GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L08306, doi:10.1029/2003GL019334, 2004

Propagation of the ‘‘Great Salinity Anomaly’’ of the 1990s around the northern North Atlantic Igor M. Belkin Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA Received 19 December 2003; accepted 18 March 2004; published 28 April 2004.

[1] Time series of T and S extending through 2001 are used to describe propagation of the ‘‘Great Salinity Anomaly’’ of the 1990s (GSA’90s). Comparison of the distance-time relations for the GSA’70s, ’80s, and ’90s reveals a substantial intensification of the large-scale circulation in the northern North Atlantic, especially in the Subarctic Gyre between Newfoundland and the Faroes. The advection rate of the GSA’70s, ’80s, and ’90s between Newfoundland and the Faroe-Shetland Channel is conservatively estimated to have been 3.5, 10, and 10 cm/s, respectively. The circulation intensification apparently occurred within a decade between the GSA’70s and ’80s. During the next decade the advection rate increased from 10 to 13 cm/s between Newfoundland and Iceland Basin. The GSA’90s was advected towards the Faroe-Shetland Channel by the northern (Iceland Basin’s) branch of the North Atlantic Current, whereas the contribution of the southern branch via the Rockall Trough INDEX TERMS: 4215 Oceanography: General: was minimal. Climate and interannual variability (3309); 4223 Oceanography: General: Descriptive and regional oceanography; 4512 Oceanography: Physical: Currents; 4532 Oceanography: Physical: General circulation; 4572 Oceanography: Physical: Upper ocean processes. Citation: Belkin, I. M. (2004), Propagation of the ‘‘Great Salinity Anomaly’’ of the 1990s around the northern North Atlantic, Geophys. Res. Lett., 31, L08306, doi:10.1029/ 2003GL019334.

1. Introduction [2] The second half of the 20th century featured a series of decadal-scale anomalies of salinity, S, temperature, T, and sea ice cover, SIC, in the northern North Atlantic. These anomalies are best defined in S, hence termed the ‘‘great salinity anomalies’’ or GSA [Dickson et al., 1988; Belkin et al., 1998]. The GSA’70s was fully described by Dickson et al. [1988]. The GSA’80s was identified and documented by Belkin et al. [1998], who, based on a limited pre-1996 data set, also noted the apparent formation of the GSA’90s in the Labrador Sea. The present study includes the latest data through 2001 that allowed the GSA’90s reconstruction. Comparison of propagation speed of the three GSAs indicates long-term intensification of the Subarctic Gyre circulation. [ 3 ] Figure 1 shows the timing (year-1900) of the GSA’90s formation in the Labrador Sea and arrival at various locations along its path. Below we describe the GSA’90s movement, with emphasis on arrival times, so that

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the anomaly’s propagation speed could be estimated and compared with other GSAs.

2. Data and Method [4] Time series of T and S extending through 2001 were used to describe the GSA’90s noted by Belkin et al. [1998]. The Annual ICES Ocean Climate Status Summaries [Turrell and Holliday, 2002a, 2002b, hereinafter referred to as TH02a and TH02b] were especially valuable. The complete set of time series of T and S used in this analysis will be presented elsewhere. Table 1 summarizes arrival times of three GSAs at key locations. It also contains travel distances to these locations from the Fylla Bank section. Using the same current schematics for all three GSAs [Dickson et al., 1988; Ellett and Blindheim, 1992; Belkin et al., 1998], an assumption was made that the large-scale circulation pattern remained basically the same between 1968 and 1997. The open-ocean limbs of the Subarctic Gyre are known to shift by a few hundred kilometers over a few years, notably the North Atlantic Current (NAC) [Belkin and Levitus, 1996] and Irminger Current [Bersch, 2002]. These shifts can be neglected at this stage given other uncertainties involved in the travel distance calculations, e.g., current meandering, and TS-events’ arrival dating, accurate to a few months at best except for several locales with frequent observations, e.g., the Faroe-Shetland Channel [Østerhus et al., 2001].

