A. Ganachaud 1 , A. Sen Gupta 2 , J. Orr, S. Wijffels, K. Ridgway, M.
Hemer, C. Maes, C. Steinberg, A. Tribollet, B. Qiu, J. Kruger
1 Oceanographer , Institut de Recherche pour le Développement,
Nouméa, New Caledonia
2 Climate modeling expert , Centre for Climate Change Research,
University of New South Wales, Sydney, Australia
SPC, March 3, 2011 1

State of the Ocean
• Provinces
• Currents
• Nutrients and oxygen
• Vertical structure
• Physical processes
Outline
Projected changes
• Currents
• Warming and stratification
• Waves & Sea level rise
SPC, March 3, 2011 2

Ocean state:
Trade Winds and Warm Pool
Classical view :
The Trade Winds pile up warm waters in the west
SPC, March 3, 2011 3

SPC, March 3, 2011
Ocean state:
Oceanic provinces
4

SPC, March 3, 2011
Ocean state: currents
North
Equatorial
Current
• Winds create two broad westward flows in the tropical
Pacific
South
Equatorial
Current
5

SPC, March 3, 2011
Ocean state: currents
• Winds create two broad westward flows in the tropical
Pacific
• ITCZ and SPCZ influence on the wind field give rise to two eastward counter currents
6

SPC, March 3, 2011
Ocean state: currents
• Winds create two broad westward flows in the tropical
Pacific
• ITCZ and SPCZ influence on the wind field give rise to two eastward counter currents
• Archipelagoes and coasts lead to strong north-south coastal currents 7

SPC, March 3, 2011
Ocean state: currents
• Ocean currents transport nutrients, oxygen and fish larvae
8

SPC, March 3, 2011
Ocean state: Temperature
Vertical Structure
Temperature across the Equator
0m
100m
500m
9

Ocean state: Temperature
Vertical Structure
0m
250m
500m
0 ° C 10 ° C 20 ° C
Temperatures
30 ° C
0m
100m
500m
SPC, March 3, 2011
Presence of stratification in the thermocline
10

Ocean state: Nutrients
Dissolved nitrate at 100m
Nutrients are mostly depleted in the euphotic zone
Oceanic upwelling or mixing is needed to transfer them to the surface layer
Replenishment by remineralization of marine snow
Similar features for phosphate & silicate
SPC, March 3, 2011 11

Ocean state: Oxygen
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
SPC, March 3, 2011 12

Ocean state: how to supply the euphotic zone ?
??
1. Upwelling (vertical current; east equator and some islands)
2. Eddies
3. Mixed layer
4. Internal tides
... against stratification
SPC, March 3, 2011 13

SPC, March 3, 2011
Ocean state: Eddies
Small-scale circulation generated spontaneously or by interaction between the large-scale flow and land
14

SPC, March 3, 2011
Ocean state: Eddies
Small-scale circulation generated spontaneously or by interaction between the large-scale flow and land
15

SPC, March 3, 2011
Ocean state: Eddies and land effects
Small-scales generated spontaneously or by interaction between the large-scale flow and land
Boundary currents
16

SPC, March 3, 2011
Ocean state: Eddies and land effects
Small-scales generated spontaneously or by interaction between the large-scale flow and land
Boundary currents
Upwelling
17

SPC, March 3, 2011
Ocean state: Eddies and land effects
Small-scales generated spontaneously or by interaction between the large-scale flow and land
Boundary currents
Upwelling
Vertical mixing for internal tides
18

SPC, March 3, 2011
Ocean state: Mixed layer
Seasonal variations of the mixed layer depth pumps deep nutrients towards the sunlit zone
19

Variability
SPC, March 3, 2011
Courtesy J. Lefèvre, IRD
Global
Warming
PDO
El Nino
Seasonal
Eddies
Internal tides
20

SPC, March 3, 2011
Global Warming
What changes were detected over the past ~50 years ?
What do IPCC projections suggest ?
21

Global Warming: Ocean Models
Temperature
(27 ° C)
Salinity (34 ppt)
Current
Laws of Physics
Courtesy A. S. Gupta
SPC, March 3, 2011
Grid Size
22

