North Atlantic Oscillation–induced changes of the upper layer circulation in the northern North Atlantic Ocean

1. Introduction

[2] During the 1990s the North Atlantic Oscillation (NAO) turned from a positive phase (1989 to 1995) with strong westerlies over the North Atlantic Ocean and strong cooling of the subpolar gyre to a negative phase (1996 and 1997) with weak westerlies and weak cooling (Figure 1). As a consequence, the sea level raised in the subpolar gyre and dropped in the subtropical gyre by about 6 cm, respectively [Reverdin et al., 1999; Esselborn, 2001]. This was partly due to an increase of the winter mixed layer temperature in the subpolar gyre and its decrease in the subtropical gyre on the order of 2°C. The deep convection in the Labrador Sea stopped after winter 1995 [Clarke et al., 2000] and the Subarctic Front, which separates the cold and low saline subarctic waters from the warmer and more saline subtropical waters, shifted westward in the Iceland Basin [Bersch et al., 1999; Curry, 2000]. The Subarctic Front is associated with the Gulf Stream north of Cape Hatteras and with the North Atlantic Current and thus runs mainly zonally except east of Newfoundland and in the Irminger and Iceland Basins, where it is meridionally orientated.

[3] Overall, the NAO high phase between 1989 and 1995 had led to a pronounced heat loss of the subpolar North Atlantic and a heat gain of the subtropical North Atlantic down to 3000 m depth [Levitus et al., 2000]. The drastic drop of the NAO index between winters 1995 and 1996 was associated with the occurrence of a blocking high-pressure cell over Scandinavia in winter 1996, so that southerly winds replaced the strong westerlies above the northeastern North Atlantic Ocean and the zero line of the wind stress curl moved southward up to 10 degrees of latitude as shown by e.g. data from the National Center for Atmospheric Research, Boulder, Colorado, USA. The centers of the Icelandic Low and the Azores High shifted westward by about 15 and 25 degrees of longitude, respectively.

[4] In this paper observations of the oceanic response to these drastic atmospheric changes in the 1990s are presented. Water mass and circulation changes of the upper layer along World Ocean Circulation Experiment (WOCE) hydrographic sections A1E and A2 (Figure 2) are described, which seem to be induced by the NAO. The sections, consisting of about 54 (A1E) and 70 (A2) stations, were recorded repeatedly in September 1991, September 1992, November/December 1994, June 1995, August 1996, August/September 1997, May 1999 (A1E) and July 1993, October 1994, May 1996, June 1997, and May 1998 (A2). The hydrographic data have WOCE accuracy [WOCE, 1994]. Water masses and transports on A1E in 1991 and A2 in 1993 are discussed by Bersch [1995] and Koltermann et al. [1999], respectively. An inverse box model was applied by Woelk [2000] to the sections recorded in 1994.

2. Water Mass Changes

[5] After the drastic drop of the NAO index in winter 1996 a westward shift of the Subarctic Front (SAF) was observed in the Iceland Basin, which led to a higher salinity of the upper 600 m along A1E between the Reykjanes Ridge and the Porcupine Bank (Figure 3). The salinity increase of the order of 0.1 is equivalent to a net evaporation rate of 4.7 mm/day over 1 year, which is about five times the estimated change of the freshwater flux at the sea surface between NAO high and low phases [Hurrell, 1995]. The temperature increase of the upper 600 m amounted to about 0.8°C, the density decrease was about 0.05 kg/m³. The thermohaline changes reached down to 1200 m depth (Figure 4) and involved the permanent pycnocline (Figure 5). The westward shift of the SAF was the result of a reduced eastward spreading of cold, low saline, and dense Subarctic Surface (SAW) and Subarctic Intermediate (SAIW) waters (θ < 7°C, S < 35.0) from the Labrador Sea and an increased northward spreading of warm, saline, and less dense Subpolar Mode Water (SPMW) and Mediteranian Water (MW; σ₀ > 27.4 kg/m³) from the subtropics. A contraction of the subpolar gyre is suggested. Observations by Pollard et al. [1999] indicate that in October/November 1996 the 35.0 isohaline of the SAF, which was found in the Iceland Basin on A1E until 1995, had shifted to 30°W near the Charlie-Gibbs Fracture Zone, about 350 km westward of A1E.

