5.3.6 Relation of Regional to Global Changes

5.3.6.1 Changes in Global Water Mass Properties

The regional analyses described in the previous sections have global organisation, as described partially in Section 5.3.1 (Figure 5.6), and as reflected in the global trend analyses in Section 5.2. The data sets used for the largest-scale descriptions over the last 30 to 50 years are reliable; different types of data and widely varying methods yield similar results, increasing confidence in the reality of the changes found in both the global and regional analyses.

The regional and global analyses of ocean warming generally show a pattern of increased ocean temperature in the regions of very thick surface mixed layer (mode water) formation. This is clearest in the North Atlantic and North Pacific and in all sectors of the Southern Ocean (Figure 5.3). There are also regions of decreased ocean temperature in both the global and regional analyses in parts of the subpolar and equatorial regions.

Both the global and regional analyses show long-term freshening in the subpolar waters in the North Atlantic and North Pacific and a salinity increase in the upper ocean (<100 m deep) at low to mid-latitudes. This is consistent with an increase in the atmospheric hydrological cycle over the oceans and could result in changes in ocean advection (Section 5.3.2). In the North Atlantic, the subpolar freshening occurred throughout the entire water column, from the 1960s to the mid-1990s (Figure 5.5 and Figure 5.6a). Increased salinity and temperature in the upper water column in the subpolar North Atlantic after 1994 are not apparent from the linear trend applied to the full time series in Figure 5.5, but are clear in all regional time series (Section 5.3.2). Freshening in the North Pacific subpolar gyre north of 45°N is apparent in both regional analyses (Section 5.3.3.2) and global analyses (Figure 5.5). Freshening of intermediate depth waters (>300 m) from Southern Ocean sources (Section 5.3.5) is apparent in both the global and regional analyses (e.g., Figure 5.5 World).

Many of the observed changes in the temperature and salinity fields have been linked to atmospheric forcing through correlations with atmospheric indices associated with the NAO, PDO and SAM. Indeed, most of the few time series of ocean measurements or repeat measurements of long sections (see Sections 5.3.2 and 5.3.4) show evidence of decadal variability. Because of the long time scales of these natural climate patterns, it is difficult to discern if observed decadal oceanic variability is natural or a climate change signal; indeed, changes in these natural patterns themselves might be related to climate change. In the North Atlantic, freshening at high latitudes and increased evaporation at subtropical latitudes prior to the mid-1990s might have been associated with an increasing NAO index, and the reversal towards higher salinity at high latitudes thereafter with a decreasing NAO index after 1990 (see Figure 3.31). Likewise in the Pacific, freshening at high latitudes and increased evaporation in the subtropics, cooling in the central North Pacific, warming in the eastern and tropical Pacific and reduced ventilation in the Kuroshio region, Japan and Okhotsk Seas could be associated with the extended positive phase of the PDO. The few detection and attribution studies of ocean changes are discussed in Section 9.5.1.

At a global scale, the observed long-term patterns of zonal temperature and salinity changes tend to be approximately symmetric around the equator (Figure 5.6) and occur simultaneously in different ocean basins (Figures 5.3 and 5.5). The scale of these patterns, which extends beyond the regions of influence normally associated with the NAO, PDO and SAM, suggests that these coherent changes between both hemispheres are associated with a global phenomenon.