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Observed and projected changes to the ocean, Part 1 Climate models, pitfalls and historical observations Presented by Alex Sen Gupta
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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
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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
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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
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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
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Historical Observations: Temperature
Ocean temperature trend ( ) oC per decade Widespread warming Natural variability can mask Global Warming Ocean temperature trend ( ) oC per decade
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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)
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Historical Observations: Oxygen
Low oxygen zones expanding Possibly related to reduced increased stratification Dissolved oxygen concentrations (eastern equatorial Pacific) Stramma et al. (2008)
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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
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Historical Observations: Acidification
Ocean CO2 build-up Carbonate/Aragonite 30% pH CO32- H+ 0.1 H2CO3 Carbonic acid HCO3 bicarbonate
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What is a climate model?
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What is a climate model? Time: 1
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What is a climate model? Time: 2
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What is a climate model? Time: 3
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What is a climate model? Time: 4
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What is a climate model? Time: 5
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What is a climate model? Time: 6
Models suggest that equatorial undercurrent will strengthen Presence of Gilbert islands reduce warming by 0.7oC
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What is a climate model? Time: 7
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What is a climate model? Time: 8
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What is a climate model? Time: 10
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What is a climate model? Time: 11
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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
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Resolution
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Resolution How an Ocean model sees the ocean?
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Resolution How an Ocean model sees the ocean?
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Resolution How an Ocean model sees the ocean?
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Resolution How an Ocean model sees the ocean?
Broad features are captured But … Cannot see small islands Cannot see fine scale circulation
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Resolution Grid box size in the different models range from about 1° to 5°
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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)
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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)
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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)
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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
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Model Bias (SST) Observations Average of all models Projected warming If cold tongue is in wrong location warming might also be in wrong location
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Conclusions Significant change has already occurred
But, need to be careful to separate climate change and natural variability
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Conclusions Climate models successfully simulate many characteristics of the climate system But they have their limitations
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