3. Formation of the GSA’90s [5] The large-scale atmospheric pressure pattern over the North Atlantic changed dramatically in 1989 when the winter NAO index switched from negative to positive state [Hurrell et al., 2003] and reached its absolute maximum on record since 1864, accompanied by the northwesterlies’ intensification over the Baffin Bay and Labrador Sea that lasted through 1995 and should have enhanced the Arctic Ocean fresh water export via the Canadian Archipelago to the Baffin Bay and Labrador Sea. Local mechanisms must also have contributed to the GSA’90s formation [Colbourne et al., 1997]. [6] The anomaly has emerged along Fylla Bank section in 1989 and completed its formation during severe winters of 1992 – 1994 when the negative air temperature monthly anomalies in West Greenland exceeded 10°C; the winters of 1992 and 1993 in Nuuk were the coldest on record since 1866 [Belkin et al., 1998]. The low-S, low-T, high-SIC, two-prong event of the early 1990s is evident in various time series from the Labrador Sea and Newfoundland Shelf area (Figure 2) [Deser et al., 2002, Figure 15; Mysak et al.,

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Figure 2. Inverted winter Davis Strait ice index (open circles) and April –July salinity anomalies at 100 m depth in the West Greenland Current (offshore of Fylla Bank; solid circles). Both curves are seasonal means normalized by their respective standard deviations [Deser et al., 2002, Figure 15]. Three GSAs are marked by the author.

Figure 1. Propagation of the GSA’90s around the northern North Atlantic. Shown are transit dates of minimum salinities (years-1900). The circulation schematic after Ellett and Blindheim [1992, Figure 6]. 1996; Colbourne et al., 1997; Reverdin et al., 1997; Belkin et al., 1998; Buch, 2002].

4. Advection of the GSA’90s Around the Northern North Atlantic 4.1. West Greenland [7] The GSA’90s emerged in 1989 over Fylla Bank top at 64N as a strong negative temperature anomaly [Buch, 2002]. Using the offshore salinity data from Fylla Bank section, Deser et al. [2002] have found a negative salinity anomaly at 100 m depth in 1989 –90 that almost rivaled the salinity anomalies in 1971 and 1982 (Figure 2). Three offshore time series from 64N, 65N and 66.5N portray a

Table 1. Arrival Time (Year) of the GSA’70s, ’80s and ’90s at Key Locations as a Function of Distance D From Fylla Bank Locationa

Db, km

Fylla Bank section Seal Island section St.27 OWS ‘‘C’’ 58N, 26W Faroe-Shetland Channel Svinøy section OWS ‘‘M’’ Gimsøy section Bjørnøya section Sørkapp section

0 1204 2066 5003 5987 7628 8197 8425 8857 9229 9797

a

GSA70c

GSA80c

GSA90d

1969 1970 1971 1974

1982 1983 1983.5 1985

1989 1990 1991

1976 1977 1977.5 1978

1986 1987

1979

1988 1988 1988

1992 1993 1994 1995 1996 1996

Location metadata are given by Belkin et al. [1998, p. 9]. Distances are computed along main currents in Figure 1, with several additional waypoints inserted to approximate the currents’ sinuous paths, especially around the Grand Banks of Newfoundland and near the Faroes. The distances are along great circles between waypoints and do not take into account the currents’ mesoscale meandering, hence should be regarded as conservative estimates of true travel distances. c After Belkin et al. [1998]. d This study. b

