Observed and projected changes to
the ocean, Part 1
Climate models, pitfalls and historical
observations
Presented by
Alex Sen Gupta
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
Outline
• Why do we care about the Ocean?
• Historical observations

Ocean temperature, stratification, sealevel, acidification, dissolved oxygen
• What is a climate model?
• Pitfalls

Resolution & model bias
Why do we care about the Ocean?
2,500,000,000
Hiroshimas !
IPCC (2007)
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
Historical Observations: Temperature
Ocean temperature trend (1950-2010) oC per decade
• Widespread
warming
Ocean temperature trend (1980-2010) oC per decade
• Natural
variability can
mask Global
Warming
Historical Observations: Temperature
Ocean salinity trend (1955-2004) oC/50yr
Cravatte et al. (2009)
• Reduced salinity over
last 50 years
• Salinity change
evidence of increased
rainfall
• Warming (and
freshening) cause
increased stratification
• Implications for
nutrients and oxygen
Historical Observations: Oxygen
• Low oxygen
zones expanding
• Possibly related
to reduced
increased
stratification
Dissolved oxygen concentrations
(eastern equatorial Pacific)
Stramma et al. (2008)
Historical Observations: Sea-Level
Sea-level change
• Global average sealevel 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
Combined TOPEX/Poseidon, Jason-1 and Jason-2/OSTM sea level fields
Historical Observations: Acidification
Ocean CO2 build-up
30%
H+
Carbonate/Aragonite
pH
CO320.1
What is a climate model?
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
•
•
Models suggest that equatorial
undercurrent will strengthen
Presence of Gilbert islands reduce
warming by 0.7oC
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
• Air Temperature
• Ocean Temperature
• Wind Speed
• Current Speed
• Cloudiness
• Water Vapour
• Rainfall
• Salinity
• Density
• Land Runoff
• Land Cover
• Ice Cover
Resolution
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
How an Ocean model sees the ocean?
• Broad
features are
captured
But …
• Cannot see
small islands
• Cannot see
fine scale
circulation
Resolution
Grid box size in the
different models
range from about
1° to 5°
Resolution
Climate Model
Surface Temperature
Satellite Observations
Surface Temperature
Gilbert Islands
• Climate models can’t see small islands
• So they don’t reproduce island process like upwelling
Karnauskas et al. (2012)
Resolution
Climate Model
Surface Temperature
Satellite Observations
Surface Temperature
Gilbert Islands
• Climate models can’t see small islands
• So they don’t reproduce island process like upwelling
Karnauskas et al. (2012)
Resolution
Climate Model
Surface Temperature
Satellite Observations
Surface Temperature
Gilbert Islands
• Models suggest that equatorial undercurrent will strengthen
• Presence of Gilbert islands reduce warming by 0.7oC
Karnauskas et al. (2012)
Model Bias
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
Model Bias (SST)
Observations
Average of all models
Projected warming
• If cold tongue is in wrong
location warming might
also be in wrong location
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