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

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Observed and projected changes to the ocean, Part
1 Climate models, pitfalls and historical
observationsPresented 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 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

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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?
2,500,000,000 Hiroshimas !
IPCC (2007)
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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

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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
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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 (1955-2004) 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
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pH
CO32-
H
0.1
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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 Model
Satellite Observations
Surface Temperature
Surface Temperature
Gilbert Islands

• Climate models cant see small islands
• So they dont reproduce island process like
  upwelling

Karnauskas et al. (2012)
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Resolution
Climate Model
Satellite Observations
Surface Temperature
Surface Temperature
Gilbert Islands

• Climate models cant see small islands
• So they dont reproduce island process like
  upwelling

Karnauskas et al. (2012)
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Resolution
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
Average of all models
Observations
Sea surface temperature

• Cold tongue extends too far to west
• Warm pool isnt warm enough
• Upwelling off south America too weak

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Model Bias (SST)
Average of all models
Observations
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