FIGURE 2.9 Spawning stock biomass (SSB) of herring in the Nordic Seas, responding to the abrupt warming of 1920s (Toresen and Ostvedt, 2000). SSB (bold line); temperature (thin line).

study, anthropogenic contributions to the earlier warming were also significant. The close proximity of the northern Atlantic focus of this warming to the dominant sinking regions of the global deep-ocean circulation makes this episode particularly relevant to global climate.

Abrupt Cooling and Freshening of the Subpolar North Atlantic, 1972-1996

Long-term measurements appropriate to climate variability studies are scarce in the ocean. Temperature records at a few coastal sites, and tide gauge records, have been sustained. Around 1948, a network of ocean weather stations was established by the International Civil Aviation Organization, mostly for atmospheric measurements. In the United States, these were manned by ships of the U.S. Coast Guard. A few, however—such as Bravo in the Labrador Sea, Mike in the Norwegian Sea, and Papa in the subpolar North Pacific—carried out deep hydrographic stations, as well as weather observations. Most were abandoned by the early 1970s; only one (Ocean Station Mike, at 65°N, 2°E in the Norwegian Sea; e.g., Gammelsrød et al., 1992) remains. The value of those time series is enormous.

Fortunately, for climate purposes, we do not always need rapidly sampled data. In the northwestern Atlantic the annual range of seasonal

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