Breaking the Linkage Between Labrador Sea Water Production and Its Advective Export to the Subtropical Gyre

JULY 2016 Z O U A N D L O Z I E R 2169

SIJIA ZOU AND M. SUSAN LOZIER Division of Earth and Ocean Sciences, Duke University, Durham, North Carolina

(Manuscript received 28 October 2015, in ﬁnal form 10 April 2016)

ABSTRACT

Deep water formation in the northern North Atlantic has been of long-standing interest because the re- sultant water masses, along with those that flow over the Greenland–Scotland Ridge, constitute the lower limb of the Atlantic meridional overturning circulation (AMOC), which carries these cold, deep waters southward to the subtropical region and beyond. It has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses. However, recent observations of Lagrangian floats have raised questions about this linkage. Motivated by these observations, the relationship between convective activity in the Labrador Sea and the export of newly formed Labrador Sea Water (LSW), the shallowest component of the deep AMOC, to the subtropics is explored. This study uses simulated Lagrangian pathways of synthetic floats produced with output from a global ocean–sea ice model. It is shown that substantial recirculation of newly formed LSW in the subpolar gyre leads to a relatively small fraction of this water exported to the subtropical gyre: 40 years after release, only 46% of the floats are able to reach the subtropics. Furthermore, waters produced from any one particular convection event are not col- lectively and contemporaneously exported to the subtropical gyre, such that the waters that are exported to the subtropical gyre have a wide distribution in age.

1. Introduction and background The exchange of heat and freshwater across the air– sea interface and the horizontal advection of heat and Labrador Sea Water (LSW) is a critical component of salt into the basin via boundary currents have been the deep, southward limb of the Atlantic meridional proposed as the two major factors that create variability overturning circulation (AMOC). Numerical simula- in LSW volume and properties (Stramma et al. 2004). tions indicate that the changes in AMOC are linked to Interannual and interdecadal variability of LSW prop- the rate of LSW production on interannual to decadal erties, measured using up to 60 years of hydrographic time scales (Böning et al. 2006; Biastoch et al. 2008; data, have been linked to changes in the North Getzlaff et al. 2005). Strong cooling during winters leads Atlantic Oscillation (NAO) index (Stramma et al. 2004; to unstable surface stratification that drives convective Curry et al. 1998; Kieke and Yashayaev 2015). A rela- overturning to depths of approximately 1500 m in the tively shallow, warm, and salty layer of LSW was pro- central Labrador Sea (Talley and McCartney 1982). The duced from the 1950s to 1970 when the NAO index was product of this overturning is a distinct water mass negative, a state characterized by a small difference (LSW) with relatively low temperature, salinity (Talley between the Azores high and Icelandic low, reduced and McCartney 1982; Pickart et al. 2002; Stramma et al. heat flux to the atmosphere, and weak westerlies in the 2004; Yashayaev 2007), and potential vorticity (PV; subpolar region (Sarafanov 2009; van Aken et al. 2011). Talley and McCartney 1982; Stramma et al. 2004) and The decreased heat flux and the relatively weak west- high concentrations of dissolved oxygen (Pickart et al. ward extension of the subpolar gyre together contrib- 2002) and chlorofluorocarbons (CFCs; Rhein et al. 2002; uted to the warm and salty intermediate water formed in Kieke et al. 2007). the Labrador Sea (Sarafanov 2009). In the early 1990s, when the NAO was positive, the strongest recorded Corresponding author address: Sijia Zou, Division of Earth and convection occurred, resulting in a thick, cold, and fresh Ocean Sciences, Duke University, Box 90227, Durham, NC 27708. LSW layer. From 1994 until 2008, weak convection and E-mail: [email protected] steady warming were observed. Enhanced LSW formation

