Meddies and decadal changes at the Azores Front from 1980 to 2000

ARTICLE IN PRESS

Deep-Sea Research II 52 (2005) 583–604 www.elsevier.com/locate/dsr2

Meddies and decadal changes at the Azores Front from 1980 to 2000 Gerold Siedlera,, Laurence Armib, Thomas J. Mu¨llera a Leibniz-Institute of Marine Science, IFM-GEOMAR, Kiel University, Germany Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0225, USA

b

Received 10 April 2004; accepted 5 December 2004

Abstract Twenty years of time series observations from the deep-sea mooring KIEL276 are used to obtain information on the frequency and propagation of meddies (Mediterranean Water eddies), on long-term changes in flow properties, and on a possible relation to the North Atlantic Oscillation. The mooring was set at the nominal position 331N, 221W at a water depth of more than 5200 m in the northern Canary Basin. It is located near the southern boundary of the Azores Current (AC), which is part of the North Atlantic subtropical gyre, and also in the large-scale Mediterranean Water (MW) tongue originating from the Strait of Gibraltar. The amplitudes of time-varying vertical quasi-geostrophic modes and the correlation of current and temperature changes at levels in the MW and the North Atlantic Central Water above are used to identify meddies. A total of 10 meddies passed the mooring during the period 1980–2000. Half of the events can be related to earlier observations. Directional changes in meddy-related velocities are used to estimate speeds and directions of meddy propagation. Directions of propagation are very homogeneous, with all the 10 meddies observed propagating with a southward velocity component within a sector of 901, and typical speeds are 2–3 cm/s. Meddy occurrence was uneven in time, with six meddies found during the first four years and only four meddies during the remaining 16 years. Decadal changes show the annual-mean and the fluctuating kinetic energy levels at the site changing from lower values in the 1980s to high values in the 1990s. This change appears to be correlated with variations in the North Atlantic Oscillation (NAO) index, with a delay in oceanic response of about 3 years. A conceptual model of AC meanders is used to identify meander signals in the upper-layer time series. The AC axis appears to be closer to the site during the 1990s than during the preceding decade and indicates a southward or southwestward displacement of the AC with increasingly positive values of the NAO index. Meddy frequency is lower when the AC gets closer from the north. A reduction in meddy occurrence in the region just south of the AC is possibly caused by the shear-induced blocking of some meddies crossing the front from the north. r 2005 Elsevier Ltd. All rights reserved.

Corresponding author. IFM-GEOMAR, Dienstgeba¨ude Westufer, Dusternbrooker Weg 20, 24105 Kiel, Germany. Tel.: +49 431 542 672;

¨

fax: +49 431 600 4152. E-mail address: [email protected] (G. Siedler). 0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2004.12.010

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1. Introduction The following study is based on a data set that is probably the longest multi-level deep-sea moored current time series ever obtained in the global ocean. Measurements were performed by the Institut fu¨r Meereskunde (now the Leibniz-Institute of Marine Science), Kiel, Germany, at site KIEL276 in the eastern North Atlantic, about halfway between the Azores and the Canary Islands, from 1980 onwards. Initial periods of these measurements were examined by Armi and Zenk (1984), Siedler et al. (1985), Zenk and Mu¨ller (1988), and Mu¨ller and Siedler (1992). The deepocean mooring site is located in a region that includes the Azores Current (AC) as part of the North Atlantic anti-cyclonic subtropical gyre, and the deep Mediterranean Water (MW) outflow including ‘‘meddies’’, which are long-lived, highsalinity lenses or eddies with anticyclonic rotation and approximate diameters of 100 km. Current variability is strongly influenced by this particular environment, and a short review of regional aspects will therefore be given first.

The position of KIEL276 is close to the mean position of the core of the eastward AC, related to the Azores Front (Ka¨se and Siedler, 1982; Gould, 1985; Ka¨se et al., 1985; Siedler et al., 1985; Sy, 1988), as part of the Gulf Stream recirculation (Klein and Siedler, 1989; Siedler and Onken, 1996). Mesoscale patterns are typical features of the Azores Front and are largely determined by baroclinic instability of the frontal jet (Alves and Colin de Verdie´re, 1999). Satellite altimeter observations show elevated mesoscale eddy energy in the Azores Current region, compared to the lower levels in the Iberian Basin to the north and in the Canary Basin to the south (Le Traon and De Mey, 1994; Hernandez and Le Traon, 1995; Tychensky et al., 1998). This is recognized in the distribution of eddy-scale energy variability in Fig. 1 (after Stammer, 1997). Unfortunately, Topex/Poseidon altimeter data only began to be available in the early 1990s, so a precise determination of AC position changes was not possible. Below the near-surface layer, the depth range between about 600 and 1300 m is dominated by the MW. The outflow of high-salinity water from the

Fig. 1. Distribution of mesoscale-eddy variability in satellite altimeter data in the eastern North Atlantic (after Stammer, 1997) with elevated energy in the Azores Current. The site of K276 is indicated.

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Mediterranean Sea through the Strait of Gibraltar compensates the excess of salt due to net evaporation in the Mediterranean Sea (e.g., Schott, 1928; Defant, 1961; Madelain, 1970; Lacombe and Richez, 1982; Bryden and Kinder, 1991; Bryden et al., 1994; Candela, 2001). After having passed the strait, the warm and high-salinity Mediterranean Water sinks down the continental slope because of its higher density compared to the surrounding Atlantic water. It is diluted continuously by mixing with surrounding water and guided by bottom topography through the Gulf of Cadiz (Zenk, 1970, 1975a; Howe, 1982; Rhein and Hinrichsen, 1993; Johnson et al., 2002). It reaches the region near Cape St. Vincent at the southwestern tip of Portugal. The high-salinity water settles below the North Atlantic Central

585

Water and usually has two cores in the vertical at approximately 700 and 1000–1400 m depths (Siedler, 1968; Zenk, 1975b; Zenk and Armi, 1990). To the west of the Gulf of Cadiz a large-scale Mediterranean Water tongue is found (Defant, 1956; Richardson and Mooney, 1975), with two main branches to the southwest and to the north (Fig. 2), forming the key source waters for the upper North Atlantic Deep Water (Wu¨st and Defant, 1936; Reid, 1978; Warren, 1981; Daniault et al., 1995; Candela, 2001). From hydrographic observations near the Mid-Atlantic Ridge it was concluded by Sy (1988) that the Mediterranean Water tongue slides westward between the two major zonal currents in the region, the North Atlantic Current in the north and the Azores Current in the south.

Fig. 2. The Mediterranean Water (MW) tongue with salinity anomalies relative to S ¼ 35:01 (after Needler and Heath (1995), Joyce (1981) and Richardson et al. (2000)), positions of meddy observations (white dots) (after Richardson et al., 2000), the approximate locations of the North Atlantic Current (NAC) and the Azores Current (AC), and the location of mooring site KIEL276. Islands indicated: Azores (Az), Madeira (Mad), Canary Islands (CanI).

