Impact of recirculation on the East Greenland Current in Fram Strait: Results from moored current meter measurements between 1997 and 2009

Deep-Sea Research I 92 (2014) 26–40

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Impact of recirculation on the East Greenland Current in Fram Strait: Results from moored current meter measurements between 1997 and 2009 L. de Steur a,b,n, E. Hansen b, C. Mauritzen c, A. Beszczynska-Möller d,e, E. Fahrbach d,1 a

NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands Norwegian Polar Institute, Fram Centre, Hjalmar Johansens gt. 14, 9296 Tromsø, Norway c Strategic Research & Innovation, DNV GL, Veritasveien 1, 1363 Høvik, Norway d Alfred Wegener Institute for Polar and Marine Research, Climate Sciences Department, Am Handelshafen 12, 27570 Bremerhaven, Germany e Institute of Oceanology PAS, Physical Oceanography Department, ul. Powstancow Warszawy 55, 80-712 Sopot, Poland b

art ic l e i nf o

a b s t r a c t

Article history: Received 8 September 2013 Received in revised form 17 May 2014 Accepted 30 May 2014 Available online 13 June 2014

Transports of total volume and water masses obtained from a mooring array in the East Greenland Current (EGC) in Fram Strait are presented for the period 1997–2009. The array in the EGC was moved along isobaths from 791N to 781500 N in 2002 to line up with moorings in the eastern Fram Strait. Analysis of the time series at the two latitudes shows that associated with the southward move, the annual mean volume transport of the EGC increased from 5.871.8 Sv to 8.772.5 Sv, mostly related with an increase in barotropic flow. This suggests a recirculation of close to 3 Sv at 781500 N as a consequence of the large-scale wind-driven cyclonic gyre in the Nordic Seas. In addition, the volume transport at 781500 N showed a clear seasonal cycle which was absent at 791N. Estimates of the wind-driven Sverdrup transport at two different latitudes show that the difference in total volume transport and seasonality can largely be explained by the wind-stress curl. However, weak transport in 2003 was only partially related with weak Sverdrup transport and coincided also with anomalously weak northerly winds. The stronger recirculation at 781500 N has also consequences for the observed Atlantic Water: there is significantly more Atlantic derived water present at the southerly latitude. In addition, the warm anomaly in Fram Strait between 2005 and 2007 doubled the amount of Recirculated Atlantic Water temporarily. Finally, we estimate that close to 2.7 Sv, or 50%, of Atlantic derived water recirculates in Fram Strait. & 2014 Elsevier Ltd. All rights reserved.

Keywords: East Greenland Current Fram Strait Current meter measurements Volume transport Recirculation Recirculating Atlantic Water (RAW)

1. Introduction The Fram Strait is the deep passage between the Arctic Ocean and the Nordic Seas: the region comprising the Greenland Sea, the Iceland Sea and the Norwegian Sea. In Fram Strait, cold and fresh Polar Water (PW) and sea ice are exported from the Arctic with the East Greenland Current (EGC) (Aagaard and Coachman, 1968a) and warm Atlantic Water (AW) enters the Arctic Ocean in the West Spitsbergen Current (WSC) (Mosby, 1962). The deep part of the strait has a maximum depth around 2650 m and is therefore the only gateway where direct exchange of intermediate and deep waters occurs between the Arctic Ocean and the Nordic Seas (Aagaard et al., 1985; Mauritzen, 1996a). As the EGC carries PW and modified AW southward along the continental shelf of n Corresonding author at: NIOZ Royal Netherlands Institute for Sea Research. P.O. Box 59, 1790 AB Den Burg, The Netherlands. E-mail address: [email protected] (L. de Steur). 1 In Memoriam.

http://dx.doi.org/10.1016/j.dsr.2014.05.018 0967-0637/& 2014 Elsevier Ltd. All rights reserved.

Greenland a considerable amount of exchange with the shelf and the deep basins, and modification of water masses take place (Aagaard and Coachman, 1968a; Rudels et al., 2002). Variations of thermohaline properties within the Arctic Mediterranean (the Arctic Ocean and Nordic Seas combined) on interannual time scales can result in buoyancy fluctuations within the subpolar North Atlantic, and subsequently leave an imprint on the North Atlantic Deep Water (Dickson et al., 1988; Aagaard and Carmack, 1989; Häkkinen, 1993). Apart from exchange of freshwater and heat between the Arctic Ocean and the Nordic Seas through the EGC and WSC, significant recirculation of AW within Fram Strait occurs between approximately 761 and 811N which makes it a very dynamical and complex region (Aagaard and Coachman, 1968b; Quadfasel et al., 1987; Bourke et al., 1988). The bulk of AW recirculates just south of 791 and is sometimes referred to as the Return Atlantic Current which joins the EGC (Paquette et al., 1985; Quadfasel et al., 1987; Bourke et al., 1988). North of 791N a portion of the WSC flows west-northwest along the Yermak plateau (Bourke et al., 1988; Rudels et al., 2000). The total amount of

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

recirculated AW in the EGC was estimated as large as 67% of the northward flowing AW if all the Yermak branch turns southward (Rudels, 1987; Manley, 1995). The recirculation of AW in Fram Strait and consequent modification through cooling and freshening compose an important source for the downstream Denmark Strait overflow (Strass et al., 1993; Mauritzen, 1996b; Rudels et al., 2002). It was found that the source waters of the Denmark Strait overflow consist of up to 64% Recirculating Atlantic Water (RAW) and that travel time of thermohaline anomalies within AW in the Nordic Seas was short, 1–3.5 years, illustrating the importance of the westward circulation in Fram Strait (Eldevik et al., 2009). In addition, it has been suggested that AW temperature anomalies circulating in the Arctic returning southward with the EGC, if not density compensated by higher salinity, may lead to a reduction of the Denmark Strait overflow (Karcher et al., 2011). Studies of the structure and properties of the EGC have a long history (Mohn, 1887; Helland-Hansen and Nansen, 1909). Aagaard and Coachman (1968a) were the first to observe significant currents in the EGC below 200 m using direct current meter measurements from a drifting ice island and they found the barotropic mode to be dominant in the EGC at  691N. The transport and structure of the EGC at 791N were first explored with moored instrumentation in the mid 1980s by Foldvik et al. (1988). Foldvik et al. (1988) concluded that, at this latitude, the EGC is not only a baroclinic, buoyancy-driven current, but that most likely wind forcing also plays an important role for driving the flow. They estimated the transport in the upper 700 m to be around 3 Sv and did not find any significant seasonal signal at this latitude. Significant mesoscale variability was present in the EGC with strong eddies passing by on time scales of weeks, however mesoscale activity was found to contribute little to the net heat fluxes (Foldvik et al., 1988). Within Fram Strait, mesoscale fluctuations in the eastern part were shown to be generated by nonlinear, wind-driven interactions. However, the relation of the regionally very strong wind field to mesoscale motions in the EGC itself remained unclear (Jónsson et al., 1992). From 1997 onwards a mooring array with 14 moorings was established across the whole deep, approximately 320 km wide, Fram Strait. Based on data from the first 2 years (1997–1999) Fahrbach et al. (2001) found that baroclinic and barotropic components in the EGC are of a similar magnitude. Their estimate of the total southward volume transport in the EGC was 13.7 7 1.7 Sv. This included an estimate of recirculation of 2.6 70.1 Sv based on the westward flow observed at two moorings in the middle of the strait. On the large scale, variability of the circulation in the Arctic Mediterranean on monthly to annual timescales was shown to be predominantly wind-driven, following contours of f =H (Isachsen et al., 2003). The transport of the EGC at 741N was found to be subject to a large seasonal cycle related to the wind-driven gyre in the Greenland Sea (Woodgate et al., 1999). In contrast to surface buoyancy or wind forcing, the role of the along-slope bottom density gradient was found to be a good candidate for increasing the magnitude of the bottom flow, and hence the barotropic flow, in the EGC at 791N (Schlichtholz and Housais, 1999; Schlichtholz, 2005). In this study we present more than a decade (1997–2009) of current measurements from the Fram Strait mooring array west of 11W and investigate the velocity field, transport and thermal properties in the EGC. The observational array is a joint effort of the Norwegian Polar Institute, concentrating on the EGC, and the Alfred Wegener Institute for Polar and Marine Research, covering the WSC and the central Fram Strait. Between September 1997 and September 2002 the moorings in the EGC were located at 791N while the moorings east of the prime meridian were at 781500 N. The fact that the moorings in the EGC were at a different latitude than the moorings in the WSC implied that a portion of the recirculation was not captured by the mooring array (Fahrbach et al., 2001). Therefore

