Observations of mesoscale variability in the Rockall Trough

Deep-Sea Research I 64 (2012) 1–8

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Observations of mesoscale variability in the Rockall Trough J.E. Ullgren a,n, M. White b a b

NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, the Netherlands Earth and Ocean Sciences, National University of Ireland, Galway, University Road, Galway, Ireland

a r t i c l e i n f o

abstract

Article history: Received 20 June 2011 Received in revised form 24 January 2012 Accepted 1 February 2012 Available online 9 February 2012

The Rockall Trough west of Ireland displays a high level of mesoscale activity for an eastern ocean boundary region. Eddies off the continental slope at 50–561N have been studied using a combination of in situ observations of current velocity and hydrography from two deep-sea moorings at the southern entrance to the Trough, and data from satellite altimetry and Argo floats. South of Rockall–Hatton Plateau, where a branch of the NAC enters the region from the west, more cyclonic eddies are found, while anticyclonic eddies dominate along the path of the Slope Current in the east. Temperature– salinity profiles from the perimeters of a cyclone and an anticyclone, respectively, show large differences on isopycnals both at the level of the subsurface salinity maximum and at intermediate depths. Anticyclonic eddies likely formed by instabilities of the Slope Current can include a parcel of salty Mediterranean Water (MW) at the intermediate level, contributing to the patchy distribution of MW in the region. & 2012 Elsevier Ltd. All rights reserved.

Keywords: North-east North Atlantic Mesoscale variability Rockall Trough

1. Background The mesoscale variability in the eastern North Atlantic is less energetic than on the western side, but a swath of higher eddy kinetic energy (EKE) follows the path of the North Atlantic Current (NAC) and enters into the Rockall Trough (Heywood et al., 1994; Fratantoni, 2001). The Rockall Trough is known as an area of relatively high mesoscale activity. In the northern Rockall Trough, interacting mesoscale features of about 100 km diameter were observed during the JASIN experiment in 1978 (e.g. Ellett et al., 1983; Pollard et al., 1983). Northwest of Porcupine Bank, a large anticyclonic eddy was observed by Booth and Meldrum (1987), who noted that eddies are also seen in satellite images from the area. The large anticyclone was dubbed ‘‘the Porcupine eddy’’ by Booth (1988). A survey in 1989 found a cold core eddy just west of the slope by Porcupine Bank, while subsurface salinity maxima were found at the slope nearby and in the Porcupine Seabight (Ellett and Turrell, 1992). A model study showed a large, quasi-stationary eddy centred on 171W, 52130N, similar to observations from a survey in 1998 (New and Smythe-Wright, 2001). South of the entrance to the Trough, at about 471N, 151W, an intense anticyclonic eddy was observed during the Tourbillon experiment in 1979 (Groupe Tourbillon, 1983). Further west, south of Rockall Plateau, Bacon (1997)

n

Corresponding author. E-mail addresses: [email protected], jenny.ullgren@gfi.uib.no (J.E. Ullgren).

0967-0637/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2012.01.015

observed two anticyclonic eddies and Bower et al. (2002) found a distinct local EKE maximum. Different mechanisms have been put forward to account for the higher eddy activity in the central Rockall Trough compared to nearby areas. Booth (1988) suggested that the origin of eddies may be baroclinic instability of the Slope Current, which flows poleward along the European continental margin. The filament of the Slope Current closest to shore is known as the Shelf Edge Current (SEC; Fig. 1). The slope west of Porcupine Bank fulfils necessary conditions for baroclinic instability, which would occur at (commonly observed) upper layer velocities in the range 5– 20 cm s  1. The Rockall Trough eddies might then be northern relatives of the slope water eddies or ‘‘Swoddies’’ in the Bay of Biscay (Pingree and Le Cann, 1992). Eddies might also originate from instabilities in the subpolar front and thus be associated with the NAC. Volkov (2005) discusses the relatively high EKE in the Rockall Trough as part of the band of high EKE that follows the NAC, but points out that the Trough is a ‘‘complex, topographically constrained region, where the bottom relief, the balance between the Eastern and Western North Atlantic Water (ENAW and WNAW) inflow from the south, and the atmospheric forcing are all factors determining the magnitude of EKE’’. Shoosmith et al. (2005) suggested that several anticyclonic eddies found translating southwestward from Goban Spur, south of Porcupine Seabight, might have been ‘‘weak meddies’’ shed from the northward flow of Mediterranean-influenced water. Eddies may play an important role in the circulation and hydrography of the Trough, by lateral stirring or by supplying energy for mixing. At the southern entrance to the Trough, y2S properties are

