ISTITUTO NAZIONALE di OCEANOGRAFIA e di GEOFISICA SPERIMENTALE - OGS Dipartimento di Oceanologia e Geofisica Ambientale

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1 ISTITUTO NAZIONALE di OCEANOGRAFIA e di GEOFISICA SPERIMENTALE - OGS Dipartimento di Oceanologia e Geofisica Ambientale CURRENT-METER MEASUREMENTS AT KM3 AND KM4 IN THE PERIOD JULY 98 DECEMBER 99 Ursella Laura

2 INDEX Introduction...3 Current measurements at KM3...6 Current measurements at KM Temperature measurements...20 References:...22 ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 2

3 Introduction Within the NEMO.RD project collaboration (De Marzo et al., 2000), the OGS has principally investigated the oceanographic characteristics of a site in the Mediterranean Sea in order to find the optimal position for an underwater telescope for high-energy astrophysical neutrinos. The underwater telescope (a kilometre cube) should be positioned at the depth of about 3000 m, just outside Capo Passero. At this scope hydrological data together with long-term time series of Eulerian marine currents are needed. Current meter moorings were positioned close to the Maltese escarpment at the sites named KM3 and KM4 (Fig. 1). Fig.1: Position of the moorings KM3 and KM4. Capo Passero Two ANDERAA currents meters were deployed at KM3 at the depths of 3300 m and 3200 m, since July The upper current-meter stopped working just after one month, while the bottom one was maintained until December Two ANDERAA ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 3

4 0 m PROGETTO NEMO: Stazione KM4 Lat N Lon E Prof m 1000 m BOA di Snodo Correntometro 350 m cavo Trappola 2000 m 10 m cavo Correntometro 100 m cavo Sganciatore Catena Zavorra 3000 m Correntometro a 2873 m Correntometro a 3237 m 3341 m Fig. 2: KM4 mooring line. The photos show two phases of the deployment, the sediment trap and the ANDERAA current-meter. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 4

5 current meters were deployed one year later at site KM4 at the depths of 2873 m and 3237 m and maintained until December Fig 2 shows the KM4 mooring line and some of the instruments. Table 1 presents further details for the positions and depths of current meters, as well as the length of time-series. Table 1. Mooring stations, positions, nominal current-meter depths and duration of time-series for each deployment period, for which good data are obtained. Station Position Bottom depth Instrument depth Deployment period KM N E 3300 m 3190 m 27/07/98 23/08/ m 27/07/98 19/12/99 KM N E 3340 m 2873 m 10/08/99 19/12/ m 10/08/99 19/12/99 Data were quality checked, i.e. spikes and instrumental failures were removed. The data were then averaged over 1 hour. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 5

6 Current measurements at KM3 The site denominated KM3 was the first one in the Ionian to be investigated as possible site for the underwater telescope for high-energy astrophysical neutrinos. Therefore, at KM3 the biggest amount of data, with respect to other neighbouring sites, is available. Fig show stick diagrams of hourly data at KM3 during the 4 deployments. There is clear evidence that the current is mainly directed toward the north/north-west, apart from some episodes, for which the current is directed toward south. Fig. 3: Stick diagram of hourly data for bottom current-meter at KM3 during the first period of deployment. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 6

7 Fig. 4: Stick diagram of hourly data for bottom current-meter at KM3 during the second period of deployment. The first period (Aug 98 Fig. 3) is the only one for which two current-meters are present at a distance of about 100 m. The current is directed toward the north and the speeds are on the order of 10 cm/s for both current-meters. The behaviour of the currents on the bottom is barotropic, at least over a layer of 100 m. During the second period (Aug 98-Feb 99 Fig.4) the current is again directed towards the north, apart from an important reversal at the beginning of November The speeds are on the same order as the previous period, even if weaker events are present The third period (Feb-Aug 99 Fig. 5) is the one in which the currents are mainly directed towards west, apart from two events, one at the beginning of March 1999 and the other at the end of June 1999, that indicate an intense northward flow. During the last period (Aug-Dec 99 Fig. 6), the currents are prevalently directed toward the north. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 7