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Meddies were first detected in the eastern North Atlantic by Armi and Stommel (1983) and then observed in detail by Armi and Zenk (1984) and Ka¨se et al. (1985). Generation off the southern and southwestern coasts of Portugal was identified (Bower et al., 1995; Serra and Ambar, 2002). It has been estimated that the overall meddy salt transport may be between 25% (Richardson et al., 1989) and more than 50% (Arhan et al., 1994) of the salinity anomaly flux that arrives from the Strait of Gibraltar. A large amount of data on meddies has been assembled by now, and a more recent summary of meddy observations by floats (Fig. 2) was provided by Richardson et al. (2000). They concluded that 90% of meddies collided with seamounts after a mean lifetime of about 1.7 years, while those meddies that do not hit a seamount can live for more than 5 years; an example being the meddy tracked for 2 years with SOFAR floats and surveyed four times by CTD and velocity profilers by Armi et al. (1989). Meddies have cores with high salinity (maximum salinities near 36.5) and high temperature (maximum temperature near 13 1C), with a typical lens diameter of 40–150 km and a depth range of 800–1400 m. They sometimes exhibit a double-core structure in the vertical, and they rotate anticyclonically (Richardson et al., 2000). Despite the temperature signal being not as well-defined as the salinity signal, meddies can be identified in moored time series from the correlation of eddy-type motion and maxima in positive temperature anomalies. Meddies also can be recognized in satellite altimeter signals (Stammer et al., 1991; Oliveira et al., 2000) and in that case will have a barotropic component in its rotational current. It can therefore be expected that meddies interact with the near-surface flow and in particular with the Azores Current. The role of the Azores Front as a boundary of two meddy regimes was discussed by Sparrow et al. (2002). They found that the propagation of meddies to the north and northwest of the front is largely controlled by the background velocity field, which has rather large speeds, while the background velocities south of the Azores Front are sufficiently small to allow an independent motion of the meddies that crossed the front in a southwesterly direction.

A laboratory experiment on these stable fluid structures was carried out by Hedstrom and Armi (1988). Injection of a homogeneous fluid into a linearly stratified and rotating background fluid produced anticyclonic lenses that could be studied for up to 600 rotation periods. Velocity measurements showed that the interior core rotates as a solid body with a decreasing, nearly axisymmetric exterior velocity field. Gill’s (1981) model predicts well both the velocity and aspect ratio vs. Rossby number of the lenses. Schultz Tokos and Rossby (1991) also found, with in situ observations using the free-falling PEGASUS, that within the radius of maximum velocity, anticyclonic solid-body rotation exists with a depth-dependent rotation period near 6 days. They also found a sharp (5 km or less) transition between the core and the outer region where the velocity decayed exponentially with radius. The observations of Armi et al. (1989) and Schultz Tokos and Rossby (1991) showed that with time, a meddy gradually was eroded from the sides. The radius of maximum velocity and the maximum velocity itself both decreased. Colin de Verdiere (1992) proposed that the southward motion of salt lenses in the Canary Basin relies on the active mixing that is observed at the periphery. The salt lens suffers a velocity divergence induced by a geostrophic adjustment process that causes in turn a squashing of the density surfaces, and the lens moves southward to conserve potential vorticity.

2. The time series The site of KIEL276 (nominal position: 331N, 221W; water depth about 5285 m) is located over an abyssal plain which is part of the northern Canary Basin. Fig. 2 presents the region including selected topographic information, the spatial range of the main MW tongue, meddy positions observed by floats after Richardson et al. (2000), approximate locations of the Azores and North Atlantic Current cores, and the position of site KIEL276. The lines of the Azores Current in the Madeira region follow the results on three southward branches of the Azores Current given by

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Klein and Siedler (1989) and Siedler and Onken (1996), with their southward branch farther west of KIEL276 between 30 and 351W being omitted here. The mooring positions were within 70.21 in latitude and longitude of the nominal position. Measurements began on 4 April 1980, and the mooring was regularly replaced by a follow-up mooring once per year. Moorings typically carried seven to nine Aanderaa current/temperature meters, with the uppermost instrument usually equipped with a pressure sensor, and with additional instruments added from time to time. More information on the KIEL276 mooring design can be found in Mu¨ller and Siedler (1992). The recovery rate was 100% from April 1980 to April 2000. In 2000 and 2001 problems occurred with the moorings, and a gap in the data series therefore exists for these 2 years. Observations

587

were resumed in February 2001. Because of this gap the present study is limited to the twenty years of continuous operation from 1980 to 2000. Instrument performance was variable, however, and the usable records are summarized in Fig. 3. One CTD cast was usually performed approximately concurrent with mooring replacement (Fig. 4). The station positions were within 70.21 in latitude and 70.21 in longitude of the nominal mooring position. Temperatures from these CTD profiles were used to check moored sensor stability and to correct the calibration using a linear temporal trend when necessary. The depths of instruments were revised, applying further corrections obtained from pressure records of the top instrument and from temperature data resulting from CTD-profiles at the start and end of the respective mooring period. When comparing with earlier KIEL276 studies, one may therefore

0 -500 -1000

Instrument depth (m)

-1500 -2000 -2500 -3000 -3500 -4000 1980

1985

1990

1995

2000

-4500 -5000 water depth -5500 0

2

4

6

8 10 12 14 Years since 01 JAN 1980

16

18

20

Fig. 3. Summary of mooring observations: solid lines ¼ current; dotted lines ¼ temperature; dash-dotted lines ¼ pressure (see also Table 1).

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588

K276 25 24 25 26

Pot. temperature / °C

20

15 NACW 27 28 10 MW

5

29

NADW

35

35.5

36 Salinity

36.5

37

37.5

Fig. 4. Potential temperature vs. salinity of the CTD casts taken near KIEL276. NACW ¼ North Atlantic Central Water, MW ¼ Mediterranean Water, NADW ¼ North Atlantic Deep Water. Isolines correspond to potential density anomaly (units: kg m3).

encounter some differences in instrument depths. As can be seen in Fig. 3, the depths of measurements changed somewhat from mooring to mooring because of minor modifications in design. Depth variations also occurred on shorter time scales because of changing inclinations of the mooring line due to varying current drag. Typical mesoscale properties of currents and temperatures are illustrated in Fig. 5 by showing a subset of the time series for the period October 1983–October 1984. Daily mean values are used, thus largely removing the barotropic and baroclinic semi-diurnal and diurnal tides and also the (nearly diurnal) inertial signals that dominate the variability in this short-period range in the region (see Dick and Siedler, 1985; Siedler and Paul,

1991). Weak mean currents and much stronger signals are recognizable on time scales of 1–3 months. The mesoscale signals often occur in phase through most of the water column, indicating a strong barotropic component. One example is seen during the period of days 510–560. Maximum amplitudes are found in the upper layers during the first 100 days of 1984, where we expect the influence of the subtropical gyre and related Azores Front meanders and shedded eddies (Siedler et al., 1985). At depths below, a meddy signal is found near day 340 at the end of 1983. A peculiar feature is recognized at 1160 m and particularly at 1660 m between day 400 and 500: a strong-current event with corresponding temperature minima. Apparently this was caused by

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20 1983

current vectors

1984

589

327 m

Speed (cm/s) & 10*dT (°C)

0 560 m

-20 -40

760 m Meddy

-60

1160 m

-80 LSW

1660 m

-100 3050 m

-120 temperature anomaly (dT)

-140

5240 m 300

350

400

450 500 Days since 01 JAN 1983

550

600

650

Fig. 5. Current vectors and temperature-anomaly (dT) time series of daily mean values for the period October 1983–October 1984. The anomaly refers to the mean over this period. The current and temperature maxima at 760 and 1160 m depths near day 340 show a meddy passing the mooring. The temperature minima and increased velocities at 1660 m between days 400 and 500 indicate Labrador Sea Water (LSW), an event only observed once in these 20 years. The current maxima between days 510 and 560 show an event with corresponding changes at all depths of observation, i.e., with strong barotropic components.

water originating from Labrador Sea Water (LSW). There was only one such event during the whole 20-year period.

