The Labrador Sea exhibits considerable variability in water properties and stratification (Figs. 24.2-24.4) and is characterized by extremely complex circulation and mixing. Time series of vertically averaged or integrated potential temperature, salinity and density can be used to provide an overview of major climatically significant changes in the LSW source observed during several decades (Fig. 24.4). When combined with the temporal evolution of the whole water column of the Labrador Sea (Fig. 24.5) and hydrographic sections across the subpolar North Atlantic (Figs. 24.2 and 24.3), the averaged properties of the 150-2,000 m layer reveal important changes in the LSW production and thickness, and the impacts of such on the heat and freshwater content, stratification and steric height throughout the whole subpolar domain.
Temporal evolution of a whole water column can be effectively visualized by producing an average or characteristic vertical profile of a given seawater property for each year or hydrographic survey and mapping a succession of such profiles in time-depth coordinates. Figure 24.5 shows such a progression of annually averaged vertical profiles constructed for the central region of the Labrador Sea for the period of 1949-2005, inclusive. Part of Fig. 24.5 is expanded in Fig. 24.8 which focuses on the recent full-depth hydrographic developments in the central Labrador and Irminger basins, discussed in Section 24.6. The method for generating the time series for analysis is as follows. The central regions of the Labrador and Irminger basins were defined by the bottom depths exceeding 3,250 and 2,830 m, respectively, and by the horizontal distance range from the AR7 line (Fig. 24.1) not exceeding 150 km. Each characteristic vertical profile was formed by robust averaging of temperature, salinity, pressure (depth) and Ao2 = 0.01 kg m-3 layer thickness (o2 is potential density anomaly referenced to 2,000db). This averaging was performed individually for each calendar year with available observations and over each o2 bin (layer), predefined by Ao2 = 0.005 kg m-3. The techniques of robust averaging, vertical interpolation and other data analyses used in the present study are documented in Yashayaev (2007b).
The 55-year record clearly shows three periods of the Labrador Sea warming (1962-1971, 1977-1983, and 1994-1997, Figs. 24.4 and 24.5). The first warming period was preceded by a fairly significant renewal of LSW that occurred during the late 1950s to early 1960s. At the end of this warming period, in 1970-1971, the Labrador Sea reached its warmest and saltiest state ever observed. The most recent warming started in 1994 and has continued throughout the following years (1994-2007). The average temperature and salinity of the upper 2,000 m layer have already returned to the high levels observed in the late 1960s and are likely to surpass these record high levels in the near future. The tendencies of warming and salinification are maintained by continuous inflow of the warm and salty waters from outside of the Labrador Sea (see Section 24.6 for more details).
On two occasions since 1960, the warming and salinification of the Labrador Sea was interrupted and offset by significant cooling and freshening caused by strong winter convection. The most remarkable event of convective watermass renewal occurred between the mid-1980s and mid-1990s and led to the development of a characteristic LSW that turned out to be the coldest, freshest, densest, deepest and most voluminous since the 1930s (Lazier et al. 2002; Yashayaev et al. 2003; Yashayaev 2007b). At the same time, the recent change in oceanographic conditions can be recognized as part of a cycle in water mass development affecting the entire subpolar domain. Periodic changes within the Labrador Sea result from the interplay of LSW and intermediate waters of a similar density from outside the basin. The warmer and saltier LSW alternatives tend to reoccupy the mid-depths as LSW production loses its vigour and is unable to compensate the loss in the LSW volume resulting from its draining out of the Labrador Sea. In Section 24.6 we discuss in more detail the "life cycle" of LSW, comprising this water's production, development, transformation and loss (due to its mixing and export).
The long-term periodic changes in the LSW properties and the subpolar hydrography as a whole can be linked to the North Atlantic Oscillation (NAO; Fig. 24.4, upper plot, note that the NAO axis is inverted). A predominance of high positive values of the NAO index is reflected in periods of cooling and freshening of the Labrador Sea, associated with renewal of the intermediate waters to 2000 m and deeper (1972-1976 and 1988-1994). In contrast, a predominance of negative NAO years from 1962 through to 1971 coincides with the period of little convective renewal of LSW, when it was becoming warmer and more saline (e.g., the 1966 section in Figs. 24.2a and 24.3a). The relationship between the NAO index and LSW production and properties is not straightforward because there are significant local processes which force the ocean on the interannual timescales. For example the thermal inertia opposes short-term fluctuations in the ocean's heat losses to the atmosphere. In addition, the local wind field is not always directly related to the strength of the westerlies over the central North Atlantic (Yashayaev 2007b).
The cycles of LSW development, evident in Figs. 24.4 and 24.5, had similar signatures of their rise and decline rates, stratification losses and vertical redistributions of freshwater. On the other hand, they varied in strength (or intensity), persistence and interannual variability within a cycle. As a result, the mixed layers formed in different years ranged in their thickness, depth, density and other characteristics. The aforementioned LSW development of the 1990s has surpassed in its prominence and outreach any other known production cycle of this water mass, forming an extreme in the recorded history of water mass renewal in the subpolar North Atlantic.
Figures 24.2 and 24.3 highlight two extreme states, the warm saline 1960s with little convective renewal, and the cold fresh mid-1990s with prolonged deep convection. The changes observed in the Labrador Sea 150-2,000 m mean temperatures and salinities between these extremes are on the order of 1 °C and 0.08, respectively, while the sea level dropped by more than 10 cm. It is notable that the density decrease and therefore expansion of the water column due to this long-term freshening could compensate only about half of the density decrease and water column contraction that would have resulted from the cooling alone. The 1970-1994 accumulation of freshwater by the Labrador Sea inferred from the full column salinity change between these years is equivalent to mixing of at least 6 m of freshwater into the water column of 1970.
In addition to the noted differences, the two extreme LSW states are responsible for markedly different vertical stratification (Fig. 24.5) and overall water mass distribution throughout the whole subpolar domain (Figs. 24.2 and 24.3). The change from the extremely low to high LSW production may have also affected the vigour of the deeper flow of ISOW (Boessenkool et al. 2006).
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