1. Observed and projected changes to the ocean, Part 1 Climate models, pitfalls and historical observations Presented by
Alex Sen Gupta

2. Authors
This presentation is based on Chapter 3 ‘Observed and expected changes to the tropical Pacific Ocean’ in the book Vulnerability of Tropical Pacific Fisheries and Aquaculture to Climate Change, edited by JD Bell, JE Johnson and AJ Hobday and published by SPC in The authors of Chapter 3 are: Alexandre S Ganachaud, Alex Sen Gupta, James C Orr, Susan E Wijffels, Ken R Ridgway, Mark A Hemer, Christophe Maes, Craig R Steinberg, Aline D Tribollet, Bo Qiu and Jens C Kruger

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

4. Why do we care about the Ocean?
The state of the ocean can effect climate and weather on seasonla simescales or from one year to the next e.g. ENSO. But the ucean also plays a big part in controlling climate change
2,500,000,000
Hiroshimas !
IPCC (2007) 4

5. Why do we care about the Ocean?
Atmospheric CO2 Concentration
What we expect
What we measure
IPCC 2007
25% of human CO2 emissions absorbed by ocean 5

6. Historical Observations: Temperature
Ocean temperature trend ( ) oC per decade
Widespread warming
Natural variability can mask Global Warming
Ocean temperature trend ( ) oC per decade

7. Historical Observations: Temperature
Reduced salinity over last 50 years
Salinity change evidence of increased rainfall
Warming (and freshening) cause increased stratification
Implications for nutrients and oxygen
Ocean salinity trend ( ) oC/50yr
Cravatte et al. (2009)

8. Historical Observations: Oxygen
Low oxygen zones expanding
Possibly related to reduced increased stratification
Dissolved oxygen concentrations (eastern equatorial Pacific)
Stramma et al. (2008)

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

10. Historical Observations: Acidification
Ocean CO2 build-up
Carbonate/Aragonite
30% pH
CO32- H+
0.1
H2CO3 Carbonic acid
HCO3 bicarbonate

11. What is a climate model?

12. What is a climate model?
Time: 1

13. What is a climate model?
Time: 2

14. What is a climate model?
Time: 3

15. What is a climate model?
Time: 4

16. What is a climate model?
Time: 5

17. What is a climate model? Time: 6
Models suggest that equatorial undercurrent will strengthen
Presence of Gilbert islands reduce warming by 0.7oC

18. What is a climate model?
Time: 7

19. What is a climate model?
Time: 8

20. What is a climate model?
Time: 10

21. What is a climate model?
Time: 11

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

24. Resolution

25. Resolution
How an Ocean model sees the ocean?

26. Resolution
How an Ocean model sees the ocean?

27. Resolution
How an Ocean model sees the ocean?

28. Resolution How an Ocean model sees the ocean?
Broad features are captured
But …
Cannot see small islands
Cannot see fine scale circulation

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

30. Resolution Climate models can’t see small islands
Satellite Observations
Surface Temperature
Surface Temperature
Gilbert Islands
Climate models can’t see small islands
So they don’t reproduce island process like upwelling
Karnauskas et al. (2012)

31. Resolution Climate models can’t see small islands
Satellite Observations
Surface Temperature
Surface Temperature
Gilbert Islands
Climate models can’t see small islands
So they don’t reproduce island process like upwelling
Karnauskas et al. (2012)

32. Resolution Models suggest that equatorial undercurrent will strengthen
Climate Model
Satellite Observations
Surface Temperature
Surface Temperature
Gilbert Islands
Models suggest that equatorial undercurrent will strengthen
Presence of Gilbert islands reduce warming by 0.7oC
Karnauskas et al. (2012)

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

34. Model Bias (SST)
Observations
Average of all models
Projected warming
If cold tongue is in wrong location warming might also be in wrong location

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

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