Spreading of the Labrador Sea Water to the Irminger and Iceland Basins Igor Yashayaev,1 Manfred Bersch,2 and Hendrik M

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L10602, doi:10.1029/2006GL028999, 2007 Click Here for Full Article

Spreading of the Labrador Sea Water to the Irminger and Iceland basins Igor Yashayaev,1 Manfred Bersch,2 and Hendrik M. van Aken3 Received 5 December 2006; revised 28 March 2007; accepted 6 April 2007; published 17 May 2007.

[1] Labrador Sea Water (LSW) is a principal convectively- McCartney, 1982], filling its intermediate reservoir with formed water mass of the subpolar North Atlantic (SPNA). water that is relatively fresh, cold, and rich in dissolved Using extensive oceanographic archives we demonstrate gases [Sy et al., 1997; Yashayaev and Clarke, 2006; Kieke et striking changes in SPNA caused by massive LSW al., 2006]. LSW regulates the large-scale dynamics of NA in production during 1987–1994 and document recent three ways. First, its volume and property changes influence salinification and warming, imminently bringing SPNA to the mid-depth circulation, mixing and signal propagation in the state last time seen in the 1960s. Two prominent LSW the subpolar basins. Second, varying LSW production classes are spreading across SPNA since the 1980s. The controls exchanges between the subtropical and subpolar first, record dense, deep, and voluminous, class has been gyres at their intermediate depths [Curry and McCartney, progressively built by intense winter convection through 2001]. Third, by contributing to the intermediate compart- 1987–1994. Even though most of this LSW has left SPNA, ments of NA, LSW affects the properties [Dickson et al., its remnants are still present there. The second, shallower, 2002] and controls the vigor (K. Boessenkool et al., On the class strengthens in 2000; over subsequent years its core relationship between the North Atlantic Oscillation and becomes slightly thicker and deepens. The anomalous deep-ocean flow speed changes during the last 230 years, signals acquired by these LSW classes in their formation submitted to Geophysical Research Letters, 2007) of the region arrive in the Irminger and Iceland basins with the lower limb of the Atlantic MOC, originating from the cold characteristic delays of two and five years for deeper LSW and dense polar overflows entering NA via the deep and a year and four years for shallower LSW. trenches in the Greenland-Scotland Ridge. The Iceland- Citation: Yashayaev, I., M. Bersch, and H. M. van Aken Scotland Overflow Water (ISOW), evolving into Northeast (2007), Spreading of the Labrador Sea Water to the Irminger and Atlantic Deep Water (NEADW), and Denmark Strait Over- Iceland basins, Geophys. Res. Lett., 34, L10602, doi:10.1029/ flow Water are the two major deep dense overflows arriving 2006GL028999. to NA from the Nordic Seas. They pass through and mix with LSW just before they enter the deep and abyssal NA reservoirs, entraining fresh water, elevated gas, and transient 1. Introduction tracer signatures of LSW, ultimately reaching the deeper [2] By governing the meridional overturning circulation layers of the Labrador Sea where their evolving properties (MOC) of the Atlantic Ocean [Dickson et al., 2002; Bryden are being monitored [Yashayaev et al., 2003]. et al., 2005], the northern North Atlantic (NA) plays an [4] The convective cooling and freshening of the active role in the ventilation of the deep layers of the whole Labrador Sea over the 1980s and early 1990s have produced world ocean. Its most remarkable intermediate water mass is LSW that by 1994 became the coldest, densest, deepest and known as the Labrador Sea Water (LSW). LSW is formed in most voluminous since the 1930s [Yashayaev et al., 2003]. the Labrador Sea through deep convection caused by high The LSW pathways shown in Figure 1a are based on the heat losses during severe winters [Lazier, 1980; Clarke and thicknesses of the LSW layer measured over three years Gascard, 1983; Gascard and Clarke, 1983]. This process, following the most massive production of this water mass which is believed to be controlled by the phase and