8 Fig. 5: Stick diagram of hourly data for bottom current-meter at KM3 during the third period of deployment. Fig. 6: Stick diagram of hourly data for bottom current-meter at KM3 during the fourth period of deployment. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 8

9 The rotary spectra has been calculated with these data (Fig.7) in order to know which are the frequency components in the signal. The peaks present in the spectra are, in order of importance, the one at low frequencies (around zero cyc/day) and the one at the inertial frequency (about 20 hours at the KM3 latitude). No tidal signal is present in the data set. The hourly data have been low-passed by a digital filter of total length of 49 h. This filter is designed in order to suppress inertial and tidal frequencies (Thompson, 1983), and in our case it suppresses only inertial frequency. Inetial freq. Fig. 7: Rotary spectra for bottom current-meter at KM3, for the three periods of deployment. Fig show the low-pass velocity vectors for the last three deployment periods at KM3. The low-pass plots are very similar to the hourly ones, being eliminated only the inertial signal (smaller that the low-frequency one) and some noise. The low-pass component of the current is also call residual current. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 9

10 Fig. 8: Stick diagram of low-pass data for bottom current-meter at KM3 during the second period of deployment. Fig. 9: Stick diagram of low-pass data for bottom current-meter at KM3 during the third period of deployment. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 10

11 Fig. 10: Stick diagram of low-pass data for bottom current-meter at KM3 during the fourth period of deployment. Within each deployment period, maximum, minimum, average and standard deviation for each residual current component has been calculated together with the vector mean amplitude and direction. These values are presented in table 2 for current-meter at KM3. Table 2: Record-length and general statistics of residual currents over the deployment periods at mooring positions KM3 Station and period Measurement Depth (m) Data Length (days) Comp. Current Minium (cm/s) Current Maximum (cm/s) Current Average (cm/s) Vector Mean (cm/s) Vector Direction (Degree N) KM E N ± ± /07/98-23/08/ E ± N ±2.83 KM E ± /08/98-11/02/99 N ±2.56 KM3 11/02/99-09/08/ E N ± ± KM3 09/08/99-19/12/ E N ± ± The speed of the vector mean is maximum during the firs period (about 5.7 cm/s) and minimum during the third one (1.92 cm/s); the direction of the vector mean stays within an ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 11

12 angle of about 30 degrees around the North, apart from the one in the third period. This is in accordance with the above observations written about hourly stick diagrams. The residual currents are visualized also by progressive vector diagrams (Fig. 11). Second period Third period Fourth period Fig. 11: Progressive Vector Diagram of current-meter data at KM3 for the last three periods of deployment. Triangles indicate the start, while squares indicate the end. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 12

13 These diagrams are constructed imagining that a water particle travels in space and time with the velocities given by current-meter data (Lagrangian representation). It permits to understand which is the general behaviour of currents in that point, and which are the speed and direction of the mean current during the considered period. Fig. 11 shows that current behaviour during the second and fourth deployment periods is similar (north-westward flow with similar intensities), while the third one is rather different, presenting a pronounced westward flow with smaller mean intensity, but with bigger variations (inversions of current). ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 13

14 Current measurements at KM4 The site denominated KM4 was studied later with respect to KM3. In fact, the project required the study of a site farther from the Maltese escarpment than KM3. Therefore only a period of measurement is available at KM4. Fig. 12 shows stick diagrams of hourly data for both current-meters at KM4. Fig. 12: Stick diagram of hourly data for both current-meter at KM4. The currents are almost barotropic over a bottom layer of about 350 m, i.e. they are almost homogeneous on the part of the water column covered by current-meters. At this site the currents are in general smaller than at KM3, but there is a bigger variation in direction, i.e. southward flow is alternated with northward one. The rotary spectra has also been calculated in this case (Fig.13). ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 14

15 Inetial freq. Fig. 13: Rotary spectra for upper (black) and bottom (red) current-meter at KM4. The frequency components are the same as for KM3. The hourly data have, therefore, been low-passed by the same digital filter of total length of 49 h (Thompson, 1983), and low-pass data have been obtained. Fig. 14: Stick diagram of low-pass data for both current-meter at KM4. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 15

