Observed and projected changes to the tropical Pacific
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 20 yrs Regions of low oxygen appear to be expanding Additional CO 2 in the surface ocean has led to reduced p. H Climate models successfully simulate many characteristics of the climate system They have limitations and must be used with care
Outline: Ocean projection for A 2 / 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 A 2 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
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 } } } Warm, mixed-layer Thermocline depth Cold, deen ocean
Vertical structure and stratification } } } 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>4 healthy conditions saturation>3. 3 marginal conditions
Future acidification increase • • Aragonite saturation is expected to fall below 3. 3 by 2040 (A 2 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 30 o. C 17 o. C
Consequences on tuna • Projected warming means temperatures become too warm in the western Pacific 30 o. C 17 o. C
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 (100 yr) 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 10 -20 m
DEPTH Oxygen replenishment at depths Dissolved Oxygen at 400 m Oxygen is abundant near the surface Higher surface and depleted near 400 m temperatures at high latitudes will generally Replenishment by high latitude lower the oxygen atmospheric input and subsurface content 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 thermocline 0 m 250 m 500 m 20°S 1000 0 m 100 m 1500 0°C 10°C 20°C Temperatures 30°C 500 m 10°S 0° 10°N 20°N
• 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): • • • 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 -200 m density
Projected Change in Surface Currents (0 -50 m) • • Large decrease in equatorial surface current Large decrease in counter currents
Vertical temperature structure Stratification in thermocline 0 m 250 m 500 m 20°S 1000 0 m 100 m 1500 0°C 10°C 20°C Temperatures 30°C 500 m 10°S 0° 10°N 20°N
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 100 m 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 100 m 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 -200 m 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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