Oceanography of the tropical Pacific Ocean Part 2 ... · on the tropical Pacific Ocean •...
Transcript of Oceanography of the tropical Pacific Ocean Part 2 ... · on the tropical Pacific Ocean •...
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Oceanography of the tropical Pacific Ocean ‐Part 2‐
Historical changes and future projections
A. Ganachaud1, A. Sen Gupta2, J. Brown3, L. Muir3,
with contributions from 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 Caledonia2 Climate modeling expert,
Centre for Climate Change Research,
University of New South Wales,
Sydney, Australia3 Climate modeling experts,
Centre for Australian Weather and Climate Research,
CSIRO Wealth from Oceans National Research Flagship
Hobart, Tasmania, Australia
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Temperature trends
Cravatte et al. 2009
Guyennon et al. 2011, submitted
trend (°C/50yr)
Time series from an in situ sensor in Noumea
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Temperature trends
trend (°C/50yr) Extension of the Warm Pool (10yr average)
19601970198019902000
Cravatte et al. 2009
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Salinity trends
trend (°C/50yr) Extension of the Warm Pool (10yr average)
1970198019902000
Cravatte et al. 2009
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Outline
State of the Ocean1. Provinces2. Currents3. Warming and stratification4. Vertical structure 5. Nutrients and oxygen6. Nutrient supply processes7. Waves and Sea Level Rise
Observed and Projected changes
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Ocean state: Trade Winds and Warm Pool
Classical view :The Trade Winds pile up warm
waters in the west
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Ocean state: Oceanic provinces ("biomes")
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Pacific Ocean circulation
Source: Fieux, M. 2010
Ocean currents transport nutrients, oxygen and fishlarvae
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Ocean state: currents
• Winds create two broadwestward flows in the tropical Pacific
SouthEquatorial
Current
NorthEquatorial
Current
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Ocean state: currents
• Winds create two broadwestward flows in the tropical Pacific
• ITCZ and SPCZ influence on the wind field give rise to two eastward countercurrents
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Ocean state: currents
• Winds create two broadwestward flows in the tropical Pacific
• ITCZ and SPCZ influence on the wind field give rise to two eastward countercurrents
• Archipelagoes and coastslead to strong north‐southcoastal currents
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Ocean state: TemperatureVertical Structure
Temperature acrossthe Equator
100m
500m
0m
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Ocean state: TemperatureVertical Structure
Presence of stratification in the thermocline
100m
500m
0m
0m
250m
500m
0°C 10°C 20°C 30°CTemperatures
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Ocean state: Nutrients
DE
PTH
Nutrients are mostly depleted in the euphotic zone
Replenishment by remineralization of marine snow
Dissolved nitrate at 100m
Oceanic upwelling or mixing isneeded to transfer them to the surface layer
Similar features for phosphate & silicate
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Ocean state: Oxygen
DE
PTH
Oxygen is abundant near the surface and depletednear 400m
Replenishment by high latitude atmospheric input and subsurface transport by ocean currents
Dissolved Oxygen at 400m
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Ocean state: how to supply the euphotic zone ?
1. Eddies2. Upwelling (vertical current; east
equator and some islands)3. Internal tides4. Mixed layer... against stratification
??
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Ocean state: Eddies
Small‐scale circulation generatedspontaneously or by interaction between the large‐scale flow and land
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Ocean state: Eddies
Small‐scale circulation generatedspontaneously or by interaction between the large‐scale flow and land
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Ocean state: Eddiesand land effects
Small‐scales generatedspontaneously or by interaction betweenthe large‐scale flow and land
Boundary currents
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Small‐scales generatedspontaneously or by interaction betweenthe large‐scale flow and land
Boundary currentsUpwelling
Ocean state: Eddiesand land effects
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Small‐scales generatedspontaneously or by interaction betweenthe large‐scale flow and land
Boundary currentsUpwellingVertical mixing for
internal tides
Ocean state: Eddiesand land effects
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Seasonal variations of the mixed layer depthpumps deep nutrientstowards the sunlitzone
Ocean state: Mixed layer
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Variability
Seasonal
El Nino
PDO
Eddies
Internaltides
Courtesy J. Lefèvre, IRD
Global Warming
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Global Warming
What changes were detectedover the past ~50 years ?
What do IPCC projections suggest ?
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•SEC strengthens in the south subtropical gyre•EAC strengthens•Little change in the north
Change in Sea Level (Roemmich, pers. Comm.)