3. Meddy occurrence We want to determine the frequency of meddy occurrence at KIEL276. While the depths of instruments differ somewhat from year to year, this fact does not pose a problem for the meddy analysis. First, low-order modes dominate the local current field (Mu¨ller and Siedler, 1992; Wunsch, 1997), with the first normal mode describing mostly Azores Front meanders and eddies and the second mode indicating MW changes. Second, meddies cover the range between 800 and 1400 m (see above). Therefore current and temperature data within ranges of several hundred meters can be combined to study the occurrence of meddies. The data set used here is summarized in Table 1. The time series of daily current vectors and temperature anomalies are presented in Fig. 6. If

records in the MW layer are very close to each other and have similar signals, only one record is shown. In the case of a weak background current field the meddy signal will dominate. If the meddy crosses the mooring with its center, we will see a maximum current in one direction and later in the opposite direction, and the temperature anomaly will have a maximum. A meddy also may drift by, just touching the mooring with the meddy’s boundary portion. In this case the current will increase for some time and the direction measured at the mooring will only moderately change during the passage. The temperature anomaly will again have a maximum, but a more moderate one. The situation is complicated by the fact that meddy signals and considerable barotropic and first-order baroclinic signals often occur at the same time. The vertical structure of the barotropic and first- and second-order baroclinic modes are presented in Fig. 7. The first-order mode and to some extent also the barotropic mode is influenced by Azores Current meanders and spin-off eddies, but the first mode is less relevant in this context because it has its zero-crossing at about 1500 m

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590 Table 1 Mooring KIEL276 data Mooring no.

Latitude (1N)

Longitude (1W)

Start date (yyyy/mm/dd)

End date (yyyy/ mm/dd)

Depths (m)

v264010 v276010 v276020 v276030 v276040 v276050 v276060 v276070 v276080 v27609a v27609b v276100 v276110 v276120 v276130 v276140 v27614lb v276150 v276160 V276170 V276180 V276190

33.10 33.17 33.08 33.19 33.17 33.20 33.16 33.14 33.11 33.09 33.09 33.11 33.10 33.15 32.92 33.00 33.14 32.96 33.00 33.00 32.99 33.00

21.83 21.85 21.88 21.90 21.92 21.92 21.96 21.96 21.92 21.88 21.88 21.90 21.91 21.89 22.14 22.00 21.98 22.02 21.96 21.99 22.00 22.01

1980/04/04 1980/10/21 1981/07/31 1982/03/09 1983/04/23 1983/10/24 1984/10/30 1985/11/21 1986/11/05 1987/11/10 1988/09/28 1989/01/18 1989/10/31 1990/09/29 1992/02/01 1993/07/16 1994/01/02 1994/09/20 1995/10/16 1996/06/30 1997/08/09 1999/01/27

1980/10/12 1981/07/23 1982/02/25 1983/04/13 1983/10/15 1984/10/21 1985/11/12 1986/10/27 1987/11/02 1987/11/10 1989/01/10 1989/10/23 1990/09/21 1992/01/23 1993/07/07 1994/08/07 1994/09/13 1995/10/11 1996/06/22 1997/08/04 1999/01/21 2000/04/10

124, 195, 243, 194, 243, 327, 327, 300, 330, 450, 650, 367, 320, 215, 270, 750, 731 240, 270, 270, 270, 270,

depth near the core depth of the Mediterranean Water. Barotropic modes can be important in the region (Mu¨ller and Siedler, 1992). The secondorder mode is dominated by the meddy signal. Time series of speed modes and temperature anomalies are combined in Fig. 8. Meddy identification was performed in two steps. The time series in Fig. 6 were inspected to find positive temperature anomaly changes and corresponding current direction and speed changes. Time series of modes and temperature anomalies were then used to confirm the earlier findings and to check for meddies that had possibly been missed during the first round. The identified meddies are marked by color in Fig. 6, and the consecutively numbered eddies are listed in Table 2. We will demonstrate the identification method by two cases. Meddy M3 occurred in mid-June 1981 (day 517–542, see Fig. 6). We find maxima in the temperature anomaly both at 995 and 1095 m and current maxima at all levels shown in Fig. 6,

376, 627, 926, 2966, 4707 499, 697, 995, 1095, 1591 550, 748, 1149, 1665, 3020 428, 629, 1032, 1535 475, 675, 1075, 1575, 2980, 5185 560, 760, 1160, 1660, 3050, 5240 562, 764, 1168, 1670, 3080 534, 736, 1040, 1142, 1644, 3047, 5235 560, 760, 1060, 1160, 1670, 3070 650, 950 1050, 1550, 3000 850, 1150, 1250, 1750, 3050 595, 800, 1100, 1200, 1700, 3050, 5185 555, 755, 1055, 1155, 1655, 3045, 5190 445, 645, 1065, 1565, 3000 1000, 1100, 3000, 5185 1050, 1150, 1650, 3050 470, 670, 970, 1120, 3030, 5275 500, 1000, 1600, 3000, 5185 5000, 1000, 1600, 3000, 5185 1000, 1600, 3000, 5185 500, 1000, 1600, 3000

but with strongest maxima at 995 and 1095 m. At these depths the directions change from north to north–northeast. There is a strong maximum in the second mode (see Fig. 8), but maxima also are found at modes 0 and 1. We conclude that there is a clear indication for a meddy, coupled with a barotropic and first-order baroclinic signal, which is possibly related to an Azores Front meander. The moderate variations in direction suggest that the meddy just touched the mooring with its boundary portion. A survey of this meddy can be found in Armi and Zenk (1984). The positions of CTD stations relative to KIEL 276 from 10 to 11 June 1981 are shown in Fig. 9 adapted from their Fig. 13. Note, however, that the least-squares fit to a paraboloid shown in their Fig. 10 for the meddy center and the associated isohalines was based on only eight stations of which only five had anomalously high salinities. There is therefore considerable error of at least 10 km in the fit, and based on our analysis the center may be displaced further away from