persis- (dashed lines). In the present article we discuss the changes tence of the North Atlantic Oscillation (NAO) [Dickson et al., in the subpolar gyre triggered by extremely deep convective 1996], turns the Labrador Sea into a main contributor of mixing in the early 1990s that led to production of this newly formed or modified intermediate waters to the exceptional LSW and document some footprints of its Atlantic MOC. The NAO index is the normalized Azores- development and transformation. to-Iceland sea level pressure difference, linked to the [5] Pickart et al. [2003] argued that LSW is likely formed strength of the large-scale zonal atmospheric transport in the southwest Irminger Sea, pointing at the fact that tracer [Hurrell et al., 2001]. observations by Sy et al. [1997] yielded very high advective [3] When produced in large quantities, LSW spreads speed and short travel times for LSW implying that it takes across the subpolar North Atlantic (SPNA) [Talley and about six months for the water to spread to the Irminger Sea, while an advective-diffusive model study by Straneo et al. [2003] showed that LSW arrives in the central Irminger 1Bedford Institute of Oceanography, Fisheries and Oceans Canada, basin two years after its formation. To explain such a Dartmouth, Nova Scotia, Canada. discrepancy in views Pickart et al. [2003] rejected the 2Institute of Oceanography, University of Hamburg, Hamburg, Germany. assumption that the sole source of LSW is the Labrador 3Royal Netherlands Institute for Sea Research, Den Burg, Netherlands. Sea and ‘‘formed’’ LSW in the Irminger Sea thus explaining the seeming too-fast spreading of this water mass. However, Copyright 2007 by the American Geophysical Union. the LSW transit times based on a short observational record 0094-8276/07/2006GL028999$05.00

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2of8 L10602 YASHAYAEV ET AL.: SPREADING OF THE LABRADOR SEA WATER L10602 by Sy et al. [1997] has not yet been questioned. Here we historic density range. All volumetric peaks in Figure 1b can provide observational evidence that the LSW transit times be grouped into two continuous progressions, each reflect- are notably longer than those suggested before, justifying ing year-to-year development and transformation of a given the advective-diffusive nature of LSW spreading and rein- LSW core. This grouping introduces an LSW class–a stating the dominant source of LSW in the Labrador Sea. sequence of LSW types linked by the same development history. 2. Data [8] The 1980-2006 AR7 repeats allowed us to build the complete history of development, evolution and decay of [6] Figures 1a and 2 (inserts) indicate hydrographic two prominent LSW classes in SPNA. The first class, stations in close proximity of repeat trans-Atlantic section LSW1987 –94 (the subscript indicates a time interval over AR7. First occupied under the aegis of the World Ocean which this LSW had likely been formed), is associated Circulation Experiment (WOCE, 1990-1997), this section with the most extraordinary documented LSW production later became the principal observational element in the (Figure 1b). Record voluminous in 1994, it has strongly Atlantic Climate Variability and Predictability (CLIVAR). diminished over the past twelve years, recently becoming Its Labrador Sea part, AR7W, has been occupied by the barely identifiable in the volumetric diagrams. The other Bedford Institute of Oceanography (Canada) since 1990, class, LSW2000, was formed in 2000. It is shallower and has while the eastern part, AR7E, has been surveyed by the lower densities than LSW1987–94.Wenamethesetwo University of Hamburg, the German Hydrographic Office, classes after the years when they achieved their coldest, and Royal Netherlands Institute for Sea Research since densest (note temporal s2 changes in each LSW) and most 1991. To portray LSW in a year typical for the low NAO voluminous states. phase and highlight the outstanding contribution of the [9] When a certain LSW class loses its volumetric 1990s to the multi-decadal history of LSW production and prominence (recent LSW1987 – 94) other criteria can be used rapid restratification