Inter-Annual Variations of Surface Currents and Transports in the Sicily
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Inter-annual variations of surface currents and transports in the Sicily Channel derived from coastal altimetry Fatma Jebri, Bruno Zakardjian, Florence Birol, Jerome Bouffard, Loïc Jullion, Cherif Sammari To cite this version: Fatma Jebri, Bruno Zakardjian, Florence Birol, Jerome Bouffard, Loïc Jullion, et al.. Inter-annual variations of surface currents and transports in the Sicily Channel derived from coastal altimetry. Jour- nal of Geophysical Research. Oceans, Wiley-Blackwell, 2017, 122, in press. 10.1002/2017JC012836. hal-01620274 HAL Id: hal-01620274 https://hal.archives-ouvertes.fr/hal-01620274 Submitted on 20 Oct 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. 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No.: 2017JC012836 Date: 12-October-17 Stage: Page: 1 Revised proofs are sent only in the case of extensive corrections upon request PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE Interannual Variations of Surface Currents and Transports in 1 10.1002/2017JC012836 the Sicily Channel Derived From Coastal Altimetry AQ1 Key Points: Fatma Jebri1,2,3,4,5 , Bruno Zakardjian1, Florence Birol5, Jer ome^ Bouffard6, Lo€ıc Jullion1, and AQ142 A 20 year time series of coastal Cherif Sammari4 3 altimetry reveals interannual variability modes of the surface 1Mediterranean Institute of Oceanography, Universite de Toulon, CNRS/INSU, IRD, UM 110, La Garde, France, 4 circulation in the Sicily Channel 2 5 The variations of its volume Mediterranean Institute of Oceanography, Aix Marseille Universite, CNRS/INSU, IRD, UM 110, Marseille, France, 3 4 transports are estimated with an Universite de Tunis El Manar, Ecole Nationale d’Ingenieurs de Tunis, Tunis, Tunisia, Institut National des Sciences et 6 empirical transport-like model Technologies de la Mer, Carthage Salammbo,^ Tunisia, 5Laboratoire d’Etudes en Geophysique et Oceanographie Spatiales, 7 Interannual variability results from OMP, Toulouse, France, 6Earth Observation Directorate, ESRIN/EOP GMQ Section, RHEA for European Space Agency, Italy 8AQ2 the Atlantic waters inflow and modulations of the mesoscale activity in coupled or compensating ways Abstract A 20 year coastal altimetry data set (X-TRACK) is used, for the first time, to gain insight into the 9 long-term interannual variations of the surface circulation in the Sicily Channel. First, a spectral along with a 10 Correspondence to: time/space diagram analysis are applied to the monthly means. They reveal a regionally coherent current 11 F. Jebri, patterns from track to track with a marked interannual variability that is unequally shared between the 12 [email protected] and Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhe- 13 [email protected] nian Current outflow northeast of Sicily. Second, an empirical altimetry-based transport-like technique is 14 proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks 15 Citation: Jebri, F., Zakardjian, B., Birol, F., and coastlines over the study area. A set of hydrographic measurements is used to validate the method. 16 Bouffard, J., Jullion, L., & Sammari, C. The inferred altimetry transports give a well-balanced mean eastward Atlantic Waters baroclinic flow of 0.4 17 (2017). Interannual variations of Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian 18 surface currents and transports in the Sicily Channel derived from coastal Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify 19 AQ1 altimetry. Journal of Geophysical the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 6 0.1 Sv) and highlights two main modes of 20 Research: Oceans, 122. https://doi.org/ variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian 21 10.1002/2017JC012836 Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards 22 to changes in the observed currents and transports. 