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J.E. Ullgren, M. White / Deep-Sea Research I 64 (2012) 1–8

NAC

MW

Fig. 1. Overview map. In the area from which altimetry data are presented in this study (50–561N, 10–201W), greytones show the mean dynamic topography for 2001–2007, and arrows the corresponding mean surface geostrophic currents. RB ¼Rockall Bank, RT ¼ Rockall Trough, PB¼ Porcupine Bank. Mooring sites A and B are marked with black crosses. Some important features of the circulation are shown by bold arrows: the North Atlantic Current (NAC), inflow of Mediterranean Water (MW), and the Shelf Edge Current (SEC; continuation shown in the northeastern corner).

characterized by strong salinity signals at intermediate depths from saline water (influenced by Mediterranean Water, MW) entering from the south and fresher Subarctic Intermediate Water (SAIW) from the west (e.g. McCartney and Mauritzen, 2001). Further north, in the central and northern Trough, the y2S profiles are much smoother (Ellett et al., 1986). Eddies are thought to contribute to the rapid northward decline of the strong salinity signals in the southern Trough (Booth and Meldrum, 1987). Most earlier works that mention eddies in the Rockall Trough have dealt with the mesoscale activity here as part of a larger regional overview (Bower et al., 2002; Lankhorst and Zenk, 2006) or focus has been on locations further north (Pollard, 1983), west (Martin et al., 1998), or south (Groupe Tourbillon, 1983) of our study region. However, the southern entrance to the Trough is an interesting region of strong hydrographic variability, where water masses meet and mix (Ullgren and White, 2010). The northward spreading of MW can be traced as far as the latitude of the Porcupine Bank (McCartney and Mauritzen, 2001), north of which its signal rapidly disappears. This latitude largely coincides with the line of zero wind stress curl, and seems to be in a transition zone between two Slope Current regimes. Along the southern European margin, the Slope Current carries waters of southern origin, and advection is stronger during periods of low North Atlantic Oscillation (NAO) index (Pingree, 2002; Pingree et al., 1999). In contrast, along the northern margin the Slope Current responds with stronger flow during periods of high NAO index, suggesting a continuation of the NAC from the west (Skagseth et al., 2004). The southern Rockall Trough is a watershed where the relative influence of southern and western water masses in the upper and intermediate layers varies strongly depending on the location of the Subpolar Front (e.g. Holliday, 2003; Ha´tu´n et al., 2005). In this paper, we combine in situ data from the southern Rockall Trough and satellite altimetry data to discuss some aspects of the mesoscale variability in the Rockall Trough.

2. Data and methods Two moorings, each equipped with a downward looking RDI 75 kHz LongRanger ADCP at a nominal depth of 150 m, an Aanderaa RCM-7 Recording Current Meter at 985 m, and eight