[6] Observations along A1E in 1999 show that with increasing NAO index the low saline subarctic water masses begin to occupy again the Iceland Basin and the region of the Rockall Plateau and the Rockall Trough (Figures 3, 4b, and 4c), indicating an eastward shift of the SAF and an expansion of the subpolar gyre. In contrast to the regions east of the Reykjanes Ridge the warm and saline anomaly was first observed in the Irminger Basin in May 1999 (Figures 3 and 4a), regarding that there were no data in 1998 along A1E. Data from the Faxafloi section at 64.3°N in the Irminger Basin show that the anomaly actually arrived in early 1998 [Mortensen and Valdimarsson, 1999]. Thus, the response in this region lagged the NAO by about 2 years, possibly due to the northward advection of the anomaly from the Iceland Basin across the Reykjanes Ridge. With the occurrence of the anomaly in the Irminger Basin there was also a pronounced increase of the temperature and salinity of the Labrador Seawater below, which emphasizes the deep reaching effect of the anomaly, as was observed in the Iceland Basin and in the Rockall Trough [Bersch et al., 1999]. Figure 3 also shows an expansion of the area with low saline subarctic waters in the Irminger Basin during the NAO high phase, which led to an eastward shift of the SAF and the associated Irminger Current above the western flank of the Reykjanes Ridge and a cutoff of the southward flow of warm and saline SPMW with the East Greenland Current.

[7] A similar temporal behaviour of the upper layer water masses as along A1E east of the Reykjanes Ridge was observed along A2 east of the Mid-Atlantic Ridge in the West European Basin, where a positive salinity anomaly occurred as well in 1996 through 1998 (Figure 6), suggesting a northward expansion of the subtropical gyre in this region. A contribution from the MW to this anomaly is indicated in the density range of σ₀ = 27.4 to 27.7 kg/m³ above the European continental slope (Figure 7). Whereas the SAF had shifted westward in the Iceland Basin, it had shifted eastward in the Newfoundland Basin in the period 1996 to 1998 and was found at 45°W in 1997, about 200 km farther east than in 1993. A corresponding southward shift of the north wall of the Gulf Stream at 70°W in 1996 and 1997 was observed by Rossby and Gottlieb [1998]. Both shifts were associated with a southward and westward spreading of cold, low saline subarctic water masses with densities σ₀ < 27.55 kg/m³ above the American continental slope (Figure 7) [Rossby and Benway, 2000].

[8] The redistribution of subarctic and subtropical water masses north of 40°N, associated with the pronounced weakening of the westerlies in 1996 and 1997, resulted in an increase of the mean salinity of the upper layer (σ₁ < 32.33 kg/m³, which is about σ₀ < 27.73 kg/m³; see Figure 7) in the eastern basins and a decrease in the Newfoundland Basin (Figure 8), especially west of 45°W above the continental slope. A reduced eastward spreading of subarctic waters with the North Atlantic Current and an intensified southward spreading of subarctic waters with the Labrador Current were suggested, while the northward spreading of subtropical waters was intensified in the northeastern North Atlantic. This led to the corresponding shifts of the SAF in the different regions and thus a change of the shape of the subpolar gyre.

