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  • View in gallery

    Mean model-derived salinity signature for July (a conservative summer month) at 1000 m during 1995–98. The 1000-m depth is chosen to resolve the MOW. (left) Important locations are shown. Subpolar water (black) < 35 psu < subtropical water (light gray) < 35.6 psu < Mediterranean outflow (dark gray). Simultaneous southward advection of low-salinity subpolar water in the WNA and northward advection of high-salinity MOW in the ENA from 1995 through 1998 is clearly evident.

  • View in gallery

    Mean semiannual salinity anomalies to the model-derived mean circulation at sections across Hudson Canyon in the NYB (solid black line) and the NEC (dashed black line) in WNA and across the BOB (dashed gray line) and the RT (solid gray line) in the ENA. The slopes (psu yr−1) of −0.21, −0.18, 0.01, and 0.004 for the NYB, NEC, RT, and BOB, respectively, are all statistically significant (95% confidence interval) for the study period.

  • View in gallery

    Climatological annual-mean model-derived salinity fields for characteristic (left) high- and (right) low-NAO phases at 1000 m. Subpolar water (black) < 35 psu < subtropical water (light gray) < 35.6 psu < Mediterranean outflow (dark gray). Concurrent southwestward outflow of low-salinity subpolar water in the WNA and northward penetration of high-salinity MOW in the ENA are observed during the low-NAO phase.

  • View in gallery

    Annual-mean normal velocity across a WNA section at 45°N (offshore of Scotian shelf) for (a) high and (b) low NAO. Similar ENA section at 53°N (offshore of Porcupine Bank) for (c) high and (d) low NAO. Section locations are shown in Fig. 3. Positive (negative) values indicate northward (southward) flow. (top) Contour lines of 0.08 mark the (a) DWBC and (b) Labrador Current waters. (d) The MOW penetration during low NAO is identifiable by the middepth 0.04 contour line along the slope.

  • View in gallery

    Schematic of circulation changes in the North Atlantic basin during positive and negative NAO phases. Thick (thin) lines signify enhanced (diminished) transport. The Labrador Current (upper 1000 m), DWBC (at >1000 m), GS–North Atlantic Current, and northward MOW (500–1500 m) are represented by southward gray, southward black, northward light gray, and northward black arrows, respectively. The SPG (cyclonic arrows in shaded region) expands (shrinks) during positive (negative) NAO phases. Note that the Labrador Current is stronger (weaker) in the upper 1000 m in WNA during low (high) NAO while a deeper DWBC is weaker (stronger) concurrently.

  • View in gallery

    Model-derived annual-mean position of the subpolar front for high (H) and low (L) NAO periods. The subpolar front is defined by the 35.1 isohaline at 1000 m (Lozier and Stewart 2008). The subpolar front for both NAO periods varied seasonally by 20 km. The 1000-m isobath is marked in bold as 1000.

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Contrasting Response of the Eastern and Western North Atlantic Circulation to an Episodic Climate Event

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  • 1 Atmospheric and Environmental Research, Inc., Lexington, Massachusetts
  • | 2 School for Marine Sciences, and School for Marine Science and Technology, University of Massachusetts—Dartmouth, North Dartmouth, Massachusetts
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Abstract

Regional observational studies in the North Atlantic have noted significant hydrographical shifts in 1997–98 because of the episodic drop in the North Atlantic oscillation (NAO) during 1996. Investigation using a basin-scale model finds that, although the western North Atlantic (WNA) witnessed unusually low-salinity water by 1997, the eastern North Atlantic (ENA) simultaneously evidenced intrusions of high-salinity water at intermediate depths. This study shows that a major source of high salinity in the ENA is from the northward penetration of Mediterranean Outflow Water (MOW) that occurred concurrently with a westward shift of the subpolar front. The authors confirm that the low-salinity intrusion in the WNA is from enhanced Labrador Current flow. Results from climatological high- and low-NAO simulations suggest that the NAO-induced circulation changes that occurred in 1997–98 are a characteristic North Atlantic basin response to different forcing conditions during characteristic high- and low-NAO periods.