Seabird SBE 37-SM MicroCAT conductivity and temperature recorders (three of which also measured pressure) placed at about 100 m intervals between the ADCP and the RCM, were deployed at the southern entrance to the Trough (Fig. 1) between October 2003 and February 2004. The western mooring (B) was then replaced with a second mooring which was finally recovered in October 2004. The moorings were placed in water depths of about 2800 m, but the instrumentation was concentrated in the upper 1000 m of the water column. The time series of velocity and hydrographic properties from set depth levels presented here are daily, de-tided values that have been corrected for mooring knock-down by linear interpolation to a regular grid with 20 m depth resolution (for more details on the mooring measurements, see Ullgren and White, 2010). Two merged, multimission satellite altimetry data products from AVISO (www.aviso.oceanobs.com; Ducet et al., 2000) were used: Absolute Dynamic Topography and Absolute Geostrophic Velocity derived from the absolute dynamic topography. Both of these gridded data sets have a 1/31  1/31 spatial resolution, and the temporal resolution is weekly. The Absolute Geostrophic Velocities are given at each (weekly) time step as vectors ðu,vÞ for each grid point in the selected area, here 50–561N, 10–201W. In this study we use data from the period January 2001– December 2007. Profile data from Argo floats that entered the region between 2001 and 2005 were downloaded from the Coriolis data centre (www.coriolis.eu.org). To detect and track eddies throughout our study area, we used the parameter Q:  2    @u @v @u Q ¼  @x @x @y based on the two-dimensional, gridded velocity field of geostrophic surface currents derived from satellite altimetry (IsernFontanet et al., 2003; Morrow et al., 2004). The term Q, the second invariant of the velocity gradient tensor, gives a measure of whether a certain domain of the flow field is dominated by rotation (Q 40) or deformation (Q o0). Apart from a factor 4 and a sign change, Q corresponds to the Okubo–Weiss parameter (Okubo, 1970; Weiss, 1991) which has also been successfully used to track oceanic eddies in altimetric velocity fields (e.g. Isern-Fontanet et al., 2006; Chelton et al., 2007). Eddy cores are defined as simply connected regions where Q is positive and larger than some threshold value Q0. For every (weekly) time step t, our algorithm selects regions in the geostrophic velocity field with Q 4 Q 0 . If the centroid of the selected region is within a search radius r of a region found in the previous image (at t1) or in any of the last n images (so as not to ‘‘lose track’’ of an eddy if it temporarily weakens to below Q0; see Morrow et al., 2004), then it is considered to be the same feature; otherwise it is counted as a new one. Features smaller than a minimum area limit, corresponding to an approximate core radius rmin, were discarded, as were very short-lived features (tracked for less than 35 days). If the centre sea level anomaly was smaller than some threshold value (set to 10 cm) referenced to the area average, the feature was also discarded. The sign of the sea level anomaly in the centre was used to define eddies as cyclonic (  ) or anticyclonic ( þ). We experimented with various values of the threshold Q0, using the standard deviation of Q as a starting point, as well as with different search radii, allowed time gaps, minimum sizes, and lifetimes. The results reported here were found using a threshold Q 0 ¼ 1:5  1011 ; searching within a radius of r¼ 50 km; allowing ‘‘breaks’’ of up to 14 days (n¼2); and discarding features smaller than r min ¼ 15 km or with a duration of less than 35 days.

J.E. Ullgren, M. White / Deep-Sea Research I 64 (2012) 1–8

3. General observations The surface geostrophic velocity field, as derived from satellite altimetry, in the southern Rockall Trough is characterized by meandering, the mean flow (Fig. 1) weak compared to the variance. The general sense of the flow is north-easterly through the Trough, with zonal velocities dominating at the mouth of the Trough, near our moorings (in particular mooring B; Fig. 1).

3

mesoscale band clearly dominates the velocity spectrum even down to 980 m depth. The current at mooring B was stronger on average and more variable than at mooring A (Fig. 2; Table 1). It was harder to recognize individual passing eddies in the record from B because of the higher variability, mostly on eddy time scales. Temperature and isopycnal depth also fluctuated strongly on these time scales.

3.1. In situ time series

3.2. Eddy kinetic energy

Time series of daily current velocity at 300 m depth, temperature, and isopycnal movement from the mooring near the continental slope at 531N (A; Fig. 1) demonstrated two typical eddy events during the ca 4.5 month of measurements. In November 2003, current vectors showed rotating motion coinciding with a rapid dip in temperature (from 11.4 1C to 10.7 1C at the 300 m level) and uplift of the sy ¼ 27:2 kg m3 isopycnal by about 100 m (Fig. 2). The eddy was present at the mooring site for about 11 days, after which the conditions returned to their previous state and remained relatively constant for the rest of the winter (November, December, January). The winter ‘‘background’’ conditions at the 300 m level were characterized by weak, predominantly northward flow, temperature of about 11.2 1C and a salinity (not shown) of 35.54. The relatively steady northward flow found at A, despite the multi-year average of surface geostrophic currents being weakly southward here, might reflect the barotropic component of the Slope Current. In the beginning of February another eddy event occurred, of the same sign but smaller in amplitude and of shorter duration. The current structure was largely barotropic in the upper kilometre of the water column, but with a minimum at the deepest measured level (980 m, Table 1). A power spectrum of velocity at the western mooring B displayed a large peak at periods between 30 and 100 days, representing the energy in the mesoscale frequency band. The

