Structure and variability of the abyssal water masses in the Ionian Sea in the period

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, , doi: /2012jc008178, 2013 Structure and variability of the abyssal water masses in the Ionian Sea in the period Manuel Bensi, 1 Angelo Rubino, 2 Vanessa Cardin, 1 Dagmar Hainbucher, 3 and Isaac Mancero-Mosquera 1 Received 3 May 2012; revised 4 December 2012; accepted 7 December 2012; published 26 February [1] This study presents aspects of the spatial and temporal variability of abyssal water masses in the Ionian Sea, as derived from recent temperature, salinity, dissolved oxygen and velocity observations and from comparisons between these and former observations. Previous studies showed how in the Southern Adriatic Sea the Adriatic Deep Water (AdDW) became fresher (ΔS 0.08) and colder (ΔT 0.1 C) after experiencing warming and salinification between 2003 and Our data, collected from October 2009 to July 2010 from two bottom moorings, one within the Strait of Otranto and the other in the northern Ionian, confirm this tendency: a bottom vein of southward-flowing AdDW, whose temperature and salinity continuously decreased during the observation time, was detected there. Typically, the vein travel time between the two stations ranged between 45 and 50 days. This gave us a temporal estimate for AdDW anomaly propagation towards the Ionian abyss from their Adriatic generation region. The density excess of the observed vein was always enough to enable its existence as a bottom-arrested current. This evidence confirms that, at that time (2009 and 2010), the Adriatic Sea was greatly contributing to the formation of Eastern Mediterranean Deep Water (EMDW), the bottom water of the Eastern Mediterranean. Hence, based on these results and on the evidence that, from 2003 to 2009, abyssal Ionian waters became saltier and warmer under the time-lagged influence of AdDW, possible future changes in the EMDW characteristics, as a response to Adriatic variability, are discussed. Citation: Bensi, M., A. Rubino, V. Cardin, D. Hainbucher, and I. Mancero-Mosquera (2013), Structure and variability of the abyssal water masses in the Ionian Sea in the period , J. Geophys. Res. Oceans, 118, , doi: /2012jc Introduction [2] The Mediterranean Sea can be considered a sort of miniature global ocean or an ocean laboratory [Bethoux et al., 1999; Malanotte-Rizzoli and Eremeev, 1999; Robinson et al., 2001]. Indeed, all major forcings of the global thermohaline circulation are present in this basin, which also helps to explain the large variability observed here [Pinardi and Masetti, 2000]. [3] During the late 1980s abrupt major changes were observed in the structure of the thermohaline circulation of the eastern Mediterranean Sea [Roether et al., 1996], and these initiated the development of a rapidly evolving 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/c, Sgonico, Trieste, Italy. 2 Dipartimento di Scienze Ambientali, Informatica e Statistica, Universita Ca Foscari di Venezia, Calle Larga Santa Marta, Dorsoduro 2137, I Venezia, Italy. 3 Institut fur Meereskunde Universitat Hamburg, Bundesstraße 53, D Hamburg, Germany. Corresponding author: M. Bensi, Dipartimento di Oceanografia, Istituto Nazionale di Oceanografia edigeofisica Sperimentale, Borgo Grotta Gigante 42/c, 34010, Sgonico (Trieste), Italy. (mbensi@ogs.trieste.it) 2012 American Geophysical Union. All Rights Reserved /13/2012JC transient state [Roether et al., 2007]. The possible origins, implications for the global and local climate, and fate of this phenomenon are not yet fully understood [Roether et al., 2007]. [4] Dense waters are formed in the Eastern Mediterranean and accumulate in the abyssal plain of the Ionian Sea (see Figure 1) [Wüst, 1961; Hopkins, 1978, 1985; Schlitzer et al., 1991; Roether et al., 1996]. During the last 20 years, various investigations of the hydrography in the Ionian Sea, which led to a better understanding of the mechanisms contributing to deep water ventilation in the area, took place [Roether and Schlitzer, 1991; Roether et al., 1996; Theocharis et al., 1999; Theocharis et al., 2002; Manca et al., 2003; Roether et al., 2007; Rubino and Hainbucher, 2007]. [5] In particular, details concerning the near-bottom route by which newly formed dense waters produced in the Southern Adriatic pass into the Ionian have recently been discussed [see, e.g., Hainbucher et al., 2006; Rubino and Hainbucher, 2007; Budillon et al., 2010; Rubino et al., 2012b; Ursella et al., 2011]. They add complexity to the results of previous investigations [see, e.g., Zore-Armanda 1969; Malanotte-Rizzoli and Hecht 1988; Kovacević et al., 1999] devoted to studying the oceanography in the approaches to the Strait of Otranto, an ~75 km wide and 931

