5.3.2 Atlantic and Arctic Oceans
The North Atlantic Ocean has a special role in long-term climate assessment because it is central to one of the two global-scale MOCs (see Box 5.1), the other location being the Southern Ocean. The long-term trends in depth-integrated Atlantic heat content for the period 1955 to 2003 (Figure 5.2) are broadly consistent with the warming tendencies identified from the global analyses of SST (see Section 18.104.22.168). The subtropical gyre warmed and the subpolar gyre cooled over that period, consistent with a predominantly positive phase of the NAO during the last several decades. The warming extended down to below 1,000 m, deeper than anywhere else in the World Ocean (Figure 5.3 Atlantic), and was particularly pronounced under the Gulf Stream and North Atlantic Current near 40°N. Long-term trends in salinity towards freshening in the subpolar regions and increased salinity in the subtropics through the mid-1990s (Figure 5.5 Atlantic and Figure 5.6a) are consistent with the global tendencies for freshening of relatively fresher regions and increased salinity in saltier regions (Section 5.2.3).