Surface geostrophic currents at site B were highly variable, but predominantly easterly and north-easterly. Rotating motions and rapid changes were common, just as in the in situ time series. Altimetry-derived and directly measured currents were strongly correlated, with linear correlation coefficients between surface geostrophic current and that measured at 300 dbar of 0.87 and 0.50 for the zonal and meridional current components, respectively. Both the zonal and meridional component of the geostrophic current had a spectral peak at about 100 days. A time series of EKE was computed from the mooring B velocity data (EKEmoorB) by removing the total mean of the yearlong record, and from the altimetry-derived surface velocity at Table 1 Mean and standard deviation of zonal (U) and meridional (V) velocities (cm s  1) from the moorings at the southern entrance to Rockall Trough in 2003–2004. Mooring

Days

A

138

B

357

Depth (m)

U

300 500 980 300 500 980

V

Mean

St. dev

Mean

St. dev

3.1 3.1 0.4 14.6 13.1 5.9

4.6 4.1 2.5 15.2 14.3 7.1

4.2 4.2  0.8  3.0  2.8  0.2

7.8 7.3 4.3 11.3 11.2 4.7

[cm/s]

50 0 −50 N

D

J

F

M

A

M

J

J

A

S

O

N

D

J

F

M

A

M

J

J

A

S

O

N

D

J

F

M

A

M

J

J

A

S

O

N

D

J

F

M

A M 2003−2004

J

J

A

S

O

[cm/s]

50 0

[°C]

−50

11 10

[m]

200 400 600

Fig. 2. Top panel: time series of daily velocity vectors at mooring A. Second panel: ditto for mooring B. Third panel: temperature at the 300 m depth level from moorings A (bold grey) and B (black). The bottom panel shows the depth of the 27.2 isopycnal at the two moorings. Note that the measurement period at A was shorter than that at B. Data from periods of strong mooring knock-down at B, when the uppermost y-S sensor was found at depths below the nominal depth of the second instrument from the top (vertical displacement of 4 70 m), have been excluded.

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J.E. Ullgren, M. White / Deep-Sea Research I 64 (2012) 1–8

at site B followed the spatially averaged EKE with a lag of two months (Fig. 3).

site B (EKEsatB) by removing running 1-year averages (Penduff et al., 2004). The seasonal cycle therefore contributes to the EKE estimate. In most of the northern North Atlantic, EKE is related to wind stress and therefore shows a seasonal cycle following that of the wind stress with a time lag of about 6 weeks, with a maximum in spring (White and Heywood, 1995; Volkov, 2005). Our time series of monthly averaged EKEmoorB did not show a typical seasonality (Fig. 3); maximum EKE at the mooring occurred in July and minimum in January. The timing of the 2004 maximum and minimum EKE at the mooring was the same in EKEsatB from the same location (Fig. 3). However, EKEmoorB was significantly higher than EKEsatB, most likely because of the limited resolution of the satellite data, which are interpolated both in time and space. Variability on smaller scales than resolved by satellite altimetry appears to contribute a considerable portion of EKEmoorB. The spatial average over the whole box 50–561N, 10– 201W showed a different temporal pattern with much lower variability (Fig. 3). A small climatology over the 6 years of EKEsatB (half a year is removed from each end of the record due to the moving averages) suggested a seasonal cycle with a maximum in June (Fig. 4). However, a large contribution to the June peak came from an extreme event in 2005; with this removed, the June EKE was only marginally higher than the second largest value that of May. The average EKE over the whole southern Rockall box was higher than average throughout the first half of 2005, with a peak in April. In respect to the positive EKE anomaly in the first half of 2005, EKE