2 ~800 m deep channel which connects the Adriatic Sea to the Ionian Sea (Figure 1). Here, an inflow of water of Ionian origin is commonly encountered on its eastern flank while an outflow of water of Adriatic origin occurs on its western flank. The main contributors to this outflow are dense waters formed on the Northern Adriatic shelf (NAdDW) and in the Southern Adriatic basin (AdDW). On average, NAdDW is a minor contributor to the Adriatic outflow [see e.g. Cardin et al., 2011]; nevertheless, it may episodically exert a profound influence on its characteristics [Vilibić and Šantić, 2008]. [6] Near the bottom in the Strait of Otranto, a part of the Adriatic outflow is known to move in almost geostrophic balance along the isobaths of the Italian shelf at m depth [Budillon et al., 2010], with a potential density typically centred between kg m 3 and 29.3 kg m 3. A second branch follows an almost meridional path, mostly determined by the local bathymetry [Hainbucher et al., 2006]. Vigorous mixing with ambient waters substantially slows both currents and also helps to differentiate the resulting characteristics of the two water masses [Hainbucher et al., 2006]. Hence, due to the small velocities characterizing the bottom-arrested currents exporting Adriatic waters [Wu and Haines, 1996, Hainbucher et al. 2006; Rubino and Hainbucher, 2007], changes in bottom waters leaving the Adriatic basin can manifest themselves only some years later in the Ionian abyss. [7] In a typical steady-state picture of the thermohaline cell of the Eastern Mediterranean, the bottom layer of the Ionian Sea is filled with waters of Adriatic origin [see, e.g., Nielsen, 1912; Wüst, 1961; Hopkins, 1978, 1985; Schlitzer et al., 1991]. However, the Aegean basin produces dense waters too (Cretan Sea Outflow Water, see Klein et al., 1999). Even though warmer than AdDW, Aegean dense waters are able to reach densities very close to those of the Adriatic outflow because they are saltier [Roether et al., 1996; Klein et al., 1999; Lascaratos et al., 1999], and a sort of competition exists between these two water masses to occupy the Eastern Mediterranean abyss [Roether et al., 2007]. Indeed, probably after 1989, the Aegean Sea started to produce a massive volume of water dense enough to replace Adriatic waters in the bottom layer of the Ionian Sea [see Roether et al., 1996], and only after 2003 did the Adriatic Sea resume being the major dense water supplier for the Eastern Mediterranean [Manca et al., 2006; Hainbucher et al., 2006; Roether et al., 2007; Rubino and Hainbucher, 2007]. [8] Recent studies [see Pinardi and Masetti 2000; Demirov and Pinardi, 2002; Gacić et al., 2010, 2011, and references cited therein] indicate that an almost decadal inversion of the general upper circulation in the northern Ionian Sea would favour changes in the thermohaline properties of the Adriatic and Aegean Seas: while a cyclonic circulation favours the ingression of salty water of Levantine origin into the Adriatic Sea, an anticyclonic circulation favours the ingression of fresher Atlantic water into the Adriatic Sea [Gacić et al., 2010]. Also, depending on the sign of such circulation, Adriatic winter convection produces saltier and warmer or fresher and colder dense waters that can spread towards the deep layers of the Ionian Sea, contributing to EMDW formation. Indeed, during the period (i.e. during the cyclonic circulation phase) a noticeable salinity and temperature increase occurred between 200 m and 1000 m depth in the Southern Adriatic [Gacić et al., 2010; Cardin et al., 2011], and this seems to have contributed to making the AdDW saltier Figure 1. CTD positions during cruises MSM13-2 (a) and MSM15-4 (c). Boxes in Figure 1b indicate the whole study area and the approaches to the Strait of Otranto. The boxes in Figure 1c refer to regions for which historical data have also been used. In Figures 1a and 1d the positions of the moorings (A, B, C, D) deployed in October 2009 are also indicated (mooring B was lost). The detailed bathymetry of this region is depicted in Figure 1d EMR = Eastern Mediterranean Ridge; WMED/EMED = Western/Eastern Mediterranean. 932

3 and warmer than ever before. Consistently, a tendency towards more saline and warm waters occurred, presumably between 2004 and 2006 [Rubino and Hainbucher, 2007], in the abyssal plain of the Ionian Sea. Afterward, under the effect of the opposite phase of the northern Ionian surface circulation (i.e. anticyclonic), the Southern Adriatic again experienced a large salinity and temperature decrease in the intermediate and deep layers down to 1000 m, starting from the middle of 2008 [Cardin et al., 2011]. [9] In this paper we delineate aspects of the spatial distribution and structure of the Ionian abyssal waters, and reconstruct some of the recent hydrological changes occurring in the Ionian abyssal plain. To this purpose, we used recent (i.e. from 2009 to 2010) temperature, salinity, dissolved oxygen and current observations, and compared them with previous observations from Our analyses, together with the knowledge acquired over recent decades in the functioning of the Eastern Mediterranean circulation, allow us to delineate possible future evolutions of the deep water mass properties in the Eastern Mediterranean Sea. Hence, we hypothesize that variations in the major features of the thermohaline circulation of the Eastern Mediterranean can be, at least partly, monitored by observing characteristics of the bottom layers of the Ionian basin. Moreover, an accurate observation of the variability of the water masses responsible for the formation of EMDW can possibly be used to predict aspects of the variability of the Eastern Mediterranean abyssal water structure. [10] The paper is organized as follows: in section 2, we describe the dataset and methods. Aspects of the temporal variability and characteristics of the Adriatic outflow are presented in section 3. In section 4, an analysis of data collected in three different regions of the Ionian Sea is described and, in sections 5 and 6, the obtained results are discussed and conclusions are presented. 