Supplemental information related to this paper is available at the Journals Online Web site: http://dx.doi.org/10.1175/2011JPO4512.s1.

Corresponding author address: Ayan H. Chaudhuri, Atmospheric and Environmental Research, Inc., 131 Hartwell Ave., Lexington, MA 02421. E-mail: achaudhu@aer.com

Abstract

Regional observational studies in the North Atlantic have noted significant hydrographical shifts in 1997–98 because of the episodic drop in the North Atlantic oscillation (NAO) during 1996. Investigation using a basin-scale model finds that, although the western North Atlantic (WNA) witnessed unusually low-salinity water by 1997, the eastern North Atlantic (ENA) simultaneously evidenced intrusions of high-salinity water at intermediate depths. This study shows that a major source of high salinity in the ENA is from the northward penetration of Mediterranean Outflow Water (MOW) that occurred concurrently with a westward shift of the subpolar front. The authors confirm that the low-salinity intrusion in the WNA is from enhanced Labrador Current flow. Results from climatological high- and low-NAO simulations suggest that the NAO-induced circulation changes that occurred in 1997–98 are a characteristic North Atlantic basin response to different forcing conditions during characteristic high- and low-NAO periods.

Supplemental information related to this paper is available at the Journals Online Web site: http://dx.doi.org/10.1175/2011JPO4512.s1.

Corresponding author address: Ayan H. Chaudhuri, Atmospheric and Environmental Research, Inc., 131 Hartwell Ave., Lexington, MA 02421. E-mail: achaudhu@aer.com

1. Introduction and methods

Large-scale atmospheric variability has a significant effect on ocean circulation patterns. A large-scale mode of natural climate variability affecting the circulation of the North Atlantic basin is the North Atlantic oscillation (NAO), which is the alternation of atmospheric mass between the subpolar low and subtropical high pressure systems (Hurrell 1995). A measurement of NAO strength is based on sea level pressure differences during winter between Iceland and the Azores. In recent decades, during the 1950s and 1960s the NAO was largely in its negative phase, whereas the 1980s and 1990s were periods when the NAO was in its high phase. NAO forcing has been shown to have significant impacts on upper and deep ocean circulation patterns in the North Atlantic (Marshall et al. 2001; Hurrell et al. 2003) as well as the biology (Henson et al. 2009).

Several observational and modeling studies have investigated the influence of the NAO on North Atlantic circulation. Eden and Willebrand (2001) show two modes of NAO response in North Atlantic circulation patterns. One is an interseasonal wind-driven barotropic response, and the other is a longer 6–8-yr baroclinic response driven by meridional overturning circulation variability. The subpolar gyre (SPG) circulation has been widely reported to respond to the NAO. The SPG has been shown to strengthen (weaken) during positive (negative) NAO phases (Bersch 2002; Hakkinen and Rhines 2004). Flatau et al. (2003) analyze surface drifters from 1992 to 1998 to find a stronger and eastward SPG and also enhanced North Atlantic Current during positive NAO. Hakkinen and Rhines (2009) study the exchange of surface drifters between SPG and the subtropical gyre (STG), similar to the study by Brambilla and Talley (2006) and report that more drifters turned southeastward (northeastward) in the eastern North Atlantic (ENA) during the positive (negative) NAO period of the 1990s. This surface behavior is attributed to Ekman drifts related to stronger westerlies in positive NAO phases. Lohmann et al. (2009) investigate the strength of the SPG to persistent positive and negative NAO forcing and indicate a nonlinear response. NAO-induced salinity changes in the eastern SPG (Herbaut and Houssais 2009) show that salinity anomalies are mainly driven by local wind stress. Furthermore, Reverdin (2010) examines 115 yr of measurements from the northeast North Atlantic and finds a significant correlation between salinity and the NAO at lags of 2–3 yr. In the southern Bay of Biscay (BOB), Pérez et al. (2000) find that low-NAO conditions correspond to high precipitation and river discharge along the Iberian coast at 42°N and that these patterns are reversed during high-NAO phases. The resultant changes in salinity of the upper and central waters of the region are found to be significantly correlated to the NAO-induced air–sea fluxes.

