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):
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•
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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
IPCC-AR-4 (2007)
The next generation of
climate models show
similar results (AR-5)
• Increased rainfall
in western Pacific
causes freshening
of surface waters
Change in Surface salinity
Change in rainfall
Vertical structure and stratification
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Warm, mixed-layer
Thermocline depth
Cold, deen ocean
Vertical structure and stratification
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Warm, mixed-layer
Thermocline depth
Cold, deen ocean
Vertical structure and stratification
Warming is surface
intensified
This leads to
widespread increase
in stratification
Projected changes in vertical currents
•
Less downwelling
Less UPWELLING
Less downwelling
•
Upwelling
along the
equator
decreases
Downwelling
on both sides
of the equator
decreases
Projected Change in Major Currents
•
•
Significant increase in Equatorial Undercurrent, New
Guinea Coastal Undercurrent and South Equatorial Current
Significant decrease in equatorial surface current
Eddies and land effects
Small-scales generated
spontaneously or by
interaction between
the large-scale flow
and land
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
High nutrient
Implication for nutrients
Warm, mixed-layer
Ocean processes needed to
bring up nutrients
• Upwelling currents
Cold, deep ocean
• Wind mixing
• Currents
Low nutrient
• wind mixing
High nutrient
Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Stratification
increases – harder to
bring nutrients
upwards
High nutrient
Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Less upwelling
bringing nutrients
upwards
High nutrient
Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Increased
undercurrent could
bring additional iron
High nutrient
Implication for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Changes in eddy
mixing. Increases and
decreases in different
places
High nutrient
Future acidification increase
Past and present
aragonite
saturation
saturation>4 healthy
conditions
saturation>3.3 marginal
conditions
Future acidification increase
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•
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 !!
Oceanic Variability will matter !
Tides (h)
Storms (day)
Ocean eddies (week)
Seasons
El Nino (2-5 years)
Decadal variations (1050 yrs and more)
Global warming (100yr)
Courtesy J. Lefèvre, IRD
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 1020m
DEPTH
Oxygen replenishment at
depths
Dissolved Oxygen at
400m
Oxygen is abundant near the surface Higher surface
and depleted near 400m
temperatures at high
latitudes will generally
Replenishment by high latitude
lower the oxygen
atmospheric input and
content
subsurface transport by ocean
currents
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
Vertical temperature structure
Stratification in
the thermocline
0m
250m
500m
20°S
1000
0m
100m
1500
0°C
10°C
20°C
Temperatures
30°C
500m
10°S
0°
10°N
20°
•
Change in Surface salinity
Increased rainfall
in western Pacific
causes freshening
of surface waters
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):
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•
•
2000: 29.6°C
2035: 30.5°C (+0.8°C);
2100: 32.2°C (+2.6°C) ;
Projected stratification
Warming is surface
intensified
This leads to
widespread increase
in stratification
Change in 0-200m density
Projected Change in Surface Currents (0-50m)
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•
Large decrease in equatorial surface current
Large decrease in counter currents
Vertical temperature structure
Stratification in
the thermocline
0m
250m
500m
20°S
1000
0m
100m
1500
0°C
10°C
20°C
Temperatures
30°C
500m
10°S
0°
10°N
20°
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
Projected Ocean
Projected Ocean
Implication for nutrients
Dissolved nitrate at
100m
Nutrients are mostly
depleted in the euphotic
zone
Replenishment by decay of
sinking organic material
DEPTH
Oceanic transport is
needed to transfer
them to the surface
layer
Similar features for phosphate & silicate
Implication for nutrients
25°
S
DEPTH
25°
S
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
Similar features for phosphate & silicate
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
Vertical structure and stratification
Warming is surface
intensified
This leads to
widespread increase
in stratification
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