To a large degree, water properties are set at the sea surface through interaction between the ocean and the overlying atmosphere (and ice, in polar regions). The water characteristics resulting from these interactions (e.g., temperature, salinity and concentrations of dissolved gases and nutrients) are transferred to various depths in the world ocean, depending on the density of the water. Warm, light water masses supply (or “ventilate”) the upper ocean at low to mid-latitudes, while the colder, denser water masses formed at higher latitudes supply the intermediate and deep layers of the ocean (see schematic in FAQ 3.1, Figure 1). The formation and subduction of water masses are important for the ocean’s capacity to store heat, freshwater, carbon, oxygen and other properties relevant to climate. In this section, the evidence for change in some of the major water masses of the world ocean is assessed.

The zonal-mean distributions of salinity, density, and temperature in each ocean basin (black contours in Figure 3.9) reflect the formation of water masses at the sea surface and their subsequent spreading into the ocean interior. For example, warm, salty waters formed in the regions of net evaporation between 10° and 30° latitude (Figure 3.4b) supply the subtropical salinity maximum waters found in the upper few hundred meters in each basin (Figure 3.9). Relatively fresh water masses produced at higher latitude, where precipitation exceeds evaporation, sink and spread equatorward to form salinity minimum layers at intermediate depths. Outflow of saline water from the Mediterranean Sea and Red Sea, where evaporation is very strong, accounts for the relatively high salinity observed in the upper 1000 m in the subtropical North Atlantic and North Indian basins, respectively.

Many of the observed changes in zonally averaged salinity, density and temperature are aligned with the spreading paths of the major water masses (Figure 3.9, trends from 1950 to 2000 shown in colours and white contours), illustrating how the formation and spreading of water masses transfer anomalies in surface climate to the ocean interior. The strongest anomalies in a water mass are found near its source region. For instance, bottom and deep water anomalies are strongest in the Southern Ocean and the northern North Atlantic, with lessening amplitudes along the spreading paths of these water masses. In each basin, the subtropical salinity maximum waters have become more saline, while the low-salinity intermediate waters have become fresher (Figure 3.9 a, d, g, j; see also Section 3.3). Strongest warming is observed in the upper 100 m, which has warmed almost everywhere, with reduced warming (Atlantic) or regions of cooling (Indian and Pacific) observed between 100 and 500 m depth.

Warming is observed throughout the upper 2000 m south of 40°S in each basin. Shifts in the location of ocean circulation features can also contribute to the observed trends in temperature and salinity, as discussed in Section 3.2. Density decreased throughout most of the upper 2000 m of the global ocean (middle column of Figure 3.9). The decrease in near-surface density (hence increase in stratification) is largest in the Pacific, where warming and freshening both act to reduce density, and smallest in the Atlantic where the salinity and temperature trends have opposite effects on density.

The remainder of this section focuses on evidence of change in globally relevant intermediate, deep and bottom water masses.

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