In the western North Atlantic (WNA), during positive NAO phases, enhanced convection in the Labrador Sea induces low-saline and colder deep Labrador Seawater formation, which reduces the volume transport of the Labrador Current and intensifies the deep western boundary current (DWBC) transport (Dickson et al. 1996; Curry et al. 1998). A concomitant response is the northward movement of the Gulf Stream (GS; Taylor and Stephens 1998). These patterns reverse during negative NAO phases when stronger Labrador Current transport, weaker deep western boundary current transport, and southward movement of the Gulf Stream are observed. In the eastern North Atlantic, positive NAO phases during the 1980s and 1990s have induced higher sea surface temperatures, causing warm-water copepod species to take precedence and cold-water copepod species to decrease, in contrast to the low NAO of the 1960s (Beaugrand et al. 2002). This regime shift to warm-water copepod species has also been attributed to overall global warming.

NAO variability has affected salinity advection changes within the North Atlantic basin as witnessed in the “Great Salinity Anomaly” events of the 1970s (Dickson et al. 1988), 1980s (Belkin et al. 1998), and 1990s (Belkin 2004). These studies have shown pulses of low-salinity water advecting as part of the subpolar circulation across the North Atlantic in 7–10-yr periods. Sundby and Drinkwater (2007) suggest that high-salinity anomalies are also witnessed in the ENA, coincident with the pulses of low-salinity water in the WNA.

Although aforementioned studies have investigated the influence of the NAO on specific regions, the basinwide response needs to be understood. Our study uses a synoptic approach to investigate salinity advection patterns at intermediate depths within the North Atlantic basin in response to NAO fluctuations. Over the 100-yr record of the NAO, the largest single-year change occurred when the NAO winter index changed from +3.96 in 1995 to a strongly negative value of −3.78 in 1996 (see online at http://www.cgd.ucar.edu/cas/jhurrell/nao.stat.winter.html). In principle, there is no unique way to define the NAO; however, several linear (e.g., principal component analysis) and nonlinear (e.g., cluster analysis) approaches have been used to quantify the NAO on daily, monthly, seasonal, and annual time scales (Hurrell and Deser 2010). In this study, we choose the NAO defined as a winter index derived from principal component analysis because it minimizes spatial extrema and also because the centers of two pressure systems are quasi-permanently located at the Iceland–Azores locations during winter (Hurrell 1995). The basin-scale response of the North Atlantic to the large drop in the NAO during 1996 is studied by analyzing simulations from an ocean general circulation model. An eddy-resolving Regional Ocean Modeling System (ROMS) model is set up for the entire North Atlantic basin (Chaudhuri et al. 2009). The spatial domain of the model extends from 15°S to 75°N and from 100°W to 20°E. The domain is implemented using a 1/6° horizontal and 50-level vertical resolution Mercator grid, with the bottom depth set to 5500 m. A 5° climatology sponge layer is prescribed toward the Arctic and the South Atlantic boundaries. Vertical mixing is determined by the generic length scale (GLS) (Umlauf and Burchard 2003) scheme. The bottom boundary conditions are parameterized by quadratic bottom drag law for the momentum equations and zero flux for tracer equations. Horizontal diffusion and dissipation are parameterized using Laplacian operators.

