temperature is typically 4°C in the upper mixed layer, 1°C at 150m, and 0.1°C or less at 1000 m. Sampling must be frequent enough so as not to be aliased by this cycle. Monitoring deep, “stable” water masses is good strategy.
Combining weather ship data, hydrographic work, particularly that of the CSS Hudson from Bedford Institute of Oceanography, Canada, and more recent moored instrumentation reveals a picture of climate change over the last 30 years that stands out in the 100-year record. There has been a persistent decline in salinity from top to bottom of the subpolar Atlantic Ocean and widespread cooling since 1972 (Figure 2.10) (e.g., Lazier, 1995). Accompanied by a surge of strong, cold winters with winds from the Canadian Arctic (and strongly positive NAO index), convection in the Labrador Sea has penetrated to great depth, yet episodically, during this period. The intense period of Labrador Sea convection produced a great outflow of the water mass, which has been traced along the western boundary of the Atlantic as far as the Antilles (Molinari et al., 1998; Smethie et al., 2000).
The positive values of the AO/NAO-index time series in this period stand out in the 150-year record. Then, suddenly, the cold winters in the northwest Atlantic reverted to an extremely mild state in early 1996. The AO/NAO indices had suddenly reversed, taking on minimums as extreme as anything seen during the century. The ocean has responded quickly, with warm, saline water invading the Labrador Sea to replace the cold, low-salinity water characteristic of intense wintertime forcing. While possible feedback coupling of the atmosphere-ocean system remains to be determined, the direct correspondence of northwest Atlantic deep convection and cold-air outbreaks from Canada is not in doubt.
The increasing supply of fresh water leading to the strong decline in salinity has several possible origins: increased flow of water and ice from the Arctic, decreased flow of saline waters northward in the Gulf Stream/North Atlantic Current system from the subtropical Atlantic, and both the increased hydrologic cycle and ice melt associated with global warming. The resemblance to the freshening seen in coupled global climate models may be misleading, because of the inaccuracy of the AO/NAO modes in many of the models. The enhanced AO/NAO mode actually increased the density and intensity of this intermediate-depth branch of the THC during this period, despite the decline in salinity. Cooling of the subpolar ocean during a general period of global warming is a dynamic negative feedback,