3. Transport Changes

[9] From the hydrographic data along A1E and A2 geostrophic transports of volume, heat, and freshwater for the upper layer (σ₁ < 32.33 kg/m³) were calculated, using a layer of no motion at a density level of σ₁ = 32.33 kg/m³, which separates the upper layer from the Labrador Seawater layer, or at the bottom for shallower stations. The derived transports for 1997 (NAO low) are compared to the mean transports for the period 1991 to 1995 (NAO high; 4 repeats of A1E and 2 repeats of A2) in Figure 9. The sections of A1E and A2 in 1997 show the largest thermohaline contrast to the sections in the NAO high period. The subdivision of the two sections shown in Figure 9 results from the extrema of the cumulative volume transport of each section (transports between the extrema were calculated and shown) and fixed segment boundaries above the Reykjanes Ridge (31°W) on A1E, the Mid-Atlantic Ridge (27°W) on A2, and at 20°W on both sections. The transport scheme reflects the characteristic circulation pattern of the northern North Atlantic. Transport values outside the two sections were derived by adding up the transports on the sections from the east, assuming conservation of volume, heat, and freshwater. This leaves the transports unbalanced in the west. The flow on the shelves is not fully resolved by the sections A1E and A2. The magnitude of the variability of the geostrophic velocity within the section segments is as large as the mean velocities, which can cause temporal transport changes for a segment due to the eddy variability. Therefore, the temporal transport changes derived from Figure 9 and ascribed to the NAO should be taken as a first guess, which is fortified by the large-scale redistribution of the upper layer water masses.

[10] In the NAO high period the volume transports across A1E and A2 summed up to 18.0 and 20.8 Sv northward, respectively (Figure 9a). About half of the 44.4 Sv transported northward by the North Atlantic Current (NAC) off the Grand Banks recirculated anticyclonically west of the Mid-Atlantic Ridge (MAR). 32.0 Sv were transported southward by the anticyclonic Mann Eddy, centered at about 44°W in the Newfoundland Basin and characterized by salinities above about 35.8 in Figure 6. East of the Mann Eddy a weak cyclonic circulation was found, which is also indicated by drifter trajectories [Krauss and Käse, 1984; Rossby, 1996]. Almost half of the total transport across A2 flowed northward east of the MAR. To balance this flow and the northward flows in the Iceland Basin and the Rockall Trough the NAC had to transport 18.6 Sv across the MAR, 9.3 Sv north of A2 and 9.3 Sv south of it. A small part of 1.8 Sv recirculated cyclonically in the Iceland Basin and entered the Irminger Basin south of A1E, whereas the major part of 16.8 Sv fed the Norwegian Current, entrained into the Iceland-Scotland overflow, and entered the Irminger Basin north of A1E. Not all of the northward flow east of 20°W enters the Rockall Trough, a part is transported anticyclonically around the Rockall Plateau into the Iceland Basin [van Aken and Becker, 1996; Pollard et al., 1999]. In the cyclonic circulation in the Irminger Basin 3.2 Sv were transported southwards by the East Greenland Current. A part of this flow feeds the Labrador Current, which transported 1.2 Sv southward above the American continental slope. Other parts recirculate in the Labrador and Irminger Basins and contribute to the formation of Labrador Seawater. North of the Grand Banks a part of the subarctic waters transported in the cyclonic circulation of the Labrador Sea flows eastward and joins the NAC [Reynaud et al., 1995]. Another part is transported anticyclonically around the Flemish Cap with a branch of the Labrador Current. Drifter measurements reveal that a part of the Labrador Current retroflects north of the Tail of the Grand Banks and joins the NAC [Rossby, 1996].

[11] In 1997 the total volume transports across A1E and A2 as well as the total transports west and east of the Reykjanes Ridge remained nearly constant, whereas the northward total transport increased west of the MAR and decreased east of it due to the shift of the maximum from the eastern flank of the MAR to its western flank (Figure 9b). So an intensified cyclonic flow occurred in the eastern Newfoundland Basin and the eastward transport across the MAR increased north of A2 and decreased south of it. Off the Grand Banks the NAC and its anticyclonic recirculation strengthened. The Labrador Current broadened and more than doubled its transport. Correspondingly, the SAF and thus the NAC shifted eastward as well as the center of the Mann Eddy (Figure 6), which was found about 125 km east of its position in 1994. In the West European Basin an anticyclonic circulation established on A2 in 1997, suggesting an occupation of this region by the subtropical gyre. On A1E the northward transport in the Iceland Basin and the Rockall Trough decreased as well as the cyclonic recirculation above the eastern flank of the Reykjanes Ridge. In the Irminger Basin the cyclonic circulation including the East Greenland Current weakened slightly.