27

the EGC moorings, all west of the Prime Meridian, were moved southward along isobaths by one-sixth degree to 781500 N in September 2002 to line up with the moorings in the eastern part of the strait. Also, new moorings were added to better cover the central part of the strait between the EGC and WSC. First results on volume and heat transports based on the mooring array at 791N were described by Fahrbach et al. (2001) and Schauer et al. (2004). Later studies by Schauer et al. (2008) and Schauer and Beszczynska-Möller (2009) address the oceanic transports through Fram Strait from the longer time series collected both before (1997–2002) and after (2002–2008) the relocation of the moored array. Time series of Polar Water properties based on the mooring data in the EGC were described by Holfort and Hansen (2005) and an analysis of the liquid freshwater flux in the EGC from 1997 to 2008 was carried out by De Steur et al. (2009). More recent results on the net volume and heat flux in the WSC up to 2010 were presented in Beszczynska-Möller et al. (2012). It was shown that the narrow core of the WSC which flows tightly along the upper continental slope (transporting  1.870.1 Sv) exhibits no seasonality while its more variable offshore branch further west (transporting between 2 and 6 Sv) features a large seasonal cycle (Beszczynska-Möller et al., 2012). Both the eastern WSC core and the off-shore branch carry a large amount of warm AW into the Arctic, accumulating up to 2.2 to 3.2 Sv. The net northward volume transport of AW in the WSC has shown no significant trend between 1997 and 2010, however, two distinct AW temperature anomalies were observed; one during 1999–2000 and one between 2005 and 2007 (Beszczynska-Möller et al., 2012). The latter warm anomaly originated from advection with the Norwegian-Atlantic Current in the Greenland Sea (Walczowski and Piechura, 2007) which propagated counterclockwise through the upper ocean of the Nordic Seas between 2003 and 2006 (Holliday et al., 2008). Here we focus on the annual and interannual variabilities of the total volume transport and thermal properties of the EGC, specifically of the intermediate water masses, between 1997 and 2009. The availability of a 12-year long mooring record allows for a new analysis of the EGCs structure and variability as presented by Foldvik et al. (1988) or Fahrbach et al. (2001). To interpret the differences in volume transport at 791N vs. 781500 N we follow a similar approach as Woodgate et al. (1999) to quantify the wind-driven (Sverdrup) transport in the EGC in Fram Strait. The difference between the transport calculated from the moorings and the estimate of the Sverdrup transport is presumably an estimate for the buoyancy driven (thermohaline) outflow of the Arctic Ocean. The structure of this paper is as follows: Section 2 describes the data set, data treatment and methods used for obtaining interpolated fields of velocity and temperature. The observed velocities and temperatures, long-term mean fields and seasonality in the EGC over the observational period are presented in Section 3. Special attention is given to the distinct differences between the structure of the EGC at 791N (1997–2002) and at 781500 N (2002– 2009). Section 4 presents and discusses the total volume transport and the role of wind forcing, and transports of different water masses including a comparison with earlier studies. Section 5 summarizes the main results.

2. Data and data treatment We focus on the variability of volume transport and temperature in the EGC, based on data obtained by the moorings starting on the shallow East Greenland Shelf in the west to the deep strait at  11W (Fig. 1). Apart from repositioning these moorings from 791N to 781500 N small changes in longitude occurred as well in September 2002, with the largest change for F9 (see Tables 2 and 3

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L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

82 oN

0

81 oN

−500

Latitude

0 F17

F14

−2

80 oN

−1000

Fram Strait

Depth [m]

79 oN 78 oN

77 oN

East Greenland Shelf

−1500

−6 F12

−2000

76 oN 12 oW

−4 F13

o

o

6W

o

0

o

6E

12 E

−2500

East Greenland Shelf

−8 F11

Longitude [m] −6000

−4000

−2000

0

−10 F10

F9

−12 −3000 8°W 7°W 6°W 5°W 4°W 3°W 2°W 1°W [cm/s]

Longitude

Fig. 1. The Fram Strait: (a) bathymetry of the Fram Strait with contours every 500 m. The mooring locations at 791N (1997–2002) are shown with magenta dots, the mooring locations at 781500 N (2002–2009) are shown with red diamonds. (b) A cross section of the mooring array and instrumentation (black squares) at 781500 N with contours of the mean velocity field of the EGC in the background. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

for the exact positions in the two periods). This was done such that the moorings remained on the same isobath and would capture a similar current structure of the EGC. At 781500 N the longitudes of the moorings were approximately at 61300 W, 51W, 41W, 31150 W, 21W and 0.81W (Fig. 1). West of 61W the broad East Greenland Shelf spans approximately 300 km with depths ranging from 100 to 300 m. From September 2003 onwards attempts were made to place additional shallow moorings here, initially at  12:51W and later on at  81W. These moorings suffered from many instrument losses but still provided good data in some years. In general the total volume transport of the EGC presented here is calculated from the southward velocities integrated from 6.51W to  11W, the latter corresponding roughly with the zero velocity contour of the EGC. Whenever possible data from the additional shallow mooring further west on the shelf are included in the analysis. Therefore two time series for the volume transport are calculated, one based on six moorings and one based on seven moorings, in order to mark the westward extension of the array from September 2003 onwards. The moorings contained a combination of Seabird SeaCATs and MicroCATs (SBE 16 and SBE 37) measuring temperature (T), pressure (P), conductivity (C), and Doppler Current Meters (Aanderaa DCM 12), Recording Doppler Current Profilers (Aanderaa RDCP 600), Acoustic Current Meters (Falmouth Scientific, Inc. 3D-ACM) or Recording Current Meters (Aanderaa RCM 7, 8, 9 or 11) measuring horizontal velocities (U, V) in the uppermost layer. RCMs and Acoustic Doppler Current Profilers (RDI ACDP) with additional pressure and temperature sensors were used to measure U, V, P and T in the intermediate and deeper waters. The uppermost instruments were ideally placed in the cold and fresh polar waters at 50–70 m (Fig. 1). The next instruments downward were located between 240 and 325 m, capturing warm RAW. Moorings F9 through F12 had instruments between 1450 m and 1550 m within cold deep water. Additional instruments were added at  500 m on mooring F10 between 2000 and 2003 and at 780 m at F9 between 2000 and 2002. From 2003 onwards there were instruments at 760 m at both F9 and F10 but not at F11 through F13. The deepest instrument at each mooring was fixed at about 10 m above the ocean floor which is between 2360 and 2600 m at moorings F11 to F9. Measurements of velocities, T and C by the RCMs were in general hourly or two hourly while T and C from the MicroCATs were measured every 15 or 30 min. The eastward (U) and northward (V) velocity components were calculated taking the average magnetic declination of each month at the mooring locations into account. The tides were removed from the

velocities and pressure using a 40-h low pass filter and daily means were calculated for U, V, P, and T. The mooring array had at times suffered from instrument losses and failures throughout the monitoring period leaving data gaps in time and space. Table 1 gives an overview of the returned or missing data per mooring per year. In order to obtain realistic volume transports of the total EGC using the same number of moorings and instruments each year, the data gaps were dealt with as follows: for the years when data was missing at one instrument location, a time series of V and T was constructed by performing a linear regression with the time series from nearby instruments with regression coefficients determined from the year before when there were data at both locations. For T the correlation between data of the nearest instrument was largest in the horizontal so the regression was carried out in the horizontal. However, for V the correlation between instrument data was largest in the vertical, and hence regression for V was carried out between instruments in the vertical. However, if a whole mooring was missing the time series for V was also determined from regression in the horizontal with neighboring moorings. The regression was performed with detided, daily averaged data. If two neighboring moorings were present (both in the year of missing data and in the year when regression coefficients are determined) the time series was reconstructed as T F12 ðzi Þ ¼ b0 þ b1  T F11 ðzi Þ þ b2  T F13 ðzi Þ;

i A f1 : ng:

ð1Þ

Here T F12 is the temperature at mooring F12, T F11 and T F13 were the temperatures at mooring F11 and F13, b0;1;2 were the coefficients determined by linear regression in the year before the year the data was missing, and zi was the depth level of the instrument. If there was only data of one neighboring mooring present, time series of V or T were obtained by T F14 ðzi Þ ¼ b0 þ b1  T F13 ðzi Þ;

iA f1 : ng:

ð2Þ

If only one instrument was missing on a mooring array (e.g. the uppermost instrument) the time series for V was determined as V F12 ðzn Þ ¼ b0 þ b1 :V F12 ðzn þ 1 Þ:

ð3Þ

After filling data gaps with the regressed fields, the data was interpolated on a vertical grid with regularly spaced coordinates of 10 m in the upper 155 m and terrain following coordinates below 155 m. Thereafter, monthly means were calculated and those were interpolated in the horizontal at a resolution of 0.251 longitude, which is approximately 5.5 km at that latitude. These final fields were used for further analysis of the total southward volume

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

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Table 1 Overview of data obtained from each mooring per year: ‘100%’ means all data was retrieved from all instruments at depths shown in Tables 2 and 3, if data from one instrument was not recovered or of bad quality, the variable and depth are given of which data was lacking. ‘X’ means the whole mooring was either lost or not recoverable. Mooring

F14

F13

F12

F11

F10

F9

1997–1998 1998–1999 1999–2000 2000–2002 2002–2003 2003–2004 2004–2005 2005–2006 2006–2008 2008–2009

100% 100% 100% V 270 m (January) 100% 100% 100% 100% V 50–70 m 100%

100% 100% 100% X X 100% X 100% V 50–70 m V 50–70 m T, V 1845 m

100% 100% 100% T 50–70 m T, V 1500 m; T, V 1845 m X X 100% 100% V 50–70 m

X 100% 100% 100% 100% 100% V 50–70 m V 50–70 m V 50–70 m 100%

V 250 m 100% 100% V, T 250 m V 50–70 m 100% V, T 250 m X 100% 100%

100% V 50–70 m X V 250 m (March) T 2585 m 100% 100% 100% 100% 100%

transport and variability of water masses in the EGC on the section. The addition of and changes in instrument depths have a big impact on the final result of vertical stratification after interpolation. Linear interpolation between temperature data at 250 m and 1500/1000 m at F11 through F13 resulted in a 01 isotherm that was too deep (  1250 m) compared to that observed in hydrographic data (varying from 800 to 1000 m). This was improved at F9 and F10 when instruments were inserted at 750 m in 2000 and 2003 however not at F11–F13. Therefore the regression method was also carried out for T at the 750 m level for moorings F9–F13 before 2003 and at F11–F13 after 2003 in order to maintain a consistent depth of the 01 isotherm. The regression coefficients of temperature between 250 m and 750 m were obtained from all hydrographic data obtained in Fram Strait during the deployment period.

3. Observed velocities and temperatures In this section daily-averaged data from some selected mooring sites, long-term mean fields and seasonality of velocities and temperatures are presented. 3.1. Time series of observed velocities and temperature Time series of velocities measured at each instrument depth for moorings F10 (eastern edge of the EGC) and F12 (central EGC) are shown as stick plots (Figs. 2 and 3). Even though substantial data gaps are present on these two sites, clear differences are seen in the data records related to the southward relocation of the moorings. At mooring site F10 the long-term mean velocity components U and V at 50–70 m depth were of similar magnitude with V ¼  8:9 cm s  1 and U ¼ 8:3 cm s  1 . In the Atlantic layer at 250–260 m depth these were V ¼  5:7 cm s  1 while U ¼  8:2 cm s  1 . At larger depths the long-term U and V were smaller, around  4 cm s  1 . In all layers the flow increased substantially at 781500 N over the whole depth. At the mooring location F12 the flow is directed much more north-south than at F10. In the upper layer the long-term mean V   10:4 cm s  1 while the westward velocity U is only  1:5 cm s  1 . In the Atlantic layer at 300–335 m the magnitudes are approximately half of that in the upper layer. Near the bottom of F12 (1800–1845 m) a slight bottom-intensified flow can be seen compared to 1500 m. Despite a large data gap at this mooring location between 2003 and 2006, the southward flow observed at 781500 N in the period after the mooring relocation is clearly much stronger than at 791N. Time series of daily temperatures are shown for F11 which has the longest continuous record of the central EGC moorings (Fig. 4). Observed temperatures in the PW ( 50–70 m) in the EGC show a mixture of the seasonal cycle related with freezing and melt of sea

ice and lateral shifts of the Polar Front, bringing warmer water westward (Holfort and Hansen, 2005). In our record up to 2009 the same was found with highest temperatures in September. The latter appeared to occur more frequently after the mooring was moved to 781500 N such that the mean temperature of PW increased from  1.17 1C to  0.89 1C. Temperatures in the underlying Atlantic layer at 250–260 m at F11 vary from 0.5 1C to 3.8 1C. Here two things can be noticed: a difference in the long-term mean temperature of at 791N and 781500 N, and on top of that the appearance of the warm AW anomaly from 2005 to 2007. The AW anomaly had an amplitude of approximately 0.7–1.0 1C at F11, and it was observed in the Atlantic layer at all mooring locations between 2005 and 2007. The warm anomaly was first observed on the eastern edge of the EGC (F9) starting in 2004, and as far west as mooring location F14 starting in 2005 (not shown). This delay suggests that a part of the warm AW anomaly recirculated in a northern branch of RAW and merged with the EGC further north of the array. In 2008 the AW temperature had decreased again to pre-2004 values. At larger depths mean temperatures are around 0.55 1C at 1455 m to  0.77 1C near the bottom at 2360 m. Considerable variability was present at 1455 m at 791N which appeared to be smaller at 781500 N. No clear trend was identified at this depth. The long-term mean temperature near the bottom at F11 appeared to show a positive trend between 2003 and 2009 of 0.007 1C year  1 in the annual mean. The trends at F10 and F9 (not shown) were 0.0085 1C year  1 and 0.01 1C year  1, respectively. Since the accuracy of the individual temperature sensors of the instruments at this depth (RCM8) is only  0.05 1C these trends cannot be claimed to be significant. However, similar conclusions have been reached by Langehaug and Falck (2012) based on hydrographic station data from Fram Strait between 1982 and 2008. Their study showed that the temperature of the deep water originating from the Norwegian Sea increased by almost  0.10 1C during the study period.