DOI: 10.1175/JPO-D-15-0210.1

Ó 2016 American Meteorological Society 2170 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46 resumed in 2008 and was attributed to strong atmo- For decades it was assumed that convection strength spheric cooling (Yashayaev and Loder 2009). Most re- in the Labrador Sea would alter deep water export from cently, observations in the central Labrador Sea in May the subpolar gyre and modify downstream properties at of 2015 revealed a strong convection during the 2014/15 intermediate depths in the North Atlantic. As for the winter (I. Yashayaev 2015, personal communication). latter, observational studies have shown a strong cor- Subsequent to its formation, LSW spreads to other parts relation between LSW thickness and property anoma- of the North Atlantic. From the identification of waters lies near Bermuda (Curry et al. 1998), with the former with a PV minimum in the basin, Talley and McCartney leading by 6 years. Similarly, Molinari et al. (1998) and (1982) identified three major spreading pathways for this van Sebille et al. (2011) both reported a 10-yr transit water mass. The first pathway they identified is a south- time of the property signals from the Labrador Sea to ward branch along the western boundary, where low PV is 26.58N in the DWBC within LSW layers. A more recent detectable as far as 208N. Another of their identified study (Pena-Molino et al. 2011) observed that water spreading pathways extends northeastward into the Ir- properties in the classical LSW layer within the DWBC minger Sea, while the third spreads eastward across the at the Line W mooring array (398N), measured in the subpolar North Atlantic. These pathways have been fur- early 2000s, reflect the intense Labrador Sea convection ther confirmed by temperature, salinity, and CFC data (Sy during the mid-1990s, indicating a 9-yr propagation time et al. 1997). Rhein et al. (2002) quantified the three LSW scale from the central Labrador Sea to Line W. As for the spreading pathways using tracer and hydrographic data. relationship between LSW production and deep water According to their results, 21% of the CFC inventory is export, some past studies indicate a linkage between found south of 538N, 20% enters the Irminger Sea, and LSW production and DWBC strength (Böning et al. 31% intrudes the eastern subpolar gyre. The rest of the 2006; Han et al. 2010), yet others show a contrary result inventory is still in the Labrador Sea. (Schott et al. 2004; Dengler et al. 2006). Relatedly, a Recent studies have focused on the spreading of LSW linkage between LSW production and AMOC strength, in a Lagrangian frame by tracking floats launched in this via fast boundary wave propagation, has been revealed water mass. Lavender et al. (2005) studied the middepth from a modeling study (Biastoch et al. 2008)within- circulation in the subpolar North Atlantic with neutrally terannual buoyancy forcing. However, when interannual buoyant profiling floats. Floats that left the Labrador wind forcing was also considered, the LSW production Sea initially drifted southeastward along the Labrador and AMOC relationship was masked by higher-frequency slope, but none followed the Deep Western Boundary variability in the AMOC anomalies. Despite this focus on Current (DWBC) beyond 448N, a current previously LSW production and its downstream impact, currently considered as the major conduit for subpolar water unanswered is whether there is a relationship between masses to reach the subtropical basin. Similarly, for the LSW production and the export of that water mass to RAFOS floats launched at LSW depths in the DWBC the subtropical North Atlantic through advection. Un- off the Labrador coast from 2003 to 2006, only 8% were derstanding this relationship, which differs from the able to enter the subtropical basin via the DWBC LSW production/DWBC and LSW production/AMOC (Bower et al. 2009). Instead, the majority of the RAFOS relationships, is the goal of this study. floats that reached the subtropical basin from the sub- Specifically, we seek to understand the extent to which polar latitudes did so by interior pathways (Bower et al. LSW production impacts the volume of newly formed 2009; Lozier et al. 2013; Lozier 2012), a finding with LSW and blended LSW that are advected across the implications for where and how the deep limb of the intergyre boundaries to the subtropical basin, and across AMOC is measured. The dynamics of these interior 308N, where LSW and the other components of the pathways, and their favorable comparison with tracer southward-flowing North Atlantic Deep Water are less spreading patterns, have recently been explored (Gary likely to recirculate back to the subpolar gyre. By doing et al. 2011, 2012). so, we aim to improve the understanding of how con- This focus on interior pathways, however, has over- vection in the Labrador Sea impacts the lower limb of shadowed another interesting feature of those observed the AMOC. floats, namely, that only 30% of the RAFOS floats were The rest of the paper is organized as follows: section 2 exported to the subtropical gyre over their 2-yr lifetime, describes the data and methods used in this paper. In despite the fact that all of the floats were launched in section 3, we describe our major results, with a focus on LSW in the DWBC. This observation raises the question the spreading pathways of LSW in a Lagrangian frame as to the mechanism that determines how much LSW is and the relationship between LSW production and its exported from the subpolar to the subtropical gyre export to the subtropical gyre. A summary is given in each year. section 4. JULY 2016 Z O U A N D L O Z I E R 2171

