Projected changes to the tropical Pacific Ocean

Based on Chapters 3 & 4

Outline

• Projected temperatures and currents: surface and vertical structure

• Implications for oceanic nutrients

• Acidification

• Food web for tuna

• Influence of temperature on tuna

Change in SST °C (2000-2100)

• 2000: 27.4°C• 2035: 28.1°C (+0.7°C)• 2100: 29.9°C (+2.5°C)

Sea surface temperature

Ocean is heat sink!

Thermocline depth

Vertical structure and stratification

Warm, mixed-layer

Cold, deep ocean

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Vertical structure and stratification

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• Most warming occurs at surface

• Leads to increase in stratification

Vertical structure and stratification

Projected changes in vertical currents

Less downwelling

Less UPWELLINGLess downwelling

Less downwelling

Implications for nutrients

• Thermocline acts as a barrier

• Limits the supply of nutrients

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient

Implications for nutrients

Increased stratification makes it harder to bring nutrients upwards

Warm, mixed-layer

Cold, deep ocean

Low nutrient

High nutrient

Why is this important?

Nutrients support the food web for tuna

Image: Marc Taquet, FADIO, IRD/IFREMER

Projected changes in major currents

Eddies

Nutrient supply by eddies

•Eddies temporarily lift the nutrient-rich waters

•Eddy activity is related to current strengths

Eddies and nutrients

Convergence zone

Similarities with ENSO

Skipjack tuna catch

Source: Lehodey et al. (1997)

Ocean acidification

Projected acidification

Saturation >4 healthy conditions

Saturation falls below 3.3 causing problems for some corals

Saturation decreases to 2.4

Sea level rise

Projection 2035 2050 2100

IPCC 8 cm 18-38 cm 23-51 cm

Semi-empirical 20-30 cm 70-110 cm 90-140 cm

Summary•I

ncreases in sea surface temperature

•Stronger stratification

•Nutrient supply reduced due to increase in stratification, with effects on tuna food web

•Slowdown of equatorial currents and upwelling

•Aragonite drops below critical levels

•Sea level rise, > 1 m cannot be ruled out