2. Data Set and Methods 2.1. Hydrographic Observations [11] Hydrographic data from conductivity, temperature and depth (CTD) casts carried out in the Ionian Sea between 2003 and 2010 were collected as part of collaborative investigations of the Eastern Mediterranean. Our analyses are centred on the data sampled between 2009 and 2010 during two cruises onboard the R/V Maria S. Merian: the first one, carried out in October 2009 (MSM13/2), covered the whole study area while the second, performed in July 2010 (MSM15/4), repeated some of the previous stations (Figure 1a, c). The analysis of such data provides information on the ongoing oceanographic situation in the entire area. The set of hydrographic data collected from 2003 to 2008 was restricted to the central Ionian (Zone A) and to the area of the Strait of Otranto (see Fig 1c). Five cruises, Poseidon 298/1-2 (May 2003) [Hainbucher et al., 2006], VECTOR-AM1 (November 2006) [Ursella et al., 2011], Meteor-71/3 (January 2007) [Rubino and Hainbucher, 2007] and SESAME IT-2 / IT-6 (March and September 2008), were analysed to outline the temporal evolution of the abyssal water mass properties of the Ionian Sea. Further information on cruise data can be found in the above-mentioned references Mooring Measurements [12] At four deep moorings deployed during the MSM13/2 cruise (Figure 1a), measurements were performed with the purpose of monitoring the local temperature, salinity and currents with a high temporal resolution within an ~100 m thick near-bottom layer. A complete scheme of each mooring line (position, bottom depth, type of instrument and depth of deployment, record length and missing data information) is presented in Table 1. Table 1. Mooring Scheme and Measurement Periods of Moorings A, C, and D Mooring Information Type of Instrument Depth (m) Record Length (hourly data) Missing data Mooring A October 14, 2009 July 9, 2010 CT SBE Depth: 852 m RCM Latitude: N T Longitude: E T T T T Mooring C October 14, 2009 July 9, 2010 CT SBE Depth: 1236 m RCM Latitude: N T Longitude: E T T T T CT SBE RCM Mooring D October 12, 2009 January 21, 2010 CT SBE Depth: 1426 m RCM Latitude: N T Longitude: E T T T T RCM

4 [13] Each mooring was equipped with RBR-TR1050 thermistors, CT (Conductivity and Temperature) SBE37 probes and current meters. Mooring A, located in the area of the Strait of Otranto at a depth of 852 m, was equipped with an RCM7 current meter positioned ~60 meters above the ocean floor. Moorings B, C and D were deployed along 39 N at depths of 2496 m, 1236 m and 1426 m respectively, (Figure 1a, d). The first two were equipped with two RCM7 current meters at the top and at the bottom of the mooring line. A similar configuration was used in mooring D but RCM8 current meters were used. In all cases, the sampling interval was set to 10 minutes for the CT and the thermistors and to 60 minutes for the current meters. The overall accuracies were within C for temperature and for salinity. The nominal speed and direction accuracies for the current meters RCM7 and RCM8 are 1 cm s 1,with5 for speeds from 5 to 100 cm s 1,and 7.5 for speeds outside this range. Moorings A and C worked properly during the whole period, which spanned 14 October 2009 to 9 July Mooring B was lost and mooring D drifted away a few months after its deployment (data are available from 12 October 2009 to 21 January 2010; see Table 1) Methods [14] Temperature and conductivity data from oceanographic cruises were obtained by means of a CTD rosette with the bottom-deep casts within ~10 12 m of the seabed. Potential temperature (θ), salinity (S) and potential density (s θ )were calculated from each original in-situ data set. Dissolved oxygen concentration (DO) was also measured using a Seabird sensor SBE 43 mounted on the CTD rosette and it was determined in parallel on samples taken at discrete depths using the Winkler method [Carpenter, 1965]. Data were calibrated and averaged every 1 dbar with overall accuracies within C for temperature, for salinity and 2% of saturation for DO. As far as dissolved oxygen concentrations are concerned, the downcast profiles were corrected with Winkler data applying a regression analysis using a second-order polynomial [Bensi and Kückler, 2009; Cardin et al., 2011]. [15] All time series obtained from mooring measurements were hourly-averaged because of the different sampling settings among temperature, salinity and currents. These averages were subsequently improved by removing outliers. Harmonic analysis [Pawlowicz et al., 2002] was then applied to all the current time series and a low-pass filter with a cutoff period at 33 hours [Flagg et al., 1976] was used to remove the influence of inertial oscillations and tides. [16] To assess the vertical structure of the temperature field, Empirical Orthogonal Function (EOF) analysis [Preisendorfer, 1988; Kundu et al., 1975] was applied to the filtered temperature time-series of all thermistors and CT deployed in each mooring. Principal Component Analysis (PCA) [Preisendorfer, 1988] was applied to all the filtered current time-series in order to study the horizontal distribution of current velocities, to assess their dispersion and to find prevalent directions in the water flow. 3. Temporal Evolution of the Adriatic Outflow Through the Strait of Otranto [17] CTD casts collected from 2003 to 2010 in the western part of the Strait of Otranto, north of 40 