et al. (2006, 2007), the configuration of ORCA025, which has 1442 3 1021 grid points and 46 vertical layers, is based on the Nucleus for European Modeling of the Ocean (NEMO) system. Vertical grid spacing increases from 6 m near the surface to 250 m at the bottom. Horizontal resolution increases with latitude, with the coarsest resolution, 27.75 km, at the equator. The model uses 2-min gridded bathymetric data (ETOPO2) from the National Geophysical Data Center (now the National Centers for Environmental Information), and initial conditions are set with a combination of temperature and salinity data derived from Levitus et al. (1998), the Polar Science Center Hydrographic Climatology version 2.1 (PHC2.1; Steele et al. 2001), and the Medatlas climatology (Jourdan et al. 1998). FIG. 1. Climatological PV (calculated as described in section 2d) As reported by Barnier et al. (2006), the climatologi- and salinity (black contours) from ORCA025 averaged over the LSW layers (700–1500 m). Modeled AR7W is designated with red cal daily mean wind stress vector, derived from ERS dots. Bathymetry shallower than 700 m is shaded gray (land masses scatterometer data (CERSAT 2002) and the NCEP– are dark gray); 1500- and 3000-m isobaths are contoured in gray. NCAR reanalysis (Kalnay et al. 1996), is used to provide the surface momentum flux. An empirical bulk param- eterization (Goosse 1997) is used to compute the surface 2. Data and methods heat fluxes and freshwater fluxes, with climatological In this section, we summarize the observational (sec- daily mean air temperatures from the NCEP–NCAR tion 2a) and model (section 2b) data used in this analysis, reanalysis; climatological monthly mean precipitation and then we compare the model fields with observations from CMAP (Xie and Arkin 1997); and monthly mean in the Labrador basin (section 2c). In section 2d,we humidity (Trenberth et al. 1989), cloud cover (Berliand give a definition for newly formed LSW, which is used and Strokina 1980) and climatological daily mean for the float launch configuration described in section 2e. wind speed from a blended ERS and NCEP–NCAR The definition of the intergyre boundaries, which sepa- reanalysis. rate the subpolar and subtropical gyres, is described in The 5-day model output used in this study is from the section 2f. model run forced with the global hindcast dataset from 1961 to 2004. Lagrangian trajectories produced from this a. Hydrographic data model output have been shown to reproduce realistic The Labrador Sea Monitoring Program of Fisheries signatures of the deep recirculation gyres in the North and Oceans Canada has been conducting oceanographic Atlantic (Gary et al. 2011). Gary et al. (2012) also observations in the Labrador Sea since 1990 along the demonstrate that ORCA025 accurately reproduces the Atlantic Repeat Hydrography Line 7 West (AR7W), distribution of CFC-11 tracers at LSW depths in the which extends from Hamilton Bank on the Labrador North Atlantic. As will be shown in section 3b, shelf to Cape Desolation on the Greenland shelf (Fig. 1). ORCA025 can reproduce the LSW spreading pattern It has been occupied annually, typically in May, allowing depicted by RAFOS floats. Additionally, the modeled for a determination of LSW vertical structure at the end LSW volume transport at 538Nof211.9 6 0.9 Sv (1 Sv [ 2 of each winter. To compare with the model’s LSW 106 m3 s 1) compares favorably with the observed properties, we use the hydrographic data collected from transport of 211.3 6 1.0 Sv based on shipboard lowered 1992 to 2004 (CDIAC 2015). ADCP measurements (Fischer et al. 2010). Therefore, we consider ORCA025 highly suitable for the purposes b. ORCA025 of our study. To conduct our study, we use ORCA025, a global c. Model–observation comparison of LSW properties ocean–sea ice model implemented on a quasi-isotropic Oceanic Remote Chemical Analyzer (ORCA) grid (a To ascertain the ability of the model to capture the tripolar grid) at eddy-permitting resolution (1/48) under variability of LSW properties in the Labrador Sea, we the framework of Drakkar, a European modeling proj- compare the model’s temperature, salinity, and density ect (Barnier et al. 2006, 2007). As described by Barnier fields at LSW depths (between 700 and 1500 m) along 2172 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46

because LSW in the ORCA025 model is saltier and thus denser than that observed (Fig. 2). e. Trajectory computation and Labrador Sea launch configuration Synthetic floats were launched along a section in the model that replicates AR7W. All launches were dynamic: floats were launched from each grid along AR7W only when properties in a particular grid met the established water mass criteria, given above as PV , 4 3 2 2 2 2 10 12 m 1 s 1 and density within [27.75, 27.84] kg m 3 (Fig. 3). A year-to-year comparison of the number of floats launched in newly formed LSW along AR7W with FIG. 2. Time series of observed (black solid lines) and modeled the total number of floats launched in newly formed (gray dashed lines) temperature, salinity, and density averaged LSW in the entire Labrador Sea yields a correlation between 700 and 1500 m along AR7W. Note that the modeled salinity is slightly higher than observed, and thus the modeled LSW coefficient of 0.93. Therefore, the number of floats is slightly denser. launched along AR7W for any 5-day period can be used as a proxy for the amount of LSW present for that AR7W to observations, using only model data contem- same period. poraneous with the observed data. As seen in Fig. 2, the Though floats are launched every 5 days from 1961 to model’s long-term trends and interannual variability 2004, only one launch is selected each year for the cal- compare favorably with those from observations: the culation of trajectories. Since we are interested in the decadal trend for observed and modeled temperatures export of newly formed LSW following its wintertime 2 are 0.058 and 0.038Cyr 1, respectively, and after de- production, we choose the launch for which the float trending the time series, the standard deviations (SDs) number is maximized. As such, float integration for each are 0.048 and 0.058C, respectively. For salinity, the trends year begins from the 5-day period when the water mass between observations and the model are 0.0009 and volume has reached its maximum for that winter. We 2 0.0003 psu yr 1, with the SD of 0.006 psu for the former note that convection during one winter may reach the and 0.006 psu for the latter after detrending. For density, depth of fossil LSW, that is, LSW formed the previous 2 2 the trend is 20.004 kg m 3 yr 1 for observations year or years. In this case, the fossil LSW would be 2 2 and 20.003 kg m 3 yr 1 for the model. After detrending, considered part of the newly formed LSW as long as it 2 2 the SD for density is 0.006 kg m 3 and 0.004 kg m 3 for shares the low PV signature. the observations and the model, respectively. For each launch site, float trajectories were calculated from the model’s three-dimensional velocity field using d. Definition of LSW Ariane, a Fortran code for trajectory computation We use both PV and density to identify LSW in the (Blanke and Grima 2010). All float trajectories were model. Assuming that relative vorticity is small com- integrated for 40 years, and for those launched after pared to planetary vorticity, PV can be approximated as 1964, velocity fields were recycled with a single discontinuity between 31 December 2004 and 1 January 1961. N2 This method has been successfully used in previous PV 5 f , g studies (Gary et al. 2011, 2012). We have also tested the validity of this method by comparing the trajectories where f is the Coriolis parameter, g is the gravitational computed from sequential years of data and those constant, and N2 is the Brunt–Väisälä frequency, which computed from data with this discontinuity (not shown). is defined as 2(g/r)(dr/dz)(Talley and McCartney The difference in trajectories between the two is in- 1982), where r is density. Thus, PV will be small for a consequential to our results. water mass that is weakly stratified in the vertical. In this f. Intergyre boundaries study, LSW is defined as the water mass in the density 2 2 2 2 range [27.75, 27.84] kg m 3 with PV , 4 3 10 12 m 1 s 1, To quantify export from the subpolar gyre and import which is the same threshold used by Talley and into the subtropical gyre, we define two boundaries, re- McCartney (1982). These density limits for LSW differ ferred to as the intergyre boundaries. These boundaries slightly from those typically used for observed classical are calculated using the model’s surface dynamic height 2 LSW ([27.74, 27.80] kg m 3; Stramma et al. 2004) anomaly DD relative to 1000 m, which is defined as JULY 2016 Z O U A N D L O Z I E R 2173