Global Warming: Ocean Models
SPC, March 3, 2011 23

Global Warming: Ocean Models
How an Ocean model sees the ocean …
Grid Size
SPC, March 3, 2011 24

Global Warming: Ocean Models
Grid Size
The grid box only contains information for the average current
SPC, March 3, 2011
Courtesy A. S. Gupta
25

Global Warming: Ocean Models
How an Ocean model sees the ocean …
Grid Size
The grid box only contains information for the average temperature
SPC, March 3, 2011
Courtesy A. S. Gupta
26

Global Warming: Ocean Models
How an Ocean model sees the ocean …
SPC, March 3, 2011
Coarse resolution  o Broad features are captured
But: o Cannot see small islands o Cannot see fine scale circulation o These features may be important for island processes... and large scale features
Courtesy A. S. Gupta
27

Global Warming: Ocean Models
How an Ocean model sees the ocean …
Grid box size in the different models range from about 1 ° to 5 °
SPC, March 3, 2011
Courtesy A. S. Gupta
28

Ocean Models: are they any good ?
Observations (HadISST)
Surface Temperature
All the models are able to capture the main features of the Pacific Ocean:
Average of 19 models
SPC, March 3, 2011
Courtesy A. S. Gupta
29

Ocean Models: are they any good ?
Observations (HadISST)
Surface Temperature
All the models are able to capture the main features of the Pacific Ocean: o Warm Pool
Average of 19 models
SPC, March 3, 2011
Courtesy A. S. Gupta
30

Ocean Models: are they any good ?
Observations (HadISST)
Surface Temperature
All the models are able to capture the main features of the Pacific Ocean: o Warm Pool o Cold Tongue
Average of 19 models
SPC, March 3, 2011
Courtesy A. S. Gupta
31

Ocean Models: are they any good ?
Observations (HadISST)
Surface Temperature
All the models are able to capture the main features of the Pacific Ocean: o Warm Pool o Cold Tongue o S. American upwelling
Average of 19 models
But in most models: o Warm pool is not warm enough o Cold tongue extends too far west
SPC, March 3, 2011
Courtesy A. S. Gupta
32

Projected changes to the circulation
Ocean currents
•SEC weakens on the equator
•EUC moves upward
•Eastward SECC weakens
•Little change in the subtropical gyres
Changes 2100/A2 versus 2000
Average over 13 IPCC projections
SPC, March 3, 2011 33

Temperature: recent changes
Warming ~1 ° C down to
100-200m
Weaker warming or even cooling below the thermocline
Enhanced stratification
Temperature change over past 50yr
(Durack & Wijffels, 2010)
Contours are average temperature
SPC, March 3, 2011 34

Temperature: recent changes
... and projections
More warming ~2 ° C down to 80m
Weaker warming or cooling below the thermocline
Even more stratification
Multi-model projected temperature change
(13 IPCC models; A2/2100 versus
1980-2000)
SPC, March 3, 2011 35

Nutrient supply recent changes
Observed changes in nutrient concentrations o Over the past 20 years: Only two time series o One suggests a decrease, the second one no trend o Too few data to be conclusive !
SPC, March 3, 2011
Phosphate concentration in the mixed layer
(Watanabe et al. 2005)
36

Nutrient supply recent changes
... and projections
What controls nutrient concentration: o Biological activity o Supply to the euphotic (sunlit) zone from deep ocean: o Stratification o Ocean currents o Turbulence in the mixed layer o Upwelling o Eddies Phosphate concentration in the mixed layer
(Watanabe et al. 2005)
SPC, March 3, 2011 37

Nutrient supply recent changes
... and projections
Projections (2100/A2): o Stratification increases by 20-30%; especially in the Warm Pool
SPC, March 3, 2011 38

Nutrient supply recent changes
... and projections
Projections (2100/A2): o Stratification increases by 20-30%; especially in the Warm Pool o The winter mixed layer shallows by
~20m; o Equatorial upwelling decreases, but region-wide 9 ° S-9 ° N upwelling remains constant.
SPC, March 3, 2011 39

