alex sen gupta

Observed and projected changes to the ocean, Part 1

Climate models, pitfalls and historical observations

Presented by Alex Sen Gupta

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

Authors

• 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

Outline

2,500,000,000

Hiroshimas !

Why do we care about the Ocean?

IPCC (2007)

What we expect

What we measure

Atmospheric CO2 Concentration

• 25% of human CO2 emissions absorbed by ocean

Why do we care about the Ocean?

IPCC 2007

Ocean temperature trend (1950-2010) oC per decade

Ocean temperature trend (1980-2010) oC per decade

• Widespread warming

• Natural variability can mask Global Warming

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)

Historical Observations: Temperature

• Low oxygen zones expanding

• Possibly related to reduced increased stratification

Dissolved oxygen concentrations (eastern equatorial Pacific)

Historical Observations: Oxygen

Stramma et al. (2008)

• 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

Combined TOPEX/Poseidon, Jason-1 and Jason-2/OSTM sea level fields

Sea-level change

Historical Observations: Sea-Level

CO32-H+

pHCarbonate/Aragonite30%

0.1

Ocean CO2 build-up

Historical Observations: Acidification

What is a climate model?

Time: 1


Time: 2


Time: 3


Time: 4


Time: 5


Time: 6


• Models suggest that equatorial undercurrent will strengthen

• Presence of Gilbert islands reduce warming by 0.7oC

Time: 7


Time: 8


Time: 10


Time: 11


Time: 11• Air Temperature• Ocean Temperature• Wind Speed• Current Speed• Cloudiness• Water Vapour• Rainfall• Salinity• Density• Land Runoff• Land Cover• Ice Cover


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 models can’t see small islands• So they don’t reproduce island process like upwelling

Resolution

Karnauskas et al. (2012)

Climate Model Satellite ObservationsSurface Temperature Surface Temperature

Gilbert Islands

• Models suggest that equatorial undercurrent will strengthen• Presence of Gilbert islands reduce warming by 0.7oC

Resolution

Karnauskas et al. (2012)

Climate Model Satellite ObservationsSurface Temperature Surface Temperature

Gilbert Islands

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

Observations Average of all models

Projected warming

• If cold tongue is in wrong location warming might also be in wrong location

Model Bias (SST)

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

alex sen gupta

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