The model simulations are conducted in two phases: 1) high- and low-NAO climatological simulations and 2) NAO annual simulations for the U.S. Global Ocean Ecosystem Dynamics (GLOBEC) years from 1993 to 1999. National Centers for Climate Prediction (NCEP) reanalysis fields (heat flux, shortwave radiation, and meridional and zonal wind stress components) from 1980 to 1993 and from 1958 to 1971 are selected because these periods witness sustained high- and low-NAO phases, respectively. The 14-yr means for each month are used to create a high- and low-NAO NCEP climatology. For example, data from the Januarys from 1980 to 1993 are averaged to create high-NAO January; similarly, January fields from 1958 to 1971 are averaged to create a low-NAO January. A comparison between the mean annual National Oceanography Centre, Southampton (NOC)–derived high-NAO and mean annual NCEP-derived net heat flux (Qnet) climatologies show differences on the order of 50–100 W m−2 in regions such as the GS and equatorial Atlantic. Similar differences are witnessed in individual months. Further investigation by evaluating individual components of the Qnet—namely, sensible heat flux, latent heat flux, shortwave radiation, and longwave radiation—on a month-by-month basis reveals that the NCEP climatology grossly overestimates the sensible and latent heat flux components. Josey (2001) reports similar discrepancies between NCEP data in comparison to shipboard observations of the fluxes and furthermore finds good agreement between observations and NOC data. NOC data are available only onward of 1980, whereas the NCEP reanalysis is available during both the selected low- (1958–71) and high-NAO (1980–93) periods. To overcome the known discrepancies between observations and NCEP Qnet (Josey 2001), a type II regression model between NCEP and NOC Qnet for the high-NAO climatology is employed. The regression model coefficients are then used to adjust the low-NAO Qnet climatology fields to generate a consistent and continuous set of atmospheric forcing fields (Chaudhuri et al. 2009). The adjusted Qnet high- and low-NAO NCEP-derived climatology fields are used to force the high and low model simulations, respectively. The momentum flux components for NCEP and NOC were comparable and did not require adjustment. The steady-state high-NAO simulation is prescribed as an initial field for the annual simulations. The forcing fields for the annual simulations are derived from NOC. The forcing is applied using mixed boundary conditions; that is, the temperature equation is relaxed to SST derived from Levitus and Boyer (1994), and an effective surface salt flux is prescribed for the salinity equation. The model shows high sensitivity to the relaxation time scale such that short relaxation times (30 days) result in subdued convection and long relaxation times (120 days) lead to weak perturbations. Sensitivity tests indicate that 90-day relaxation scales provide optimal parameterization. Further details of the model are provided in Chaudhuri et al. (2009). In addition, a brief summary of model skill is presented in the supplementary materials (available at the Journals Online Web site: http://dx.doi.org/10.1175/2011JPO4512.s1). Salinity fields generated by the model are presented as part of our analysis.

2. Results

Ocean salinity is a relatively conservative tracer and its variation in space and time because of riverine outflow, saltwater advection, evaporation, precipitation, and sea ice conversion provides a quantitative estimate of changes in circulation patterns. Simulated salinity fields are presented from 1995 to 1998 to illustrate the variation of salt as an indicator of the basin-scale response (Fig. 1) to NAO fluctuations. Results at a depth of 1000 m are presented to resolve the possible flow of the Mediterranean Outflow Water (MOW). During 1995, the three main sources of water in the North Atlantic basin—namely, the subpolar water (<35 psu, black in Fig. 1), the subtropical water (>35 and <35.6 psu; light gray in Fig. 1), and the MOW (>35.6 psu; dark gray in Fig. 1)—are in their characteristic positive NAO state, representative of a long positive NAO period from 1988 to 1995. The low-saline subpolar water is transported by the Labrador Current (Lazier and Wright 1993) into the WNA. The Labrador Current flows southeastward from the Hudson Strait to the tail of the Grand Banks, where it turns southwestward but does not penetrate farther downstream. The subtropical water largely composed of the Gulf Stream and the North Atlantic Current displays typical eastward flow, carrying heat and salt across the basin to the ENA and subarctic regions. The highly saline MOW exits the Strait of Gibraltar with a salinity of 38.4 psu and progresses both westward and northward. The model experiences both mean and meddy-driven flow (Sparrow et al. 2002) in the Gulf of Cadiz; however, the circulation transitions mainly to mean flow as the MOW traverses north toward the Bay of Biscay. At the Bay of Biscay, the Mediterranean outflow mean salinity is 35.65 psu (Iorga and Lozier 1999). Similar circulation patterns are observed in 1996 during the onset of the episodic NAO drop (Fig. 1). Instead, in 1997, the salinity patterns show considerable changes on both sides of the North Atlantic basin. Although low-saline subpolar water penetrates farther south on the western side, simultaneously and in contrast, the saltier MOW traverses farther north on the eastern side. By 1998, the low-saline subpolar water is seen to arrive in the vicinity of the Northeast Channel (NEC) in the WNA and saltier MOW is observed reaching northward of Porcupine Bank in the ENA. Coincidentally, the subpolar front is observed to progressively shift westward from 1996 (not shown but characteristic high- and low-NAO response discussed in Fig. 6), in agreement with Bersch (2002) and Hátún et al. (2005).