N, reveal a noticeable variability in the characteristics of the local bottom waters. We have to note that in this part of the basin the local seasonal variability can be large, because the area is subject to intense mixing and spatial thermohaline variability due to the presence of strong vertical shear and mesoscale activity [Kovacević et al., 1999; Ursella et al., 2011]. Moreover, the influence of the NAdDW also varies seasonally: it is higher during winter and spring and lower during summer and autumn [Vilibić and Šantić, 2008]. Hence, observed variations cannot be ascribed unambiguously to variations in the producing mechanisms of the AdDW. With such caveats, the Adriatic outflow was warmest in It got progressively colder (max ΔT 0.3 C) until and warmer again from Salinity, in contrast, increased until 2007 and then reversed (max ΔS 0.04) to values similar to those observed in 2003 (Table 2). A consequence of this evidence is that, very likely, during 2003 the AdDW was not dense enough to reach the Ionian abyssal plain (Table 2). [18] This variability is in agreement with the average trend of temperature and salinity observed in the Southern Adriatic in the layer between 200 m and 1000 m depth, which is mostly the layer producing AdDW. In particular, a noticeable decline in the LIW signature was detectable (Δθ 0.1 C, ΔS 0.08) from 2007 onwards [Cardin et al., 2011], coincident with the reversal of the northern Ionian upper-layer circulation which occurred between 2006 and 2007 [Gacić et al., 2010; Cardin et al., 2011]. Due to the quoted variability, we used additional Eulerian measurements (temperature, salinity and currents), collected over the Otranto sill and in the northern Ionian at Moorings A, C, and D (see Figure 1a, d) between October 2009 and July 2010, in order to render our statistics more robust. [19] Data from mooring A (mooring positions are depicted in Figure 1a, d) capture a decrease in the salinity from to 38.70, especially evident after January 2010 (Figure 2a). Temperature also decreased from Cto13.40 C during this period, but with a less pronounced trend (Figure 2a). A similar but less noticeable trend was observed in temperature Table 2. Thermohaline Properties of the Core of Adriatic Waters Outflowing the Strait of Otranto Between 2003 and 2010 a Year/month Cruise θ ( C) Salinity s θ (kg m -3 ) DO (ml l -1 ) 2003/May POSEIDON / / /29.18 n/a 2006/Nov VECTOR-AM /Jan M 71/ /Oct SESAME-IT / / /Oct MSM / /July MSM / / /4.75 a Data are retrieved from punctual CTD casts. Ranges are given when a large variability between adjacent stations is encountered. 934

5 Figure 2. Time series of potential temperature (θ), salinity (S) and bottom currents recorded by the CT SBE37 and the current meter Aanderaa RCM7 on mooring A (left) in the Strait of Otranto and on mooring C (right) in the Ionian Sea between October 2009 and July The data shown here have been filtered with a 33-h lowpass Hamming filter. and salinity data acquired at mooring C (125 km distant from mooring A; see Figure 1d), starting from the middle of March 2010 (Figure 2b). Current measurements in the Strait of Otranto (mooring A) and in the northern Ionian Sea (mooring C) revealed average values of ~6.6 cm s 1 and 1.8 cm s 1, respectively. Topographic constraints, together with mixing with adjacent water masses [Jungclaus and Backhaus, 1994; Rubino et al., 2003], are probably responsible for the observed bottom velocity differences. Using the parameterization for turbulent mixing presented by Jungclaus and Backhaus [1994], we can give an estimate for the slowdown of the AdDW along its path towards the abyssal plain of the Ionian Sea. According to our assessment, the propagation time of the AdDW from the Adriatic Sea to zone A of the Ionian Sea is larger than years. [20] While EOF analysis reveals substantial homogeneity for the waters of Adriatic origin observed at moorings A and C (Figure 3a, b), with the first mode by far the strongest one (92 93% of the total energy), the bottom water at mooring D (northeastern Ionian Sea) shows a departure from the homogeneity attributed to the first mode (83% of the total energy). The shape of the corresponding first-mode EOF (EOF1) profile (Figure 3c) shows that the strongest contribution to the whole EOF1 comes from the deepest temperature timeseries. Indeed, near-bottom temperature at mooring D did not always show coherent variability along the vertical Figure 3. The two strongest modes from the EOF decomposition of the temperature profile in the moorings A, C and D. Notice how the first mode, EOF1, has a prevalent barotropic character at mooring A and C while the variance distribution along depth at mooring D differs more from moorings A and C. The sum of EOF1 and EOF2 (99.2% in A, 99.1% in C and 97.3% in D) demonstrates that the most of the variance is contained in the first two modes. 935