FIG. 3. PV along the model’s AR7W on (left) 26 Mar 1968 and (right) 1 Mar 1990, when the maximum volume of LSW was formed for each year; 75 floats were launched on the former date and 521 on the latter. Black dots indicate 2 2 2 float launch positions. White contours denote where PV is smaller than 4 3 10 12 m 1 s 1, and black dashed lines 2 indicate where density is between 27.75 and 27.84 kg m 3. To avoid surface intensification of floats induced by de- creasing model vertical resolution with depth, floats were released at a fixed vertical interval, 200 m, which is the maximum vertical resolution in the upper 2600 m of the model.

ð P sfc North Atlantic are summarized in sections 3b and 3c, DD 5 (a 2 a ) dp, S,T,P 35,0,P respectively. In section 3d, the relationship between P 1000 LSW formation and blended LSW export is studied. a a where S,T,P is the in situ specific volume and 35,0,P is a. Interannual variability of LSW formation and its the specific volume of seawater at a standard tempera- link to NAO ture and salinity. Variables S, T, and P represent in situ salinity, temperature, and pressure, respectively As mentioned above, we have chosen to quantify the (Knauss 1997). Dynamic height anomaly DD is com- amount of LSW formed each year in the model with the puted for each grid in the basin domain at each time step number of floats launched in newly formed LSW. To and then averaged over all 44 years to produce a mean ascertain the representativeness of the float number as a value DD at each grid. We choose the DD contour of proxy for LSW formation, we compare the time series of 2 9m2 s 2 as the subpolar boundary, north of which is considered to be the subpolar region, and the DD con- 2 tour of 11 m2 s 2 as the subtropical boundary, south of which is the subtropical basin (Fig. 4). As will be shown below, some floats that cross the subtropical boundary are carried back to the subpolar basin at one or more times during their 40-yr integration. Therefore, we set another boundary in the subtropics, 308N, from which we do not expect the floats to return to the subpolar basin. As shown by previous studies (Bower et al. 2009) and also shown below, floats that have been exported south of 308N are likely to join the southward-flowing DWBC. By 238N, almost all floats are within the DWBC. For this reason, we also use 238N as the latitude where LSW is primarily contained within the DWBC.

3. Results FIG. 4. The 44-yr (1961–2004) mean surface dynamic height 2 anomaly (DD) in the North Atlantic. Black contours (9, 11 m2 s 2) Section 3a describes the interannual variability of indicate the intergyre boundaries. The black dashed line across LSW formation and its relation to NAO. Detailed 308N serves as another model boundary; 700-, 1500-, and 3000-m pathways and the ages of newly formed LSW in the isobaths are contoured in gray. 2174 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46

FIG. 5. Interannual variability of ﬂoat number anomaly (solid red) and LSW thickness anomaly (dashed blue) derived from ORCA025 when the strongest convection takes place each year. The winter NAO index (gray bars) is also shown from 1961 to 2004. The winter NAO index is computed by averaging the monthly NAO index from December to March each year. Data are from NOAA/NWS/CPC (2015).