Fig. 1.
Fig. 1.

Mean model-derived salinity signature for July (a conservative summer month) at 1000 m during 1995–98. The 1000-m depth is chosen to resolve the MOW. (left) Important locations are shown. Subpolar water (black) < 35 psu < subtropical water (light gray) < 35.6 psu < Mediterranean outflow (dark gray). Simultaneous southward advection of low-salinity subpolar water in the WNA and northward advection of high-salinity MOW in the ENA from 1995 through 1998 is clearly evident.

Citation: Journal of Physical Oceanography 41, 9; 10.1175/2011JPO4512.1

The impact of these considerable and contrasting changes in salinity on both sides of the North Atlantic is felt farther downstream as the low-saline subpolar water in the WNA and saltier MOW in ENA mix with the underlying and overlying water columns, respectively. Our model results confirm enhanced salinity in the ENA during 1995–98 across the Rockall Trough (RT; Fig. 2). A 0.02-psu increase in salinity and a 0.2°C increase in temperature from 1996 to 1998 across the Rockall Trough have been reported by Holliday (2003) based on CTD observations. Our model estimates suggest a slightly higher increase of 0.03 psu in salinity (Fig. 2) and a 0.2°C increase in temperature (not shown). Farther south, model-derived salinity estimates across the Santander section (González-Pola et al. 2005) within the southwestern corner of the Bay of Biscay show a salinity increase of 0.01 psu from 1996 to 1998 (Fig. 2). Model results are in agreement with an increase of 0.005 psu yr−1 reported from CTD observations with a peak in 1997 (González-Pola et al. 2005). Two possible reasons for the salinity increase are enhanced advection and/or water mass property exchange. A clear correlation does not emerge between transport across the Santander section and the rate of change in salinity from the model. Water mass properties across the Santander section are known to be influenced by local freshwater and heat fluxes in the upper and central waters (Somavilla et al. 2009). These upper masses are presumably mixing with the MOW at intermediate depths especially during winter, thus providing a convective source of salinity variability. Investigating changes in MOW tracer properties as suggested by Millot et al. (2006) is beyond the scope of this study; however, an extensive study has been reported by Lozier and Sindlinger (2009).

Fig. 2.
Fig. 2.

Mean semiannual salinity anomalies to the model-derived mean circulation at sections across Hudson Canyon in the NYB (solid black line) and the NEC (dashed black line) in WNA and across the BOB (dashed gray line) and the RT (solid gray line) in the ENA. The slopes (psu yr−1) of −0.21, −0.18, 0.01, and 0.004 for the NYB, NEC, RT, and BOB, respectively, are all statistically significant (95% confidence interval) for the study period.