of the mid depths during the last to refine its volumetrically defined s2 range. Among these decade, we compare AR7 occupations with a composite are salinity (Figure 2), temperature [Lazier et al., 2002] and section built as 1966 replica of AR7, which mainly includes potential vorticity [Talley and McCartney, 1982] minima, the most extensive SPNA survey before WOCE led by John oxygen maximum [Clarke and Coote, 1988] and the an- Lazier (CSS Hudson, 1966). thropogenic tracers [Azetsu-Scott et al., 2003]. Even when thinning remnants of highly modified LSW1987 – 94 formed 3. Identification of LSW weak volumetric maxima, the mentioned extrema remained within the volumetrically defined s2 ranges, thus confirm- [7] All distinguishable instances of LSW form distinct ing or endorsing the volumetric identification of strongly peaks in volumetric density (s2—potential density anomaly transformed LSW. referenced to 2000 dbar) and temperature-salinity (T-S) [10] We disagree with the approach taken by Kieke et al. censuses of each basin-survey. The s2-T-S range, height, [2006], introducing ‘‘classical’’ and ‘‘upper’’ LSW that are and shape of such a peak identify a unique LSW, while the defined by potential density ranges of 27.74–27.80 and peak’s maximum points to the core of this water mass. The 27.68–27.74 kg m3, respectively. If combined, these two AR7 T-S projections will be published separately; here we layers fill most of the top 2000–2200 m of the Labrador Sea introduce a s2 layer volumetric approach. More compact leaving no room for other intermediate waters arriving form than its T-S companion, the s2 method is equally effective outside of the Labrador Sea to replace LSW. We argue that for the LSW identification-both methods produced identical LSW does change its density (Figure 1b), occasionally s2 ranges for the LSW cores discussed in the paper. The ‘‘crossing’’ the 27.74 kg m3 boundary. On the other hand, essence of the s2 volumetric approach can be expressed by the density range defining classical LSW is excessively plotting basin-mean thickness of individual s2 layers (Ds2 = 3 broad, indicating that this water was never thinner than 0.01 kg m )inthes2-time coordinates. Such a diagram for 1000 m (at odds with our LSW1987 –94 findings), and thus the Labrador Sea is presented in Figure 1b. It was con- systematically overestimating the LSW thicknesses and structed by averaging layer thicknesses from individual production rates. We also find it inappropriate to label hydrographic stations (in a single year or survey) weighted the densest and deepest LSW unique for the 1990s as by the distance or area represented by these stations. At each ‘‘classical’’. station of a basin-survey each LSW was defined by a s2 3 [11] The AR7 section plots (e.g., Figure 2) and volumet- range (±0.017 kg m ) centered at the volumetric core ric inventories (e.g., Figure 1b) demonstrate that a universal (Figure 1b; Labrador Sea, e.g., 36.916j1990, 36.940j1993). definition of a given LSW class (e.g., LSW ) meant to These individual density ranges collectively form an LSW 1987 – 94

Figure 1. (a) Map of the subpolar North Atlantic showing major topographic features (the color legend relates elevation/ depth, m). White-rimmed circles indicate hydrographic stations occupied between 1987 and 2005 along the trans-Atlantic section AR7. Dashed lines represent thickness of the layer defined by the (s2—potential density anomaly referenced to 3 2000 dbar) range best confining the core of deep LSW in 1995–1997 (36.92 < s2 < 36.95 kg m ; this mapping was based on the 1995–1997 hydrographic profiles, which positions are indicated in the figure by white dots). Yellow arrow-headed lines follow the LSW spreading and recirculation pathways as inferred from the LSW thickness and vertical section plots; red-arrow headed lines indicate the spreading of Icelandic Slope Water. (b) A s2-time plot showing average thickness (in m) 3 3 of Ds2 =0.01kgm layers in the Labrador Sea. For the construction of this plot overlapping s2 layers set by 0.002 kg m (s2) have been used. Volumetric peaks characteristic of the LSW1987 –94 and LSW2000 classes are labeled.

