Observed and projected changes to the ocean, Part 1 Climate models, pitfalls and historical observations (Chapter 3, Ganachaud et al., 2012) Alex Sen Gupta

Why do we care about the Ocean? Historical observations – Ocean temperature, stratification, sea- level, dissolved oxygen, acidification What is a climate model? Pitfalls – Resolution & model bias

2,500,000,000 Hiroshimas ! Why do we care about the Ocean?

What we expect What we measure Atmospheric CO 2 Concentration – 1/4 of human CO 2 emissions absorbed by ocean Why do we care about the Ocean?

– Widespread warming Ocean temperature trend ( ) o C/decade Historical Observations: Temperature

Ocean temperature trend ( ) o C/decade Ocean temperature trend ( ) o C/decade – Widespread warming – Natural variability can mask Global Warming Historical Observations: Temperature

– Reduced salinity over last 50yr – Salinity change evidence of increased rainfall – Warming (and freshening) cause increased stratification – Implications for nutrients and oxygen Ocean salinity trend ( ) o C/50yr Cravatte et al. 2009

– Low oxygen zones expanding – Possibly related to reduced increased stratification Historical Observations: Oxygen Dissolved oxygen concentrations (eastern equatorial Pacific)

– Global average sea-level increase ~20cm – Very rapid sea-level rise in Western Pacific over last 20 yrs – Related to natural variability PDO), not reflective of long term trend Historical Observations: Sea-level Combined TOPEX/Poseidon, Jason-1 and Jason-2/OSTM sea level fields Sea-level change

Historical Observations: Acidification CO 3 2- H+H+ pH Carbonate/Aragonite 30% 0.1 Ocean CO2 Build-up

What is a climate model?

Time: 1 What is a climate model?

Time: 2 What is a climate model?

Time: 3 What is a climate model?

Time: 4 What is a climate model?

Time: 5 What is a climate model?

Time: 6 What is a climate model?

Time: 7 What is a climate model?

Time: 8 What is a climate model?

Time: 10 What is a climate model?

Time: 11 What is a climate model?

Time: 11 What is a climate model? Air Temperature Ocean Temperature Wind Speed Current Speed Cloudiness Water Vapour Rainfall Salinity Density Land Runoff Land Cover Ice Cover

Resolution

How an Ocean model sees the ocean …

Resolution How an Ocean model sees the ocean …

Resolution How an Ocean model sees the ocean …

Resolution – Broad features are captured But … – Cannot see small islands – Cannot see fine scale circulation

Grid box size in the different models range from about 1° to 5° Resolution

Gilbert Islands Climate models can’t see small islands So they don’t reproduce island process like upwelling Resolution Gilbert Islands Climate Model Satellite Observations Surface Temperature

Models suggest that the equatorial undercurrent will strengthen Presence of Gilbert islands reduce warming by 0.7 o C Gilbert Islands Resolution Climate Model Satellite Observations Surface Temperature

Model Bias Sea surface temperature Observations Average of all models Cold tongue extends too far to west Warm pool isn’t warm enough Upwelling off south America too weak

Conclusions Significant change has already occurred But, need to be careful to separate climate change and natural variability

Conclusions Climate models successfully simulate many characteristics of the climate system But they have their limitations

Observations Average of all models Projected warming – If cold tongue is in wrong location warming might also be in wrong location Model Bias Sea surface temperature