[12] For the estimation of the heat transport (the term temperature transport is more exactly because it is calculated relative to 0°C without assuming no net volume transport, but it is rather uncommon) the summerly raised temperature of the surface layer (upper 100 m) of the two sections was replaced by a constant value taken from 100 m depth at each station. Most of the changes in volume transport are also reflected by the changes in heat transport (Figures 9c and 9d), because the latter is strongly dominated by the former. But there was a remarkable difference. In 1997 a drastic reduction of the northward total heat transport across A2 of about 24 percent occurred, 0.07 PW west of the MAR and 0.15 PW east of it, which removed the convergence of the heat transport between A1E and A2 in the NAO high period. The reduced northward heat transport west of the MAR is in contrast to the increased volume transport. The reduction resulted from a strengthened southward heat transport of the NAC recirculation, which was not compensated by an increased northward heat transport of the NAC off the Grand Banks. Regarding the increased volume transport of the NAC, it is indicated that colder water was transported northward with the NAC. East of the MAR the northward heat transport reduced due to the anticyclonic circulation occuring in 1997 and the shift of the maximum northward heat transport from the eastern flank of the MAR to the western flank. The latter ensured that the maximum eastward heat transport across the MAR occurred north of A2 in 1997, in contrast to the NAO high period. There was only a slightly enhanced northward total heat transport across A1E east of the Reykjanes Ridge in 1997, associated with the warm and saline anomaly.

[13] The freshwater transport was calculated relative to a salinity of 35.0 and is shown only in regions where the salinity is less than 35.0 (Figures 9e anf 9f), reflecting the circulation of the low saline arctic and subarctic water masses in the subpolar gyre, which determine its size and shape. In the NAO high period there was a strong cyclonic circulation of freshwater in the Iceland Basin and enhanced transports of freshwater in the East Greenland Current and the NAC crossing the MAR, as is suggested by an eastward expansion of the subpolar gyre. In 1997 the cyclonic circulation of the low saline waters reduced strongly in the Iceland Basin, whereas it intensified in the western Newfoundland Basin, where the freshwater transport of the Labrador Current as well as that of the NAC increased considerably. The latter pointed to an enhanced contribution by the Labrador Current retroflection joining the NAC. Less freshwater was transported eastward across the MAR. Overall, the northward total freshwater transport across A2 increased fivefold in 1997, which enhanced the accumulation of freshwater in the subpolar gyre strongly. This seemed to occur predominantly in the western Newfoundland Basin and in the southern Labrador Sea.

4. Discussion

[14] A redistribution of cold, low-saline subarctic waters and warm, saline subtropical waters in the northern North Atlantic with the NAO was also found by Levitus [1989a, 1989b], comparing the pentads 1970–1974 (NAO high) and 1955–1959 (NAO low). The temperature and salinity changes were of the order of 0.5°C and 0.05 psu, respectively, occurring in the upper 600 m, and were accompanied by a density decrease (dropping of isopycnals) in the Irminger, Iceland, and West European Basins and a density increase (lifting of isopycnals) in the Newfoundland Basin in the NAO low period. Above the eastern flank of the Reykjanes Ridge a northerly flow was found in this period [Langseth and Boyer, 1972; Wegner, 1973]. As in the 1990s, the thermohaline changes pointed to a deformation of the subpolar gyre shape and corresponding shifts of the SAF with the NAO. Contours of the steric sea level for the two pentads reflected a contraction of the subpolar gyre in the east and an expansion east and south of Newfoundland in the NAO low period [Levitus, 1990]. Additionally, Figure 31 by Levitus [1989a] reveals that under NAO high conditions the SAIW with a density of about σ₀ = 27.5 kg/m³ spread mainly eastward with the NAC instead of southward with the Labrador Current.