16 Maximum, minimum, average and standard deviation for each residual current component has been calculated together with the vector mean amplitude and direction. These values are presented in table 3 for current-meters at KM4. Table 3: Record-length and general statistics of residual currents over the deployment periods at mooring positions KM4 Station and period Measurement Depth (m) Data Length (days) Comp. Current Minium (cm/s) Current Maximum (cm/s) Current Average (cm/s) Vector Mean (cm/s) Vector Direction (Degree N) KM E ± N 10/08/99-19/12/ E N ± ± ± The maximum and minimum values are very similar to those at KM3, but the average in time of each of the components is lower, even if there are bigger fluctuations around these value. The vector means are very small in amplitude and the related flows are directed westward. The residual currents are visualized also by progressive vector diagrams. Fig. 15: Progressive Vector Diagram of current-meter data for upper (left) and bottom (right) currentmeters at KM4. Triangles indicate the start, while squares indicate the end. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 16

17 It is clear from Fig. 15 that the flow is barotropic, with inversions in direction at the same times for the upper and bottom current-meters. However, the upper flow is more energetic than the bottom one. Finally, a comparison between KM3 and KM4 data during the same period has been done. Fig.16 and Fig. 17 show the hourly and low-pass data in the period August-December 1999, respectively. Southward pulses at KM3 correspond to southward pulses at KM4, even if the former are stronger than the latter. Fig. 16: Stick diagram of hourly data for current-meters at KM3 and KM4 during the last period of measurements. As already said, currents at KM3 are prevalently directed towards the north, north-west, while this is not the case of KM4. This is also seen in figure 18, where the vector mean and the variance elypses are depicted for both current-meters and for different measurement periods. This is probably explained by the fact that current meters at KM3 are located just outside the Maltese escarpment and the bathymetric constraint plays an important role in the dynamics. On the contrary KM4 is located far enough from the escarpment not to be influenced in such a way. Moreover, the expected currents in the bottom layer at this latitude should be directed towards ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 17

18 the south, due to the presence of Adriatic Deep Waters exiting the Otranto Strait. This unexpected behaviour is due to the recent changes happened in the Eastern Mediterranean in the last 10 years (Manca B. et al., 2002), for which the deep layer is now occupied by more dense water (saltier and warmer with respect to the Adriatic one) coming from the Aegean Sea. The Adriatic dense water is confined to the depth of 1500 m. Fig. 17: Stick diagram of low-pass data for current-meters at KM3 and KM4 during the last period of measurements. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 18

19 Fig. 18: Vector mean and variance elypses calculated for current-meters at KM3 and KM4 during the last period of measurements. The probability content of the elypses, is 68%, that corresponds to one standard deviation in one-dimensional case. KM3 KM3 F2 KM3 F3 KM3 F4 KM4 ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 19

20 Temperature measurements Current-meters are also equipped with temperature sensors. The temperature series are available for the same periods in which currents are present. Fig. 19 shows temperature in the bottom layer at KM3, while Fig.20 shows temperature for both current-meters at KM4. Fig. 19: Temperature in the bottom layer at KM3 for the entire period of study. Raw data are depicted in red, while a moving average of length 15 days is superimposed in black. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 20

21 a) b) Fig. 20: Temperature at the depth of 2873 m (a) and of 3237 m (b) for KM4. Raw data are depicted in red, while a moving average of length 15 days is superimposed in black. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 21

22 References: DE MARZO, C., for the NEMO.RD Collaboration, Feasibility studies for a Mediterranean neutrino observatory The NEMO.RD Project. Proceedings of TAUP conference Nuclear Physics B, P.S. 87: 433. THOMPSON, R.O.R.Y., 1983: Low-pass filters to suppress inertial and tidal frequencies. J. Phys. Oceanogr., 13: MANCA, B. B., URSELLA L. and SCARAZZATO P., 2002: New development of Eastern Mediterranean circulation based on hydrological observations and current measurements. Accepted for publication on a special issue of PSZN Marine Ecology. ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE O.G.S. PAGINA 22