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Figure 2

4of8 L10602 YASHAYAEV ET AL.: SPREADING OF THE LABRADOR SEA WATER L10602 be used through its development and transformation history record–LSW1987 – 94. In 1994 this water mass appears as should not be based on a static range of a certain variable. the most prominent feature of the intermediate layers, filling Intensifying and deepening convective mixing, followed by the entire central part of the Labrador Sea basin within the water mass dislodging and diluting do change the properties depth range of 5002400 m (Figures 1b and 2b). This of LSW and, as a result, alter its core and boundaries. This means that within the Labrador and Irminger basins and to means that any criteria involving the LSW properties must some degree in the Iceland Basin the well-mixed body of account for possible changes in such properties. The volu- the fresh LSW1987 – 94 has penetrated to the depths earlier metric method used in our study automatically adjusts to a occupied by NEADW. As a result, this LSW exceeded both specific LSW core, following its year-to-year dynamics and vertically and horizontally any other water mass seen in transformation and revealing spatiotemporal changes in this SPNA since the 1930s. LSW. [15] As time progressed, temperature, salinity and density stratifications re-established above the thinning patch of 4. Salinity on the Trans-Atlantic Section North of LSW1987 –94. Such isolation of LSW1987 – 94 was a result of a 50°N substantial decrease in the net annual heat loss from the Labrador Sea to the atmosphere after 1994 [Lazier et al., [12] The compilation of composite trans-Atlantic hydro- 2002; Yashayaev and Clarke, 2006]. By 2004 (Figure 2c), graphic sections for 1966, 1994, and 2004 (Figure 2) most of the excess volume of LSW1987 – 94 has disappeared introduces all principal water masses of SPNA, reflecting and the water columns have restratified above its deep core transitions from their saltiest (and warmest) state of the mid- causing stratification in the whole SPNA to change. The late 1960s to extremely fresh (and cold) phase of 1994 and saltier and warmer remnants of LSW1987 – 94 could still be then to generally saltier (and warmer) conditions of the recognized in 2006 (not shown). In the Labrador Sea, its recent years. A distinct intermediate salinity minimum present signature is a weak local salinity minimum between associated with LSW can be recognized in all sections 34.88 and 34.90, and the slightly increased spreading of the crossing the subpolar basins. However, the depth, thickness, isotherms between 2.9 and 3.1°C (not shown). By 2004 the and properties of this water change from basin to basin, patch of LSW1987 –94 in the Iceland Basin had also started to within basins and between surveys. lose its volume and gain salt. [13] The year 1966 (Figure 2a) is characteristic for record [16] In 2004, the Labrador and Irminger Seas also exhibit salty and warm SPNA of 1964–1971 (since the 1930s) two new (to 1994) features. The first feature is a homog- [Yashayaev et al., 2003; Yashayaev and Clarke, 2006]. This enous layer between 400 and 1300 m. This water corre- year is also central in the 1960–1971 low NAO phase. The sponds to the volumetric density class LSW2000 (Figure 1b). winter of 1965–1966 was particularly mild in the Labrador It was massively formed by winter convection in 2000 and Sea [Lazier, 1980]. Thus it was unlikely that significant has been then modified by mixing and moderate convection convective renewal of LSW occurred during the mid-late during subsequent years [Yashayaev and Clarke, 2006]. 1960s, causing the LSW lying at the intermediate depths to Even though some shallow mixed layers could be found remain isolated from the upper layer and become saltier and in the Labrador and Irminger Seas since 1997, it was only warmer through its mixing with surrounding waters. In the winter of 1999–2000 when this water developed into a 1966 deep LSW could be identified in the Labrador Sea as distinct and homogeneous LSW class, since then maintain- a nearly homogeneous layer with salinities between 34.88 ing its integrity. The increase in winter convection in 2000 and 34.90 (in the distance range 700 240 km). A coincides with five-year high NAO, pointing at high heat retrospective analysis suggests that the last significant LSW losses in the Labrador Sea and explaining why the con- renewal of the 1960s occurred in the winter of 1962–1963 vectively formed water spread deeper and wider in 2000 [Lazier, 1980]. than in the 5 previous years. [14] A particularly large change occurred between the [17] The second new feature of the 2004 section 1966 and 1994 surveys. Indeed, in 1966 the deep LSW core (Figure 2c) is the relatively salty and warm intermediate was everywhere saltier, warmer and shallower than in any layer separating LSW1987– 94 and LSW2000 within the hydrographic survey of SPNA during the 1990s. This Labrador and Irminger Seas. It is the core of saltier and change is a result of production of an exceptionally cold, warmer water arriving to replace the deeper LSW. This fresh, dense, deep, and vertically homogeneous LSW class water originates from Icelandic Slope Water (ISW) seen by strong winter convection between the late 1980s and the near the Reykjanes Ridge. ISW in its turn is formed through early-mid 1990s [Lazier et al., 2002]. This water was, in a direct linear mixture of the original Iceland-Scotland fact, the most voluminous LSW in its historic 70-year Overflow Water with the overlying Atlantic thermocline

Figure 2. Salinity on the trans-Atlantic section AR7 compiled from (a) 1966, (b) 1994, and (c) 2004 oceanographic surveys. Corresponding water sampling and profiling sites are indicated in the insert maps. Hatched lines contour constant potential density of 24.72, 24.74 (violet) and 24.77, 24.79 kg m3 (yellow). Densities shown in these section plots are referenced to the sea surface (note that the densities used in Figures 1 and through the rest of the paper are referenced to 2000 dbar). The most prominent feature in these sections is the Labrador Sea Water (LSW, LSW2000, and the corresponding vertical double-headed arrows) formed in the Labrador Sea through deep convection during severe winters. ISOW, NEADW, and DSOW indicate the relatively salty Iceland-Scotland Overflow Water, subsequently developing into the Northeast Atlantic Deep Water and the column-coldest Denmark Strait Overflow Water.