6 Figure 4. (a) Stick diagram of horizontal currents at mooring D (top RCM and bottom RCM) and (b) time series of insitu temperature obtained from the thermistors. (see e.g. the event which occurred at the end of December 2009; Figure 4). Hence, the Adriatic signal seems to be confined to a thin layer (~20 30m) close to the bottom. We observed (Figure 4) strong pulses of waters significantly colder than the average (maximum measured anomaly ΔT 0.16 C, associated with the arrival of Adriatic outflow), intermittently extending to all thermistors along the mooring. Currents at moorings C and D fundamentally differed in their vertical structure: at C (not shown) the velocity decreased with depth while at D it increased with depth (Figure 4a). [21] Cross-correlation between the first EOF modes in A and C for all six thermistors of each mooring shows the existence of a positive average time-lag throughout the observation period (Figure 5a), demonstrating the propagation of the AdDW signal from the Strait of Otranto towards the northern Ionian. It peaks in a broad interval, extending from 1010 to 1440 hours, where the cross-correlation coefficient is > 0.3 (confidence interval ) and it reaches its maximum peak (0.4117) at ~1240 hours (~ 51 days). When computed between the nearbottom temperature series alone, the maximum peak goes slightly over 0.5 (not shown). So, in general, low values of the cross-correlation coefficient are obtained: this is due to the non-stationarity of the events, as the cross-correlation coefficients are obtained by averaging the whole series. When restricted to specific events, the local correlation reaches values as high as 0.8. The obtained time-lag is statistically significant, particularly during the episode of strong southward currents observed at A at the end of March In this case, the major cooling event, first apparent at mooring A, reached mooring C with a delay of ~45 days (Figure 2; Figure 5b). [22] At mooring A, the strongest and temporally longest episode of Adriatic outflow lasted ~2 weeks, from the end of March to 15 April 2010 (Figure 2a; Figure 6a). Peak velocities reached ~24 cm s 1 and a comparison between PCA and EOF time series reveals that strongest southward (northward) pulses were linked to the largest negative (positive) temperature variations (Figure 6a, b). Note that the average bottom flow was mainly directed along the principal axis in the Strait of Otranto during the whole period (Table 3), but some quick reversal phases were also observed (Figure 2a). They are probably associated with the passage of internal eddies, as recently reported by Ursella et al. [2011]. Furthermore, Rubino et al. [2012a], based on an analysis of Eulerian measurements south of the Sicily escarpment, do not exclude the possibility that vortices formed in the Strait of Otranto could be detectable in the abyssal plain of the Ionian Sea. [23] The PCA (Table 3) shows that most of the variability in the water flow at the Strait of Otranto (mooring A) occurred along an inclination of 138 azimuth, thus being mostly influenced by the local features of the bottom topography. Likewise, the main variability at mooring C (D) was oriented around ( )azimuth.thefirst component (PC1) captures this main variability with >70% of total energy in most of the cases, while the second component (PC2) contains the remaining. The PC1 at mooring C (Figure 6c, d) documents an episode characterized by large velocity amplitudes (velocities up to cm s 1 ) in the bottom layer at the beginning of May As mentioned above, this fact might indicate the arrival of an energetic pulse of Adriatic water. Indeed, similarly to what was observed at mooring A, this current pulse at mooring C was associated with a sudden decrease in temperature (Δθ 0.07 C) and salinity (ΔS 0.005) (see Figure 2b), which produced a temporary density increase of ~0.012 kg m 3 at the bottom. Figure 5. The cross-correlation between EOF1 signals from moorings A and C (upper panel) shows that the strongest signal of the Adriatic bottom waters at mooring A is leading by ~50 days as indicated by the maximum value. Lower panel: the actual EOF1 series showing the temperature variability for moorings A (left-hand axes) and C (right-hand axes). 4. Deep Ionian Waters Analysis [24] The spatial distribution of the abyssal water masses in the Ionian Sea during October 2009 and July 2010 was analysed using CTD casts collected in the central Ionian, over the Eastern Mediterranean Ridge and along the Greek slope (see Figure 1). The results revealed no substantial differences between the properties of the abyssal waters detected in the first and in the second cruise. Therefore we focus our analysis 936

7 Figure 6. Comparison between current velocity (PC1) and temperature (EOF1) signals at (a) mooring A and (c) mooring D. The arrow in Figure 6a indicates the strongest current event observed in the Strait of Otranto in April Figure 6b and 6d (referring to moorings A and C, respectively) show the corresponding normalized series with a 1 standard deviation threshold applied. Table 3. Basic Statistics of the Current Velocity Data Sets in the Original Geographic Coordinates as Well as in Principal Component Decomposition, the Latter Including the Orientation of the Principal Angle and the Explained Variance of Each Component Geographical coordinates Principal Components Mean St.deviation Mean St.dev. Orientation Explained variance (cm s 1 ) (cm s 1 ) (cm s 1 ) (cm s 1 ) ( Azymuth) (%) Mooring A East PC RCM North PC Mooring C East PC top RCM North PC Mooring C East PC bottom RCM North PC Mooring D East PC top RCM North PC Mooring D East PC bottom RCM North PC mainly on the data coming from the MSM13/2 cruise (October 2009), which offers a more detailed spatial distribution Central Ionian Sea [25] The distribution of DO, temperature, salinity and density observed in the central Ionian abyss during MSM13/2 (Figure 7) highlights the presence of ventilated bottom waters of Adriatic origin. In the northern part of the section (station 851), where the maximum depth is ~2500 m, the AdDW was characterized by θ C, S 38.73, DO 4.55 ml l 1 and s θ kg m 3, while in the southern part of the section, where the maximum depth is ~3500 m (station 874), it was characterized by θ C, S 38.73, DO 4.55 ml l 1 and s θ kg m 3. [26] The fact that maximum DO values were observed near the bottom at stations 851 and 874 clearly demonstrates that AdDW reached that region. However, AdDW properties observed here were different from those observed in the Strait of Otranto because, during their descent towards the Ionian abyss, intense mixing affecting the branches of AdDW led to progressive warming and salinification as well as to a reduction in the DO [Manca et al., 2003; Hainbucher 937