3.2. Mean structure of the EGC at 791N and 781500 N The above results show that the relocation of the moorings in the EGC had apparent consequences for the observed velocity field in the EGC. Therefore the long-term mean and standard deviation of the measured velocities and temperatures are provided at all instrument locations for the two locations 791N (1997–2002) and 781500 N (2002–2009) separately (Tables 2 and 3). Mean values from the additional shelf mooring added in 2003 are also given (Table 4). At mooring sites F10, F11 and F12 a general increase in southward velocity was recorded at all depths. The largest changes occurred in the upper core of the EGC at F12 where V increased with  75% when the mooring was moved from 791N to 781500 N. At F11, F13 and F14 southward velocities in the upper layer increased with  35%, 40% and 50% respectively, while a small

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L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

F10 at 2°W 5 50−70 m

Velocity [cm/s]

0 −5 −10 −15 −20 1997

1998

1999

2000

2001

2002

2003 2004 Year

2005

2006

2007

2008

2009

2010

2001

2002

2003 2004 Year

2005

2006

2007

2008

2009

2010

2001

2002

2003 2004 Year

2005

2006

2007

2008

2009

2010

2001

2002

2003 2004 Year

2005

2006

2007

2008

2009

2010

4 250−265 m

Velocity [cm/s]

0 −4 −8 −12 −16 1997

Array relocation 1998

1999

2000

Velocity [cm/s]

5 1500−1540 m 0 −5 −10 1997

1998

1999

2000

Velocity [cm/s]

5 2550−2650 m 0 −5 −10 1997

1998

1999

2000

Fig. 2. Stick plots of daily mean velocities at mooring site F10 at all four depths. Velocities are detided and shown every seventh day. Negative velocities indicate south/ westward flow. The annual mean value of the north-south velocity component V is plotted on top (black diamonds). September 2002, the point in time when the relocation of the mooring array took place, is indicated with the vertical dotted line.

decrease was seen in the Atlantic layer at F13 and F14 near the shelf break. The westward velocity at all depths of F10 showed a clear increase, at some depths mean westward velocities more than doubled at 781500 N. The opposite, a decrease of the westward velocity, was measured at F13 and F14. This shows that after the moorings were relocated a stronger westward barotropic recirculation, a part of the large cyclonic gyre in the Nordic Seas (see e.g.

Isachsen et al., 2003), and an increase in the observed southward flow in the EGC was measured at 781500 N. During both periods a slight bottom-intensified flow was observed at F12 and F13, the two shelf-slope moorings. This became slightly stronger at 781500 N. No trends in velocities were observed at any of the mooring locations. Considerable southwestward velocities were present on the shelf at F17, contributing to the southward transport of freshwater over the shelf (Table 4). The move of the array

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

31

F12 at ~4°W 5 50−70 m

Velocity [m/s]

0 −5 −10 −15 −20 1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2005

2006

2007

2008

2009

2010

2005

2006

2007

2008

2009

2010

2005

2006

2007

2008

2009

2010

Year 4 300−335 m

Velocity [m/s]

0 −4 −8 −12 −16 1997

1998

1999

2000

2001

2002

2003

2004

Year Velocity [m/s]

4 1500−1540 m 0

Array relocation

−4 −8 1997

1998

1999

2000

2001

2002

2003

2004

Year Velocity [m/s]

4 1800−1845 m 0 −4 −8 1997

1998

1999

2000

2001

2002

2003

2004

Year Fig. 3. Stick plots of daily mean velocities at mooring site F12 at all four depths. Velocities are detided and shown every seventh day. Negative velocities indicate south/ westward flow. The annual mean value of the north-south velocity component V is plotted on top (black diamonds). September 2002, the point in time when the relocation of the mooring array took place, is indicated with the vertical dotted line.

had also consequences for the observed temperature. At almost all instrument locations the temperatures measured at 781500 N were higher than at 791N. An exception occurred in the upper layer at F12, F13 and F14 where the PW exhibited slightly lower temperatures after the southward move. Cross sections of the mean velocity and temperature fields based on the time series where data gaps were filled through the regression method are shown for the two latitudes 791N and

781500 N (Fig. 5). The EGC is clearly visible as a baroclinic core above 800 m with cold PW in the top 200 m overlying the warm AW with increasing temperatures going eastward (see also Foldvik et al., 1988; Fahrbach et al., 2001; Schauer et al., 2004; Beszczynska-Möller et al., 2012). Below that southward velocities in the cold intermediate and deep waters are significantly smaller. There is a distinct difference in the structure of the EGC at the two latitudes. At 791N the velocities over the whole depth are smaller,

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L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

F11 at ~ 3°W 3

50−70 m

Array relocation

Temperature [C]

2

1

0

−1

−2 1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2005

2006

2007

2008

2009

2010

2005

2006

2007

2008

2009

2010

2005

2006

2007

2008

2009

2010

Year 4

250−260 m

Temperature [C]

3.5

3 2.5

2 1.5

1 0.5 1997

1998

1999

2000

2001

2002

2003

2004

Year

Temperature [C]

−0.4

1455−1465 m

−0.5 −0.6 −0.7 1997

1998

1999

2000

2001

2002

2003

2004

Year

Temperature [C]

−0.65 −0.7

2360 m

−0.75 −0.8 −0.85 −0.9 1997

1998

1999

2000

2001

2002

2003

2004

Year Fig. 4. Daily mean temperatures at mooring site F11 at all four depths. On top the annual mean values are plotted (black diamonds).

i.e. the EGC is more baroclinic and the AW temperature is lower. At 781500 N the EGC is stronger, more barotropic and the Atlantic Water temperatures are higher. The increase in observed AW temperature is partially due to a larger contribution of warmer

recirculated RAW at 781500 N but also due to the exceptional warm anomaly which occurred between 2005 and 2007. In Section 4.2 we come back to this and estimate the volume associated with the AW temperature anomaly and its possible impacts further downstream.

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

33

Table 2 Mean and standard deviation of detided velocities (U, V) and temperature (T) per instrument at each depth for moorings F14 (west) through F9 (east) for the period September 1997 through August 2002 at 791N. Nominal instrument depths are given in meters. Units for U and V are (cm s  1) and for T (1C). Mooring Longitude

F14 6.831W

F13 5.31W

F12 4.251W

F11 31W

F10 21W

F9  01W

Depth U V T

50–70 m  2.2 7 5.4  3.0 7 7.2  1.69 7 0.12

50–70 m  2.4 74.9  4.2 76.1  1.63 7 0.18

50–70 m  0.0 7 7.8  9.0 7 8.6  1.617 0.17

50–70 m  5.9 7 8.1  8.4 7 8.0  1.177 1.13

75 m  4.8 7 6.9  7.1 77.3 0.09 7 1.26

75 m  11 78.5  0.2 78.7 1.137 1.54

Depth U V T

270 m  1.0 7 1.8  3.1 73.4 0.57 70.29

240 m 0.0 7 3.1.  1.3 7 4.1 0.93 7 0.83

300 m  0.6 7 5.6  3.5 7 5.6 1.357 0.51

250 m  3.5 7 5.9  5.1 76.7 1.81 70.58

255 m  5.7 7 6.0  3.3 7 4.9 2.017 0.47

250 m  7.5 7 6.8  0.5.7 7.0  1.917 0.64

Depth U V T

– – – –

1000 m 0.8 7 1.2  2.17 2.5  0.127 0.05

– – – –

– – – –

– – – –

780 m  5.5 74.4  0.9 74.7 0.09 7 0.21

Depth U V T

– – – –

– – – –

1500 m  0.7 7 1.7  1.8 7 3.1  0.477 0.05

1455 m  1.2 7 3.2  2.3 7 4.0  0.54 7 0.07

1510 m  2.17 3.8  1.1 73.2  0.65 7 0.06

1500 m  3.7 73.5  0.3 73.6  0.717 0.05

Depth U V T

– – – –

– – – –

1800 m  0.5 7 2.3  2.9 7 4.3  0.60 7 0.07

2360 m  1.17 4.9  1.5 7 7.0  0.79 7 0.02

2565 m  2.2 7 4.5  0.3 7 4.4  0.83 7 0.04

2450 m  3.8 75.2 1.8 7 4.6  0.84 70.04

Table 3 Same as Table 2 but now for the period September 2002 through August 2009 when the mooring array was located 781500 N. Note that the lateral mooring positions had also changed slightly. Mooring Longitude

F14 6.51W

F13 51W

F12 41W

F11 3.251W

F10 21W

F9 0.81W

Depth U (cm s  1) V (cm s  1) T (1C)

50–70  1.2 74.9  4.2 7 6.8  1.717 0.09

50–70  1.2 7 8.2  6.3 7 10.5  1.707 0.30

50–70 m  2.6 7 7.2  15.6 7 8.1  1.65 7 0.24

50–70 m  5.5 78.0  11.4 79.5  0.89 71.36

65 m  10.7 7 7.9  9.5 78.7 1.46 7 1.43

60 m  9.7 710.0  0.17 9.5 2.30 7 1.30

Depth U (cm s  1) V (cm s  1) T (1C)