float number with the LSW thickness in the model 508N every 3 months from 2003 to 2005. To verify that (Fig. 5). The model LSW thickness is computed as the ORCA025 can accurately reproduce this pattern, test average depth along AR7W of the water that has PV , floats were launched within the model with a similar 2 2 2 4 3 10 12 m 1 s 1 and density between 27.75 and observational design, namely, model floats were 2 27.84 kg m 3 on the same day of the float release. The launched at 508N at 700 and 1500 m every 3 months from two variables yield a strong correlation (r 5 0.91), 1961 to 2004 and then integrated forward for 2 years. An leading us to conclude that the number of floats example of the float trajectories within 2 years is shown launched when the maximum amount of newly formed in Fig. 6. Of the floats launched over many releases, LSW is present is a good indicator of the interannual 27% 6 6% were able to reach the southern tip of the variability in LSW thickness, which has been the tradi- tional measure. Actually, we believe that the float number provides a more accurate estimate of LSW formation since the number of floats released depends not just on the vertical extent of the newly formed water, but on its lateral extent as well. As can be seen from Fig. 5, Labrador Sea convection has strong interannual to interdecadal variability, which has been previously linked to the winter NAO index (Kieke and Yashayaev 2015; Sarafanov 2009; van Aken et al. 2011; Kieke et al. 2007; Rhein et al. 2011). A negative NAO winter index has been linked to weaker LSW production from the 1960s to 1970s, while a per- sistent positive NAO since the 1980s has resulted in strong convective activity and greater LSW formation. 5 FIG. 6. Test float trajectories launched at LSW depths in years The float number anomaly is correlated with NAO (r from 2000 to 2002 with 2-yr lifetimes. Initial launch locations for all 0.62), as is the model’s LSW thickness anomaly (r 5 108 floats (72 at 700 m and 36 at 1500 m) at 508N are shown in red, 0.66). Both correlations are significant at the 95% con- and the final positions are indicated with black dots. Colors rep- fidence level based on a t test. resent the normalized temperature anomaly (8C) along the path of each float, computed following Bower et al. (2009):(T 2 Ti)/dTmax, b. Pathways of newly formed LSW where Ti is each float’s initial temperature, and dTmax is the maximum temperature difference: 38C for floats launched at 700 m and Bower et al. (2009) have shown the 2-yr spreading 0.728C for those launched at 1500 m. The 700-, 1500-, 3000-m iso- pathways of RAFOS floats launched in the DWBC at baths are contoured in gray. JULY 2016 Z O U A N D L O Z I E R 2175

FIG. 7. Probability maps of trajectories 40 years after launch on 1 Mar 1990: (a) never exported floats (17% of total), (b) recirculated floats (15%), (c) exported floats (42%), and (d) in-between floats (26%). The probability map is created by first dividing the North Atlantic into 0.25830.258 grids, counting the number of times floats pass through each grid (including repetitions), and then dividing the number of passes in each grid by the total float passes over all grids (Gary et al. 2012). Black solid lines represent the intergyre boundaries. Black dots indicate final float positions; 700- and 1500-m isobaths are contoured in gray.

Grand Banks (438N), with 7% 6 6% following the crossed only once during their lifetime, i.e., they DWBC continuously. The majority of floats (73% 6 never returned to the subpolar gyre, and those that 6%) drifted eastward and northeastward within the were repeatedly exported, i.e., they crossed the subpolar gyre, which compares favorably to the ob- intergyre boundaries several times before they ended served percentage (70%; 28/40) of RAFOS floats that their mission in the subtropical basin); and took this route (Bower et al. 2009). Thus, based on this 4) in between—these floats ended their mission located comparison, we consider the model capable of approx- between the subpolar and subtropical boundaries. imating the spreading pathways of LSW. An example of the spatial distribution of each tra- To describe the pathways of the simulated float trajectory category is revealed by a probability map (Fig. 7) jectories after the AR7W launch, floats are placed in constructed from pathway positions for 40 years after four categories: the AR7W launch in 1990. Probability maps from other 1) never exported—these floats circulated solely within launch years yield qualitatively the same maps. In gen- the subpolar basin during their entire lifetime and eral, floats are confined north of 308N during the 40 years did not cross either intergyre boundary; of integration. At the end of those 40 years, 31% of them 2) recirculated—these floats crossed the subtropical reside in the subpolar gyre: 17% circulated in the sub- boundary once or more, but they recirculated back polar basin the entire time, while 14% returned to the and ended their mission in the subpolar gyre; subpolar basin after one or more exports to the sub- 3) exported—these floats crossed the subtropical tropical gyre. Only 46% of the floats ended up in the boundary and at the end of their mission were in subtropical gyre 40 years after launch, of which 7% the subtropical gyre (this category includes those that crossed the intergyre boundaries only once and 17% 2176 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46