8 Figure 7. Vertical distribution of (a) θ, (b) DO, (c) S, and (d) s θ along the cross NW-SE section in the central Ionian Sea. Data are from the cruise MSM13-2 performed in October The position of the CTD stations is indicated at the top x-axis. et al., 2006]. Moreover, one has to consider that different pathways yield different mixing with the ambient waters: as a result the bottom temperatures were slightly different at the two stations. [27] The abyssal part of the Ionian Sea (zone A of Figure 1) was characterized by the presence of a layer of relatively cold and fresh water (θ C, S 38.72, s θ <29.20 kg m 3 and DO 4.4 ml l 1, which corresponds to 76% of the saturation value, hereafter named old EMDW ), located at a depth of ~3000 m (Figure 8). This water overlaid a bottom layer (at depths ~4000 m) dominated by slightly warmer and saltier waters (θ C, S , s θ >29.20 kg m 3 and DO ~ 4.6 ml l 1, which corresponds to 82% of the saturation value, hereafter named new EMDW ). The observed DO values show that the bottom layer was filled by more recent water. Our data are consistent with recent observations taken in the northwestern part of the Ionian Sea [Sparnocchia et al., 2011]. In accordance with Rubino and Hainbucher [2007], we excluded the possibility that the observed bottom water could have originated in the Aegean Sea. [28] Further evidence of the presence of waters of Adriatic origin in the Ionian Sea can be seen in Figure 9, where the bottom distributions of θ, S and DO in October 2009 are delineated. This figure (Figure 9d) also shows the bifurcation that the AdDW undergoes south of the Strait of Otranto, mainly due to the bottom topography constraint [Hainbucher et al., 2006]. [29] Table 4 summarizes the temporal variability of the Ionian abyssal water mass between 2003 and 2010 obtained by the analysis of data collected during six oceanographic cruises. The large discontinuity between 2003 and 2007 derives from the fact that, during this period, new EMDW filled the Ionian abyssal plain under the influence of AdDW which was saltier and warmer than before [Rubino and Hainbucher, 2007] Eastern Ionian Sea [30] To characterize the deep circulation in the transition zone between the Ionian and the Aegean Seas we analysed the distribution of the observed thermohaline properties along the northern part of the Eastern Mediterranean Ridge (right side of each panel in Figure 10), which is the principal entrance of modified Aegean waters to the Ionian Sea [Roether et al., 2007]. To this purpose, Cretan Deep Water (CDW; see Figure 10) was defined as characterized by θ >13.50 CandS > Our data show that CDW occupies the layer between 1000 m and 2000 m depth, and its properties are in the following ranges: θ C, S , s θ kg m 3 and DO ml l 1. Beneath this water mass, a different one characterised by θ C, S 38.74, s θ kg m 3 and DO 4.4 ml l 1, probably resulting from mixing between EMDW and CDW, was found. At a depth between 2500 m and 3000 m (stations from 883 to 886), along the southern part of the Eastern Mediterranean Ridge (left side of each panel in Figure 10), the usual route for EMDW flowing towards the Levantine Basin [Roether et al. 2007], the data reveal the presence of a core of water characterized by θ C, S 38.73, s θ kg m 3 and DO ml l 1. This water, due to its thermohaline properties, and particularly to 938

9 Figure 8. Vertical distribution of (a) θ, (b) DO, (c) S, and (d) s θ in the central abyssal plain of the Ionian Sea obtained from the MSM13/2 cruise. CTD station numbers are indicated at the top. its relatively high values of DO, is probably a branch of EMDW of Adriatic origin which reached the eastern part of the Ionian Sea on its way to the Levantine basin but was not dense enough to occupy the deepest part of this region Greek Slope Section [31] A hydrographic section along the Greek slope (Figure 11) shows that the CDW, in October 2009, propagated into the northern Ionian following an along-slope almost geostrophic circulation on its way toward the Adriatic basin, maintaining its position in the layer between 1000 m and 2000 m. In the southern part of this section (i.e., at the right side of each panel in Figure 11), embedded in this circulation, Cretan Intermediate Water is also evident between 100 m and 300 m depth. This water mass is characterised by θ 14.9 C, S , s θ kg m 3 and DO ml l 1.In the northern part of this section, in contrast, the Cretan Intermediate Water is mixed with the underlying LIW (present down to 600 m depth), giving rise to a less oxygenated, less saline and colder water mass, which overflows the Otranto sill and enters the Adriatic Sea. Note also the presence