Citation: Journal of Physical Oceanography 41, 9; 10.1175/2011JPO4512.1

Coincident freshening along the WNA is observed from sections at the Northeast Channel and Hudson Canyon in the New York Bight (NYB; Fig. 2). Across the Northeast Channel, a monotonic decrease in model-derived salinity is observed at a rate of 0.18 psu yr−1 for the study period (Fig. 2). Freshening trends on the order of 0.5 psu are also noticed by Houghton and Fairbanks (2001) in the Northeast Channel and Georges Bank based on 6-monthly hydrographic surveys for the GLOBEC period (1995–98). Farther south of the Northeast Channel, the Hudson Canyon exhibits a similar decrease in salinity, with the exception of a slight increase in mid-1996. Model-derived salinity time series (Fig. 2) show reasonable agreement with hydrographic observations reported by Mountain (2003) for the New York Bight region. The sustained pulse of low-saline water thus progresses southward and onto shallower regions. The salinity changes across the WNA differ by an order magnitude from those of the ENA because the spatial salinity gradients are stronger in the WNA. Although MOW results in smaller salinity changes in comparison to the subpolar water, the low-salinity trend in the WNA and the high-salinity trend in the ENA during 1997–98 are statistically significant (Fig. 2).

3. Discussion

Although the influence of the 1996 low-NAO event on changes in circulation patterns of the North Atlantic basin during 1997–98 is clearly established, whether these changes are a singular response to an episodic low-NAO event or part of a regular oceanic response to fluctuations in the state of the NAO needs to be understood. The possible response of the North Atlantic basin to a decadal-scale near-stationary NAO pattern (Kushnir 1994) could be different from the response to annual-scale nonstationary extreme NAO events: for example, the extents of zonal winter wind stress are different for decadal high-NAO (1980–93) periods and an extreme high-NAO year such as 1989; similarly, decadal low-NAO (1958–71) periods have different momentum flux patterns in comparison to an extreme low-NAO year such as 1996. However, to a first order, it is reasonable to investigate the oceanic response to the two climatological atmospheric states because these choices are relevant and complementary for the depiction of changes in the North Atlantic.

To address the same, we present the mean salinity structure at 1000 m for the basin during characteristic high- and low-NAO simulations (Fig. 3). The mean high- and low-NAO salinity fields are created by averaging 3-day interval salinity fields from the climatological high- and low-NAO simulations for the entire 12-month period respectively. Results indicate that in the ENA, while the northern limb of the MOW extends into the Bay of Biscay during high-NAO phases (Fig. 3a), the MOW penetrate farther northward, south of Porcupine Bank, during low-NAO phases (Fig. 3b). Concurrently, in the WNA, although the Labrador Current inflow reaches south of the Tail of the Grand Banks during high-NAO phases (Fig. 3a), the inflow is seen to extend farther southwestward, toward the Scotian Shelf, during the low-NAO phases (Fig. 3b).

Fig. 3.
Fig. 3.

Climatological annual-mean model-derived salinity fields for characteristic (left) high- and (right) low-NAO phases at 1000 m. Subpolar water (black) < 35 psu < subtropical water (light gray) < 35.6 psu < Mediterranean outflow (dark gray). Concurrent southwestward outflow of low-salinity subpolar water in the WNA and northward penetration of high-salinity MOW in the ENA are observed during the low-NAO phase.