40 years, an indication of the long residence time for LSW in the subpolar basin. The percentage of floats between boundaries, however, remains relatively constant after 25 years, indicating a smaller residence time scale for this region. c. The age of the exported LSW So far, we have seen that only 46% of the floats launched along AR7W end up in the subtropical gyre after 40 years, which indicates a long advective time scale for LSW to reach the subtropical gyre. A calculation and mapping of LSW age confirms this expectation (Fig. 9, left). The youngest LSW is found in the Labrador and Irminger Seas, as well as along the DWBC, through FIG. 8. The percentage of floats in the subpolar gyre (blue), in the which floats reach the subtropical gyre within 15 years subtropical gyre (red), and in the area bounded by the intergyre (the fastest float took less than 3 years). The average age boundaries (black) as a function of time after the launch across of floats at the intergyre boundary region is 22 6 10 years, AR7W. The solid curves designate the percentage averaged over all 44 releases while the colored dots represent the percentages for which is also evident in the cross-sectional plot of age each release. along AR7W in Fig. 9 (right). As discussed above, the majority of the floats launched in the central basin along AR7W took 20 years on average to first cross the ended up south of 308N. The floats that reached the subtropical boundary. However, floats launched in the subtropical gyre did so by a number of pathways, in- DWBC, which account for less than 5% of the total cluding those in the central and eastern North Atlantic launched, took less than 15 years to be exported to the basin. The central basin pathways were described pre- subtropical gyre. viously by recent studies focused on interior pathways At 308N, where floats are less likely to recirculate back (Bower et al. 2009; Lozier et al. 2013) of the subpolar to to the subpolar gyre, the age is 30 6 8 years. The age subtropical export. The eastern basin pathway is along distribution along AR7W of the floats that reach 308Nis the eastern flank of the Reykjanes Ridge, in agreement similar to that shown in Fig. 9 (right), but with greater with prior observational studies (Kieke et al. 2009; ages. The spatial distribution of LSW age shown in Fig. 9 Rhein et al. 2015). This pathway is similar to the (left) is consistent with the age of simulated LSW from southward pathway of Iceland–Scotland Overflow Wa- Gary et al. (2012). Using a CFC observational dataset, ter (ISOW) revealed from both observational and Rhein et al. (2015) show a similar LSW age distribution: modeling studies (Lankhorst and Zenk 2006; Xu their youngest waters are in the Labrador and Irminger et al. 2010). Seas, age increases eastward from south of the Grand Repeating the above analysis for all 44 launches Banks to the Rockall Trough near the intergyre (1961–2004) reveals that there is relatively little vari- boundary region, and age in the western subtropical ability over time (Fig. 8): on average, 34% of the total gyre increases steadily to the south. In Rhein et al. floats stayed within the subpolar basin 40 years after (2015), the age of young LSW (age , 40 years) is 16 launch, 46% were able to reach the subtropics, and 20% years when it crosses the intergyre boundary and 22–24 stayed between the two boundaries. Despite the signif- years at 308N. Both of these ages fall in the range derived icant change in the number of floats launched each year, from Lagrangian floats in this study. the fractions remain relatively constant (67%) except Considering the long residence time for LSW in the for the years 1961, 1969, and 1971, when fewer than 10 subpolar basin, the volume export at the subtropical floats were launched along AR7W, a paucity that in- boundary or at 308N for any particular year is expected dicates weak convective activity in the Labrador Sea to be composed of waters with many different ages. The during those years. We consider these floats too few to results in Fig. 10 confirm this expectation. In 2003, floats draw conclusions on their preferred pathways, but given exported across the subtropical boundary, as well as the pathways from the other years, we feel confident in floats arriving at 308N, are of various ages and, impor- our assessment that there is little interannual variability tantly, there is no distinguishable difference in the in LSW pathways. number of floats from one age to the next, even though The decrease (increase) in the percentage of floats in the number of the floats launched varies significantly the subpolar (subtropical) gyre persists over the full with time. Also, the number of floats from any given JULY 2016 Z O U A N D L O Z I E R 2177

FIG. 9. (left) Average age of floats for 44 releases of 40 years: the whole domain is divided into 0.25830.258 grids, and the age for each grid is computed by averaging the time elapsed since launch for each particle. Repeated visits by the same float to the same grid are included in the age calculation. To avoid biasing the average, only when the box has more than 100 float occurrences is the mean age computed. The black solid lines indicate the subpolar boundary (north) and the subtropical boundary (south); the black dashed line indicates 308N. Initial launch locations are shown in red; 700-, 1500-, and 3000-m isobaths are shown in gray. (right) The average age of all floats from 44 releases that cross the subtropical boundary in 40 years as a function of initial position along AR7W. Black dashed contours show 2 where climatological density is between [27.75, 27.84] kg m 3; white contours show the area with PV smaller than 4 3 2 2 2 10 12 m 1 s 1. launch year that contributes to the 2003 export is quite Sea has been estimated to be ;2Sv(Pickart and Spall small (;2%) compared to the number of floats launched 2007); thus, the contribution of the transformed water each year, which, as discussed above, is a measure of the mass to the amount of LSW exported each year is convective strength in the Labrador Sea. Similar results 0.04 Sv, a negligible quantity compared to the 11.3 6 are observed in other years’ export. In the Eulerian 1.0 Sv of LSW within DWBC at 538N(Fischer et al. frame, the vertical diapycnal mass flux in the Labrador 2010). In other words, from both an Eulerian and

FIG. 10. Histogram of the number of ﬂoats from each launch year exported south of the subtropical boundary (light blue bars) and 308N (dark blue bars) in 2003. Initial launch number is plotted with a black solid line; its average is denoted by the red dashed line. On average, only 2% of ﬂoats released each year along AR7W contribute to the 2003 LSW export across the subtropical boundary. 2178 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46

FIG. 11. Positions of simulated floats launched in the Labrador Sea when they crossed the 2 2 subtropical boundary in 2003 (black dots), with annually averaged PV (m 1 s 1) shown in 2 2 2 color. White contours outline the area where PV is smaller than 12 3 10 12 m 1 s 1, and the 2 black dashed line indicates density between [27.75, 27.84] kg m 3. When they reached the subtropical boundary, over 70% of floats (launched from 1961 to 2002) were located within the area bounded by the two density contours. The PV map and float distributions shown here for 2003 are representative of other years.