Ocean currents
Observed changes to the circulation
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•SEC weakens on the equator•EUC moves upward•Eastward SECC weakens•Little change in the subtropical gyres
Changes 2100/A2 versus 2000Average over 13 IPCC projections
Ocean currents
Projected changes to the circulation
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Temperature: recent changes at the surface
Surface Temperature trend over past 50yr
Cravatte et al., 2009
Tropical warming of 0.1°C/decade and Warm Pool expansion
Climatological average
Warm Pool
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Temperature: recent changes ... and surface projections
Multi-model projected temperature change(13 IPCC models; A2/2100 versus
1980-2000)
Warming ~2.5°CWeaker warming in the
southeast Pacific
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Temperature: recent changes ... and surface projections
Multi-model projected temperature changeadded to 20th century observations
"Warm pool" as per 28°C definition
Interannual variability
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Temperature: recent changes at the surface
Surface Temperature trend over past 50yr
Cravatte et al., 2009
Tropical warming of 0.1°C/decade and Warm Pool expansion
Climatological average
Warm Pool
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Temperature: recent changes at depths
Temperature change over past 50yr(Durack & Wijffels, 2010)
Contours are average temperature
Warming ~1°C down to 100‐200m
Weaker warming or coolingbelow the thermocline
‐> Enhanced stratification
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o Surface water becomes ‘lighter’ more than deep water, due to: Surface warming Surface freshening
o Increased stratification inhibits mixing
Observed change in density with depth
Warm, ‘light’ water
Cold, ‘heavy’ water
Nutrient supply recent changes ... and projections
Courtesy A. S. Gupta
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Temperature: recent changes ... and projections
Multi-model projected temperature change(13 IPCC models; A2/2100 versus
1980-2000)
More warming ~2°C down to 80m
Weaker warming or cooling belowthe thermocline
Even more stratification
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oOver the past 20 years: Only two time series
oOne suggests a decrease, the second one no trend
oToo few data to be conclusive ! (need continued observations!)
Nutrient supply recent changes
Phosphate concentration in the mixed layer(Watanabe et al. 2005)
Observed changes in nutrientconcentrations
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Nutrient supply recent changes ... and projections
Phosphate concentration in the mixed layer(Watanabe et al. 2005)
What controls nutrient concentration:
oBiological activityoSupply to the euphotic (sunlit)
zone from deep ocean:oStratificationoOcean currentsoTurbulence in the mixed layeroUpwellingoEddies
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oStratification increases by 20-30%; especially in the Warm Pool
oThe winter mixed layer shallows by ~20m;
oEquatorial upwelling decreases, but region-wide 9°S-9°N upwelling remains constant.
oEquatorial UnderCurrent reinforced
Nutrient supply recent changes ... and projections
Projections (2100/A2):
Suggest reduced nutrient supply and therefore biological activity
}Reinforcement possible in the PEQD(Polovina et al.)
}
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o More oxygen data than nutriento Major decrease of dissolved oxygen in
the remineralization zone with westward extension of the oxygen minimum zone (OMZ)
Dissolved oxygen recent changes
Stramma et al. 2008
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Dissolved oxygen concentration is expected to continue to decline due to high latitude ocean warming
Low oxygen areas are expected to expand
Dissolved oxygen recent changes ... and projections
Stramma et al. 2008
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Two more actors: Waves and Sea Level
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Wave climate
Projected increase (or decrease)to significant wave height Wang and Swail, 2006
•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
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Sea level rise
•As water warms it expands•As ice-sheets and glaciers melt they increase ocean volume
+20 cm over 100yrsIncreasing sea level:•Alters oceanic ecosystems/habitat•Changes shape of coastlines•Changes nature and extent of mangrove
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Sea level rise
•New published estimate suggest IPCC AR-4 was too conservative: +80 cm to +1.4m possible•Regional deviations from global mean are of O(5cm)+20 cm
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Conclusions on the tropical Pacific Ocean
• Ocean is warming, withenhanced signal in the upper 100‐200m
• More stratification limitsnutrient supply;
• Mixed layer reach is reduced• Some ocean currents
projected to change;• Equatorial divergence region
projected to shrink
• Ocean is warming, withenhanced signal in the upper 100‐200m
• More stratification limitsnutrient supply;
• Mixed layer reach is reduced• Some ocean currents
projected to change;• Equatorial divergence region
projected to shrink
• Dissolved oxygen decreases; ocean becomes more aciditic
• Sea level has risen by ~20 cm;
• Sea level could rise by another 80‐140 cm
• Dissolved oxygen decreases; ocean becomes more aciditic
• Sea level has risen by ~20 cm;
• Sea level could rise by another 80‐140 cm