3.3. Eddy tracking The Q parameter algorithm for eddy tracking in geostrophic velocity fields detected a total of 35 eddies in the area 50–561N, 10–201W, between the summers of 2001 and 2007 (Fig. 5). While eddies were widely distributed over areas with water depths Z 2500 m, eddy formation and eddy tracks were concentrated in certain locations. There appeared to be no systematic advection of eddies towards the entrance of the Trough neither from the south nor the west. Rather, eddies formed along the slope or made their first appearance over the deep water in the southwestern part of the study area. Eddies that first appeared some 1/10–1/51 from the boundary might have entered from outside the area rather than being formed just inside it; the distance from the boundary can be an artifact due to the minimum size limitation imposed. The eddy detection algorithm does not distinguish between first occurrences due to advection or local formation. However, such ambiguity only applies to a few cases. The advection of eddies did not form a clear pattern, but there was some preference for motion towards the northeast along the continental slope, following the mean flow. South of about 521N (and east of about 191W) there was a tendency to east–west flow, bringing most eddies formed over the northern Porcupine Abyssal Plain in an offshore direction.

600 moor 300 m moor 400 m moor 500 m moor 600 m sat site B sat area average

EKE (cm2 s−2)

500 400 300 200 100 0

2002

2003

2004

2005

2006

2007

Fig. 3. Time series of monthly average EKE from different depth levels at mooring B (grey lines), as well as surface geostropic currents interpolated to site B (black line) and averaged over the area 50–561N, 10–201W (dash-dotted). Year labels start at January for each year.

200

EKE [cm2 s−2]

150

100

50

0 1

2

3

4

5

6 7 Month

8

9

10

11

12

Fig. 4. Monthly satellite-derived EKE at site B averaged over six years (2002–2007). Grey, all data included; black, excluding June 2005.

J.E. Ullgren, M. White / Deep-Sea Research I 64 (2012) 1–8

5

−1 00 0

56

−2 50 0

55

54

53 −4

00 −5

00

0

52

−20

51

50 20

00

18

16

14

12

10

Fig. 5. Cyclonic (blue) and anticyclonic (red) eddies tracked in geostrophic velocity fields using the Q parameter. The first point of each trajectory is marked by a dot. Depth contour interval: 500 m.

Ten out of the 35 eddies were formed within a small area inside the central Rockall Trough (northeastern part of study area, see Fig. 5). Anticyclones strongly dominated this site: nine of 10 eddies formed here were anticyclones, and half of all the detected anticyclonic eddies (nine out of 19) were formed in this ‘‘corner’’ north of Porcupine Bank, where the steep continental slope changes orientation by almost 451 from east/west to northeast/ southwest. Nearby, but somewhat further to the southwest, Booth (1988) found enhanced eddy activity, in particular one anticyclonic eddy which he described as likely formed by baroclinic instability of the Slope Current. West of about 181W and at the southern entrance to the Trough, near our mooring sites, there was preferential forming of cyclones – see e.g. the cluster of four cyclones formed just north of mooring B (Fig. 5). Eddy cores were on average somewhat bigger in the west than the east, with a weak but statistically significant correlation (r ¼ 0.33) between eddy core radius and longitude. This indicates the existence of different populations of eddies, with different formation mechanisms. We speculate that – as suggested by both Paillet (1999) and Booth (1988) – there is a tendency for the relatively warm Slope Current in the east to spin out warm-core eddies, and that coldcore eddies are preferentially formed at the extension of the NAC branch entering from the west.

4. A specific case The level of eddy activity in the southern Rockall Trough was unusually high in the first half of 2005, reflected in EKE computed from surface geostrophic velocities (Fig. 3). The spatially averaged EKE reached a maximum in April, but at site B the 2005 EKE peak occurred in June (Fig. 3). Dynamic topography showed the presence of a relative high and a low feature at the southern entrance to Rockall Trough at the time of the largest EKE peak, to each side of site B (Fig. 6). The southward flow at the western side of a cyclone and east of an anticyclone produced the strong geostrophic currents at site B in June 2005; maximum geostrophic current velocity of 40.8 cm s  1 southward occurred on 15 June 2005. Two Argo floats were circulating in the area at the time, and were caught up in the circulation around the two features (Fig. 6).