Citation: Journal of Physical Oceanography 41, 9; 10.1175/2011JPO4512.1

Annual-mean velocity profiles taken offshore of the Scotian Shelf for the high- (Fig. 4a) and low- (Fig. 4b) NAO phases reveal an interesting pattern. Although the subsurface flow weakens and the deeper flow intensifies during the high-NAO phase (Fig. 4a), a stronger subsurface and weaker deep flow is seen during the low-NAO phase (Fig. 4b). This subsurface and deep flow anticorrelation is likely due to enhanced convection in the Labrador Sea during high-NAO phases, which intensifies the DWBC and conversely diminished convection during low-NAO phases intensifies the subsurface Labrador Current (Dickson et al. 1996; Dickson 1997; Curry et al. 1998). In fact, the 1000-m level chosen for most of our analysis appears to be within the transition zone of the observed anticorrelation. Model-derived annual transport in the upper 1000 m is estimated to be 5.8 Sv (1 Sv ≡ 106 m3 s−1) during the low-NAO phase and 3.4 Sv during the high-NAO phase. In the ENA, annual-mean velocity across a section offshore of Porcupine Bank for high (Fig. 4c) and low NAO (Fig. 4d) show a clear northward intermediate flow with a maximum near 1300 m during the low-NAO phase, whereas no such signal is present during the high-NAO phase. These results thus suggest that the NAO-induced episodic 1996–97 event is not a singularity but part of a broader NAO-induced atmospheric response.

Fig. 4.
Fig. 4.

Annual-mean normal velocity across a WNA section at 45°N (offshore of Scotian shelf) for (a) high and (b) low NAO. Similar ENA section at 53°N (offshore of Porcupine Bank) for (c) high and (d) low NAO. Section locations are shown in Fig. 3. Positive (negative) values indicate northward (southward) flow. (top) Contour lines of 0.08 mark the (a) DWBC and (b) Labrador Current waters. (d) The MOW penetration during low NAO is identifiable by the middepth 0.04 contour line along the slope.

Citation: Journal of Physical Oceanography 41, 9; 10.1175/2011JPO4512.1

The substantial change in ocean circulation on both sides of the North Atlantic can be attributed to the variations in wind-induced momentum and buoyancy fluxes into the ocean due to the large NAO drop in 1996. Figure 5 presents a schematic of the North Atlantic basin response to characteristic NAO forcing based on our model simulations and the observations presented above. The GS and its extension, the North Atlantic Current, are the main sources of saline water to the Nordic Seas. Curry and McCartney (2001) and de Coëtlogon et al. (2006) demonstrate that the Gulf Stream has a stronger (weaker) northeastward transport during positive (negative) NAO periods because of linear adjustment of wind stress curl, baroclinic Rossby wave propagation, and meridional overturning circulation (Fig. 5). Therefore, weaker Gulf Stream and North Atlantic Current transport after the 1996 low result in a reduction of subtropical water inflow into the ENA, as evidenced by Orvik et al. (2001). They note a decrease in volume transport in the Atlantic Inflow by 1997 in the ENA from moored current meters, vessel-mounted ADCP, and SeaSoar-CTD observations during 1995–99. This response may have been transient, but Orvik and Skagseth (2003) find that high inflow events related to Norwegian Atlantic Slope Current coincide with high-NAO events and vice versa with a lag of 15 months. In deeper waters, Bersch (2002) report a contraction in the SPG in 1996–97 coinciding with the low-NAO years in the 1990s from analysis of hydrographic data. The subpolar front retreats westward during 1996, coincident with a strong weakening of the westerlies as determined by the NAO (Bersch 2002; Hátún et al. 2005). Results from our characteristic high- and low-NAO simulations show an eastward (westward) movement of the subpolar front during high- (low-) NAO periods (Fig. 6). Bersch et al. (2007) suggest a shrinking SPG advects subtropical waters northward in the ENA, which is balanced by enhanced southward advection in the WNA. Although we agree that a shrinking SPG does provide a pathway for enhanced northeastward advection of subtropical water, we suggest that the incoming subtropical water in itself has weaker transport. In addition, we propose that the weaker subtropical water inflow augmented by a westward shift of the subpolar front in 1996 provides a passage for farther northward penetration of MOW at intermediate depths by 1997 (Fig. 5). Lozier and Stewart (2008) have also suggested that the expanded SPG during positive NAO periods may act as a barrier to northward advection of Mediterranean outflow during positive NAO periods and vice versa (Fig. 5).

Fig. 5.
Fig. 5.