270 m 0.7 71.9  2.17 4.9 0.80 70.42

240 m 0.0 7 4.4  0.6 7 6.5 1.59 71.03

335 m  0.9 7 6.0  7.0 7 6.7 1.78 70.76

260 m  2.6 75.8  7.4 7 7.8 2.30 7 0.71

255 m  9.5 76.5  7.17 7.2 2.177 0.53

250 m  8.6 78.3 0.6 7 7.7 2.34 7 0.64

Depth U (cm s  1) V (cm s  1) T (1C)

– – – –

1010 m 1.5 71.7  3.1 73.9  0.17 70.08

– – – –

– – – –

770 m  6.3 74.6  5.8 74.8  0.137 0.20

770 m  5.5 75.2  0.247 4.5 0.00 70.21

Depth U (cm/s) V (cm/s) T (1C)

– – – –

– – – –

1540 m 0.2 7 1.3  3.0 7 4.2  0.487 0.04

1465 m  0.8 72.2  2.7 75.0  0.56 70.08

1510 m  4.9 74.0  4.6 74.3  0.65 70.05

1500 m -3.27 4.6  0.17 4.0  0.667 0.05

Depth U (cm s  1) V (cm s  1) T (1C)

– – – –

– – – –

1845 m 1.4 72.0  4.4 7 6.0  0.63 7 0.07

2360 m  0.8 73.6  1.8 7 8.1  0.777 0.04

2650 m  4.8 73.7  5.5 74.4  0.81 70.03

2600 m  1.6 7 5.1  0.17 3.2  0.81 70.03

Table 4 Same as Table 2 for the additional shelf mooring F17 for the period September 2003 through August 2009. Mooring Longitude

F17 81W

Mean bin or sensor depth (m)

U (cm s  1) V (cm s  1) T (1C)

4.17 0.8  6.6 72.9  1.56 7 0.27

65–85 65–85  100

3.3. Seasonality Based on 1 year of data Foldvik et al. (1988) concluded that there is no significant seasonal signal present in the volume

transport of the EGC at 791N. The same was found by Fahrbach et al. (2001) based on the mooring data from the whole Fram Strait at 791N between 1997 and 1999; they found a clear seasonal cycle in the WSC but not in the EGC. With a much longer time series presently at hand than discussed by Foldvik et al. (1988) and Fahrbach et al. (2001), and since the EGC moorings had been relocated, we take a new look at the seasonality of velocities and temperature. For both periods the mean velocities (Fig. 6) and mean temperatures (Fig. 7) for January–February– March (upper panels) and July–August–September (lower panels) are shown separately. At latitude 791N (left panels) no clear difference is seen between winter and summer. This changes drastically at the latitude 781500 N where a much stronger barotropic flow is seen around 21W (F10) in winter compared to

34

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

78°50N: 2002-2009

2

0

2

−500

0

−500

0

−1000

−2

−1000

−2

−4

−1500 −2000 −2500

−6

Mean V −6

Depth [m]

Depth [m]

79°N: 1997-2002 0

−8 −5

−4

−3

−2

−2000 −2500

−10 −1 [cm/s]

−4

−1500

−6

Mean V −6

−8 −5

−4

0

3

−500

2

−1000

1

−1500

0

−2000

−1

−2000

−2

−2500

Mean T

−2500 −6

−5

−4

−2

−10 −1 [cm/s]

Longitude

−3

−2

−1 [°C]

Depth [m]

Depth [m]

Longitude

−3

0 −500

3

−1000

1

−1500

0

2

−1

Mean T −6

−5

Longitude

−4

−3

−2

−1

[°C]

−2

Longitude

Fig. 5. Cross sections of mean southward velocity (upper panels) and temperature (lower panels) at latitude 791N (1997–2002) (left panels) and at 781500 N (2002–2009) (right panels).

2

−500

0

−1000

−2 −4

−1500 −2000 −2500

−6

Jan−Feb−Mar

78°50N: 2002-2009 Depth [m]

Depth [m]

79°N: 1997-2002 0

−8

5°W

4°W

3°W

2°W

2

−1000

−2

0 −4

−1500 −2000 −2500

−10 6°W

0 −500

1°W [cm/s]

−6

6°W

5°W

0

2

−500

0

−1000

−2 −4

−1500

−2500

−6

Jul−Aug−Sep

−8

5°W

4°W

3°W

3°W

2°W

−10 1°W [cm/s]

2°W

0 −500

2

−1000

−2

0 −4

−1500 −2000 −2500

−10 6°W

4°W

Longitude

Depth [m]

Depth [m]

Longitude

−2000

−8

Jan−Feb−Mar

1°W [cm/s]

−6 −8

Jul−Aug−Sep 6°W

5°W

Longitude

4°W

3°W

2°W

−10 1°W [cm/s]

Longitude

Fig. 6. Cross sections of mean southward velocity shown separately for winter months (upper panels) and summer months (lower panels) at latitude 791N (left panels) and at 781500 N (right panels).

78°50N: 2002-2009 3

0

3

−500

2

−500

2

−1000

1

−1000

1

−1500

0

−1500

0

−2000

−1

Jan−Feb−Mar

−2500 6°W

5°W

4°W

3°W

2°W

Depth [m]

Depth [m]

79°N: 1997-2002 0

−2000 −2500

−2 1°W [cm/s]

6°W

5°W

Longitude

4°W

3°W

2°W

1°W [cm/s]

−2

Longitude

0

3

0

3

−500

2

−500

2

−1000

1

−1000

1

−1500

0

−1500

0

−2000

−1

−2000

−2 1°W [cm/s]

−2500

Jul−Aug−Sep

−2500 6°W

5°W

4°W

3°W

2°W

Longitude

Depth [m]

Depth [m]

−1

Jan−Feb−Mar

−1

Jul−Aug−Sep 6°W

5°W

4°W

3°W

2°W

1°W [cm/s]

−2

Longitude

Fig. 7. Cross sections of mean temperature shown separately for winter months (upper panels) and summer months (lower panels) at latitude 791N (left panels) and at 781500 N (right panels).

summer (right panels Fig. 6). Also at the shelf at 61W a stronger seasonal cycle is seen with maximum southward flow in winter. The strongest seasonal cycle in east-west velocity (U) was found at 21W (F10) with largest westward flow in winter/spring and

smallest in summer (not shown). The amplitude of the seasonal cycle in U at 781500 N increased slightly from that at 791N illustrating the importance of the recirculating branch at that latitude.

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

35

−2

Southward shift of the array Additional mooring added

Volume transport (Sv)

−4 −6 −8 −10 −12 −14

Volume transport pre−2002 6 moorings Volume transport post−2002 6 moorings Volume transport post−2003 7 moorings Annual mean 6 moorings Annual mean 7 moorings

Monthly means −16 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Year 0

Fig. 8. Monthly mean volume transport at latitudes 791N (green line) and at 78150 N (cyan line) and the annual mean volume transports based on 6 moorings F9 through F14 (magenta line). The second period also shows the monthly mean and annual mean volume transport based on seven moorings F9 through F17 (blue and red lines). F17 was added to the array on the shelf in 2003. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

Seasonality in temperature distribution is mostly visible in the Atlantic layer in both the first and the second period (Fig. 7). Highest temperatures occur in summer and AW extends further west while cold PW extends less far east in summer than in winter. In the deeper layers there was no clear seasonal signal present.