of a less oxygenated layer (DO < 4.2 ml l 1 ) centred at ~1000 m depth. [32] Around 1500 m depth, attached to the slope south of the Strait of Otranto (stations 856 and 869 in Figure 11), CTD data reveal the presence of a thin layer characterised by θ < 13.5 C, S < 38.74, s θ kg m 3 and DO ml l 1.It is Adriatic water sinking towards the abyssal layers of the Ionian Sea, as it has properties similar to those observed in the bottom layer of the Strait of Otranto (station 860; not shown). Also, nine months later, in July 2010, Adriatic waters were detectable in the same area (station 655; see Figure 1c). Very likely, they were produced in the Southern Adriatic during winter These waters, however, were colder (Δθ = 0.1 C), fresher (ΔS= 0.01), heavier (Δs θ = kg m 3 ) and more ventilated (ΔDO = +0.1 ml l 1 ) than in October Discussion [33] The analyses described in this paper can be useful to unveil aspects of the inter-annual variability in the Ionian abyssal plain and in the Strait of Otranto. Between 2003 and 2010, two large discontinuities in the characteristics of Adriatic deep waters occurred. After 2003, AdDW became warmer and saltier [Rubino and Hainbucher, 2007], but it again became fresher and colder after 2007 [Cardin et al., 2011]. Such variability seems consistent with a concomitant reversal of the Ionian near-surface circulation, which has been shown to be able to produce thermohaline changes in the Adriatic Sea [Gacić et al., 2010; Cardin et al., 2011]. In accordance with this almost decadal mode of variability, phases characterized by cyclonic sub-basin circulation alternate with anticyclonic ones. While a cyclonic circulation favours the entrance of waters of Levantine origin into the Adriatic, with a consequent local increase in heat and salt contents, an anticyclonic circulation favours the entrance of waters of Atlantic origin into the Adriatic Sea, with a consequent reduction in the local heat and salt contents. As a matter of fact, a switch from a cyclonic to an anticyclonic surface circulation took place around 2006 in the Ionian 939

10 Figure 9. 3-D view of the horizontal distribution of (a) potential temperature ( C), (b) salinity and (c) oxygen saturation (%) at the bottom in October Panel d shows a schematic representation of the general bottom cyclonic circulation in the Ionian Sea, deduced as described in the text, together with the positions of the CTD stations (white dots in Figures 9a 9c and red dots in Figure 9d). Table 4. Average Thermohaline Properties in the Abyssal Stations (Depths >4000 m) of the Ionian Sea (Zone A of Figure 1) Year/month Cruise Stations θ ( C) Salinity s θ (kg m 3 ) DO (ml l 1 ) 2003/May POS /January M n/a 2008/March S-IT /September S-IT /October MSM /July MSM Sea [Gacić et al., 2010] and, consequently, the Southern Adriatic experienced an abrupt temperature and salinity decrease down to 1000 m depth [Gacić et al., 2010; Cardin et al., 2011]. [34] Our data show unambiguously that in October 2009, as well as in July 2010, the Ionian abyssal plain (zone A of Figure 1) was occupied by two types of EMDW of Adriatic origin: a relatively cold and fresh water (θ C, S 38.72, s θ < 29.20kgm 3 and DO 4.4 ml l 1 ), located at a depth of ~3000 m, and a slightly warmer, saltier and more oxygenated water (θ C, S , s θ > 29.20kgm 3 and DO 4.6 ml l 1 ; see Figure 8) at the very bottom (depth ~ 4000 m). The difference in the oxygen values between these abyssal waters prompts their classification as old and new EMDW. Importantly, their different thermohaline properties can be ascribed to the variability of the AdDW production in previous years. Indeed, if we consider that at least 2 3 years are needed for Adriatic waters to be converted into Ionian abyssal waters [Wu and Haines, 1996], and that the observed abyssal characteristic variability is consistent with that observed in the Adriatic basin, we can confidently conjecture that the new EMDW 940

11 Figure 10. Vertical distribution of (a) θ, (b)do,(c)s,and(d)s θ observed along the S-N section on the Eastern Mediterranean Ridge during October The old EMDW is visible at the bottom in the southern part of the section. The positions of the CTD stations are indicated on the top x-axis. derives from AdDW produced between 2003 and The spatial distribution of CTD data also reveals that, so far, new EMDW occupies mainly the western flank of the Ionian abyssal plain, while its eastern part is still mainly filled by old EMDW. The paucity of the existing data does not allow for a detailed explanation of the observed phenomenon: we just note that topographic constraints, together with the effect of friction and turbulent mixing (which is a complex function of flow velocity and water characteristics) crucially contribute to determining the bottom-arrested current actual route [Hainbucher et al., 2006]. Additional measurements performed in the Strait of Otranto between October 2009 and July 2010 by means of moored instruments confirm that the Adriatic outflow was freshening and cooling (ΔS 0.03 and Δθ 0.1 C) during that period. Its core was confined to the western deepest part of the Strait as a highly variable, almost barotropic, vein of dense water. Energetic pulses of AdDW, causing sudden temperature anomalies as large as 0.5 C in the bottom layer, were observed. Bottom current averages approached 6.6 cm s 1. However, dense water (s θ >29.20kgm 3 )detectedatthe beginning of April 2010 reached