Observed and projected
changes to the tropical Pacific
Ocean, Part 2
Presented by
Alexandre Ganachaud and Alex Sen Gupta
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
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
Outline
• Projected temperatures and currents:

surface and vertical structure
• Implications for oceanic nutrients and
oxygen
• Acidification
• Influence on tuna distribution
Projected changes to CO2
Projected changes to CO2
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
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
New models are similar
IPCC-AR-4 (2007)
IPCC-AR-5
Vertical structure and stratification
Thermocline depth
Vertical structure and stratification
• Warming is surface
intensified
• This leads to
widespread increase
in stratification
Change in 0-200m density
Projected change in currents
• Increase in Equatorial Undercurrent , New Guinea
Coastal Undercurrent and South Equatorial Current
(SEC); Decrease in SEC at surface near equator
Projected changes in vertical currents
• Upwelling
along the
equator
decreases
Less downwelling
Less UPWELLING
Less downwelling
• Downwelling
on both sides
of the equator
decreases
Implications for nutrients
Dissolved nitrate at 100 m
Depth
• Nutrients are mostly depleted
in the euphotic zone
• Replenishment by decay of
sinking organic material
• Oceanic transport is
needed to transfer
nutrients to the
surface layer
Similar features for phosphate & silicate
Implications for nutrients
25°
S
25°
N
Dissolved nitrate at 100 m
Dissolved nitrate at 100 m
Depth
• Nutrients are mostly depleted
in the euphotic zone
• Replenishment by decay of
sinking organic material
• Oceanic transport is
needed to transfer
nutrients to the
surface layer
Similar features for phosphate & silicate
Nutrient supply to sun-lit depths
??
• Upwelling (vertical
current; east equator
and some islands)
• Eddies
• Vertical mixing from wind
• Mixing from tides
• ... against stratification
Eddies and land effects
• Small-scales generated
spontaneously or by
interaction between
the large-scale flow
and land
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
Future acidification increase
• Past and present
aragonite
saturation
Future acidification increase
2040
2100
• Aragonite saturation
is expected to fall
below 3.3 by 2040
• Aragonite saturation
expected to
decrease to 2.4 in
2100
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)
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
Possible consequences for tuna
• Skipjack preferred temperature habitat
30oC
17oC
Possible consequences for tuna
• Projected warming means temperatures
become too warm in the western Pacific
30oC
17oC
Oceanic variability will matter!
• Tides (h)
• Storms (day)
• Ocean eddies (week)
• Seasons
• El Nino (2-5 years)
• Decadal variations (1050 years and more)
• Warming (100yr)
Source: J. Lefèvre, IRD
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