4. Volume transport of the EGC at 791N and 781500 N 4.1. Wind-driven transport The monthly mean, full depth, net volume transport of the EGC west of 11W in Fram Strait was determined based on the full, i.e. regressed, time series of instruments (Fig. 8). The monthly mean southward (¼negative) transports at 791N (1997–2002; green line) varied from  2.1 Sv to 10.4 Sv while at 781500 N (2002–2009; cyan and blue lines) the transports were larger, ranging from  4 Sv to 14.7 Sv. The difference between the cyan and blue lines is related to the extension of the array with an additional shelf mooring F17 at 81W from September 2003 onwards. At 791N the long-term mean volume transport was  5.871.8 Sv while at 781500 N the long-term mean was  8.772.5 Sv (integrated up to 6.51W). This difference in transport estimates indicates that the recirculation in Fram Strait between these two latitudes adds approximately 3 Sv to the EGC. Incorporation of the shelf mooring at 81W adds approximately 0.7 Sv to the southward transport leading to a mean transport of 9.472.7 Sv at 781500 N between 2003 and 2009. Foldvik et al. (1988) calculated a transport of  3 Sv in the upper 700 m of the EGC at 791N between 1984 and 1985. Based on the data presented here we obtain  3.4 70.7 Sv and  4.1 71 Sv for the transport in the upper 700 m at the two different latitudes. The transport determined for the full-depth EGC between 1997 and 1999 at 791N by Fahrbach et al. (2001) was  11.1 71.7 Sv. This included an estimate of  2.6 70.1 Sv of recirculation in the strait. If we add the estimate of recirculated transport from Fahrbach et al. (2001) to our full-depth long-term mean at 791N we obtain 8.4 Sv which is very close to the long-term mean transport at 781500 N of  8.7 Sv. We note that adding the recirculation of 2.6 Sv to our mean transport between 1997 and 1999 results in only 9.2 Sv and hence smaller southward transport than obtained by Fahrbach et al. (2001) who found  11.1 Sv. This difference is most likely related to the fact that different methods

were used to fill in data gaps, i.e. using the annual mean value at an instrument location (Fahrbach et al., 2001) versus the method based on a regression with nearby instruments (this study). An increase in volume transport in the EGC based on these mooring data in Fram Strait was noted earlier in De Steur et al. (2009) but was not explained there. Here we show that the apparent increase in southward transport as well as an increase in seasonality at 781500 N can be attributed to the southward move of the western moorings in Fram Strait which therefore captured a larger barotropic return flow (Figs. 5 and 8). The differences in volume transport observed at the two latitudes are further investigated in light of the prevailing wind forcing. The windstress curl over Fram Strait was calculated from monthly mean surface wind fields from ECMWF ERA-Interim data. The 5-month running mean wind-stress curl was determined over the region 81W–81E, 781N–801N and compared with the 5-month running mean volume transport of the EGC (Fig. 9 – upper panel). The 5month low-pass filtered volume transport during the first period at 791N did not show any seasonal signal but this changed dramatically at 781500 N from September 2002 onwards. The largest (smallest) southward transport occurred in winter (summer) when the wind stress curl was maximum (minimum). The correlation between wind-stress curl over Fram Strait and volume transport was significant at 781500 N. Besides, significant correlations were found on time scales with the zonal wind component at 791N on time scales greater than 6 months and with the meridional wind component over Fram Strait at 781500 N. From the above we infer that the transport in the EGC at 791N is hardly driven by wind-stress curl, at least on the seasonal or longer time scales, and consists mostly of a buoyant flow as first shown by Wadhams et al. (1979). One sixth degree to the south the EGC merges with the recirculating branch of the wind-driven gyre of the Nordic Seas and hence shows a stronger barotropic transport and a significant seasonal cycle. The cyclonic wind-stress curl in the Nordic Seas is responsible for a northward Sverdrup transport which must be compensated by a southward return flow in the EGC (see also Woodgate et al., 1999). A difference in windstress curl at two different latitudes in Fram Strait induces a difference in the Sverdrup transport which is given by Z Sverdrup transport ¼

1

ρ0 β

curl τ dx

ð4Þ

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

Transport [Sv]

Running mean WCRL

−4

x10-7 1 0.8 0.6 0.4 0.2 0

Southward shift of the array

−6 −8 −10 −12 1997

Wind stress curl

36

Volume transport EGC at 79°N Volume transport EGC at 78°50’N 1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Sverdrup transport [Sv]

Year 2

Calculated around 78.75°N

0 −2

Calculated around 81.75°N −4 −6 −8 1997

Sverdrup transport at 81.75°N Sverdrup transport at 78.75°N 1998

1999

2000

2001

2002

Annual means 2003

2004

2005

2006

2007

2008

2009

2010

Year Fig. 9. Upper panel: ECMWF mean wind stress curl (WCRL, light gray) in Fram Strait averaged over the region 81W–81E, 781N–801N and the volume transport at 791N (dark gray) and at 781500 N (black) observed in the EGC. All three time series are a 5-month running mean based on monthly means. Lower panel: 5-month running mean Sverdrup transport calculated from the wind stress curl around latitudes 81.751N and 78.751N for the periods 1997–2002 (dark gray) and 2002–2009 (black) respectively. The annual means are shown with circles in both panels (green and cyan). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

Here τ is the wind stress given by ρair Cd juju with u being the wind velocity and CD is the drag coefficient which we select to be 1:3  10  3 . Due to the coarse resolution (1.51 latitude) of the gridded ECMWF data we choose the latitudes 78.751N and 81.751N at which we calculate the mean wind stress curl to compare the Sverdrup transport (Fig. 9 – lower panel). The wind-driven transport north of 811N for 1997–2002 was much smaller (  0:6 71:8 Sv) than the transport south of 801N post-2002 (  37 4 Sv). However, the Sverdrup transport determined at 81.751N did show a small seasonal cycle which was not seen in the calculated volume transport at 791N. At 78.751N very weak wind-stress forcing was responsible for weak Sverdrup transport in 2003, in agreement with the exceptionally low volume transport obtained from the mooring data in 2003. However, an almost similarly weak Sverdrup transport was obtained in 2009 which was not observed in the mooring data. Hence, the calculated winddriven Sverdrup transport does not fully explain observed variations in the EGC at 781500 N. We note that in 2003 atypically weak northerly winds prevailed (on average twice as weak as normal) which was not the case in 2009 even though the wind-stress curl was small. In addition, other processes, e.g. density or surface pressure variations, could have contributed too. The monthly mean seasonal cycle is shown for both the observed transport in the EGC and the calculated Sverdrup transport for pre- and post-2002 (Fig. 10). At the southern latitude around 78.751N the seasonal amplitude increases significantly relative to 81.751N. As shown before the transport prior to September 2002 features hardly any seasonal cycle which is also the case for the Sverdrup transport calculated at 81.751N (Fig. 10 – left panels). The southward transport based on the mooring data post-2002 shows a seasonal peak to peak difference of 6 Sv which can be fully explained by the seasonal wind-driven Sverdrup transport at that latitude (Fig. 10 – right panels). The maximum observed southward transport however occurs in February, somewhat earlier than the peak in the Sverdrup transport which is in March. From the above we estimate that the average buoyant flow of the EGC may be around 5.2 Sv, i.e. 5.8 Sv observed in the EGC at 791N minus 0.6 Sv calculated Sverdrup transport. At 781500 N the

wind-driven part is approximately 3 Sv. It should be kept in mind that these quantities scale with the choice of CD which may vary between 1:0  10  3 and 2:0  10  3 . Hence for a choice of CD ¼ 1:0  10  3 the Sverdrup transport is 23% smaller while for C D ¼ 2:0  10  3 the Sverdrup transport is 50% larger than determined above with C D ¼ 1:3  10  3 . 4.2. Transports of water masses The above results show that the variability of the EGC at 781500 N is closely related to the strength of the wind-stress curl. The resulting recirculation is most pronounced as an increase in the depth-averaged flow of the EGC. Of specific interest here are the upper and intermediate Atlantic derived water masses that merge with the Arctic outflow of the EGC. Further south the Denmark Strait Overflow Water (DSOW) is fed by these waters which mix and cool on their way south along the Greenland and Iceland seas (Strass et al., 1993; Mauritzen, 1996a,b; Rudels et al., 2002; Eldevik et al., 2009; Mauritzen et al., 2011). Based on the moored instrument data in the EGC at 791N Foldvik et al. (1988) estimated the amount of Atlantic derived water with T 4 0 1C above 700 m depth to be 2 Sv. Here we quantify the transports of Atlantic derived waters again based on the data sets at the two different latitudes. We also quantify the other water-mass transports in the EGC. Since salinity data at intermediate to large depths were of bad quality or lacking we adapt the simplified definitions from Rudels et al. (2008) and use depth levels instead of density contours (Table 5). These depth levels are chosen as close as possible to the density contours based on available hydrographic data. The Atlantic Water (AW) above 500 m depth consists of Recirculating Atlantic Water (RAW) and Arctic Atlantic Water (AAW) which are distinguished from each other by the 2 1C isotherm. RAW recirculates directly in Fram Strait while the upper AAW has been far into the Arctic Ocean or has recirculated further north of Fram Strait. Below the upper Atlantic waters dense Atlantic Water is present (dAW) comprising denser Atlantic Water from both the Nordic Seas and the Arctic Ocean. The deep water comprises both Arctic deep waters and Nordic Sea deep water. The transport of Polar Water and the Atlantic derived

