Observed and projected changes to the
ocean, Part 1
Climate models, pitfalls and historical observations
(Chapter 3, Ganachaud et al., 2012)
Alex Sen Gupta
• Why do we care about the Ocean?
• Historical observations
– Ocean temperature, stratification, sealevel, dissolved oxygen, acidification
• What is a climate model?
• Pitfalls
– Resolution & model bias
Why do we care about the Ocean?
2,500,000,000
Hiroshimas !
Why do we care about the Ocean?
Atmospheric CO2
Concentration
What we
expect
What we
measure
– 1/4 of human CO2 emissions absorbed by ocean
Historical Observations: Temperature
Ocean temperature trend (1950-2010) oC/decade
– Widespread
warming
Historical Observations: Temperature
Ocean temperature trend (1950-2010) oC/decade
– Widespread
warming
Ocean temperature trend (1980-2010) oC/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 50yr
– 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)
Historical Observations: Sea-level
Sea-level change
– Global average sea-level
increase ~20cm
– Very rapid sea-level rise
in Western Pacific over
last 20 yrs
– 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
30%
H+
Carbonate/Aragonite
pH
CO320.1
Ocean CO2 Build-up
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
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
Resolution
Climate Model
Surface Temperature
Satellite Observations
Surface Temperature
Gilbert Islands
• Models suggest that the equatorial undercurrent will strengthen
• Presence of Gilbert islands reduce warming by 0.7oC
Model Bias
Sea surface temperature
Observations
Average of all models
• Cold tongue extends too far to west
• Warm pool isn’t warm enough
• Upwelling off south America too weak
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
Model Bias
Sea surface temperature
Observations
Average of all models
Projected warming
– If cold tongue is in
wrong location
warming might also
be in wrong
location