strong cold event in 1989 – 1990, accompanied by a low-S anomaly [Buch, 2002]. These time series also reveal a previously unreported GSA of the late 1990s: a pronounced low-S anomaly in 1996 – 1997, apparently caused by enhanced inflow of the Polar Water [Buch, 2002]. We suggest that the low-S anomaly of 1996– 1997 has its origin in the maximum efflux of the Arctic sea ice via Fram Strait to the Greenland Sea in 1994– 1995 [Vinje et al., 1998; Kwok and Rothrock, 1999]. This sea ice pulse was likely accountable for a 100 m thick, low-S (S < 34.65) surface layer anomaly in the Greenland Sea in April – May 1996 (K. Hatten, personal communication). 4.2. Labrador and Newfoundland Shelves [8] The first wave of the GSA’90s was observed in 1990– 1991 along the Seal Island line off Labrador and the Bonavista line off Newfoundland (TH02b). It passed St.27 off St. John’s in 1991 [Colbourne and Foote, 2000] when the T-anomaly in the 0 – 176 m layer dropped to its lowest value on record since 1950, while the corresponding negative S-anomaly was only comparable with the GSAs of 1970 and 1984. The twin-minimum S-anomaly persisted over the Newfoundland Shelf from 1991 through at least 1995 [Colbourne and Foote, 2000; Smith et al., 2001]. The twin-minimum negative S-anomaly that peaked at St. 27 in 1992 and 1995 could be related to a similar twin-minimum S-anomaly upstream, off Fylla Bank, which peaked two-tothree years earlier, in 1989 and 1993 (Figure 2). The NAO switch that preceded and possibly triggered the GSA’90s had two spikes, in 1989 and 1992 – 95 [Hurrell et al., 2003], reflected in the twin-minimum Tair anomaly in Nuuk in 1989 and 1993 – 94 [Buch, 2002]. 4.3. The Irminger Current [9] The Irminger Current transported the GSA’90s towards Iceland. The anomaly’s arrival at Faxaflo´i section west of Iceland, along 64.5N, in 1993 manifested as a drop of the mean S of the 0– 200 m layer from 35.10 to 34.97 (TH02a). The low-S conditions persisted through 1997. 4.4. North Iceland [10] Carried by the North Iceland Irminger Current, the GSA’90s arrived in 1995 at Siglunes section along 19W (TH02b). The GSA’90s was evident in both T and S at 50 m

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depth, with T < 0.5°C and S < 34.6. The low-S signal persisted through 1997. Along Langanes section across the East Icelandic Current, a low-S signal was observed in 1998, when S at 25 m depth fell from 34.75 to 34.55 (TH02b). This signal, however, may have represented the next GSA, of the late 1990s, noted above. 4.5. Faroese Waters [11] The GSA’90s passed via the Faroe Bank Channel SW of Faroe in 1993 – 1994 and was recorded in the Faroe Current north of Faroe in 1993 – 1995 (TH02b). The attendant T-anomalies peaked in 1994– 95. The Faroe Bank Channel TS-indices of the GSA’90s were 7.6°C and 35.18 in the 100 – 300 m layer. The Faroe Current TS-indices of the GSA’90s determined for the current’s core were 6.7°C and 35.16 (TH02b). 4.6. Faroe-Shetland Channel [12] The GSA’90s peaked here in 1993 – 1995 when the S-anomaly of the surface Atlantic water in the Slope Current was comparable with that of the GSA’80s and ’70s, approximately 0.02, 0.03 and 0.04, respectively (TH02b). 4.7. Norwegian Sea [13] The GSA’90s passed through Svinøy-NW section in 1993 – 1994, when the depth-averaged T and S above the slope decreased to 7.7°C and 35.17 respectively (TH02b). This anomaly arrived at OWS ‘‘M’’ (66N, 2E) in 1994 – 1995, when the annual mean T150 and S150 fell, respectively, to 6.1°C and 35.08 [Aure, 1999]. In 1995– 1996 the GSA’90s passed through Gimsøy-NW section with the depth-averaged above-slope T and S of 5.8°C and 35.12, respectively (TH02b). In 1995 – 1997 this anomaly was observed along Sørkapp-W section with the depth-averaged above-slope T and S of 3.6°C and 35.03, respectively (TH02b). The decadal-scale GSAs are superimposed on a long-term cooling and freshening in the Norwegian Sea, ascribed mainly to increased freshwater supply from the East Icelandic Current [Blindheim et al., 2000]. As pointed out by Blindheim et al. [2000, p. 655], ‘‘As a result, temperature and salinity in some of the time series were lower in 1996 than during the Great Salinity Anomaly in the 1970s.’’ It should be noted, however, that the extremely low salinity of 1996 may have been caused by the abovementioned new GSA that has just formed in the Greenland Sea.