peak velocities as high as 24 cm s 1. A comparison between the mooring observations collected in the Strait of Otranto and in the northern Ionian at 39 N highlights the spatial link between the circulation patterns observed there and in the Southern Adriatic. We identified the existence of a time lag of days for the propagation of signals of Adriatic waters from the Strait of Otranto to the northern part of the Ionian Sea. At mooring C, bottom current velocities were greatly reduced due to their mixing with waters of Aegean origin (θ >13.50 CandS >38.74), which dominate the northern Ionian basin in the layer m. They showed average values of ~1.8 cm s 1 and peak velocities of ~10 cm s 1. This information can be used to asses the time needed for the AdDW to reach the Ionian abyss. From our assessment [see, e.g., Jungclaus and Backhaus, 1994; Hainbucher et al., 2006; Rubino and Hainbucher, 2007] this time seems to exceed 2 years. 6. Conclusions [35] Based on the previous analyses and considerations we conjecture that it is possible, starting with the observed trends, to attempt a forecast of possible developments of the structure of the Ionian abyssal water masses during the coming years. As noted, following the anomaly initiated in the Southern Adriatic after 2003, bottom waters encountered in the abyssal layers of the Ionian basin became warmer and saltier between 2003 and 2007 [Rubino and Hainbucher, 2007]. This implies a rather direct but delayed link between characteristics of the intermediate layer of the Southern Adriatic and bottom waters of the Ionian abyss. Through July 2010, the abyssal region of zone A had not yet been affected by AdDW formed in the Southern Adriatic after However, our data also reveal that a strong interaction between the Adriatic outflow and waters of Aegean origin positioned above them occurs, especially in the northern Ionian. This fact might imply that even longer times than 941

12 Figure 11. Vertical distribution of (a) θ, (b) DO, (c) S, and (d) s θ along the Greek slope section that follows the preferential pathway of the Aegean waters towards the Adriatic Sea in October The position of the CTD stations is indicated on the top x-axis. those discussed above are required to observe major changes in the Ionian Sea. [36] Finally, our observations about the recent changes occurring in the Adriatic outflow, as well as the small density difference observed between abyssal waters of Adriatic and Aegean origin in the Ionian Sea, also render it not improbable that even a moderate future decrease in the AdDW density (or a moderate density increase in the CDW) could trigger a new shift in the source of EMDW. This last conjecture is made even stronger by the recent results of Gacić et al. [2011], who pointed out that the Eastern Mediterranean Transient could be an (irregularly) repeating phenomenon, as part of its cause seems to be connected with cyclical inversions of the Ionian circulation. The water volumes involved in possible future shifts will, however, be crucial in distinguishing between minor local variability and major climatic changes. [37] Acknowledgements. The ship time of RV MARIA S. MERIAN and the financial support for the journey of scientists and transport of equipment was provided by the German Research Foundation (DFG) within the core program METEOR/MERIAN. The investigation was partly supported by the projects VECTOR (VulnErabilità delle Coste e degli ecosistemi marini italiani ai cambiamenti climatici e loro ruolo nei cicli del carbonio mediterraneo) and SESAME (Southern European Seas: Assessing and Modelling Ecosystem Changes). The captain and the crew of the R/V M. S. Merian are acknowledged for their collaboration during both cruises with special thanks to Ilse Buns, Norbert Verch, and Andreas Welsch. We are also thankful to Claudio Zanolla for providing the multi-beam bathymetry charts. Isaac Mancero Mosquera contributed to this work with the support of the Programme for Training and Research in Italian Laboratories of the Abdus Salam International Centre for Theoretical Physics, Trieste. List of Acronyms AdDW Adriatic Deep Water CDW Cretan Deep Water CTD Conductivity, Temperature, Depth CT Conductivity Temperature sensors DO Dissolved oxygen concentration EMDW Eastern Mediterranean Deep Water EMR Eastern Mediterranean Ridge EOF Empirical Orthogonal Function LIW Levantine Intermediate Water NAdDW North Adriatic Deep Water PCA Principal Component Analysis RCM Recording Current meter References Bensi, M., and S. Kückler (2009), Progetti VECTOR e SESAME: metodologia di elaborazione dei dati ctd raccolti nelle crociere di febbraio e marzo Technical report 2009/100 OGA 17 OCE. IN-OGS, Trieste [Available from OGS-B.go Grotta Gigante 42/c Sgonico (Ts), Italy, mbensi@ogs.trieste.it]. Bethoux, J. P., B. Gentili, P. Morin, E. Nicolas, C. Pierre, and D. Ruiz-Pino (1999), The Mediterranean Sea: a miniature ocean for climatic and environmental studies and a key for the climatic functioning of the North Atlantic, Prog. Oceanogr., 44(1-3), , doi: /s (99) Budillon, G., N. L. Bue, G. Siena, and G. Spezie (2010), Hydrographic characteristics of water masses and circulation in the Northern Ionian Sea, Deep Sea Res. Topical Stud. Oceanogr., 57, Cardin, V., M. Bensi, and M. Pacciaroni (2011), Variability of water mass properties in the last two decades in the Southern Adriatic Sea with 942

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