Nutrient supply recent changes
... and projections
Projections (2100/A2): o Stratification increases by 20-30%; especially in the Warm Pool o The winter mixed layer shallows by
~20m; o Equatorial upwelling decreases, but region-wide 9 ° S-9 ° N upwelling remains constant.
All factors suggest reduced nutrient supply and therefore biological activity
SPC, March 3, 2011 40

Dissolved oxygen recent changes
Stramma et al. 2008 o More oxygen data than nutrient o Major decrease of dissolved oxygen in the remineralization zone with westward extension of the oxygen minimum zone (OMZ)
SPC, March 3, 2011 41

Dissolved oxygen recent changes
... and projections
Stramma et al. 2008
Dissolved oxygen concentration is expected to continue to decline due to high latitude ocean warming
Low oxygen areas are expected to expand
SPC, March 3, 2011 42

Acidification: recent changes
Change in surface pH for last 10yrs
When additional CO
2 dissolves in the ocean, ocean acidity rises.
This makes it more difficult for corals and certain phytoplankton species to grow
-0.06 pH units
([H + ]decreased by 30%)
From: Pacific Science Association, 2007
SPC, March 3, 2011 43

Acidification: recent changes
... and projections
Change in surface pH for last 10yrs
•Another decrease of
0.2 to 0.3 pH units is expected
•The aragonite saturation horizon is expected to shallow to
150m depth
From: Pacific Science Association, 2007
SPC, March 3, 2011 44

SPC, March 3, 2011
Two more actors:
Waves and Sea Level
45

Projected increase (or decrease) to significant wave height
Wang and Swail, 2006
SPC, March 3, 2011
Wave climate
•Too few observations to determine CC trends
•The wave "climate" is related to ENSO and other climate signals
•Only few projections; not resolved by IPCC models
46

Sea level rise
•As water warms it expands
•As ice-sheets and glaciers melt they increase ocean volume
+20 cm over 100yrs
Increasing sea level:
•Alters oceanic ecosystems/habitat
•Changes shape of coastlines
•Changes nature and extent of mangrove
SPC, March 3, 2011 47

SPC, March 3, 2011
Sea level rise
+20 cm
•New published estimate suggest IPCC AR-4 was too conservative: +1 m to +2 m possible
•Regional deviations are of
O(5cm)
48

Conclusions on the tropical Pacific Ocean
• Ocean is warming, with enhanced signal in the upper 100-200m
• Stratification limits nutrient supply; mixed layer reach is reduced
• Some ocean currents have changed and will change;
Equatorial divergence region projected to shrink
• Dissolved oxygen decreases; ocean becomes more aciditic
• Sea level has risen by
~20 cm; could rise by another 1-
2 m
SPC, March 3, 2011 49

SPC, March 3, 2011
QuickTime™ and a
decompressor are needed to see this picture.
50

SPC, March 3, 2011
Extra information
51

Nutrient supply recent changes
... and projections
Multi model change in density with depth
Warm, ‘light’ water
1990 o Surface water becomes ‘lighter’ more than deep water, due to:
 Surface warming
 Surface freshening o Increased stratification inhibits mixing
2090
Cold, ‘heavy’ water
SPC, March 3, 2011
Courtesy A. S. Gupta
52

Nutrient supply recent changes
... and projections
Multi model change in density with depth
Light available
Biology
Low nutrients
1990 o Surface water becomes ‘lighter’ faster than deep water, due to:
 Surface warming
 Surface freshening o Increased stratification inhibits mixing
2100
Barrier to mixing o Less mixing means less nutrients can be brought to surface
No light
No biology
High nutrients
SPC, March 3, 2011
Courtesy A. S. Gupta
53

Ocean Models – are they any good?
Observations (HadISST)
ENSO
Most models shows
ENSO-like behavior.
Average of 18 models
But
Different degrees of realism (too strong, too weak, too periodic, wrong season)
Almost all models have
ENSO warm patch too far west
SPC, March 3, 2011 54

Ocean Models – are they any good?
Observations (CARS06) Surface Salinity
Averaging across the different models the general observed pattern is captured
Average of 13 models
SPC, March 3, 2011 55

Pacific Ocean circulation
SPC, March 3, 2011
Source: Fieux, M. 2010
56