5. Comparison of Propagation Speed of the GSA’70s, ’80 and ’90s Confirms the Subarctic Gyre Spin-Up [14] The advection rate of the three GSAs can be estimated from their respective distance-time relations (Figure 3). The GSA’70s was the slowest: It took 10 years for this anomaly to cover the 10,000-km distance between Fylla Bank and Svalbard, whereas the GSA’80s and ’90s completed this journey in 6 to 7 years. The GSA’70s also featured a fairly constant propagation speed, approximately 3 cm/s. The GSA’80s and ’90s had a much higher speed, especially in the Labrador-Irminger Gyre. A major difference between the 1980s and 1990s is that the GSA’80s advection was uniformly faster than the one of the GSA’70s, whereas the

Figure 3. Comparison of propagation speed of the GSA’70s, ’80s, and ’90s based on Table 1 data. See color version of this figure in the HTML. GSA’90s advection was very fast only between Newfoundland and the Faroes. The acceleration of the Subarctic Gyre is evident from a comparison of travel times between the Grand Banks of Newfoundland and Faroe-Shetland Channel: the GSA’70s, ’80s, and ’90s crossed the North Atlantic in five, two and two years, respectively. Given the approximately 5600-km distance, it means that the advection rate increased from 3.5 to 10 cm/s between the 1970s and 1980s. A further increase occurred between the 1980s and 1990s based on two observations of the GSA’90s, one in 1991 at St.27 off St. John’s, Newfoundland [Colbourne and Foote, 2000; Smith et al., 2001]; another in 1992 in the Iceland Basin along the WOCE A1E line [Bersch, 2002]; hence the propagation speed is approximately 4000 km/year or 13 cm/s. The circulation spin-up inferred from the GSAs propagation corroborates the observed interdecadal increase in the intensity of the North Atlantic gyre circulation [Curry and McCartney, 2001; Dickson et al., 2001] associated with the unprecedented amplification of the NAO [Hurrell et al., 2003].

6. Discussion [15] Several recent works presented fresh views of the North Atlantic upper layer circulation based on drifter data [Poulain et al., 1996; Valdimarsson and Malmberg, 1999; Fratantoni, 2001; Orvik and Niiler, 2002; Reverdin et al., 2003] and hydrography [Hansen and Østerhus, 2000; Bersch, 2002; Holliday, 2003]. Some authors show two major branches of the NAC east of the Mid-Atlantic Ridge, one via the Iceland Basin, another via the Rockall Trough [e.g., Fratantoni, 2001; Orvik and Niiler, 2002], whereas others view the Rockall Trough branch of the NAC as much less significant or even dissociated from the NAC and originated in the NE Atlantic [e.g., Krauss, 1986; Hansen and Østerhus, 2000; Reverdin et al., 2003]. The repeat WOCE hydrography across the Iceland Basin and southern approaches to the Rockall Trough allowed a reliable estimation of the poleward transport partition between these

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two branches. In both NAO states, high and low, the Iceland Basin branch clearly dominated in the baroclinic volume and was a sole conduit for the freshwater poleward transport [Bersch, 2002, Figure 9]. The GSA’90s trajectory in space and time is entirely consistent with the Bersch [2002, Figure 3] data that shows the GSA’90s advection in 1992 – 1995 via the Iceland Basin where the anomaly was observed annually at 26W, while no freshening was observed in the Rockall Trough.

7. Conclusions [16] The GSA’90s was fundamentally similar to the GSA’70s and ’80s, yet special enough to warrant its detailed study and comparison with other GSAs. Identification of differences in formation, structure and propagation of various anomalies provides observational benchmarks and constraints for modeling studies [Ha¨kkinen, 2002a, 2002b; Marsland et al., 2003]. In particular, the above comparison of distance-time relations of three consecutive decadal-scale anomalies of the 1970s, 1980s and 1990s has revealed a significant increase in their propagation speed after the 1970s, especially between Newfoundland and the Faroes. Since these anomalies were advected along limbs of the Subarctic Gyre, the observed increase suggests a substantial intensification of the Subarctic Gyre in the 1980s and 1990s. The circulation intensification of the 1980s encompassed the entire northern North Atlantic, whereas in the 1990s the circulation intensification was confined within the Subarctic Gyre between Newfoundland and the Faroes. This suggests that the strength of coupling between the Subarctic Gyre and the Nordic Seas circulation could vary on a decadal scale. The coupling was strong in the 1980s and weak in the 1990s. [17] Acknowledgments. This study stemmed from my previous research funded by NOAA and my current study of ocean fronts funded by NASA grants NAG 53736 and NAG 512741. The support of both agencies is greatly appreciated.

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I. M. Belkin, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA. ([email protected])

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