Upper North Atlantic Deep Water (UNADW) is formed by deep convection in the Labrador Sea between Canada and Greenland, so is also known as Labrador Sea Water (LSW). It is the shallowest component of the NADW, located above the overflow water masses that supply the Lower North Atlantic Deep Water (LNADW). AR4 Chapter 5 assessed the variability in water mass properties of LSW from the 1950s. Recent studies have confirmed the large interannual-to-multi-decadeal variability of LSW properties and provided new information on variability in formation rates and the impact on heat and carbon (Section 3.8.1) uptake by the deep ocean.
During the 1970s and 1980s and especially the 1990s the UNADW has been cold and fresh. In Figure 3.9A it is the strong freshening signal from the 1960s to the 1990s that dominates the trend. This freshening trend reversed in the late 1990s (Boyer et al., 2007; Holliday et al., 2008; see Section 22.214.171.124). Estimates of the LSW formation rate decreased from about 7.6 to 8.9 Sv in 1997–1999 (Kieke et al., 2006) to roughly 0.5 Sv in 2003–2005 (Rhein et al., 2011), and since 1997, only less dense LSW was formed compared to the high NAO years before. There is, however, evidence that formation of denser LSW occurred in 2008 (Våge et al., 2009; Yashayaev and Loder, 2009), but not in the following years (Yashayaev and Loder, 2009; Rhein et al., 2011).
The strong variability in the formation of UNADW affected significantly the heat transfer into the deep North Atlantic (Mauritzen et al., 2012). Substantial heat entered the deep North Atlantic during the low NAO years of the 1960s, when salinity was large enough to compensate for the high temperatures, and dense LSW was still formed and exported to the subtropics.
- ↑ 3 The formation rate of a water mass is the volume of water per year that is transformed into the density range of this water mass by surface processes (for instance cooling), eventually modified through ocean interior processes (for instance mixing). Formation rates are reported in Sverdrups (Sv). 1 Sv equals 106 m3 s–1.