[15] There were several observations which indicate that the shifts of the SAF between Cape Hatteras and south of Newfoundland are induced by the NAO. Comparing the hydrographic data from Atlantis cruise 229 [Fuglister, 1960] in November 1956 (NAO low) with the data from Oceanus cruise 133/7 [Fukumori et al., 1991] in May 1983 (NAO high), there was a much smaller amount of cold, low saline water in the upper 1000 m above the continental slope south of Newfoundland at about 52°W in 1983 and the SAF was found about 1.5 degrees of latitude farther north. In 1997 the amount of cold, low saline water had increased again and the SAF had shifted southward [Joyce et al., 1999]. A similar southward shift of the SAF at 55°W was observed between 1976 and 1977 [McCartney et al., 1980; Schmitz and McCartney, 1982], when the NAO index dropped from +1.4 in winter 1976 to −2.1 in winter 1977. At about 62°W, southeast of Nova Scotia, Petrie and Drinkwater [1993] reported a reduction of cold, low saline shelf and slope waters between the 1960s (NAO low) and the 1970s (NAO high) and a corresponding decrease of the Labrador Current transport. Using atmospheric and oceanic time series, Joyce et al. [2000] concluded that if the NAO index is low, the Gulf Stream is in a southerly position in the region between 50° and 70°W as is the line of zero wind stress curl. Rossby [1999] found that at least on the annual timescale a varying overflow of shelf waters to the slope region contributes to the shifts of the Gulf Stream. In the region between 65° and 75°W the position of the SAF seems to lag the NAO by about two years [Taylor and Stephens, 1998]. Corresponding temperature variations were observed down to 400 m depth [Molinari et al., 1997].

[16] Whereas the SAF shifts southward with decreasing westerlies south of the Grand Banks and eastward east of the Grand Banks, it shifts northward and westward north of about 50°N. The northward shifts are best documented by the upper layer temperature record from the Ocean Weather Station Charlie at 52.7°N, 35.5°W [Levitus et al., 1994]. Comparing hydrographic data from 1966 [Lazier, 1973] with a climatology of Reynaud et al. [1995], a pronounced northwestward shift of the ‘Northwest Corner‘ of the SAF in the Labrador Basin can be deduced for the NAO low period in the 1960s. In 1966 the temperature in the upper layer at OWS Charlie was at the 1960s maximum, after the NAO index had stayed below −1.7 between 1962 and 1966. Between 1966 and 1973 the NAO index increased strongly and cold, low saline subarctic waters spread eastward into the Iceland Basin and toward the Rockall Trough [Wade et al., 1997]. A strong decrease of the sea surface salinity in the Irminger Basin in this period was reported by Taylor and Stephens [1980]. At OWS Charlie the upper layer temperature decreased to a pronounced temperature minimum in 1973.

[17] The eastward expansion of the subpolar gyre seems to affect also the warm and saline inflow of Atlantic Water to the Norwegian Sea through the Faeroe-Shetland Channel. Between 1990 and 1995 a decrease of the salinity and an increase thereafter were observed in the Faeroe-Shetland Channel [Turrell, 1997]. This corresponds to the temporal evolution of the upper layer salinity in the eastern part of section A1E (Figure 3) and the changes of the freshwater transport (Figures 9e and 9f). A similar behaviour was found in the 1970s with the occurrence of a pronounced salinity minimum in the Faeroe-Shetland Channel in 1975 through 1977 at the end of the NAO high period [Dooley et al., 1984]. A reduced flow through the Rockall Trough after 1996 was suggested by the trajectories of 88 floats in the northeastern North Atlantic, which showed only little tendency to pass through the Rockall Trough but were mainly advected northward through the Iceland Basin [Bower et al., 2000].

[18] Possibly water mass anomalies advected from the Labrador Current west of Newfoundland to the eastern basins contribute to the redistribution of subarctic waters. A timescale of about 2 years was deduced by Reverdin et al. [1997] from the variability of the upper layer temperature and salinity at Ocean Weather Stations and standard sections between the late 1940s and the late 1980s. Dickson et al. [1988] reported the advection of a large low-saline anomaly in the subpolar gyre in the 1970s under NAO high conditions. Similarly, Belkin et al. [1998] described the eastward advection of a cold and fresh anomaly from the Labrador Current in the first half of the 1980s under NAO high conditions. Colbourne et al. [1994] showed that the area of arctic water with temperatures <0°C in the Labrador Current increased with the NAO index as was the case in 1990 and 1991. Thus, possibly this anomaly contributed to the cold, low saline anomaly occurring in the Iceland Basin between 1992 and 1995 (Figure 3). Drifter measurements in 1996 and 1997 suggest an advection timescale of about 1 year for this pathway [Malmberg and Valdimarsson, 1999].

