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

Observed and projected changes to the tropical Pacific Ocean , Part 2. Presented by. Alexandre Ganachaud and Alex Sen Gupta. Authors.

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

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  1. Observed and projected changes to the tropical Pacific Ocean, Part 2 Presented by Alexandre Ganachaud and Alex Sen Gupta

  2. 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

  3. 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

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

  5. Projected changes to CO2

  6. Projected changes to CO2

  7. 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 27.4°C in 2000 to 28.1°C (+0.7°C) in 2035 and 29.9°C (+2.5°C) in 2100

  8. 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 27.4°C in 2000 to 28.1°C (+0.7°C) in 2035 and 29.9°C (+2.5°C) in 2100

  9. New models are similar IPCC-AR-4 (2007) IPCC-AR-5

  10. Vertical structure and stratification Thermocline depth

  11. Vertical structure and stratification • Warmingis surface intensified • This leads to widespreadincrease in stratification Change in 0-200m density

  12. Projected change in currents • Increase in Equatorial Undercurrent , New Guinea Coastal Undercurrent and South Equatorial Current (SEC); Decrease in SEC at surface near equator

  13. Projected changes in vertical currents • Upwelling along the equator decreases • Downwelling on both sides of the equator decreases Less downwelling Less UPWELLING Less downwelling

  14. Implications for nutrients • Nutrients are mostly depleted in the euphotic zone • Replenishment by decay of sinking organic material Dissolved nitrate at 100 m Depth • Oceanic transport is needed to transfer nutrients to the surface layer Similar features for phosphate & silicate

  15. Implications for nutrients 25°S 25°N • Nutrients are mostly depleted in the euphotic zone • Replenishment by decay of sinking organic material Dissolved nitrate at 100 m Dissolved nitrate at 100 m Depth • Oceanic transport is needed to transfer nutrients to the surface layer Similar features for phosphate & silicate

  16. Nutrient supply to sun-lit depths • Upwelling (vertical current; east equator and some islands) • Eddies • Vertical mixing from wind • Mixing from tides • ... against stratification ??

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

  18. 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 ??

  19. Future acidification increase • Past and present aragonite saturation

  20. Future acidification increase • Aragonite saturation isexpected to fallbelow 3.3 by 2040 • Aragonite saturation expected to decrease to 2.4 in 2100 2040 2100

  21. 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)

  22. 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

  23. Possible consequences for tuna • Skipjack preferred temperature habitat 30oC 17oC

  24. Possible consequences for tuna • Projected warming means temperatures become too warm in the western Pacific 30oC 17oC

  25. 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

  26. 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

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