Fig. 6. Tracks of two Argo floats, ID AC-6900275 (black) and ID C-6900276 (white), from April through October 2005 overlaid on average dynamic height for June 2005. Large solid dots mark the selected vertical profiles, 28 May for C-6900276 and 26 June for AC-6900275; along the track of AC-6900275, two other, secondary profiles are also selected, 5 August (bold circle) and 4 October (thin circle) 2005. Arrows (first one bold) show the direction of travel.

The time resolution of Argo floats means there is a risk of aliasing, as it cannot be ruled out from Argo location data that the float might have completed a full loop around a mesoscale feature (with for example a diameter of 55 km and a swirl speed of 20 cm s  1 – both reasonable, cf. Shoosmith et al., 2005) during the 10 day reporting interval. The trajectories should therefore be interpreted with caution. One float, with ID 6900275 arrived from the south and did a large, anticyclonic loop from June to November 2005 (hereafter, we refer to this float with a prefix of AC, for anticyclonic, added to its ID number). The float trajectory described one and a half full circle with a diameter of about 135 km, centred at about 521N,171300 W (Fig. 6). A year later, float AC-6900275 returned and did another loop which overlay the track from 2005. Anticyclonic circulation thus appeared to be a recurring feature here, as also indicated by the mean dynamic topography (Fig. 1). Four anticyclonic eddies were detected in the vicinity (Fig. 5). The anticyclonic eddy seen in June 2005 was remarkable because it did not form as part of the quasi-permanent dynamic high south of 531N where it was found when the Argo float circled it. Instead, it was traced back to a high found north of Porcupine Bank already in early 2005 that moved first northwestward across the Trough and then southward – occasionally weakening and then strengthening again – until it eventually merged with the larger anticyclonic area in the south (Fig. 7). The other float, ID 6900276 (hereafter with prefix C for cyclonic) had earlier circulated in the bands of zonal current often found around 52–531N (e.g. New and Smythe-Wright, 2001), and passed site B in June 2005 from the northwest. It made half a loop around the cyclonic eddy, and exited towards the northeast along the continental slope. The cyclonic eddy (centred at 53113N, 1618W) was clearly seen in dynamic heights and detected in the Q field at the time. The automatic eddy tracking algorithm first picked up a cyclonic eddy here in April, then centred at 531200 N, 15151W, when it had pinched off from a larger low dynamic height feature stretching in from the west between 521 and 531N (Fig. 7). Each of the two Argo floats sampled the perimeter of one of the two eddies flanking mooring B in June 2005. For a comparison of water properties between the anticyclone and the cyclone, we

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J.E. Ullgren, M. White / Deep-Sea Research I 64 (2012) 1–8

Fig. 7. The earlier development of the cyclone (C) and anticyclone (AC) around which the Argo floats (cf. Fig. 6) circulated in June 2005. The Q parameter contours of the selected features (Q 0 ¼ 1:5  1011 ; bold black) are overlain on dynamic topography maps, one snapshot per month from February through July 2005. By July, AC had joined the larger area of anticyclonic circulation south of 531N (south of mooring B, grey cross). The dotted black contour is the (Q) outline of a neighbouring low (cyclone), before C was ‘‘terminally’’ pinched off; the connection highlights the western origin of C. Q contours here have not been subjected to the search radius or life span tests of the eddy detection algorithm, so eddies weakening below the detection limit between successive plots does not affect the marking in this figure.

selected a profile from each float; from float AC-6900275 we selected two additional, later profiles, since that float kept circulating longer in the region, and because the anticyclonic feature later developed further. The primary selected profiles from float AC-6900275 and C-6900276 are separated in time by about a month (26 June from AC-6900275, 28 May 2005 from C-6900276). The first selected profile from AC-6900275 was taken just as the anticyclonic eddy had arrived from the north and was in the process of merging with the larger high. The water in the anticyclone was warmer and saltier throughout the upper 2000 m of the water column (Fig. 8). The largest salinity difference between the profiles ( 4 0:28) was found at the 1050 dbar level, where the potential temperature profiles also differed strongly, by 42:1 1C. In the upper 100 dbar a local maximum in salinity difference, of about 0.26, coincided with the largest temperature difference between the profiles, of 2:3 1C. There were thus marked differences in water properties between the two eddies both at the upper and MW levels. The strongest positive salinity anomaly was found in the first profile of AC6900275, during the merging of the anticyclone from the north. With the caveat that the strength of the anomaly in part depends on the location of the profile in relation to the eddy core, the warm, salty water sampled here – ultimately of southern origin – appears to have been brought back south by the migrating eddy, rather than contained in the larger anticyclonic region south of 531N.