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Figure 3. Anomalies of (left) potential temperature and (right) salinity averaged over time- and distance-dependent density ranges representing the two most prominent LSW classes – (bottom) LSW1987 – 94, 1987 to 2005 (inclusive), and (top) LSW2000, 1999 to 2005 (inclusive). Distance indicates the position along the composite AR7 section, with its origin in the central Labrador Sea. Eastward (right) pointing arrows indicate how record cold and fresh classes of LSW spread across the ocean, reaching first the Irminger Sea and then the Iceland Basin. Westward (left) pointing arrows indicate how a bulk of anomalously warm and saline water appeared near the western flank of the Reykjanes Ridge in 2000 and over subsequent years spread west along AR7, to the central and then western parts of the Irminger and Labrador basins. Anomalies were computed for hydrographic profiles indicated by dark dots. A distance-dependent long-term mean reference state used to compute these anomalies was comprised of three segments, derived for each basin separately to prevent any influence of adjacent basins. Each basin-wide norm was constructed by grouping all individual property values in 40 km distance bins; calculating medians in each group; and subsequent fitting a polynomial function of distance to these medians to achieve continuity of the reference state in each basin.

water near the Faroes, without interfering LSW [van Aken is consistent with the recent decline in NAO and winter and de Boer, 1995]. That ISW then follows the slopes of convection. Iceland and the Reykjanes Ridge until it enters the Irminger Sea through the Charlie-Gibbs Fracture zone. From the 5. Transit of LSW Anomalies to the Irminger Sea western slope of the Reykjanes Ridge the ISW intrudes into and Iceland Basin the centre of the Irminger gyre, forming a relatively thin, but salty and warm layer, now prominently seen between the [20] Figure 3 shows potential temperature (Figure 3, left) LSW1987 – 94 and LSW2000 cores. This characteristic salinity and salinity (Figure 3, right) anomalies in the two LSW maximum is typically 140–200 m deeper than its temper- classes — LSW1987–94 (Figure 3, bottom) and LSW2000 ature companion. (Figure 3, top) and adjacent to them warmer and saltier [18] In 2004 a clear LSW2000 salinity minimum was first waters. The distance is measured along the composite AR7 observed in the Iceland Basin. section and referenced to the center of the Labrador Sea; [19] Summarizing the long-term changes in the LSW fractional year reflects actual date of each station. properties based on hydrographic sections backed by time [21] As already stated in our discussion of Figure 1b, the series [Dickson et al., 2002; Yashayaev et al., 2003; development of deep convection in the Labrador Sea Yashayaev and Clarke, 2006], we report a strong salinity between 1987 and 1994 resulted in consecutive annual and temperature contrast between the 1966 and 1994 increases in the LSW density during these years. A series sections (these two extreme years set boundaries for the of papers describes the mechanisms of such buoyancy loss historic ranges of the LSW properties). We also state that and documents the temperature and salinity changes accom- the intermediate layers of SPNA are now approaching the panying the intensification of winter convection in the salinity and temperature levels of 1966 (typical for record Labrador Sea [Lazier et al., 2002; Yashayaev et al., 2003]. salty and warm state of the mid 1960s – early 1970s). This

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[22] Between 1987 and 1994 LSW1987 – 94 was rapidly [25]1.TheLSW1987–94 temperature anomalies were changing within its source region in the Labrador Sea, record low in the Labrador Sea in 1994, while the coldest cooling by about 0.45°C (Figure 3, left bottom), becoming LSW invaded the entire Irminger Basin only by 1996, 0.06 kg m3 denser and doubling in volume (Figure 1b). implying a two-year delay. Since the bulk of LSW typically Note that during all these years except the last, 1994, arrives to the Irminger Sea between 500 and 700 km, we LSW1987 – 94 was steadily becoming saltier (Figure 3, right primarily use this distance range to define the LSW transit bottom, Labrador