Panoramic View ofSubpolar Changes

The compilation of composite trans-Atlantic hydrographic sections presented in Figs. 24.2 and 24.3 introduces all principal water masses of the subpolar North Atlantic evolving from their warmest and saltiest state of the mid-to-late 1960s to the extremely cold and fresh phase of 1994 and then to the generally warmer and saltier conditions of the recent years. The differences between the two extreme states have been thoroughly analyzed in several recent publications (Yashayaev 2007a, b; Yashayaev et al. 2007a; Boessenkool et al. 2006). Here we review and expand those analyses by considering two other trans-Atlantic sections (2001 shown in Figs. 24.2c and 24.3c, and 1962 shown in Fig. 24.14) and by discussing temperature changes.

A distinct salinity minimum at the intermediate depths is associated with LSW and can be recognized in all sections crossing the subpolar basins (Figs. 24.3 and 24.14). At the same time, this water mass exhibits substantial changes in its characteristics, thickness and depth from basin to basin, within basins and between surveys. Here we make a basic assumption that the Labrador Sea largely leads in this process: the changes seen in its main reservoir (Figs. 24.2 and 24.3, 24.14) spread to the other subpolar basins, presumably via advective-diffusive exchange (Straneo et al. 2003). Next we describe the observations that provide evidence to support this assumption. The full-depth transoceanic sections demonstrate how well the changes in the characteristic LSW layers are coordinated across the subpolar region.

1966 is in the middle of a prolonged phase of negative NAO (Fig. 24.4, 1960-1971) and period of record warm and salty Labrador Sea conditions (Figs. 24.4 and 24.5). The winter of 1965-1966 was particularly mild in the Labrador Sea region (Lazier 1980), so it is unlikely that there was significant convective renewal of LSW during the mid- and late 1960s. This caused the LSW lying at the intermediate depths to remain isolated from the upper layer, become warmer and saltier through its mixing with surrounding waters and also become advectively replaced by those waters. In 1966, deep LSW could be identified in the Labrador Sea as a nearly homogeneous layer with salinities between 34.88 and 34.90 (Fig. 24.3a, the distance range —700 to -240km). A retrospective analysis suggests that the last significant renewal of LSW occurred in the winter of 19621963 (Lazier 1980; Yashayaev et al. 2003).

A particularly large change occurred between the 1966 and 1994 surveys, with freshening occurring at virtually all depths across the subpolar gyre. In 1966 the deep LSW core was everywhere saltier, warmer and shallower than in any hydro-graphic survey of the subpolar North Atlantic during the 1990s. The change by 1994 was a result of production of large volumes of an exceptionally cold, fresh, dense, deep and vertically homogeneous LSW class by strong winter convection of the late 1980s-early 1990s (Lazier et al. 2002; Yashayaev 2007b). This LSW is, in fact, the most voluminous LSW observed in the historic record and is discussed in more detail later. In 1994 this water mass was the most prominent feature of the intermediate layers, filling the entire central part of the Labrador Sea basin within the depth range of 500-2,400 m (Figs. 24.2b and 24.3b). This means that within the Labrador and Irminger basins, and to some degree in the Iceland Basin, the well-mixed body of fresh LSW had penetrated to the depths previously occupied by more saline NEADW. As a result, this LSW exceeded both vertically and horizontally any other water mass seen in the subpolar North Atlantic since the beginning of the International Ice Patrol survey in the 1930s. As time progressed, temperature, salinity and density stratification re-established above the thinning patch of LSW (Figs. 24.2-24.5, 24.7 and 24.8). The isolation of LSW was a result of a substantial decrease in the net annual heat loss from the Labrador Sea to the atmosphere after 1994 (Lazier et al. 2002; Yashayaev 2007b).

By 2001, most of the excess volume of LSW had disappeared from the Labrador and Irminger Seas and the water column had restratified above its deep core. However, 7 years after the last convective renewal of LSW took place, remnants of this water mass could still be easily identified in these two basins inside the 1,5002,200 m depth range. These remnants were warmer and saltier than at the time of formation in 1993-1994 (Figs. 24.2c and 24.3c). In the same year the Iceland Basin did not show any significant increase in salinity and decrease in the volume of LSW. Indeed, a fairly extensive patch of LSW with salinities similar to those observed in 1994 could still be seen in this basin; a newly formed vintage of dense LSW was still arriving there. However, during these years the low-salinity layer of the Iceland Basin became denser, consistent with the LSW source changes (details follow). In the same year the Iceland Basin did not show any significant increase in salinity and decrease in the volume of LSW relative to 1994 (as shown further in this chapter the LSW of the Iceland Basin was fresher between 1994 and 2001 and saltier after 2001).

By 2004 the Iceland Basin's reservoir of LSW had also begun to lose its volume while gaining heat and salt. This suggests that it took no longer than 10 years after the cessation of the very deep convection in the Labrador Sea, for this LSW to start disappearing from the eastern parts of AR7 (Figs. 24.2d and 24.3d). The other water columns continued to restratify above the deep LSW core causing stratification in the whole subpolar North Atlantic to change. At the same time, the remnants of LSW that had become warmer and saltier over the years could still be recognized in 2004 (and even in 2007, not shown) by their thinned and weakened salinity and potential vorticity minima.

The intermediate depth ranges of the 2001 and 2004 AR7 sections exhibit two recently developed hydrographic features not seen in 1994 (Figs. 24.2 and 24.3). The first feature is a new LSW class seen as a homogenous low-salinity layer in the 400-1,300 m depth range. This water was massively formed in the winter of 2000, was modified via lateral and possibly convective mixing during subsequent years, and can still be identified via a volumetric analysis (Section 24.5). Even though some mixed layers could be found in the Labrador and Irminger Seas during 19971999, it was only in the winter of 1999-2000 when a distinct and homogeneous LSW class, maintaining its integrity in space and time, was produced. The 2000's increase in winter convection coincides with a local high of the NAO index, meaning higher levels of winter-time heat losses and explaining why in the year 2000 the convectively formed water spread deeper and wider in the Labrador Sea than during the preceding pentad. What makes the 2004 AR7 section particularly interesting for our investigation of the 2000's LSW class is that it was the first time when this water mass was definitely observed in the Iceland Basin, identified by a distinct salinity minimum within the depth range of 1,000-1,300 m.

The second feature that appeared between 1994 and 2001 is a relatively warm and salty intermediate layer separating the two LSW layers in the Labrador and Irminger Seas. This is a modified core of the Icelandic Slope Water (ISW), which is usually found near the Reykjanes Ridge and is now spreading towards the basin's centers to replace the deep LSW class. ISW is known to be formed through a direct linear mixing process blending the original Iceland-Scotland Overflow Water (ISOW) with the overlying Atlantic thermocline water near the Faroes, not involving LSW (van Aken and de Boer 1995). ISW then follows the slopes of Iceland and the Reykjanes Ridge until it enters the Irminger Sea. From the western slope of the Reykjanes Ridge the ISW intrudes into the center of the Irminger gyre, forming a relatively thin, but noticeably salty and warm layer. This characteristic salinity maximum is typically 140-200 m deeper than its temperature companion (discussed in Section 24.6.4).

While the vertical average of properties is commonly used to provide an overview of the temporal evolution of a water mass, a more detailed analysis of the development of LSW is required to understand the processes that determine the variability. In Section 24.5 we discuss a new approach which gives greater insight into those processes by distinguishing between classes of LSW formed at different times, and allowing their temporal and spatial changes to be accurately tracked.

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