ARTICLE IN PRESS G. Siedler et al. / Deep-Sea Research II 52 (2005) 583–604

591

Speed / (cm/s) & 10*dT

(A) 0

376 m 926 m

-50

Speed / (cm/s) & 10*dT

243 m 1149 m

1591 m M1 M2

-100 1980 0

100

0

200

300

M3

400

M4

500

629 m 1032 m

1665 m

600

700

243 m

560 m

1075 m

-50 1535 m

-100 1982

800

Speed / (cm/s) & 10*dT

499 m 995 m 1095 m

M5 900

1000

1100

0

1160 m 1660 m M6

1575 m 1200

1300

1400

562 m

534 m 1040 m 1142 m

-50 1168 m 1670 m

-100 1984 1500

1600

1700

1644 m

1800

1900

2000

2100

Days since 01 JAN 1980, yearly grid

Speed / (cm/s) & 10*dT

(B) 0

1060 m 1160 m 1670 m M7

-50 -100 1986

Speed / (cm/s) & 10*dT

2200

2300

2400

0 -50 -100 1988 3000

Speed / (cm/s) & 10*dT

450 m 950 m 1050 m

760 m

3100

0

2500

1550 m 2600

2700

2800

2900

650 m

367 m

555 m

1150 m 1250 m 1750 m

1100 m 1200 m 1700 m

1055 m 1155 m 1655 m

3200

3300

3400

3500

3600

215 m 965 m 1065 m

-50

1565 m

-100 1990 3700

3800

3900 4000 4100 Days since 01 JAN 1980, yearly grid

4200

4300

Fig. 6. Time series of daily current vectors (blue, north is upwards) and temperature anomalies (black, in 0.1 1C). The uppermost series has the correct speed scale, series below are displaced by 20 cm s1 each. Thick lines and numbers M1 to M10 mark meddy occurrence (see Table 2).

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592

Speed / (cm/s) & 10*dT

(C) 0

270 m 1000 m 1100 m

-50 -100

Speed / (cm/s) & 10*dT

1050 m

1992 4500

4600

732 m

-50 -100

4700

4800

4900

5000

5100

240 m 970 m 1120 m

270 m 1000 m

1600 m

1994 5200

Speed / (cm/s) & 10*dT

1650 m M9

M8

4400

0

750 m

5300

0

5400

5500

5600

5700

5800

270 m 1000 m

270 m 1000 m

1600 m

1600 m

-50 -100

1996 5900

6000

6100 6200 6300 Days since 01 JAN 1980, yearly grid

6400

6500

Speed / (cm/s) & 10*dT

(D) 0

270 m 1000 m

-50

6600 Speed / (cm/s) & 10*dT

1600 m M10

-100 1998 6700

6800

6900

7000

7100

7200

7300

0 -50 -100 2000 7400

7500

7600

7700

7800

7900

8000

Days since 01 JAN 1980, yearly grid

Fig. 6. (Continued)

KIEL276 than shown. A more likely lens path past the mooring, based on the mooring velocity data and our present knowledge of the radial velocity structure vs. temperature anomaly, is also shown. It is worth noting that the north-northeastward direction in the progressive vector diagram in Fig. 17 of Armi and Zenk does not give the meddy

propagation direction, but is dominated by the anticyclonic meddy current. The second example is meddy M10, which occurred in early 1999 (day 6999–7040). The temperature anomaly curve at 1000 m has a distinct maximum, and the currents are maximal at that depth with directions changing from

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593

KIEL276: quasi-geostrophic modes 0 F1 -1000

F2

F0

Depth / m

-2000

-3000

-4000

-5000

-6000 -3

-2

-1

0

1

2

3

Fig. 7. Vertical structure of quasi-geostrophic dynamical modes (F0 ¼ barotropic; F1 ¼ first baroclinic, F2 ¼ second baroclinic).

northwest through north and east to southeast. Mode 2 is strong, but both modes 0 and 1 are involved. This is a meddy that crossed the mooring almost with its center. Several moderate maxima in temperature anomaly are seen in Fig. 8, with their peaks just touching the dash-dotted line corresponding to 2.5 times the standard-deviation. Because of the low values in Mode 2 at the same time, they were not considered meddy signals. The same argument applies for the large temperature anomaly maximum at the end of 1987. There is another large maximum at the end of the temperature curve towards the end of 1993. Because of the break in records at that time, it is uncertain whether this signal was related to a meddy. This maximum was therefore not considered a meddy signal. The clear correlation of signals in the case of identified meddies makes sure that meddies are really observed. Because of a certain arbitrariness in the method, however, we cannot be completely sure that meddies with weak signals at the site are not missed. The number of found meddies there-

fore should be considered a lower limit of meddy occurrence. The sequence of directions of currents measured at the mooring provides information on the propagation direction of each anticyclonic meddy. Rough estimates of the propagation directions of the proposed meddy detections are presented in a schematic anticyclonic meddy in Fig. 10. The first example above, meddy no.3, has only a small directional change from north to north-northeast. This suggests a meddy propagation in a southsoutheastward direction (Fig. 10). The arrow was reproduced in Fig. 9. The second example above, meddy no.10, shows first northwestward, then northward and finally east-southeastward motion, suggesting a meddy propagation direction to the southwest (Fig. 10). A total of 10 meddies were identified during the time span of 20 years. Durations of the meddy signals range from 9 to 41 days. Propagation speeds were estimated from these durations by assuming a meddy diameter of 100 km and a reduction of the path length across the meddy

ARTICLE IN PRESS G. Siedler et al. / Deep-Sea Research II 52 (2005) 583–604

594

10

Mode 0

0

Modal amplitude/(cm/s) & 5*dT/K

Mode 1

-10

Mode 2

-20

M1 - 4

5

6

7

8

9

10

dT -30

-40

1980 -50

0

1985 1000

2000

1990

1995

3000 4000 5000 Time/d since 01 JAN 1980, yearly grid

6000

7000

Fig. 8. Time series of barotropic (Mode 0), first-order baroclinic (Mode 1) and second-order baroclinic (Mode 2) dynamical modes and temperature anomalies (temperature—median) at MW levels. The dash-dotted lines give 2.5  the standard deviation. The gaps in 1993 and 1994 result from insufficient vertical coverage in records during those periods. Meddy events are given by colors and numbers M1 to M10 corresponding to Fig. 6.

Table 2 Meddy occurrence Meddy ID

Days since 01.01.1980

Duration (days)

Dir

spd (cm/s)

Corresponding event in earlier publications

1 2 3 4 5 6 7 8 9

324–360 370–389 517–542 564–601 1004–1016 1422–1456 2501–2525 4543–4583 4999–5008

36 19 25 37 12 34 24 40 9

SSE S SSW SSE SSE SSW SW SW SSW

1.8 2.9 2.8 2.2 2.2 3.1 3.0 2.7 6.4

Armi Armi Armi Armi

10

6999–7040

41

SW

2.7

and and and and

Zenk Zenk Zenk Zenk

(1984): (1984): (1984): (1984):

event event event event

A B D, lens 3 E

Richardson and Tychensky (1998), Tychensky et al. (1998): Meddy 3

Time of meddy occurrence, duration of meddy crossing and direction (dir) of meddy propagation estimated from velocity direction change, approximate meddy propagation speed (spd) obtained when assuming a meddy extension of 100 km and a reduction of the path length across the meddy corresponding to Fig. 10 (for accuracy see text).

