1. Observed and projected changes to the tropical Pacific Ocean, Part 2 (Chapter 3, 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. All models show warming
Change in Surface temperature
( ) 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
Validation process;
: 27.4°C
: 28.1°C (+0.7°C); model spread +/-0.3°C
: 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 stratification8
Vertical structure and stratification }
Warm, mixed-layer }
Thermocline depth }
Cold, deen ocean

9. Vertical structure and stratification9
Vertical structure and stratification }
Warm, mixed-layer }
Thermocline depth }
Cold, deen ocean

10. Vertical structure and stratification
1010
Vertical structure and stratification
Warming is surface intensified
This leads to widespread increase 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
Pacific Current system
Very stable; the currents are projected to remain similar in the future
However their strength is projected to change, especially near the equator.
For instance, the equatorial undercurrent, in blue … Iron

13. Eddies and land effects
Small-scales generated spontaneously or by interaction between the large-scale flow and land
Amount of eddies related to strength of currents

14. Implication for nutrients
Warm, mixed-layer
Thermocline acts as a barrier between surface and deep ocean
Surface ocean nurtient depleted (biological activity)
Deep ocean nutrient rich (decay of sinking material)
Cold, deep ocean
Low nutrient
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

15. Implication for nutrients
Warm, mixed-layer
Ocean processes needed to bring up nutrients
Upwelling currents
Wind mixing
Currents
wind mixing
Cold, deep ocean
Low nutrient
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

16. Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Stratification increases – harder to bring nutrients upwards
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

17. Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Less upwelling bringing nutrients upwards
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

18. Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Increased undercurrent could bring additional iron
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

19. Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Changes in eddy mixing. Increases and decreases in different places
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
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 (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 ( 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 m

28. Oxygen replenishment at depths
Dissolved Oxygen at 400m
Oxygen is abundant near the surface and depleted near 400m
Replenishment by high latitude atmospheric input and subsurface transport by ocean currents
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. Vertical temperature structure
Stratification in the thermocline 0m
250m
500m
20°S 10°S ° °N 20°N
1000 0m
100m
1500
0°C °C °C °C
Temperatures
500m

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

32. All models show warming
Change in Surface temperature
( ) 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
3333
Projected stratification
Warming is surface intensified
This leads to widespread increase 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. Vertical temperature structure
Stratification in the thermocline 0m
250m
500m
20°S 10°S ° °N 20°N
1000 0m
100m
1500
0°C °C °C °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 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
Dissolved nitrate at 100m
DEPTH
Nutrients are mostly depleted in the euphotic zone
Replenishment by decay of sinking organic material
Oceanic transport is needed to transfer them to the surface layer
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
Similar features for phosphate & silicate

40. Implication for nutrients
DEPTH
Dissolved nitrate at 100m
Nutrients are mostly depleted in the euphotic zone
Replenishment by decay of sinking organic material
Oceanic transport is needed to transfer them to the surface layer
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
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
4343
Vertical structure and stratification
Warming is surface intensified
This leads to widespread increase in stratification
Change in 0-200m density

45. 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.

46. 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