Lagrangian perspective, convection strength in the launches were conducted at 238N, where southward-moving Labrador Sea and newly formed LSW export are not floats are concentrated within the DWBC and between 2 tightly coupled, as discussed further below. isopycnals of [27.75, 27.84] kg m 3 (Fig. 13). Again, the time series of blended LSW export to the south of 238N is derived d. Convection in the Labrador Sea and the export from the number of floats that crossed 238Neachyear. of blended LSW to the subtropics Figure 14 (top) shows the lead–lag correlation co- The above analyses make clear that there is no efficients between the convection strength in the Labrador coherent arrival of LSW with the same age. Thus, to Sea, given by the float number launched each year along further investigate the relationship between LSW AR7W, and the blended LSW export to the subtropical production and its export, we no longer identify floats by gyre. The coefficient reaches a maximum of 0.63 with the their launch year in the Labrador Sea. Instead, we seek blended LSW export lagging by 3 years, yet after detrend- to understand the relationship between LSW production ing, no significant correlation is observed. A maximum and the blended LSW export to the subtropical gyre, correlation coefficient of 0.55 is found between convection regardless of age. To identify the locations of blended and the export volume across 238N at 0 lag–lead; a 0.38 LSW near the subtropical boundary, we locate all coefficient is computed after detrending the two time series, AR7W launched floats when they are crossing the with convection leading the export by 3 years. boundary (Fig. 11). In Fig. 11, floats are concentrated Since the advective time scale for floats from AR7W 2 2 2 within the area where PV is ,12 3 10 12 m 1 s 1 and to the subtropical gyre (238N) is on average 22 (30) years, 2 density is between [27.75, 27.84] kg m 3 west of the Mid- the high correlations (before detrending) are not con- Atlantic Ridge. These property ranges are chosen as the sidered to result from a causal relationship between criteria for the new float launches into blended LSW. convection strength and LSW advective export. Rather, Floats were then released on each 5 January north of the we suggest that the fast response of blended LSW export subtropical boundary (Fig. 12) from 1961 to 2004, and the to LSW formation results from a boundary density blended LSW export for each year is the total number of anomaly induced by convection in the Labrador Sea, the floats that crossed the subtropical boundary by the end of signal of which can be propagated southward quickly that year. We consider this time series an indication of ex- though boundary waves. This supposition is supported port variability for blended LSW, rather than LSW linked by a modeling study that showed an overall strength- to water mass formation in a given year. Similar dynamic ening of the AMOC within 1–2 years after LSW JULY 2016 Z O U A N D L O Z I E R 2179

FIG. 12. Initial (blue dots) and final (red dots) locations of floats FIG. 13. As in Fig. 12, but for blended LSW across 238N (black launched on 5 Jan 2003 that were able to reach the subtropical dashed line). The inset shows the positions of those AR7W basin within 1 year. This float distribution map differs only slightly launched floats when crossing 238N, superposed on the 44-yr mean from year to year. One-tenth of the total data points were randomly meridional velocity, with black contours outlining the density be- 23 selected for plotting. Black solid curve indicates the subtropical tween [27.75, 27.84] kg m . This distribution sets the criteria for boundary. The black dashed box (north of the subtropical bound- the blended LSW launch north of 238N (PV at this location is not ary) indicates the dynamic launch area: longitude [758W, 268W], distinguishable and thus is not used to identify LSW). Again, the 2 2 2 latitude [358N, 508N], PV [0, 12 3 10 12]m 1 s 1, and density float distribution map and velocity field are representative of all 2 [27.75, 27.84] kg m 3. Only a few floats launched outside the box other years. made it to the subtropics within a year. The 700-, 1500-, and 3000-m isobaths are contoured in gray.

travel near the western boundary, yet these floats ac- convection (Biastoch et al. 2008). Also demonstrated count for less than 5% of the total. in this study is that the AMOC response to convection An analysis of the age of LSW when it crosses into is primarily on decadal time scales. The fact that there the subtropical gyre shows that it is a combination of are negligible correlations after detrending (which waters formed years or even decades prior to the year dampens decadal variability) also validates our as- of the crossing. We show that floats launched in a sessment that convective variability in the Labrador particular winter contribute only marginally to future Sea cannot explain the downstream variability of LSW volume exports. Thus, we conclude that the LSW export through advection. contribution of a particular winter’s convection to the total LSW export in any given subsequent year is too small to appreciably impact the volume of 4. Summary that export. In this study, we use trajectories of synthetic floats We extend our analysis to include blended LSW ex- launched in the Labrador Sea in an ocean general cir- port, by which the water mass is defined by its hydro- culation model to simulate the spreading pathways of graphic properties only, not by its age. We find no newly formed LSW. We show that only 46% 6 7% of linkage between LSW formation and the export of the LSW formed during winter is able to reach the blended LSW across the subtropical boundary or 308N subtropical boundary after 40 years. The rest of the through advection. Rather, the water mass export in this water mass largely recirculates within the subpolar gyre layer appears to respond to Labrador Sea convection via or is resident in the area between the intergyre bound- fast western boundary waves with a time lag of no more aries. The exported floats primarily enter the subtropics than 1–2 years. via interior pathways that extend from the western The relatively long time for LSW to reach 308N (30 boundary to the eastern basin of the North Atlantic, years on average) stands in contrast to the arrival time of though not all of those pathways indicate a direct route LSW from the Labrador basin to the subtropical gyre for export. Some floats that cross into the subtropical based on property correlations: Curry et al. (1998) gyre recirculate back to the subpolar basin more than show a high correlation between LSW thickness and once before ending up in the subtropical gyre, which temperature anomalies at 328N near Bermuda with the lengthens the average time scale for the floats to be former leading by only 6 years, while van Sebille et al. exported. The mean age of floats when they first reach (2011) estimate that LSW reaches 268N (at Abaco) in the subtropical boundary is 22 6 10 years and it takes 9 years based on classical LSW salinity anomalies. Thus, 30 6 8 years for them to reach 308N. The youngest floats, left unanswered in our study is the question as to how with ages less than 15 years, are those that originate or property signals observed in the Labrador Sea are 2180 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46