[19] Whereas the NAO-induced shifts of the SAF and the line of zero wind stress curl are in phase in the western North Atlantic, they are out of phase in the eastern North Atlantic, where under NAO low conditions the SAF retreats westward and northward, while the line of zero wind stress curl moves southward. The latter allows a huge area of warm and saline SPMW be incorporated in the subpolar cyclonic circulation by northward Ekman and Sverdrup transport anomalies. As an instantaneous response to a drop of the NAO index a barotropic cyclonic anomaly centered at 45°N, 40°W developed in a numerical model by Eden and Willebrand [2000]. After 3 years the spin down of the subpolar and subtropical gyres dominates in the model, with only small changes in the total volume transport across 48°N.

[20] Using the method of Hall and Bryden [1982], Lorbacher [2000], and Lorbacher and Koltermann [2000] calculated a decrease of the meridional heat transport across section A2 in the total water column between 1994 and 1997 by about 45 percent. A significant amount was contributed by the baroclinic change in the upper layer, as is also shown in Figures 9c and 9d. Häkkinen [1999] concludes that the NAO is the dominant forcing for the variability of the meridional heat transport at interannual and longer timescales, predominantly in the region of the Gulf Stream and the NAC.

5. Conclusions

[21] Hydrographic data indicate that the NAO induces a redistribution of cold, low-saline subarctic waters and warm, saline subtropical waters in the upper layer of the northern North Atlantic. During periods with weak westerlies the eastward transport of subarctic waters with the NAC and the Irminger Current is reduced and the SAF shifts westward in the Irminger and Iceland Basins and northward in the Labrador and West European Basins, suggesting a contraction of the subpolar gyre. At the same time the southward transport and accumulation of subarctic waters east and south of the Grand Banks of Newfoundland is enhanced and the SAF shifts eastward and southward in this region. Conditions are vice versa in periods with strong westerlies. Data from the 1990s along WOCE sections A1E and A2 indicate that the redistribution of water masses occurs within about 2 years. In the Irminger Basin a delayed westward retreat of the SAF was observed after the drastic drop of the NAO index in 1996, compared to the quasi-instantaneous shift in the Iceland Basin.

[22] Overall, the shape of the subpolar gyre and hence its circulation change with the NAO, as it is deduced here from baroclinic transports in the period 1991 to 1995 (NAO high) and in 1997 (NAO low). Whereas under NAO low conditions the cyclonic circulation reduces in the Irminger and Iceland Basins, it strengthens in the Newfoundland Basin, where the Labrador Current and its retroflection and the cyclonic circulation in the eastern part intensify, with a strong northward flow above the western flank of the MAR. The NAC off the Grand Banks shifts eastward and its volume transport increases due to an enhanced contribution by the Labrador Current retroflection. The zonal transport of the NAC across the MAR decreases south of about 47°N, so that the overall anticyclonic recirculation west of the MAR is increased. The latter, the colder water masses of the NAC, and the occurrence of an anticyclonic circulation in the West European Basin, resulting probably from a northward expansion of the subtropical gyre, lead to a considerable reduction of the northward total heat transport across 47°N in the upper layer. In contrast, the northward total heat transport across 55°N increased slightly.

[23] It is an open question whether the increased heat content of the upper layer of the northeastern North Atlantic in 1996 and 1997 contributes significantly to the increase of the NAO index after 1996. If the anomalous heat content of about 3.5 GJ/m² [Bersch et al., 1999] is released to the atmosphere during 1 year this would nearly double the climatological total heat flux of about 60 W/m². This suggests that the out-of-phase shifts of the atmospheric and oceanic fronts associated with the changing extents of the subarctic and subtropical regimes in the northeastern North Atlantic could play an important role for the forcing of the NAO. Additionally, the large salt content of the anomaly favours deep convection in the Labrador Sea, driving the meridional overturning circulation.