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

EGC volume transport

78°50N: 2002-2009 Volume transport [Sv]

Volume transport [Sv]

79°N: 1997-2002 2 0 −2 −4 −6 −8 −10 −12

37

−14 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

2

EGC volume transport

0 −2 −4 −6 −8 −10 −12 −14 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Month

2 0 −2 −4 −6 −8 −10 −12

Sverdrup transport

−14 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Sverdrup transport [Sv]

Sverdrup transport [Sv]

Month 2 0 −2 −4 −6 −8 −10 −12

Sverdrup transport

−14 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Month

Month 0

Fig. 10. Upper panels: mean seasonal cycle of the volume transport observed in the EGC at 791N (left) and at 78150 N (right). Standard deviations from the mean are indicated with dashed lines. Lower panels: mean seasonal cycle of the Sverdrup transport calculated at two different latitudes 81.751N and 78.751N for the periods 1997– 2002 (left) and 2002–2009 (right) respectively.

Table 5 Water mass definitions and volume transports for Polar Water (PW), Atlantic Water (which comprises Recirculating Atlantic Water (RAW) and Arctic Atlantic Water (AAW)), dense Atlantic Water (dAW), Intermediate Water (IW) and Deep Water (DW). Definitions are adapted from Rudels et al., 2008 and comprise depths instead of density surfaces. Watermass Definition

Transport at 791N

Transport 781500 N

PW

T o 0 1C and depth o 500 m

 1.27 7 0.47 Sv  0.96 70.29 Sv (up to 6.51W) and  1.45 7 0.47 Sv (up to 81W)

AW

2 1C o T and depth o 500 m 2 1C o T and depth o 500 m 0 1C o T o 2 1C and depth o 500 m

 1.6 7 0.4 Sv

 2.3 70.63 Sv

 0.16 7 0.16 Sv

 0.5 70.43 Sv

 1.44 70.38 Sv

 1.8 70.58 Sv  1.78 70.49 Sv

with RAW and AAW dAW

0 1C o T o 2 1C and 500 m o depth

 1.017 0.35 Sv

IW

T o 0 1C and 500 m o depth o 1400 m

 1.15 7 0.37 Sv  1.54 70.56 Sv

DW

T o 0 1C and 1400 m o depth

 0.85 7 0.93 Sv

 2.197 1.1 Sv

water masses are shown in Fig. 11 and the long-term mean volume transports of all water masses at the two latitudes are given in Table 5. The mean volume transport of PW determined up to 6.51W was 1.27 70.47 Sv at 791N, identical to what was found by Foldvik et al. (1988) but smaller than the 1.8 Sv suggested by Aagaard and Greisman (1975). The large mean PW transport at 791N is for a large part due to anomalously large transport of PW of 1.5 and 1.7 Sv in 2001 and 2002 which was also associated with a larger freshwater flux those years. In 2001 this was related with a cold

and fresh Arctic anomaly while in 2002 this was due to an anomalously strong upper ocean southward flux (see lower panel of Fig. 9 where a strong wind-driven Sverdrup transport is visible in 2002). At 781500 N the transport of PW based on the four moorings in the EGC was lower, namely 0.96 70.29 Sv. At this latitude likely more PW flows over the shelf than at 791N. When data from the newly deployed shelf mooring at 81W is incorporated (as of September 2003) the mean transport of PW was found to be 1.45 70.47 Sv confirming the presence of a substantial flux of low salinity Arctic water on the shelf. The net transport of RAW in the EGC varied from 0.1 Sv to 1 Sv. In general a larger amount of RAW was observed at 781500 N for two reasons. First, larger transport was found at the southerly latitude because of the stronger recirculation here. Second, a warm Atlantic anomaly from the Nordic Seas passed through Fram Strait between 2005 and 2007 (Walczowski and Piechura, 2007; Holliday et al., 2008; Beszczynska-Möller et al., 2012) such that RAW occupied a larger area of the section. This led to a doubling of RAW (0.82 Sv) between 2005 and 2007 relative to the other years (Fig. 11). The transport of AAW in the upper 500 m varied from roughly 1.35 Sv to 2.35 Sv and interannual variations in AAW occurred at both latitudes. The mean transport of dAW increased substantially from 1.01 70.35 Sv at 791N to almost 1.8 70.58 Sv at 781500 N. The sum of all transports of Atlantic derived waters (RAW, AAW and dAW) at 791N between 1997 and 2002 was 2.62 70.73 Sv southward, which is larger than the estimate of Foldvik et al. (1988) of 2.0 Sv at 791N. At 781500 N the total longterm mean transport of all Atlantic derived waters was 4.11 71.07 Sv, hence an increase of 1.56 Sv relative to 791N. With respect to the colder and deeper water masses the transport of IW increased from 1.15 70.37 Sv at 791N to 1.54 7 0.56 Sv at 781500 N and transport of DW more than doubled from 0.85 70.93 Sv at 791N to 2.19 71.1 Sv at 781500 N (Table 5). Aagaard and Greisman (1975) estimated the total amount of Atlantic derived water returning with the EGC around 5.7 Sv based

38

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

0

Transport [Sv]

−0.5

−1

Additional shelf mooring −1.5

−2 PW

−2.5

−3

RAW AAW AW dAW

Southward shift of the array

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Year Fig. 11. Time series of the annual mean transport of PW, total upper AW (sum of RAW and AAW), and dAW, see definitions in Table 5. The solid blue line indicates transport of PW up to 6.51W and the blue dashed line is the transport of PW including the new western shelf mooring at 81W as of 2003. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

on the assumption that there is no net transport through Fram Strait. We find a total of 4.11 71.07 Sv at 781500 N. The recirculation at 781500 N adds approximately 1.5 Sv of Atlantic derived water to the EGC relative to 791N. However, the total recirculation of Atlantic derived waters in the whole Fram Strait (including that at or just north of 791N) is larger because Atlantic derived water at 791N has also recirculated here. The latter is estimated to be the sum of RAW at 791N (0.16 Sv), the portion of AAW with T 4 11 at 791N (0.75 Sv) – assuming that AAW with T o 11 has circulated into the Arctic proper – and presumably one third of the dAW at 791N (0.3 Sv), adding up to approximately 1.2 Sv. Based on these assumptions the total recirculation of Atlantic derived water in Fram Strait would amount to 2.7 Sv. Beszczynska-Möller et al. (2012) calculated the mean transport of AW with T 4 2 1C in the WSC to be 37 0.2 Sv. Their estimate of Atlantic derived water with 01 o T o 21 was around 1.4 Sv (Fig. 3 of Beszczynska-Möller et al., 2012) amounting to a total of 4.4 Sv of Atlantic derived waters with T 4 01 in the WSC. However, the central part of Fram Strait between 11W and 51E where the bulk of the recirculation occurs was not included in that estimate. The transport of Atlantic derived water in the area between 11W and 51E is highly variable and annual averages between 2002 and 2009 range from 0 to 1 Sv to the north. Accordingly the northward transport of Atlantic water with T 4 01 east of 11W is likely between 4.4 and 5.4 Sv. Hence our estimate of 2.7 Sv recirculated Atlantic waters in the EGC is in agreement with Rudels (1987) and Marnela et al. (2013), that approximately 50% of Atlantic water recirculates in Fram Strait. Between 2005 and 2007 the amount of RAW on the section at 781500 N was larger due to the warm anomaly in Fram Strait. Related with the higher temperatures and the fact that the warm Atlantic anomaly was not density compensated, the depth of isopycnals changed, potentially affecting the strength of the Denmark Strait Overflow (DSO) further south (Karcher et al., 2011). Here we estimate that change related to the direct recirculation of AW in Fram Strait based on the mooring data. Since there are no continuous density data at intermediate depths from the instrument records we determine the average depth of the thermohaline 0.5 1C from the mooring data between 51W and 21W (Fig. 12). This thermohaline corresponds approximately with the isopycnal σ ¼ 28:0 kg m  3 based on the available hydrographic data in the EGC

