5.3 Regional Changes in Ocean Circulation and Water Masses
Robust long-term trends in global- and basin-scale ocean heat content and basin-scale salinity were shown in Section 5.2. The observed heat and salinity trends are linked to changes in ocean circulation and other manifestations of global change such as oxygen and carbon system parameters (see Section 5.4). Global ocean changes result from regional changes in these properties, assessed in this section. Evidence for change in temperature, salinity and circulation is described globally and then for each of the major oceans. Two marginal seas with multi-decadal time series are also examined as examples of regional variations.
The upper ocean in all regions is close to the atmospheric forcing and has the largest variability; it is also the best sampled. For these reasons, Section 5.2 mainly assessed upper-ocean observations for long-term trends in heat content and salinity. However, there are important changes in heat and salinity at intermediate and abyssal depths, restricted to regions that are relatively close to the main sources of deep and intermediate waters. These sources are most vigorous in the northern North Atlantic and the Southern Ocean around Antarctica. This is illustrated well in salinity differences shown for the Atlantic (1985–1999 minus 1955–1969) and Pacific (1980s minus 1960s) in Figure 5.6. Striking changes in salinity are found from the surface to the bottom in the northern North Atlantic near water mass formation sites that fill the water column (Section 5.3.2); bottom changes elsewhere are small, being most prevalent at the under-sampled southern ends of both sections. At mid-depth (500 to 2,000 m), the Atlantic and southern end of the Pacific section show widespread change, but the North Pacific signal is weaker and shallower because it has only weak intermediate water formation (and no deep water formation). Changes in intermediate and deep waters can ultimately affect the ocean’s vertical stratification and overturning circulation; the topic of the overturning circulation in the North Atlantic is considered in Section 5.3.2.
The observed changes in salinity are of global scale, with similar patterns in different ocean basins (Figure 5.6). The subtropical waters have increased in salinity and the subpolar surface and intermediate waters have freshened in both the Atlantic and Pacific Oceans during the period from the 1960s to the 1990s and in both hemispheres in each ocean. The waters that underlie the near-surface subtropical waters have freshened due to equatorward circulation of the freshened subpolar surface waters; in particular, the fresh intermediate water layer (at ~1,000 m) in the SH has freshened in both the Atlantic and Pacific Oceans. In the Northern Hemisphere (NH), the Pacific intermediate waters have freshened, and the underlying deep waters did not change, consistent with no local bottom water source in the North Pacific. In the central North Atlantic, the intermediate layer (approximately 900–1,200 m) became saltier due to increased salinity in the outflow from the Mediterranean that feeds this layer.
Figure 5.6. Meridional sections of differences in salinity (psu) of the a) Atlantic Ocean for the period 1985 to 1999 minus 1955 to 1969 and b) Pacific Ocean for the World Ocean Circulation Experiment (WOCE) 150°W section (1991–1992) and historical data from 1968 plus or minus 7.5 years. Contours are the mean salinity fields along each section and show the key features. The salinity differences are differences along isopycnals that have been mapped to pressure surfaces. The Atlantic section is along the western side of the Atlantic Ocean and the Pacific section is along 150°W. The two figures are redrafted from Curry et al. (2003) and Wong et al (2001). Water masses shown include Antarctic Intermediate Water (AAIW), Circumpolar Deep Water (CDW), North Atlantic Deep Water (NADW), Mediterranean Water (MW), Labrador Sea Water (LSW), Denmark Strait Overflow Water (DSOW) and North Pacific Intermediate Water NPIW). The areas shaded in grey represent the seafloor and oceanic crust.