Projected changes to the tropical Pacific Ocean

Based on.....

Outline Projected temperatures and currents: surface and vertical structure Implications for oceanic nutrients Ocean acidification Sea-level rise

Sea surface temperature Change in SST °C (2000-2100) 2000: 27.4°C 2035: 28.1°C (+0.7°C) 2100: 29.9°C (+2.5°C) The ocean is a heat sink!

Vertical structure and stratification 5 Vertical structure and stratification } Warm, mixed-layer } Thermocline depth } Cold, deep ocean

Vertical structure and stratification 6 Vertical structure and stratification } Warm, mixed-layer } Thermocline depth } Cold, deep ocean

Vertical structure and stratification 7 Vertical structure and stratification Most warming occurs at surface Leads to increase in stratification

Projected changes in vertical currents Less downwelling Less downwelling Less UPWELLING Less downwelling

Implications for nutrients Warm, mixed-layer Thermocline acts as a barrier Limits the supply of nutrients Cold, deep ocean Low nutrient Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface High nutrient

Implications for nutrients Warm, mixed-layer Cold, deep ocean Low nutrient Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface Increased stratification makes it harder to bring nutrients upwards High nutrient

Why is this important? Nutrients support the food web for tuna

Projected changes in major currents Pacific Current system Very stable; the currents are projected to remain similar in the future However their strength is projected to change, especially near the equator. For instance, the equatorial undercurrent, in blue … Iron

Eddies Amount of eddies related to strength of currents

Nutrient supply by eddies Eddies can temporarily lift nutrient-rich waters Eddy activity is related to current strength

Eddies and nutrients

Convergence zone

Importance of ENSO for fisheries Skipjack tuna catch Source: Lehodey et al. (1997)

Ocean acidification

Projected acidification Saturation >4 = healthy conditions Saturation falls below 3.3, causing problems for 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 Increases in sea surface temperature Stronger stratification Nutrient reduced due to increase in stratification Slowdown of equatorial currents and upwelling Aragonite drops below critical levels Sea level rise, > 1 m cannot be ruled out