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Fig. 9. Station profiles of salinity S, potential temperature y and density anomaly s1 and fitted distribution of maximum salinity after Armi and Zenk (1984). The thin arrow presents the approximate mean current measured at KIEL276 (see Fig. 6) and the thick arrow is a guess on the path of the outer part of meddy no.3 past the mooring (see Fig. 10).

NNW

N

NNE

NW

NE

WNW

ENE

W

E

WSW

ESE

8 3 10 6

SE

SW

4 SSW

SSE

S

2

7

9

1

5

Fig. 10. Schematic presentation of an anticyclonic meddy. Open arrows give the actual meddy currents, solid arrows give propagation directions of meddies estimated from current direction changes (see Fig. 6), and meddy numbers correspond to those in Table 2.

corresponding to Fig. 10. The typical resulting speeds are 2–3 cm/s (Table 2). These are approximate numbers only because there is some arbitrariness in selecting the start and end of meddy events and because of the assumption of a 100 km meddy diameter. However, nine out of 10 meddies lead to these numbers, while only meddy no.9, with a duration much shorter than the other events, gives a higher speed. The estimated propagation directions are remarkably uniform, all approximately between southwestward and southward. When searching in earlier publications for meddy occurrence near KIEL276, a correspondence between meddy observations with floats, hydrography and altimeter is found in several cases and is documented in Table 2. The occurrence of meddies is not uniform with time at KIEL276 (Fig. 11). Six meddies were found during the 4 years from 1980 to 1983, while only four meddies were identified during the following 16 years from 1984 to the beginning of 2000. The

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596

1980

1985

M1234

0

5

1000

6

1990

7

2000

1995

8

3000

4000 days

9

5000

10

6000

7000

Fig. 11. Occurrence of ten meddies (M1–10) at Site KIEL276 from 1980 to 2000. The width of the lines corresponds to the duration of the meddy signal.

probability for observing a meddy during the whole period of 20 years at the single-point mooring is lower (4%) than the more regional estimate from hydrography (8%) by Armi and Zenk (1984). The difference in meddy frequency between the first and second part of the time series suggests a possible relation to decadal changes in the region.

4. Long-term changes The large differences in meddy frequency between the first and the second decade of observations leads to the question whether these might be related to decadal changes in the general circulation. We want to check whether annual mean kinetic energy and fluctuating kinetic energy (FKE) levels indicate decadal changes (we use the term FKE instead of the usual Eddy Kinetic Energy (EKE) to indicate that mesoscale changes can be caused by eddies, Azores Front meanders or Rossby waves). In order to determine timemean currents we need to obtain a basic time series without gaps. This can be achieved by using dynamical modes. Mu¨ller and Siedler (1992) showed that the dominance of the barotropic and the first and second baroclinic dynamical modes at KIEL276 permits an interpolation in the vertical by time-varying dynamical modes. They described the method and its reliability. Mode F1 is most important for the upper 1000 m and mode F2 essential for the MW levels (see Fig. 7). The resulting time series for our 20-year data set (Fig. 12) indicates decadal changes, with stronger fluctuating currents during the 1990s than before during the 1980s.

The annual means of the east and north components at three levels (Fig. 13) obtained from the interpolated time series show medium-strong signals during the first years up to 1984, then lower values up to 1990, and particularly strong signals from 1990 to 2000. Energy levels change consistently in-phase from the main thermocline (450 m) down to the deep sea (3000 m). The fluctuating kinetic energy at 450 m (Fig. 14) even more clearly demonstrates this major difference in energy levels found between the first and the second decades of the observations. It is conceivable that such changes can be caused either by a variation of mean gyre flow amplitude and shear and a corresponding change in instability with resulting changes in FKE levels, or by a displacement of the core of the gyre, i.e., by a change in the larger-scale current patterns. It is also possible that both effects contribute together to the observed longterm variations. Farther west, at the ‘‘Betatriangle’’, Armi and Stommel (1983) also found evidence of gyre scale variability in hydrographic and CTD data obtained over a 2 year time period. The Azores Current is known to meander and to shed eddies (e.g., Pingree, 1997; Tychensky et al., 1998; Pingree et al., 1999). It can well be expected that the Azores Front axis approaches and withdraws from site KIEL276 at certain times. Hydrographic surveys showed the Azores Current mostly in the north of the site (Ka¨se et al., 1985; Siedler et al., 1985; Rudnick, 1996), as does the altimeter distribution in Fig. 1. We attempt to identify meanders affecting the moored time series by using a simple conceptual model (Fig. 15). If the front meanders to the right, the Azores Current will move cyclonically around cold water from the north, while it will enclose warm water

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597

KIEL276, interpolated, 7-d averages 250 m 0

450 m

Speed / (0.05 cm/s)

-1000

700 m -2000

1100 m -3000

1500 m -4000 1980

1985

1990

1995

3000 m -5000 0

1000

2000

3000 4000 5000 Time / d since 01 JAN 1980

6000

7000

Fig. 12. Seven-day averages of currents, interpolated in the vertical with the use of time-varying dynamical modes. The gap in 1994 is due to an insufficient number of records for the determination of modes during that time. The distance from 0 to 1000 at the ordinate corresponds to a velocity change from 0 to 50 cm/s. The time series for each depth level are off-set by 1000.

from the south in anti-cyclonic motion when meandering to the left. If we assume that the meanders propagate westward corresponding to baroclinic Rossby waves with equivalent time and space scales (e.g., Magaard and Mysak, 1986), the following sequences can be expected: In a cyclonic meander the current at the mooring will first have a southward component; the direction will then change counter-clockwise to a northward component and the temperature will have a minimum in between. In the anti-cyclonic meander the sequence is reversed, first a northward component, then a clockwise directional change to a current with southward component and with a temperature maximum in between. A similar pattern would occur with eddies shed by the Azores Current and traveling westward. The model

assumes that the mean eastward current velocity off the main axis of the Azores Current does not exceed the westward meander (eddy) propagation speed. We search for events with strong variable currents in the 7-day averages of near-surface velocity and temperature and such correspondences in current/temperature changes in Fig. 16. No clear indication for warm water events are found at the upper levels, all the five events marked show cold water anomalies with cyclonic motion. This result suggests that the Azores Front is usually north of the mooring site, with mesoscale signals at KIEL276 mostly caused by eddies during that time. But there appears to have been a shift in the front’s location from a more northerly position in the 1980s to a position close

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598

10 East component

450 m

u/(cm/s)

5

1100 m 0 3000 m

-5

1980 0

2

2

4

6

8

1990 10

12

16

18

20

16

18

20

North component 3000 m

0 v/(cm/s)

14

1100 m

-2 -4

450 m

-6 -8

0

2

4

6

8

10

12

14

Years since 01 JAN 1980 Fig. 13. Annual means of east (u) and north (v) velocity components near the three given depth levels.

