5.3.3.2 Intermediate and Deep Circulation and Water Property Changes
Since the 1970s, the major mid-depth water mass in the North Pacific, North Pacific Intermediate Water (NPIW), has been freshening and has become less ventilated, as measured by oxygen content (see Section 5.4.3). The NPIW is formed in the subpolar North Pacific, with most influence from the Okhotsk Sea, and reflects changes in northern North Pacific surface conditions. The salinity of NPIW decreased by 0.1 and 0.02 psu in the subpolar and subtropical gyres, respectively (Wong et al., 2001; Joyce and Dunworth-Baker, 2003). An oxygen decrease and nutrient increase in the NPIW south of Hokkaido from 1970 to 1999 was reported (Ono et al., 2001), along with a subpolar basin-wide oxygen decrease from the mid-1980s to the late 1990s (Watanabe et al., 2001). Warming and freshening occurred in the Okhotsk Sea in the latter half of the 20th century (Hill et al., 2003). The Okhotsk Sea intermediate water thickness was reduced and its density decreased in the 1990s (Yasuda et al., 2001).
In the southwest Pacific, in the deepest waters originating from the North Atlantic and Antarctica, cooling and freshening of 0.07°C and 0.01 psu from 1968 to 1991 was observed (Johnson and Orsi, 1997) and attributed to a change in the relative importance of Antarctic and North Atlantic source waters and weakening bottom transport. Bottom waters in the North Pacific are farther from the surface sources than any other of the world’s deep waters. They are also the most uniform, in terms of spatial temperature and salinity variations. A large-scale, significant warming of the bottom 1,000 m across the entire North Pacific of the order of 0.002°C occurred between 1985 and 1999, measurable because of the high accuracy of modern instruments (Fukasawa et al., 2004). The cause of this warming is uncertain, but could have resulted from warming of the deep waters in the South Pacific and Southern Ocean, where mid-depth changes since the 1950s are as high as 0.17°C (Gille, 2002; see Figure 5.8), and/or from the declining bottom water transport into the deep North Pacific (Johnson et al., 1994).