There are a variety of reasons to look at the seasonal cycle in greater depth: The Southern ocean experiences substantial variability, whether it be seasonal, regional, or interannual (from year to year). S.O. important for oxygen uptake and also changes the most according to this model green = change between climate forcing(red) and control(black) simulations climate change control difference

Research Questions How well do current models represent the variability of oxygen in the Southern Ocean? How will oxygen concentrations in the Southern Ocean change with increased CO2? Approach: Use observations and coupled climate models

ARGO Profiling Floats Floats take profile measurements of top 2000 meters every ten days A subset have oxygen sensors 17,521 profiles Oxygen data has recently been recalibrated using World Ocean Atlas http://aquarius.umaine.edu/images/sci_argo_float.jpg floats go under the ice which is how we get this winter data

GFDL climate model simulations CM2-1deg and CM2.6: Pre-industrial and climate change CO2 simulations Coupled to miniBLING biogeochemical model Subsampling of model data Allows for more representative comparison between the models and observations Biology Light Iron Nutrient and Gas Model (miniBLING) Biogeochemical model with oxygen, carbon and phosphate tracers CM2-1deg coupled climate model http://soccom.princeton.edu/

Zonally and time-averaged depth profiles both models underpredict the amount of dissolved oxygen, but CM2-1deg is generally worse increased CO2 simulation showed less o2 in the water column oxygen minimum zone tends to get shallower moving south CM2.6 is better at modeling oxygen profiles in the pacific ocean but shows no clear improvement in the Indian and Atlantic Oceans. region of maximum oxygen change with increased CO2 gets shallower moving south

Zonally averaged seasonal cycle comparison (depth = 50 meters) lots of variability, not a strong seasonal cycle when looking at the subsampled data data is incomplete- the Pacific Ocean at 45deg only has data from 1 month! can’t say what the seasonal cycle is given the results of the subsampled data contrast with strong seasonal cycle seen before the data was subsampled for the most part, both models overlap with the observational data within one standard deviation

20 year time-mean change in oxygen (µmol/kg) (Control - climate change) most similar at the surface both show broad decreases in oxygen with climate change regardless of depth biggest difference between the two models is arguably in the Pacific

Percent change between subsampled control and climate change simulations CM2-1deg Highest change in oxygen between 1026 and 1028 density surfaces Generally positive changes mean that oxygen levels are predicted to decrease on these specific density surfaces similar changes around 55 degrees south, around the region of the ACC CM2.6 (higher resolution) is able to resolve eddies eddys have an effect on stratification which could narrow the change of oxygen to smaller density ranges. could be why CM2.6 shows sharp peaks where oxygen decreases by a large amount

Summary CM2-1deg tends to underpredict the amount of dissolved oxygen compared to observations Higher resolution CM2.6 is somewhat better at modeling oxygen concentrations but still underpredicts oxygen levels High variability in the seasonal cycle Decrease in dissolved oxygen in climate change simulation Maximum decrease between density surfaces 1026.5 and 1027.5 kg/m 3

Increased ventilation/ less respiration Decreased ventilation/ more respiration AOU changes greater under the thermocline AOU change due to stratification change (time since water was at the surface)and respiration change consumption is shifted in the water column solubility decreases, is especially important near the surface also near the NADW, which gets warmer according to the model AOU negative near bottom convection in wedell seas renewing bottom waters so H2o is newer? Water temperature increases Little or no change in water temperature