to the site in the 1990s, with meanders to the right of the Azores Current main axis touching the mooring from time to time. While integral time scales of current and temperature variability during the 20 years of observation are typically about 20 days or less, they amount to 40–50 days during periods of clear meander signals. The question arises what might be the cause of a decadal variation in the position of the Azores Front? The most significant atmospheric variability on decadal scale in the North Atlantic is the North Atlantic Oscillation (NAO, see Hurrell et al., 2003). Sea-surface atmospheric pressure differences between the centers of the Icelandic Low and the Azores High vary on multi-year to decadal scales, best recognized during the winter months. Wind speeds and directions change correspondingly. An increasing NAO index, defined, e.g., by the difference in normalized surface

pressure anomalies between Lisbon/Portugal and Reykjavik/Iceland, means stronger meridional pressure gradients and therefore increasing westerly winds between the Icelandic Low and the Azores High. Both thermohaline circulation and wind-driven gyre transports can play a role in the interaction of ocean and atmosphere on these decadal scales, with the sequence of events controlled by the response of the oceanic heat transport to changes in atmospheric forcing (Marshall et al., 2001). On time scales shorter than decadal, the influence of the ocean back to the atmosphere is much weaker. The changes in wind stress due to the NAO will affect Ekman transports and pressure fields in the sub-surface ocean, and a number of models have shown that meridional shifts in circulation patterns can be expected (see Visbeck et al., 2003).

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599

KIEL276, 450 m, FKE 120 1980

1990

100

FKE(cm**2/s**2)

80 181-d averages, 91-d overlapping (solid line) 60 91-d cumulative (dotted line) 40

20

0

0

2

4

6

8 10 12 Years since 01 JAN 1980

14

16

18

20

Fig. 14. Fluctuating kinetic energy (FKE): half-year means (solid line) and cumulative 3-months means (dotted line).

Fig. 15. Schematic Azores Current loops assuming eastward mean current and westward propagation of meanders. Cyclonic (anti-clockwise) meanders in the south hold cold water from the north while anti-cyclonic (clockwise) meanders in the north hold warm water from the south.

Can there be a relation with the observed changes at site KIEL276? The NAO forcing may either change the transports in the subtropical gyre (resulting in a variation of the Azores Current transports) or a displacement of the northeastern boundary of the subtropical gyre (resulting in a displacement of the Azores Front), or both. Curry and McCartney (2001) demonstrated that the baroclinic mass transport, obtained from potential energy anomalies between the Labrador Sea and Bermuda (or the centers of the subpolar and subtropical gyres), has a similar temporal structure as the NAO index. Their results suggest gyre transport changes that are related to NAO atmospheric pressure changes.

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600 20 0 -20 20

376 m

1980 0

100

0 -20

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300

400

800

900

1000

1100

1500

1600

1700

Speed / (cm/s) & 10*dT

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2200

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3100

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367 m

555 m

3300

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3500

4000

4100

4200

4300

750 m 4500

4600

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5200

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1996 5900

20

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0

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1992

20 -20

2100

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3700

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2000

2600

3200

1990

20

1900

650 m

0 -20

1400

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2500

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1986

0 -20

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1800

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700

562 m

1984

20

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243 m

0 -20

500

629 m

1982

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1998 6600

6700

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7000

7100

7200

7300

0 -20 -40

2000 7400

7500

7600

7700

7800

7900

8000

Days since 01 JAN 1980, yearly grid Fig. 16. Time series of currents and temperatures in the main thermocline. Cold water with cyclonic change indicating the passage of an Azores Front meander is marked in red.

The winter (December to March) NAO index time series for the period covering our moored measurements (after Hurrell, 2004) in Fig. 17 shows similarities with the mean kinetic energy (MKE) and the fluctuating kinetic energy (FKE) time series at KIEL276: moderately high values in the early 1980s, low values during most of the second half of the 1980s, high values during the first half of the 1990s, and after a short decrease rather high values again later in the 1990s. The

pattern suggests a correspondence of NAO and KIEL276 observations, which infer a displacement of the Azores Current axis towards the mooring site with an increasingly positive NAO, corresponding to a change of the gyre pattern in the northeastern part of the subtropical gyre. Observed sea-surface temperature patterns also seem to display a displacement to the southwest in the Azores Current region (Eden and Jung, 2001, their Fig. 4), and the covariance of wind speed and the

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MKE/(cm**2/s**2)

(A)

FKE/(cm**2/s**2)

(B)

601

30 MKE

20 10 5*NAO 0 1975 -10 -5

1980

1985

0

5

1990 10

1995 15

20

100 FKE 50 0

10*NAO

-50 -5

0

5

10

15

20

TIA/(Tg/s)

(C) 10 5 0

2*NAO

-5 Transport index anomaly (TIA) relative to 59 Tg/s -10 -5

0

5

10

15

20

Years since 01 JAN 1980 Fig. 17. Three-year running means of the North Atlantic Oscillation Index (NAO) (after Hurrell, 2004; Curry and McCartney, 2001) given in all three diagrams by the dotted line with different scaling, and (A) mean kinetic energy (MKE), (B) fluctuating kinetic energy (FKE); and (C) the baroclinic transport index anomaly (TIA) (after Curry and McCartney, 2001, see text).

NAO displays an intermediate maximum along the axis of the Azores Current (Visbeck et al., 2003, their Fig. 3). Curry and McCartney (2001) used 3-year running means for their comparison of the NAO and mass transport. The 1980–2000 part of their Fig. 1 is also given in Fig. 17, and for easier comparison we have applied the same averaging. Please note that the NAO is scaled to have similar amplitudes as the comparison curves in the three subfigures. The similarities of the NAO index and MKE/FKE are certainly not less distinct than the similarities between the NAO and the transport index. We conclude that there appear to exist changes both in the baroclinic transport and in the spatial pattern of the subtropical gyre, which are

associated with the NAO. The deviations particularly in the late 1980s in all three cases in Fig. 17 suggest a delay of the oceanic response to the NAO by about three years, which agrees well with model results that suggest a lag of three to five years of the baroclinic ocean response to the atmospheric NAO signal (Eden and Jung, 2001). With the Azores Current being in the north of KIEL276 during the 1980s and closer to the mooring in the 1990s, we find that the probability of detecting meddies is high when the Azores Front is well to the north of the mooring, and low when the front is closer to the mooring. The data of Bower et al. (1995) do not suggest anything anomalous at the meddy source region for one of the periods discussed here. We therefore suspect

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that the Azores Front acts as a barrier, reducing the probability of finding meddies close to the southern boundary of the front.

5. Conclusions A total of 10 meddies were identified during the 20 years of record at site KIEL276, and in half of the cases the signals could be well related to earlier observations. Meddy occurrence was uneven in time, with six meddies during the first 4 years and only four meddies during the remaining 16 years. Meddies propagated slowly with typical speeds of 2–3 cm/s, and directions of propagation were very homogeneous, with all the 10 meddies identified propagating with a southward velocity component within a sector of 901. Long-term variations of the upper-ocean flow at the site suggest that the AC moved closer to the mooring site from the north or northeast during the 1990s, which was correlated with an increasingly positive NAO index. The oceanic response to NOA forcing has a delay of about three years. Together with the findings of Curry and McCartney (2001), our results indicate an effect of the NAO gyre flow forcing on both the magnitude and the position of the AC. The probability of detecting meddies is high when the Azores Front is well to the north of the mooring, and low when the front is closer to the mooring. Ruddick (1987) suggested that lenses could break up if the strain due to larger-scale shear is sufficiently strong. The strain on meddies in the Azores Front shear may be strong enough to block some meddies crossing the front from north to south, and only a certain number of them will reach the region to the south of the front, thus reducing the frequency of occurrence.