at 781500 N up to 2009. The depth of the 0.5 1C thermohaline between 2003 and 2009 showed an annual mean depression in 2007 of about 85 m and monthly depressions down to 100 m at the end of 2006 (Fig. 13). Thereafter the thermohaline veered back and returned to shallower depths than before 2005. Scaled with the results from Karcher et al. (2011) a depression of 85–100 m may lead to a reduction in DSO of 17–23% on annual to monthly time scales. In addition, the delayed effect of the warm anomaly that entered the Arctic in 2006 and will return through Fram Strait can influence the isopycnal depth and hence the DSO with a lag of one to two decades (Karcher et al., 2011).

5. Summary Results were presented from velocity and temperature data from the mooring array in the East Greenland Current in Fram Strait west of 11W between 1997 and 2009. In September 2002 these moorings were relocated from 791N to 781500 N which had large consequences for the observed flow regime of the EGC. Relative to earlier studies of Fahrbach et al. (2001) and De Steur et al. (2009) the results presented here were based on an improved method to deal with data gaps in the time series: these were filled through a regression with time series of (neighboring) instrumental records at the same latitude. This resulted in updated transport estimates relative to earlier studies. In addition, by considering the time series as two separate time series, i.e. at 791N and 781500 N, this method leaves us with a better interpretation of the observed variations. The main conclusions of this study are summarized as follows:

 Compared to 791N stronger velocities are found at 781500 N in the EGC at all depths. The increase in both southward but also westward flow was particularly large at 21W leading to larger mean volume transport of 8.7 72.5 Sv at 781500 N compared to 5.8 Sv7 1.8 Sv at 791N. This explains the apparent but unaccountable increase in volume transport in the EGC noted earlier by De Steur et al. (2009). In addition, a larger seasonal cycle was observed compared to earlier studies from 791N. The strong presence of a barotropic component with a significant seasonal cycle in the EGC at 781500 N is due to a larger contribution of

L. de Steur et al. / Deep-Sea Research I 92 (2014) 26–40

39

−500

Monthly mean depth T=0.5°C

Interface depth [m]

−550

Annual mean depth T=0.5°C

−600

−650

−700

−750

−800 2002

2003

2004

2005

2006

2007

2008

2009

2010

Year Fig. 12. Monthly and annual mean depths of the isotherm T ¼ 0.5 1C based on the mooring data which coincides approximately with the potential density contour σ ¼ 28:0 kg m  3 .











recirculation at this latitude associated with the cyclonic gyre of the Nordic Seas. The temperature of AW in the EGC observed at 781500 N is in general higher than at 791N due to more direct recirculation of AW. Also, the warm AW anomaly that occurred in Fram Strait increased the temperature of returning AW on the mooring section in the EGC significantly between 2005 and 2007. At 781500 N the temperature of the deep water in western Fram Strait showed an increasing trend in agreement with results from hydrography between 1982 and 2008 (Langehaug and Falck, 2012). The strong correlation of the total transport with the winddriven Sverdrup transport at 781500 N confirms that the EGC at the southerly latitude is for a large part wind driven. The difference in observed transport between 791N and 781500 N can for a large part be accounted for by the difference in windstress curl at the different latitudes. The buoyant flow of the EGC at 781500 N was estimated to be 5 Sv with no obvious seasonal signal while the wind-driven contribution is approximately 3–4 Sv with a seasonal amplitude close to 3 Sv. At 781500 N the southward transport of PW was up to 1.4 Sv. The southward transport of Atlantic derived water (the sum of RAW, AAW and dAW) was on average 4.1 Sv at 781500 N while this was approximately 2.6 Sv at 791N. We confirm earlier studies that 50% of the Atlantic derived water recirculates around and north of 781500 N. The effect of direct recirculation of the warm AW anomaly in Fram Strait between 2006 and 2007 was a deepening by 85 m of the 0.5 1C thermohaline, corresponding roughly with the σ ¼ 28:0 kg m  3 isopycnal. This may reduce the DSO by 17– 23%. Larger changes may occur at a longer delay related to the return of the AW anomaly after circulation through the Arctic Ocean.

5.1. Recommendations The Arctic Ocean is at present undergoing rapid and large-scale changes and is very sensitive to ongoing and potential future climate change. Observations of heat, freshwater and volume fluxes between the Arctic and Subarctic are key to our understanding of the response of the ocean to climate variability and feedbacks therein. Therefore a long-term observational program in

or near Fram Strait is essential. Potentially a reduction in the amount of moorings across the Fram Strait, a so-called skeleton array with instruments at key locations in the EGC and WSC, can be sufficient to observe critical variations in thermohaline properties of the in- and outflows.

Acknowledgments The data collection and processing was carried out at the Norwegian Polar Institute and Alfred Wegener Institute for Polar and Marine Research and was funded by the DAMOCLES project, financed by the European Union in the 6th Framework Programme for Research and Development (Project no. 018509). Prior to that the long-term mooring program in Fram Strait had been funded by EU programs VEINS and ASOF-N and by national Norwegian and German funding. The analysis of data from the EGC moorings was first carried out at the Norwegian Polar Institute under abovementioned projects and was continued at the Royal Netherlands Institute for Sea Research. Many thanks go out to all who were involved in the data collection, particularly all technical support from the Norwegian Polar Institute and the Alfred Wegener Institute, and the crews of R/V Lance and R/V Polarstern for their support during the field campaigns in Fram Strait. We like to thank an anonymous reviewer for very useful comments which helped to improve this paper. References Aagaard, K., Carmack, E.C., 1989. The role of sea ice and other fresh water in the Arctic circulation. J. Geophys. Res. 94, 14485–14498. Aagaard, K., Coachman, L.K., 1968a. The East Greenland Current north of Denmark Strait: Part I. Arctic 21, 181–200. Aagaard, K., Coachman, L.K., 1968b. The East Greenland Current north of Denmark Strait: Part II. Arctic 21, 267–290. Aagaard, K., Greisman, P., 1975. Towards new mass and heat budgets for the Arctic Ocean. J. Geophys. Res. 80, 3821–3827. Aagaard, K., Swift, J.H., Carmack, E.C., 1985. Thermohaline circulation in the Arctic Mediterranean Seas. J. Geophys. Res. 90, 4833–4846. Beszczynska-Möller, A., Fahrbach, E., Schauer, U., Hansen, E., 2012. Variability in Atlantic water temperature and transport at the entrance to the Arctic Ocean 1997–2010. ICES J. Mar. Sci. Bourke, R.H., Wiegel, A.M., Paquette, R.G., 1988. The westward turning branch of the West Spitsbergen Current. J. Geophys. Res. 93, 14065–14077. De Steur, L., Hansen, E., Gerdes, R., Karcher, M., Fahrbach, E., Holfort, J., 2009. Freshwater fluxes in the East Greenland Current: a decade of observations. Geophys. Res. Lett. 36.

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