Sea, 1988–1993). This change in the time to the Irminger Sea. LSW1987 – 94 salinity means a loss in the LSW buoyancy in [26] 2. The sustained cooling of LSW1987 – 94 in the Iceland addition to that caused by the multiyear cooling of Basin ended only in 1999, five years after this water reached LSW1987 – 94 (Figure 3, left bottom, Labrador Sea, 1987– its coldest point in the Labrador Sea. Since 1999, this deep 1994). To remind, this LSW cooling was a result of extreme LSW of the Iceland Basin shows a slight warming. heat losses to the atmosphere during the severe winters of [27] Not only extreme points were used to find the LSW the early 1990s [Lazier et al., 2002]. These matching transit times. We have also analyzed the shifts or transitions tendencies in the temperature and salinity effects on the between extensive warm and cold LSW phases and docu- LSW density explain the rapid increase in the LSW density mented the basin-to-basin transit of these phases. Here are (Figure 1b). The noted increase in the LSW1987 – 94 salinities some characteristic points of these warm-to-cold and cold- was due to mixing with the saltier NEADW that is entrained to-warm LSW1987 – 94 phase transitions: in the Labrador into LSW every time when convection gets deeper. The Sea cooling in 1990 and warming in 1998; in the Irminger progressive deepening of the lower margin of LSW and Sea cooling in 1992 and warming in 2000; in the Iceland therefore of winter convection was observed between 1988 Basin cooling in 1995 and warming in 2005. The delays in and 1993 [Yashayaev and Clarke, 2006], supporting our the noted transitions confirm our estimates of transit times explanation of the salinity increase. The last occurrence of to the Irminger and Iceland basins (2 and 5 years, intense convection 1993/1994 [Lazier et al., 2002] did not respectively). seem to reach deeper than in 1993, simply because in the [28] It was beyond our expectation to discover double spring of 1994 the depth of the mixed layer was not salinity minima in the LSW1987 – 94 series for the Irminger noticeably deeper than in the spring of 1993 [Yashayaev and Iceland basins. However bi-minimum patterns similar and Clarke, 2006]. Therefore, it is most likely that winter to that seen in the Labrador Sea (but of weaker magnitude) convection of 1994 did not bring up enough of salty can be recognized in the two other basins. Focusing only on NEADW from below LSW to overcompensate the freshen- the second low in the LSW1987 – 94 salinities, we consecu- ing effect of convective entrainment of the upper low– tively see it in the Labrador, Irminger and Iceland basins in saline waters. This has resulted in a single year disruption of 1994, 1996, and 1999. This again confirms the two and five the LSW salinity increase. Until 1993 this salinity increase year transits of LSW1987 – 94 to the two latter basins. was maintained by the NEADW entrainment and after the [29] In addition to the spreading times, Figure 3 (bottom) cessation of LSW1987 – 94 convective renewal in 1994 by also illustrates changes in the spatial extent of the LSW1987–94 mixing with other intermediate waters (saltier than LSW). bodies in the Labrador and Irminger Seas. LSW1987 –94 That 1993 to 1994 salinity decrease by almost 0.01 has expands spatially when convection intensifies and contracts formed the second minimum in the bi-minimum LSW1987–94 when the deep intermediate layers lose LSW and restratify. salinity record. However, newly formed LSW was colder in Between 2000 and 2003 LSW1987 – 94 withdrew from the 1994 than it was in 1993, thus not disrupting or even eastern part of the Irminger Sea leaving it to warmer and slowing the tendency of its cooling heading to the all-time saltier waters, which include ISW, Subpolar Mode Water temperature low seen in 1994. Since 1994 the deep reservoir [McCartney and Talley, 1982] and also strongly modified of LSW1987– 94 has mainly remained isolated from the LSW. For almost a decade now, the LSW1987 –94 layers of winter mixed layer [Lazier et al., 2002], steadily becoming the Irminger and Labrador Seas have been showing sus- warmer and saltier and showing only slight density changes tained warming and salinification. These signals can be (Figures 1a, 2b, 2c, and 3). tracked back to the western flank of the