Acknowledgements The 20-year time series was obtained by the joint work of the staff of the earlier Department of Marine Physics of the Institut fu¨r Meereskunde Kiel, including Walter Zenk, and the captains and crews of several research vessels. We want to thank

all those people who contributed to the data set. Funding for some periods was provided as part of various projects by the Deutsche Forschungsgemeinschaft, Germany, and through the institutional funding of the Institut fu¨r Meereskunde at Kiel University by the Federal Ministry of Science and Technology and by the State of SchleswigHolstein, Germany.

References Alves, M.L.G.R., Colin de Verdie´re, A., 1999. Instability dynamics of a subtropical jet and applications to the Azores Front Current system: Eddy-driven mean flow. Journal of Physical Oceanography 29, 837–864. Arhan, M., Colin de Verdie´re, A., Memery, L., 1994. The eastern boundary of the subtropical North Atlantic. Journal of Physical Oceanography 24, 1295–1316. Armi, L., Stommel, H., 1983. Four views of a portion of the North Atlantic subtropical Gyre. Journal of Physical Oceanography 13, 828–857. Armi, L., Zenk, W., 1984. Large lenses of highly saline Mediterranean water. Journal of Physical Oceanography 14, 1560–1575. Armi, L., Hebert, D., Oakey, N., Price, J., Richardson, P., Rossby, T., Ruddick, B., 1989. Two years in the life of a Mediterranean salt lens. Journal of Physical Oceanography 19, 354–370. Bower, A.S., Armi, L., Ambar, I., 1995. Direct evidence of meddy formation off the southwestern coast of Portugal. Deep-Sea Research I 42 (9), 1621–1630. Bryden, H.L., Kinder, T.H., 1991. Steady two-layer exchange through the Strait of Gibraltar. Deep-Sea Research 38 (Supplement 1), S445–S463. Bryden, H.L., Candela, J., Kinder, T.H., 1994. Exchange through the Straits of Gibraltar. Progress in Oceanography 33, 201–248. Candela, J., 2001. Mediterranean Water and Global Circulation. In: Siedler, G., Church, J., Gould, J. (Eds.), Ocean Circulation and Climate. Academic Press, San Diego, pp. 419–429. Colin de Verdiere, A., 1992. On the southward motion of Mediterranean salt lenses. Journal of Physical Oceanography 22, 413–420. Curry, R.G., McCartney, M.S., 2001. Ocean Gyre circulation changes associated with the North Atlantic oscillation. Journal of Physical Oceanography 31, 3374–3400. Daniault, N., Maze´, J.P., Arhan, M., 1995. Circulation and mixing of the Mediterranean water west of the Iberian Peninsula. Deep-Sea Research 41, 1685–1714. Defant, A., 1956. Die Ausbreitung des Mittelmeerwassers im Nordatlantischen Ozean. Deep-Sea Research Supplement 3, 465–470.

ARTICLE IN PRESS G. Siedler et al. / Deep-Sea Research II 52 (2005) 583–604 Defant, A., 1961. Currents in a Strait. In: Physical Oceanography I. Pergamon Press, New York, pp. 513–543 (Chapter XVI). Dick, G., Siedler, G., 1985. Barotropic tides in the Northeast Atlantic inferred from moored current meter data. Deutsche Hydrographische Zeitschrift 38, 7–22. Eden, C., Jung, T., 2001. North Atlantic interdecadal variability: oceanic response to the North Atlantic oscillation (1865–1997). Journal of Climate 14, 676–691. Gill, A.E., 1981. Homogeneous intrusions in a rotating stratified fluid. Journal of Fluid Mechanics 103, 275–295. Gould, W.J., 1985. Physical oceanography of the Azores front. Progress in Oceanography 14, 167–190. Hedstrom, K., Armi, L., 1988. An experimental study of homogeneous lenses in a stratified rotating fluid. Journal of Fluid Mechanics 191, 535–556. Hernandez, F., Le Traon, P.-Y., 1995. Mapping mesoscale variability of the Azores Current using TOPEX/POSEIDON and ERS 1 altimetry, together with hydrographic and Lagrangian measurements. Journal of Geophysical Research 100 (C12), 24995–25006. Howe, M.R., 1982. The Mediterranean outflow in the Gulf of Cadiz. Oceanography and Marine Biology Annual Review, 37–64. Hurrell, J.W., 2004. http://www.cgd.ucar.edu/jhurrell/nao. stat.winter.html. Hurrell, J.W., Kushnir, Y., Ottersen, G., Visbeck, M., 2003. An overview of the North Atlantic oscillation. In: Hurrell, J.W., Kushnir, Y., Ottersen, G., Visbeck, M. (Eds.), The North Atlantic Oscillation. Climatic Significance and Environmental Impact. Geophysical Monograph 134, American Geophysical Union, Washington DC, pp. 1–35. Johnson, J., Ambar, I., Serra, N., Stevens, I., 2002. Comparative studies of the spreading of Mediterranean Water through the Gulf of Cadiz. Deep-Sea Research II 49, 4179–4193. Joyce, T.M., 1981. Influence of the mid-Atlantic Ridge upon the circulation and the properties of the Mediterranean Water southwest of the Azores. Journal of Marine Research 39, 31–52. Ka¨se, R.H., Siedler, G., 1982. Meandering of the subtropical front south-east of the Azores. Nature 300 (5889), 245–246. Ka¨se, R.H., Zenk, W., Sanford, T.B., Hiller, W., 1985. Currents, fronts and eddy fluxes in the Canary Basin. Progress in Oceanography 14, 231–257. Klein, B., Siedler, G., 1989. On the origin of the Azores Current. Journal of Geophysical Research 94, 6159–6168. Lacombe, H., Richez, C., 1982. The regime of the Strait of Gibraltar. In: Nihoul, J.C.J. (Ed.), Hydrodynamics of SemiEnclosed Seas. Elsevier, Amsterdam, pp. 13–73. Le Traon, P.Y., De Mey, P., 1994. The eddy field associated with the Azores front east of the mid-Atlantic Ridge as observed by the GEOSAT altimeter. Journal of Geophysical Research 99, 9907–9923. Madelain, F., 1970. Influence de la topographie du fond sur l’e´coulement Me´diterrane´en entre le De´troit de Gibraltar et le Cap Saint-Vincent. Cahiers Oceanographiques 22, 43–61.

