Observed and projected changes to the tropical Pacific Ocean, Part 2

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Observed and projected changes to the tropical
Pacific Ocean, Part 2
Presented by
Alexandre Ganachaud and 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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Conclusions from Part 1

• The tropical Pacific has warmed, although natural
  variability can alter rates on decadal timescales
• Sea level has risen, with particularly high rates
  in the west over the last 20 years
• Additional CO2 in the surface ocean has led to
  reduced pH
• Regions of low oxygen appear to be expanding
• Climate models successfully simulate
  characteristics of the climate system but must be
  used with care

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Outline

• Projected temperatures and currents
• surface and vertical structure
• Implications for oceanic nutrients and oxygen
• Acidification
• Influence on tuna distribution

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Projected changes to CO2
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Projected changes to CO2
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Ocean warming

• Change in SST from (2000-2100) averaged over 20
  models

• All models show warming and agree on aspects of
  warming pattern
• Pacific basin SST projected to increase from
  27.4C in 2000 to 28.1C (0.7C) in 2035 and
  29.9C (2.5C) in 2100

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Ocean warming

• Change in SST from (2000-2100) averaged over 20
  models

• All models show warming and agree on aspects of
  warming pattern
• Pacific basin SST projected to increase from
  27.4C in 2000 to 28.1C (0.7C) in 2035 and
  29.9C (2.5C) in 2100

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New models are similar
IPCC-AR-4 (2007)
IPCC-AR-5
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Vertical structure and stratification
Thermocline depth
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Vertical structure and stratification

• Warming is surface intensified
• This leads to widespread increase in
  stratification

Change in 0-200m density
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Projected change in currents

• Increase in Equatorial Undercurrent , New Guinea
  Coastal Undercurrent and South Equatorial Current
  (SEC) Decrease in SEC at surface near equator

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Projected changes in vertical currents

• Upwelling along the equator decreases
• Downwelling on both sides of the equator decreases

Less downwelling
Less UPWELLING
Less downwelling
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Implications for nutrients
Dissolved nitrate at 100 m
Depth

• Nutrients are mostly depleted in the euphotic
  zone
• Replenishment by decay of sinking organic material

• Oceanic transport is needed to transfer nutrients
  to the surface layer

Similar features for phosphate silicate
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Implications for nutrients
25S
25N
Dissolved nitrate at 100 m
Dissolved nitrate at 100 m
Depth

• Nutrients are mostly depleted in the euphotic
  zone
• Replenishment by decay of sinking organic material

• Oceanic transport is needed to transfer nutrients
  to the surface layer

Similar features for phosphate silicate
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Nutrient supply to sun-lit depths

• Upwelling (vertical current east equator and
  some islands)
• Eddies
• Vertical mixing from wind
• Mixing from tides
• ... against stratification

??
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Eddies and land effects

• Small-scales generated spontaneously or by
  interaction between the large-scale flow and land

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Nutrient supply conclusions

• Reduction of upwelling at equator
• Eddies probable changes
• Reduction of vertical mixing from winds
• Internal tides no change
• Stratification increase acts as a stronger
  barrier to nutrient supply

??
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Future acidification increase

• Past and present aragonite saturation

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Future acidification increase

• Aragonite saturation is expected to fall below
  3.3 by 2040
• Aragonite saturation expected to decrease to 2.4
  in 2100

2040
2100
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Conclusions changes to physical ocean

• Large, consistent projected changes to SST
• Higher rainfall in western Pacific reduced
  salinity
• Increased stratification, geater in west
• Significant slowdown of equatorial currents and
  upwelling acceleration of Equatorial
  Undercurrent
• Sea level rise 80 cm to 1.4 m
  possible(!decadal variations)

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Conclusions changes to the chemical ocean

• Nutrient supply from deep layers reduced due to
  greater stratification away from the equator
• Oxygen below mixed layer (100 m) reduced due to
  decreased input from higher latitudes.
• Aragonite drops below critical threshold within
  the next few decades

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Possible consequences for tuna

• Skipjack preferred temperature habitat

30oC
17oC
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Possible consequences for tuna

• Projected warming means temperatures become too
  warm in the western Pacific

30oC
17oC
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Oceanic variability will matter!

• Tides (h)
• Storms (day)
• Ocean eddies (week)
• Seasons
• El Nino (2-5 years)
• Decadal variations (10-50 years and more)
• Warming (100yr)

Source J. Lefèvre, IRD
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Outlook IPCC models AR-5

• - Improved realism but similar results in new
  models
• - ENSO projections still uncertain
• Earth System Models with biology
• New experiments including decadal prediction