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Alex Sen Gupta

Observed and projected changes to the tropical Pacific Ocean , Part 2 ( Chapter 3, Ganachaud et al., 2012). Alex Sen Gupta. Conclusions from Part I. The tropical Pacific has warmed, although natural variability can alter rates on decadal timescales

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Alex Sen Gupta

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  1. Observed and projected changes to the tropical Pacific Ocean, Part 2 (Chapter3, Ganachaud et al., 2012) Alex Sen Gupta

  2. Conclusions from Part I • The tropical Pacific has warmed, although natural variability can alter rates on decadal timescales • Sea level has risen, with particularly high rates in the western Pacific over the last 20yrs • Regions of low oxygen appear to be expanding • Additional CO2 in the surface ocean has led to reduced pH • Climate models successfully simulate many characteristics of the climate system • They have limitations and must be used with care

  3. Outline:Ocean projection for A2 / 2100 • Projected temperatures and currents: surface and vertical structure • Implications for oceanic nutrients • Acidification • Influence on Tuna distribution

  4. Relatively high emissions scenario Relatively low emissions scenario

  5. Change in Surface temperature (2000-2100) average over 20 climate models • All models show warming • Most models agree on aspects of the spatial pattern of warming • Pacific basin SST (for A2 Scenario): • 2000: 27.4°C • 2035: 28.1°C (+0.7°C); model spread +/-0.3°C • 2100: 29.9°C (+2.5°C) ; model spread +/-0.6°C

  6. IPCC-AR-4 (2007) The next generation of climate models show similar results (AR-5)

  7. Increased rainfall in western Pacific causes freshening of surface waters Change in Surface salinity Change in rainfall

  8. Vertical structure and stratification } Warm, mixed-layer } Thermocline depth } Cold, deenocean

  9. Vertical structure and stratification } Warm, mixed-layer } Thermocline depth } Cold, deenocean

  10. Vertical structure and stratification Warmingis surface intensified This leads to widespreadincrease in stratification

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

  12. Projected Change in Major Currents • Significant increase in Equatorial Undercurrent, New Guinea Coastal Undercurrent and South Equatorial Current • Significant decrease in equatorial surface current

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

  14. Implication for nutrients Warm, mixed-layer Thermocline acts as a barrierbetween surface and deepocean Surface oceannurtientdepleted (biologicalactivity) Deepoceannutrientrich (decay of sinkingmaterial) Cold, deepocean Lownutrient High nutrient

  15. Implication for nutrients Warm, mixed-layer Ocean processes needed to bring up nutrients • Upwelling currents • Wind mixing • Currents • wind mixing Cold, deepocean Lownutrient High nutrient

  16. Implication for nutrients Warm, mixed-layer Cold, deepocean Lownutrient Stratification increases – harder to bring nutrients upwards High nutrient

  17. Implication for nutrients Warm, mixed-layer Cold, deepocean Lownutrient Less upwelling bringing nutrients upwards High nutrient

  18. Implication for nutrients Warm, mixed-layer Cold, deepocean Lownutrient Increased undercurrent could bring additional iron High nutrient

  19. Implication for nutrients Warm, mixed-layer Cold, deepocean Lownutrient Changes in eddy mixing. Increases and decreases in different places High nutrient

  20. Future acidification increase • Past and present aragonite saturation • saturation>4 healthy conditions • saturation>3.3 marginal conditions

  21. Future acidification increase • Aragonite saturation is expected to fall below 3.3 by 2040 (A2 scenario), possibly jeopardising some corals. • Aragonite saturation expected to decrease to 2.4 in 2100

  22. Conclusions 1: Projected changes to the physical ocean • Large, consistent projected changes to surface temperature • Increased precipitation in western Pacific and reduced salinity • General increase in stratification, enhanced in the west • Significant slowdown of equatorial currents and upwelling; acceleration of Equatorial Undercurrent • Nutrient supply from deep layers is likely to reduce due to increase in stratification, away from the equator • Aragonite drops below critical threshold within a few decades • Sea level rise: over 1 meter cannot be ruled out; influence on habitat

  23. Consequences on tuna • Skipjack preferred temperature habitat extends across Pacific 30oC 17oC

  24. Consequences on tuna • Projected warming means temperatures become too warm in the western Pacific 30oC 17oC

  25. Thank you !!

  26. Oceanic Variability will matter ! • Tides (h) • Storms (day) • Ocean eddies (week) • Seasons • El Nino (2-5 years) • Decadal variations (10-50 yrs and more) • Global warming (100yr) Courtesy J. Lefèvre, IRD

  27. Mixed layer • Seasonal variations of the mixed layer depth pumps deep nutrients towards the sunlit zone • Higher stratification will limit this effect • Future Mixed layer is projected to shoal by 10-20m

  28. Oxygen replenishment at depths • Oxygen is abundant near the surface and depleted near 400m • Replenishment by high latitude atmospheric input and subsurface transport by ocean currents DEPTH Dissolved Oxygen at 400m Higher surface temperatures at high latitudes will generally lower the oxygen content

  29. Nutrient supply by ocean eddies • Eddies temporarily lift the nutrient-rich waters • Eddy activity is related to current strengths; some changes could happen but no conclusion so far

  30. Stratification in the thermocline Vertical temperature structure 0m 250m 500m 20°S 10°S 0° 10°N 20°N 1000 0m 100m 1500 0°C 10°C 20°C 30°C Temperatures 500m

  31. Increased rainfall in western Pacific causes freshening of surface waters Change in Surface salinity

  32. Change in Surface temperature (2000-2100) average over 20 climate models • All models show warming • Most models agree on aspects of the spatial pattern of warming • Warm Pool SST (warmest 10% of Pacific region): • 2000: 29.6°C • 2035: 30.5°C (+0.8°C); • 2100: 32.2°C (+2.6°C) ;

  33. Projected stratification Warmingis surface intensified This leads to widespreadincrease in stratification Change in 0-200m density

  34. Projected Change in Surface Currents (0-50m) • Large decrease in equatorial surface current • Large decrease in counter currents

  35. Stratification in the thermocline Vertical temperature structure 0m 250m 500m 20°S 10°S 0° 10°N 20°N 1000 0m 100m 1500 0°C 10°C 20°C 30°C Temperatures 500m

  36. 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

  37. Projected Ocean

  38. Projected Ocean

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

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

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

  42. Nutrient supply: conclusions Reduction of upwelling at the equator Eddies: probable changes Reduction of vertical mixing from winds Internal tides: no change Stratification increase acts as a stronger barrier ??

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

  44. Conclusions 2: Projected changes to the chemical ocean • Sea level rise: +80 cm to +1.4 m possible; influence on habitat (decadal variations) • Oxygen below the mixed layer (~100 m) is likely to reduce due to decreased input from higher latitudes.

  45. Outlook for new IPCC model generation: 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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