603

Magaard, L., Mysak, L.A., 1986. Ocean waves: classification and basic features. In: Su¨ndermann, J. (Ed.), Ozeanographie. Landolt-Bo¨rnstein, New Series 3c, Springer, Berlin, pp. 1–16. Marshall, J., Johnson, H., Goodman, J., 2001. A study of the interaction of the North Atlantic oscillation with the ocean circulation. Journal of Climate 14, 1399–1421. Mu¨ller, T.J., Siedler, G., 1992. Multi-year current time series in the eastern North Atlantic Ocean. Journal of Marine Research 50, 63–98. Needler, G.T., Heath, R.A., 1995. Diffusion coefficients calculated from the Mediterranean salinity anomaly in the North Atlantic Ocean. Journal of Physical Oceanography 5, 173–182. Oliveira, P.B., Serra, N., Fiuza, A.F.G., Ambar, I., 2000. A study of meddies using simultaneous in-situ and satellite observations. In: Halpern, D. (Ed.), Satellites, Oceanography and Society. Elsevier, New York, pp. 125–148 (Chapter 7). Pingree, R.D., 1997. The eastern subtropical gyre (North Atlantic): flow rings recirculations structure and subduction. Journal of the Marine Biological Association of the United Kingdom 77, 573–624. Pingree, R.D., Garcia-Soto, C., Sinha, B., 1999. Position and structure of the subtropical/Azores Front region from combined Lagrangian and remote sensing (IR/altimeter/ SeaWiFS) measurements. Journal of the Marine Biological Association of the United Kingdom 79, 769–792. Reid, J.L., 1978. On the mid-depth circulation and salinity field in the North Atlantic Ocean. Journal of Geophysical Research 83, 5063–5067. Rhein, M., Hinrichsen, H.H., 1993. Modification of Mediterranean Water in the Gulf of Cadiz, studied with hydrographic, nutrient and chlorofluoromethane data. Deep-Sea Research 40, 267–291. Richardson, P.L., Mooney, K., 1975. The Mediterranean outflow—A simple advection diffusion model. Journal of Physical Oceanography 5, 476–482. Richardson, P.L., Walsh, D., Armi, L., Schroder, M., Price, J.F., 1989. Tracking three meddies with SOFAR floats. Journal of Physical Oceanography 19, 371–383. Richardson, P.L., Bower, A.S., Zenk, W., 2000. A census of Meddies tracked by floats. Progress in Oceanography 45, 209–250. Richardson, P.L., Tychensky, A., 1998. Meddy trajectories in the Canary Basin measured during the Semaphore Experiment., 1993–1995. Journal of Geophysical Research 103, 25029–25045. Ruddick, B.R., 1987. Anticyclonic lenses in large-scale strain and shear. Journal of Physical Oceanography 17, 741–749. Rudnick, D.L., 1996. Intensive surveys of the Azores Front, 2, inferring the geostrophic and vertical velocity fields. Journal of Geophysical Research 101, 16291–16303. Schott, G., 1928. Die Wasserbewegungen im Gebiete der Gibraltarstrasse. Journal du Conseil International pour l’Exploration de la mer III (2) Copenhagen.

ARTICLE IN PRESS 604

G. Siedler et al. / Deep-Sea Research II 52 (2005) 583–604

Schultz Tokos, K., Rossby, T., 1991. Kinematics and dynamics of a Mediterranean salt lens. Journal of Physical Oceanography 21, 879–892. Serra, N., Ambar, I., 2002. Eddy generation in the Mediterranean undercurrent. Deep-Sea Research II 49, 4225–4243. Siedler, G., 1968. Die Ha¨ufigkeitsverteilung von Wasserarten im Ausstrombereich von MeeresstraXen. Kieler Meeresforschungen 24, 59–65. Siedler, G., Onken, R., 1996. Eastern Recirculation. In: Krauss, W. (Ed.), The warmwatersphere of the North Atlantic Ocean. Gebru¨der Borntraeger, Berlin, Stuttgart, pp. 339–364 (Chapter 11). Siedler, G., Paul, U., 1991. Barotropic and baroclinic tidal currents in the eastern basins of the North Atlantic. Journal of Geophysical Research 96, 22259–22271. Siedler, G., Zenk, W., Emery, W.J., 1985. Strong-current events related to a subtropical front in the Northeast Atlantic. Journal of Physical Oceanography 15, 885–897. Sparrow, M., Boebel, O., Zervakiz, V., Zenk, W., CantosFiguerola, A., Gould, W.J., 2002. Two circulation regimes of the Mediterranean outflow revealed by Lagrangian measurements. Journal of Physical Oceanography 32, 1322–1330. Stammer, D., 1997. Global characteristics of ocean variability estimated from regional TOPEX/POSEIDON altimeter measurements. Journal of Physical Oceanography 27, 1743–1769. Stammer, D., Hinrichsen, H.H., Ka¨se, R.H., 1991. Can meddies be detected by satellite altimetry? Journal of Geophysical Research C4 (96), 7005–7014. Sy, A., 1988. Investigation of large-scale circulation patterns in the central North Atlantic: the North Atlantic Current, the Azores Current, and the Mediterranean Water plume in the area of the Mid-Atlantic ridge. Deep-Sea Research 35, 383–413.

Tychensky, A., Le Traon, P.-Y., Hernandez, F., Jourdan, D., 1998. Large structures and temporal change in the Azores Front during SEMAPHORE experiment. Journal of Geophysical Research 103, 25009–25027. Visbeck, M., Chassignet, E.P., Curry, R., Delworth, T., Dickson, B., Krahmann, G., 2003. The ocean’s response to North Atlantic oscillation variability. In: Hurrell, J.W., Kushnir, Y., Ottersen, G., Visbeck, M. (Eds.), The North Atlantic Oscillation. Climatic Significance and Environmental Impact. Geophysical Monograph 134, American Geophysical Union, Washington DC, pp. 113–146. Warren, B.A., 1981. Deep Circulation of the World Ocean. In: Warren, B.A., Wunsch, C.I. (Eds.), The Evolution of Physical Oceanography, Scientific Surveys in Honor of Henry Stommel. MIT Press, Cambridge, Massachussetts, pp. 6–41. Wu¨st, G., Defant, A., 1936. Atlas zur Schichtung und Zirkulation des Atlantischen Ozeans. Schnitte und Karten von Temperatur, Salzgehalt und Dichte. In:s Wissenschaftliche Ergebnisse der Deutschen Atlantischen Expedition auf dem Forschungs—und Vermessungsschiff ‘‘Meteor’’ 1925–1927, 6 Atlas, 103 plates. Zenk, W., 1970. On the temperature and salinity structure of the Mediterranean water in the Northeast Atlantic. DeepSea Research 17, 627–631. Zenk, W., 1975a. On the Mediterranean outflow west of Gibraltar. ‘‘Meteor’’ Forschungsergebnisse A 16, 23–34. Zenk, W., 1975b. On the origin of the intermediate doublemaxima in the T/S profiles from the North Atlantic. ‘‘Meteor’’ Forschungsergebnisse A 16, 35–43. Zenk, W., Armi, L., 1990. The complex spreading pattern of Mediterranean water off the Portuguese continental slope. Deep-Sea Research 37, 1805–1823. Zenk, W., Mu¨ller, T.J., 1988. Seven-year current meter record in the eastern North Atlantic. Deep-Sea Research 35, 1259–1268.