FIG. 14. (top) Cross-correlation coefficients as a function of lead–lag for the launched float number anomalies along AR7W and exported float number anomalies across the subtropical boundary. Black solid line indicates the coefficient before detrending and the dashed line indicates the value after detrending. The values at 95% confidence level are shaded with light gray. Negative values along the x axis indicate that convection is leading export. (bottom) As in the top panel, but for the float number anomalies launched and exported number anomalies across 238N. transmitted to the subtropical gyre in such a relatively Blanke, B., and N. Grima, 2010: ARIANE v2.2.6. Laboratoire de short time scale, if not through advection. This question Physique des Oceans. [Available online at http://www.univ-brest.fr/ forms the basis for a future study. lpo/ariane/.] Böning, C. W., M. Scheinert, J. Dengg, A. Biastoch, and A. Funk, 2006: Decadal variability of subpolar gyre transport and its Acknowledgments. The authors gratefully acknowl- reverberation in the North Atlantic overturning. Geophys. edge the U.S. National Science Foundation (Award Res. Lett., 33, L21S01, doi:10.1029/2006GL026906. OCE-1259103) for the support of this work. S. Zou Bower, A. S., M. S. Lozier, S. F. Gary, and C. W. Böning, 2009: thanks the ARIANE group for trajectory computation Interior pathways of the North Atlantic meridional over- ö turning circulation. Nature, 459, 243–247, doi:10.1038/ code, A. Biastoch and C. B ning for ORCA model data, nature07979. S. Gary for code and suggestions, and N. Foukal and CDIAC, 2015: CLIVAR repeat section AR07W. Carbon Dioxide F. Li for helpful discussions. International Analysis Center. Subset used: 1992–2004, accessed 29 January 2015. [Available online at http://cdiac.ornl.gov/ oceans/RepeatSections/clivar_ar07w.html.] REFERENCES CERSAT, 2002: Mean Wind Fields (MWF product) volume 1—ERS-1, ERS-2 & NSCAT user manual. Rep. C2-MUT- Barnier, B., and Coauthors, 2006: Impact of partial steps and mo- W-05-IF, Department of Oceanography from Space, mentum advection schemes in a global ocean circulation Ifremer, 54 pp. model at eddy permitting resolution. Ocean Dyn., 56, 543–567, Curry, R. G., M. S. McCartney, and T. M. Joyce, 1998: Oceanic doi:10.1007/s10236-006-0082-1. transport of subpolar climate signals to mid-depth subtropical ——, and Coauthors, 2007: Eddy-permitting ocean circulation waters. Nature, 391, 575–577, doi:10.1038/35356. hindcasts of past decades. CLIVAR Exchanges, Vol. 12, No. 3, Dengler, M., J. Fischer, F. A. Schott, and R. Zantopp, 2006: Deep International CLIVAR Project Office, Southampton, United Labrador Current and its variability in 1996–2005. Geophys. Kingdom, 8–10. Res. Lett., 33, L21S06, doi:10.1029/2006GL026702. Berliand, M. E., and T. G. Strokina, 1980: Global Distribution of Fischer, J., M. Visbeck, R. Zantopp, and N. Nunes, 2010: In- the Total Amount of Clouds. Hydrometeorological Publishing terannual to decadal variability of outflow from the Lab- House, 71 pp. rador Sea. Geophys. Res. Lett., 37, L24610, doi:10.1029/ Biastoch, A., C. W. Böning, J. Getzlaff, J. M. Molines, and 2010GL045321. G. Madec, 2008: Causes of interannual-decadal variability in Gary, S. F., M. S. Lozier, C. W. Böning, and A. Biastoch, 2011: the meridional overturning circulation of the midlatitude Deciphering the pathways for the deep limb of the meridional North Atlantic Ocean. J. Climate, 21, 6599–6615, doi:10.1175/ overturning circulation. Deep-Sea Res. II, 58, 1781–1797, 2008JCLI2404.1. doi:10.1016/j.dsr2.2010.10.059. JULY 2016 Z O U A N D L O Z I E R 2181

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