Chapter 30
The Ocean
As heat has accumulated in the Ocean there has been a 4% increase in thermal stratification of the upper layers in most ocean regions (0 to 200 m, 40-year record) north of 40°S (WGI AR5 Section 3.2.2). Increasing thermal stratification has reduced ocean ventilation and the depth of mixing in many ocean sub-regions (medium confidence; WGI AR5 Section 3.8.3). This in turn reduces the availability of inorganic nutrients and consequently primary productivity (medium confidence; Section 6.3.4). In the STG, which dominate the three major ocean basins (Section 30.5.6), satellite-derived estimates of surface chlorophyll and primary production decreased between 1999 and 2007 (Box CC-PP). In contrast,
however, in situ observations at fixed stations in the North Pacific and North Atlantic Oceans (Hawaii Ocean Time-series (HOT) and Bermuda Atlantic Time-series Study (BATS)) showed increases in nutrient and chlorophyll levels and primary production over the same period, suggesting that other processes (e.g., ENSO, PDO, North Atlantic Oscillation (NAO), winds, eddies, advection) can counteract broad-scale trends at local scales (Box CC-PP). The continued warming of the surface layers of the Ocean will very likely further enhance stratification and potentially limit the nutrient supply to the euphotic zone in some areas. The response of upwelling to global warming is likely to vary between regions and represents a complex interplay between local and global variables and processes (Box CC-UW).
(a) Climatological-mean sea surface salinity (1955–2005)
(b) Annual mean evaporation–precipitation (1950–2000)
30.3.1.6. Thermal Stratification
30
Evaporation–precipitation average (m/yr-1)
Practical Salinity Scale of 1978 32
33
34
35
36
37
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(c) The 58-year (2008 minus 1950) sea surface salinity change
–2
–0.1
0
0.1
0.2
–2
–1
0
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(d) The 30-year (2003–2007 average centered at 2005, minus the 1960–1989 average centered at 1975) sea surface salinity difference
∆ Practical Salinity Scale of 1978 –0.5 –0.4 –0.3
–3
∆ Practical Salinity Scale of 1978 0.3
0.4
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white areas = areas where calculations were not carried out gray stippling = change is not significant at the 99% confidence level
–0.25 –0.5 –0.15 –0.1 –0.05
0
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0.1 0.15
0.2
0.25
white areas = areas where calculations were not carried out gray stippling = change is not significant at the 99% confidence level
Figure 30-6 | (a) The 1955–2005 climatological-mean sea surface salinity (Antonov et al., 2010) color contoured at 0.5 Practical Salinity Scale 1978 (PSS78) intervals (black lines). (b) Annual mean evaporation-precipitation averaged over the period 1950–2000 (National Centers for Environmental Prediction (NCEP)) color contoured at 0.5 m yr–1 intervals (black lines). (c) The 58-year (2008 minus 1950) sea surface salinity change derived from the linear trend (PSS78), with seasonal and El Niño-Southern Oscillation (ENSO) signals removed (Durack and Wijffels, 2010) color contoured at 0.116 PSS78 intervals (black lines). (d) The 30-year (2003–2007 average centered at 2005, minus the 1960–1989 average centered at 1975) sea surface salinity difference (PSS78) (Hosoda et al., 2009) color contoured at 0.06 PSS78 intervals (black lines). Contour intervals in (c) and (d) are chosen so that the trends can be easily compared, given the different time intervals in the two analyzes. White areas in (c) and (d) are marginal seas where the calculations are not carried out. Regions where the change is not significant at the 99% confidence level are stippled in gray. Figure originally presented as WGI AR5 Figure 3.4. All salinity values quoted in the chapter are expressed on the Practical Salinity Scale 1978 (PSS78) (Lewis and Fofonoff, 1979).
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