5. Discussion The eddies observed in the southern Rockall Trough had distinctive y–S properties on isopycnals (Fig. 8). Some eddies might have entered from outside the study region, but the majority appeared to be locally formed (Fig. 5). That most southern Rockall Trough eddies are local in origin and often nearstationary, rather than systematically migrating, is in concert

with the observation by Read et al. (2010) that the NAC consists of long lasting, slow moving eddies. To allow local formation of eddies with distinct y–S characteristics, strong temperature and salinity gradients are needed. Marked thermohaline gradients indeed exist in the southern Rockall Trough, both between the warm, saline Slope Current and surrounding upper waters (Booth and Ellett, 1983), and between the fresher SAIW in the northwest and saline MW from the southeast at the intermediate level (e.g. Ullgren and White, 2010). The radii of eddy cores found by automatic tracking were mostly between 20 and 30 km (27 km on average), while other data fields and float tracks suggested peripheral radii of up to about 65 km. The sizes compare well with observations by Booth (1988). Eddies formed by the Slope Current might be expected to have signature warm, saline water in the upper few hundred metres. Argo float profiles in an anticyclone northwest of Porcupine Bank during the strong eddy current event in summer 2005 showed positive salinity anomalies in the upper layer, at depths between about 75 and 300 m, compared to the neighbouring cyclone. A strong y–S anomaly was also found at the 1000 dbar level, indicating MW influence. One possible explanation is that an eddy formed in the Slope Current from a water column containing MW at the intermediate level, so that the eddy shed had both Slope Current and MW characteristics. Another possibility is that an eddy formed in the upper layer entrapped a parcel of MW, as in the case of an eddy further south studied in the Tourbillon experiment (Arhan and ColindeVerdie re, 1985); or conversely, that a meddy entrained slope water. Paillet et al. (2002) observed a northern meddy at about 451N that was relatively weak, being formed from more dilute MW than southern meddies. Shoosmith et al. (2005) considered four southwestward moving anticyclones near Goban Spur, between 471 and 501N, as possible weak meddies. MW appears as isolated parcels even farther north (Ullgren and White, 2010). The meddy type of anticyclonic eddy is common along the Iberian margin but gives way to swoddies in the Bay of Biscay (Pingree and Le Cann,

J.E. Ullgren, M. White / Deep-Sea Research I 64 (2012) 1–8

16 14

0 AC 275:1 AC 275:2 AC 275:3 C 276

26.2

200 400

26.6

12

E

10

27.6

8

MW

27.4

800 1000 1200 1400

28

6

P (dbar)

θ (°C)

600

W NA

27

1600 4

1800

4

28.

2 34.8

7

35

35.2

35.4

35.6

S

35.8

C 276 AC 275:1 AC 275:2 AC 275:3

2000 34.8 35 35.2 35.4 35.6 S

Fig. 8. y2S diagram (a) and salinity profile as a function of pressure (b) for the selected float profiles from 2005, from the cyclonic eddy sampled by float C-6900276, and at different points along the track of float AC-6900275 around the anticyclonic circulation (1, 26 June; 2, 5 August and 3, 4 October 2005). The dark grey line in (a) represents ENAW as defined by Harvey (1982); dashed lines show the extent of the ENAW envelope. A salinity maximum on the sy ¼ 27:6 kg m3 is characteristic of MW, also labelled in (a).