Schematic of circulation changes in the North Atlantic basin during positive and negative NAO phases. Thick (thin) lines signify enhanced (diminished) transport. The Labrador Current (upper 1000 m), DWBC (at >1000 m), GS–North Atlantic Current, and northward MOW (500–1500 m) are represented by southward gray, southward black, northward light gray, and northward black arrows, respectively. The SPG (cyclonic arrows in shaded region) expands (shrinks) during positive (negative) NAO phases. Note that the Labrador Current is stronger (weaker) in the upper 1000 m in WNA during low (high) NAO while a deeper DWBC is weaker (stronger) concurrently.

Citation: Journal of Physical Oceanography 41, 9; 10.1175/2011JPO4512.1

Fig. 6.
Fig. 6.

Model-derived annual-mean position of the subpolar front for high (H) and low (L) NAO periods. The subpolar front is defined by the 35.1 isohaline at 1000 m (Lozier and Stewart 2008). The subpolar front for both NAO periods varied seasonally by 20 km. The 1000-m isobath is marked in bold as 1000.

Citation: Journal of Physical Oceanography 41, 9; 10.1175/2011JPO4512.1

In the WNA, Dickson et al. (1996) and Curry et al. (1998) suggest that severe winters during positive NAO years bring cold Arctic air into northern Canada. This climate signal enhances the convection in the Labrador Sea, which induces low-saline and colder deep Labrador Seawater formation. While volume transport in the Labrador Current decreases (Fig. 5), the deep western boundary current that transports the Labrador Seawater intensifies. Instead, during negative NAO periods, weaker winds induce shallower convection and formation of saltier and warmer deep Labrador Seawater. The transport patterns are reversed such that the Labrador Current transport intensifies (Figs. 4a,b) and the deep western boundary current transport weakens. Han and Tang (2001) compute volume transport in the western Labrador Sea using Ocean Topography Experiment (TOPEX)/Poseidon altimeter data from 1993 to 1998 and find significant positive correlation with the NAO index. They confirm that the sea level was lowest during 1997–98, complemented by the increased baroclinic Labrador Current flow, and attribute the response to NAO-induced strong cyclonic wind stress curl forcing.

This study suggests that a relevant source of observed high salinity at intermediate depths in the ENA during 1997–98 is the Mediterranean outflow and that the simultaneous source of freshening in the WNA is the transport of the Labrador Current. We hypothesize that a shrinking (expanding) SPG, coincident with a westward (eastward) shift of the subpolar front and further augmented by a weaker (stronger) North Atlantic Current flow, allows for enhanced (diminished) northward penetration of MOW within the ENA during negative (positive) NAO phases. Simultaneously, stronger (weaker) winds in the WNA during positive (negative) NAO phases induce deeper (shallower) convection in the Labrador Sea as well as diminish (enhance) the Labrador Current transport. Our results also suggest that the NAO-induced episodic 1996–97 event is not a singularity but part of a broader NAO-induced atmospheric response. Thus, the mechanisms discussed in this paper suggest that salinity anomalies are conversely interlinked between the eastern and western North Atlantic. However, further investigation is required to understand whether these mechanisms complement or oppose the Great Salinity Anomaly study or other studies such as out-of-phase signals observed between east and west parts of the subpolar gyres (Reverdin et al. 1997). The present analysis provides a new framework at intermediate depth in which to think of these earlier data analyses for the upper ocean. In conclusion, we note that the present year (2010) has witnessed strong low-NAO conditions (−4.62), and we expect the North Atlantic circulation patterns to respond in a manner similar to the 1996 event in a couple of years.

Acknowledgments

This work is supported by NASA’s Interdisciplinary Science (IDS) Program under Grant NNG04GH50G, GLOBEC NWA Program under Grant NSF OCE 0535379, and NSF GLOBEC PRS Program under Grant OCE-0815679. We are grateful to two anonymous reviewers for their insightful comments, which significantly helped improve the manuscript. We also thank Dr. Igor Belkin for reviewing this paper.

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