Reykjanes Ridge, [23] The single temperature and dual salinity minima where in 2000 a strong positive anomaly was first seen described above combined with other characteristic points replacing a strong cold and fresh anomaly associated with in the LSW1987 – 94 development and transformation LSW1987 –94 (this transition is also reflected in the sections observed within the formation region of this water mass in Figure 2, west of Reykjanes Ridge). (in the Labrador Sea) serve as effective markers or tags, [30] LSW2000 has been sporadically renewed during its which arrival can be sought in the other Atlantic basins. For short history, resulting in patchier temperature and salinity example, the buildup and following rapid decline of anomaly fields than those for the deeper water (Figure 3). LSW1987 – 94 class expressed in the increase and then decrease Even if our sections and volumetric estimates suggest that in the corresponding density layer thickness (Figure 1b) have LSW2000 and its anomalies arrive from the Labrador Sea, in had their 2-year delayed imprint in an analogous volumetric some years this water could be remixed and its signals compilation for the Irminger Sea. altered outside of the formation region. What assures us in [24] Linking the LSW1987 – 94 signals observed in the this LSW spreading east is that since 2000, when it was first LSW formation region with the signals arriving in the voluminously appeared in the Labrador Sea (Figure 1b), this Irminger Sea and Iceland Basin (Figure 3, compare with water mass is steadily becoming warmer and saltier, show- LSW pathways in Figure 1a) one can come to the following ing its record low temperature and salinity in 2000. The conclusions: corresponding developments of LSW2000 in the Irminger Sea and the Iceland Basin (Figure 3, top) suggest that it

7of8 L10602 YASHAYAEV ET AL.: SPREADING OF THE LABRADOR SEA WATER L10602 took, respectively, a year and four years for this water mass References to reach each of these basins in order. These transit times are Azetsu-Scott, K., E. P. Jones, I. Yashayaev, and R. M. Gershey (2003), by about a year less than those for LSW1987 – 94. Note the Time series study of CFC concentrations in the Labrador Sea during deep and shallow convection regimes (1991–2000), J. Geophys. Res., westward spreading of the warmer and saltier anomaly in 108(C11), 3354, doi:10.1029/2002JC001317. the LSW2000 layer from the flanks of the Reykjanes Ridge, Bryden, H. L., H. R. Longworth, and S. A. Cunningham (2005), Slowing of to the Labrador Sea very similar to that in the LSW1987 –94 the Atlantic meridional overturning circulation at 25°N, Nature, 438, layer. Finally, since the year of its first massive production 655–657. Clarke, R. A., and A. R. Coote (1988), The formation of Labrador Sea (2000) the core of this water shows progressive deepening Water. Part III: The evolution of oxygen and nutrient concentration, in both Labrador and Irminger Seas. J. Phys. Oceanogr., 18, 469–480. Clarke, R. A., and J.-C. Gascard (1983), The formation of Labrador Sea Water. Part I: Large-scale processes, J. Phys. Oceanogr., 13, 1764–1778. 6. Discussion and Concluding Remarks Curry, R. G., and M. S. McCartney (2001), Ocean gyre circulation changes associated with the North Atlantic Oscillation, J. Phys. Oceanogr., 31, [31] Temperature and salinity anomalies measured in the 3374–3400. Dickson, B., I. Yashayaev, J. Meincke, B. Turrell, S. Dye, and J. Holfort cores of LSW1987 – 94 and LSW2000 (Figure 3) advect to the (2002), Rapid freshening of the deep North Atlantic Ocean over the past east reaching the Irminger Sea in two years and one year, four decades, Nature, 416, 832–837. respectively, and arriving in the Iceland Basin in five and Dickson, R. R., J. R. N. Lazier, J. Meincke, P. Rhines, and J. Swift (1996), four years after their formation in the Labrador Sea. It is not Long-term coordinated changes in convective activity of the North Atlan- surprising that these transit times are at least double those tic, Prog. Oceanogr., 38, 241–295. Gascard, J.