1992); a recent study by Carton et al. (in press) indicates that meddies and swoddies may meet and interact there. Swoddies are eddies with water of slope current origin, whose generation are often associated with topographic features such as promontories and canyons (Le Cann et al., 2005). The Porcupine Bank is a major topographic feature which itself is incised by a large, deep canyon near 521N. Whilst the Slope Current is principally barotropic, it has a baroclinic component (Pingree et al., 1999; Huthnance et al., 2002). A scaling exercise by Booth (1988) suggested that for typical values for slope topography and current speed west of Porcupine Bank, baroclinic instability of the Slope Current is a likely cause of eddy formation. The Rockall Trough is a region of unusually deep winter mixing (Ellett et al., 1986; Meincke, 1986). Maximum EKE occurred in May–June, 2–3 months after the peak in winter mixed layer extent, when seasonal restratification has developed significantly (Ullgren, 2007; Holliday et al., 2000). The contrast between the core of the Slope Current and the weakly stratified spring conditions offshore could provide the available potential energy in the upper water column to be released during baroclinic instability (cf. Chanut et al., 2008; Katsman et al., 2004). The anticyclone observed in June 2005 originally formed north of Porcupine Bank in early 2005 and moved around the southern Rockall Trough before eventually merging with a larger region of anticyclonic flow south of 531N. Both cyclonic and anticyclonic eddies are commonly observed in the southern Rockall Trough (e.g. Booth, 1988; Ellett and Turrell, 1992; Ullgren and White, 2010). Where upper layer flow from the south (SEC; see Fig. 1) meets inflow from the west (NAC) at about 531N, a large standing eddy has been observed (e.g. Ellett et al., 1986). New et al. (2001) suggested that large, quasi-stationary eddies seen in model output at about the latitude of Porcupine Bank may block the northward penetration of MW. Our observations show that these eddies can also incorporate MW (Fig. 8). As the eddies tend to separate from the continental slope – eddy and float trajectories south of about 521N showed preference for zonally oriented, offshore flow – this might contribute to the dispersion of MW at these latitudes. Eddies also formed along a band broadly following the 4000 m contour (Fig. 5), between about 181 and 201W. Cold-core eddies

dominated this region, where meanders or instabilities of the NAC are a likely source of mesoscale variability. Site B did not show the strong seasonality in EKE with a winter/spring maximum described by e.g. Volkov (2005). A small local maximum in December 2003, secondary to the maximum in July 2004 (Fig. 3), might echo the semiannual EKE signal observed in nearby regions of the North Atlantic (White and Heywood, 1995). EKE in our study region was unusually high in 2005, perhaps linked to strong northerly winds that prevailed over the eastern North Atlantic in winter 2004–2005 (ICES, 2006). The lack of clear seasonality in parts of our study area might result from the different eddy formation mechanisms at play, associated with the NAC and the Slope Current with their respective seasonal signals.

6. Summary Several possible origins of the mesoscale activity in the Rockall Trough have been proposed: baroclinic instability of the Slope Current, weak meddies shed from a poleward undercurrent, or instability of the NAC (Booth, 1988; New and Smythe-Wright, 2001; Shoosmith et al., 2005; Volkov, 2005). We have studied a close-up of the southern Rockall Trough and find evidence that all of these formation mechanisms are active to some extent. Some eddies, mostly cyclones, in the western part of our study area (near 201W) are likely to be formed by instabilities at the easternmost extension of the NAC. Closer to the continental slope, warm-core eddies are more abundant and appear to be associated with the Slope Current (Paillet, 1999; Pingree and Le Cann, 1992). If such eddies form south of about 531N, over the northern Porcupine Abyssal Plain where anticyclonic eddies were a recurring phenomenon, they may contain – or entrain – MW at the deeper level. This could contribute to the patchy distribution of MW, as isolated lenses of MW have been found detached from the continental slope (Ullgren and White, 2010). The tendency for such parcels to be transported offshore may contribute to the disappearance of the MW signature north of Porcupine Bank (McCartney and Mauritzen, 2001). However, we cannot rule out that these MW lenses are northern meddies that interact with the slope waters, rather than passively being entrained. Northwest of

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Porcupine Bank, a ‘‘hotspot’’ of anticyclone formation was found, most likely linked to the Slope Current and local topography. The observation that Rockall Trough eddies carry water with distinctive water mass properties means they can play an important role in the heat and salt budget of the region, but their contribution remains to be quantified.

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