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Alex Sen Gupta

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

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Alex Sen Gupta

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  1. Observed and projected changes to the ocean, Part 1 Climate models, pitfalls and historical observationsPresented 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 2011. 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? 2,500,000,000 Hiroshimas ! IPCC (2007)

  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

  6. Historical Observations: Temperature Ocean temperature trend (1950-2010) oC per decade • Widespread warming • Natural variability can mask Global Warming Ocean temperature trend (1980-2010) 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 (1955-2004) 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

  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

  23. Resolution

  24. Resolution How an Ocean model sees the ocean?

  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? • Broad features are captured But … • Cannot see small islands • Cannot see fine scale circulation

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

  29. Resolution Satellite Observations Climate Model 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)

  30. Resolution Satellite Observations Climate Model 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 Satellite Observations Climate Model 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)

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

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

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

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

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