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

• 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

Outline:Ocean projection for A2 / 2100

• Projected temperatures and currents: surface and vertical structure

• Implications for oceanic nutrients

• Acidification

• Influence on Tuna distribution

Relatively high emissions scenario

Relatively low emissions scenario

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

The next generation of climate models show similar results (AR-5)

IPCC-AR-4 (2007)

• Increased rainfall in western Pacific causes freshening of surface waters

Change in Surface salinity Change in rainfall

Thermocline depth

Vertical structure and stratification

Warm, mixed-layer

Cold, deen ocean

}

}

}

Thermocline depth


Warm, mixed-layer

Cold, deen ocean

}

}

}

Warming is surface intensified

This leads to widespread increase in stratification


Projected changes in vertical currents

• Upwelling along the equator decreases

• Downwelling on both sides of the equator decreases

Less downwellingLess UPWELLING

Less downwelling

Projected Change in Major Currents

• Significant increase in Equatorial Undercurrent, New Guinea Coastal Undercurrent and South Equatorial Current

• Significant decrease in equatorial surface current

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

Eddies and land effects

Implication for nutrients

Thermocline acts as a barrier between surface and deep ocean

Surface ocean nurtient depleted (biological activity)

Deep ocean nutrient rich (decay of sinking material)

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient


Ocean processes needed to bring up nutrients

• Upwelling currents

• Wind mixing

• Currents

• wind mixing

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient


Stratification increases – harder to bring nutrients upwards

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient


Less upwelling bringing nutrients upwards

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient


Increased undercurrent could bring additional iron

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient


Changes in eddy mixing. Increases and decreases in different places

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient

Future acidification increase

Past and present aragonite saturation

saturation>4 healthy conditions

saturation>3.3 marginal conditions

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

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

Consequences on tuna•Skipjack preferred temperature habitat extends across Pacific

30oC 17oC

Consequences on tuna• Projected warming means temperatures become

too warm in the western Pacific

30oC 17oC

Thank you !!

Courtesy J. Lefèvre, IRD

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)

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

DE

PT

H

Dissolved Oxygen at 400m

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

Higher surface temperatures at high latitudes will generally lower the oxygen content

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

100m

500m

0m

0m

250m

500m

0°C 10°C 20°C 30°CTemperatures

Vertical temperature structure

Stratification in the thermocline

1000

1500

20°S 10°S 0° 10°N 20°N

• Increased rainfall in western Pacific causes freshening of surface waters

Change in Surface salinity

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) ;



Projected stratification

Change in 0-200m density

Projected Change in Surface Currents (0-50m)

• Large decrease in equatorial surface current• Large decrease in counter currents

100m

500m

0m

0m

250m

500m

0°C 10°C 20°C 30°CTemperatures

Vertical temperature structure

Stratification in the thermocline

1000

1500

20°S 10°S 0° 10°N 20°N

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

Projected Ocean

Projected Ocean

DEPTH

Dissolved nitrate at 100m

Oceanic transport is needed to transfer them to the surface layer

Similar features for phosphate & silicate


Nutrients are mostly depleted in the euphotic zone

Replenishment by decay of sinking organic material

DE

PT

H

Dissolved nitrate at 100m

Oceanic transport is needed to transfer them to the surface layer

Similar features for phosphate & silicate


Nutrients are mostly depleted in the euphotic zone

Replenishment by decay of sinking organic material

25°S 25°S

??

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

??

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




Change in 0-200m density

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.

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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Alex Sen Gupta