-C., and R. A. Clarke (1983), The formation of Labrador Sea suggested by Sy et al. [1997], because the latter were based water. Part II: Mesoscale and smaller-scale processes, J. Phys. Oceanogr., on a rather short observational record insufficient to register 13, 1780–1797. the true arrival of LSW into the two eastern basins. Our Hurrell, J. W., Y. Kushnir, and M. Visbeck (2001), The North Atlantic Oscillation, Science, 291(5504), 603–605. records do not show any evidence of the deeper LSW class Kieke, D., M. Rhein, L. Stramma, W. M. Smethie, D. LeBel, and W. Zenk (LSW1987 – 94) formed outside the Labrador Sea. This fact (2006), CFC inventories and formation rates of Upper Labrador Sea reinstates the true source of this water in the Labrador Sea. Water, 1997–2001, J. Phys. Oceanogr., 36, 64–86. Lazier, J. R. N. (1980), Oceanographic Conditions at OWS Bravo 1964– The volume of LSW is notably larger in the Labrador Sea 1974, Atmos. Ocean, 18(3), 227–238. than elsewhere, meaning that its fresh and cold anomalies, Lazier, J. R. N., R. M. Hendry, R. A. Clarke, I. Yashayaev, and P. Rhines especially when associated with larger volumes, may result (2002), Convection and restratification in the Labrador Sea, 1990–2000, in comparable or even larger anomalies downstream. This Deep Sea Res., Part I, 49(10), 1819–1835. McCartney, M. S., and L. D. Talley (1982), The subpolar mode water of the explains why the observed LSW anomalies ‘‘preserve’’ their North Atlantic Ocean, J. Phys. Oceanogr., 12, 1169–1188. magnitude after leaving the Labrador Sea. Pickart, R. S., F. Straneo, and G. W. K. Moore (2003), Is Labrador [32] In addition to showing new vintages of LSW spread- Sea Water formed in the Irminger Sea?, Deep Sea Res., Part I, 50, 23–52. ing across the ocean, Figure 3 reveals the source and Straneo, F., R. S. Pickart, and K. Lavender (2003), Spreading of Labrador spreading of the recent warming and salinification of the Sea Water: An advective-diffusive study based on Lagrangian data, Deep mid depths. The westward pointing arrows in this figure Sea Res., Part I, 50, 701–719. Sy, A., M. Rhein, J. R. N. Lazier, K. P. Koltermann, J. Meincke, A. Putzka, (Figure 3) underline the essence of the three-dimensional and M. Bersch (1997), Suprisingly rapid spreading of newly formed exchange between the Labrador and Irminger basins main- intermediate waters across the North Atlantic Ocean, Nature, 386, tained by the cyclonic circulation within each basin. After 675–679. most of LSW has drained from the Labrador Sea, the Talley, L. D., and M. S. McCartney (1982), Distribution and circulation of 1987 – 94 Labrador Sea Water, J. Phys. Oceanogr., 12, 1189–1205. anomalously warm and salty waters entering the Labrador- van Aken, H. M., and C. J. de Boer (1995), On the synoptic hydrography of Irminger gyre from the east and southeast (e.g., ISW intermediate and deep water masses in the Iceland Basin, Deep Sea Res., arriving from the Reykjanes Ridge) become noticeable Part I, 42, 165–189. Yashayaev, I., and R. A. Clarke (2006), Recent warming of the Labrador and their pathway can be mapped (Figure 3). This indicates Sea, AZMP Bull., 5, 12–20. that the Irminger-Labrador gyre receives waters from mul- Yashayaev, I., J. R. N. Lazier, and R. A. Clarke (2003), Temperature and tiple sources and passes their anomalous features in both salinity in the central Labrador Sea, ICES Mar. Sci. Symp., 219, 32–39. eastward (LSW) and westward (e.g., ISW) directions. H. M. van Aken, Royal Netherlands Institute for Sea Research, P.O. Box [33] Acknowledgments. The authors thank three anonymous 59, NL-1790 AB Den Burg/Texel, Netherlands. reviewers for valuable comments and suggestions and express their M. Bersch, Institute of Oceanography, University of Hamburg, Bundesstr. gratitude to Allyn Clarke, John Lazier, Jens Meincke, Bob Dickson, and 53, D-20146 Hamburg, Germany. many others who over five decades surveyed, explored, and monitored the I. Yashayaev, Bedford Institute of Oceanography, Fisheries and Oceans subpolar basins. The 2004 A1E hydrographic data are courtesy of Detlef Canada, 1 Challenger Drive, P.O. Box 1006, Dartmouth, NS, B2Y 4A2, Quadfasel and John Mortensen, and were collected with funding by the Canada. (yashayaevi@ mar.dfo-mpo.gc.ca) Bundesminister fu¨r Bildung und Wissenschaft (German CLIVAR) and the EU Commission (ASOF-E). Finally, many friends and colleagues of the authors contributed to this study by sharing their best skills and spirits on land and, especially, at sea.

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