23 Received 27 FEB 2017 Accepted 19 SEP 2017 25 24 Accepted article online 28 SEP 2017 26 27 1. Introduction 28 The Sicily Channel (hereafter SC) plays a central role in the Mediterranean Overturning Circulation by con- 29 trolling the water mass exchanges between the Eastern and Western subbasins of the Mediterranean and 30 by linking their respective large-scale circulations. The water masses exchanges and flows between the two 31 subbasins are, as first approximation, driven by a two-layer system: surface fresh Atlantic Waters (AW) flow 32 eastward whereas more saline intermediate and bottom waters are formed in the eastern basin, i.e., the 33 Levantine Intermediate Water (LIW) and the Eastern Mediterranean Deep Waters (EMDW), flowing westward 34 (Astraldi et al., 1996, 1999). This dynamics of water masses is modulated by its regional mesoscale variations, 35 wind forcing, mixing processes (e.g., Lermusiaux & Robinson, 2001; Manzella et al., 1988; Stansfield et al., 36 2003) and largely constrained by the complex topography of the SC (e.g., Astraldi et al., 1999; Napolitano 37 et al., 2003). Several recent studies following the detection in the 1990s of the Eastern Mediterranean Tran- 38 sient (e.g., Roether et al., 2014) have highlighted the SC as a key area to follow major changes in the Medi- 39 terranean Overturning Circulation (e.g., Gasparini et al., 2005; Malanotte-Rizzoli et al., 2014). 40AQ3 We focus here on the surface circulation that regulates the AW path toward the Ionian Sea and Levantine 41 basin. In the pioneering works, the AW was always considered flowing South East from Sicily toward the 42 Libyan coasts (Lacombe & Tchernia, 1972; Nielsen 1912; Ovchinnikov, 1966). Recent studies using drifters 43 (Poulain & Zambianchi, 2007), numerical simulations (Beranger et al., 2004) or coastal altimetry (Jebri et al., 44 2016) have depicted more complex circulation patterns (Figure1a) with much of the AW feeding two main 45 F1 streams, the Atlantic Ionian Stream (AIS) flowing south-eastward close to Sicily (Buongiorno Nardelli et al., 46 VC 2017. American Geophysical Union. 2001; Robinson et al., 1999) and the Atlantic Tunisian Current (ATC) flowing southward over the Tunisian 47 All Rights Reserved. shelf (Sammari et al., 1999). The ATC may also have secondary currents branches over the Gulfs of Gabes 48 JEBRI ET AL. SICILY CHANNEL INTERANNUAL VARIBILITY 1 ID: kannanb Time: 13:09 I Path: //chenas03/Cenpro/ApplicationFiles/Journals/Wiley/JGRC/Vol00000/170402/Comp/APPFile/JW-JGRC170402 J_ID: JGRC Customer A_ID: JGRC22508 Cadmus Art: JGRC22508 Ed. Ref. No.: 2017JC012836 Date: 12-October-17 Stage: Page: 2 Journal of Geophysical Research: Oceans 10.1002/2017JC012836 Figure 1. (a) Scheme of the main Atlantic waters circulation features at the upstream and over the Sicily Channel based on Jebri et al. (2016). Acronyms are listed in Table 1. The altimetry tracks (044, 059, 135, 161, 222, and 237) from the T/P 1 J1 1 J2 mission and crossing the study area are shown in green dashed lines. The 200 m isobath (pink solid line) is from ETOPO2v1 global gridded database. (b) Mean absolute geostrophic cross-track velocities computed from the selected TP 1 J1 1 J2 altimetry tracks over the period 1993–2013. JEBRI ET AL. SICILY CHANNEL INTERANNUAL VARIBILITY 2 ID: kannanb Time: 13:09 I Path: //chenas03/Cenpro/ApplicationFiles/Journals/Wiley/JGRC/Vol00000/170402/Comp/APPFile/JW-JGRC170402 J_ID: JGRC Customer A_ID: JGRC22508 Cadmus Art: JGRC22508 Ed. Ref. No.: 2017JC012836 Date: 12-October-17 Stage: Page: 3 Journal of Geophysical Research: Oceans 10.1002/2017JC012836 and Sidra, i.e., the Atlantic Libyan Current (ALC) (e.g., Jebri et al., 2016). This complex circulation has a 49 marked seasonal component mainly driven by the AW transport seasonality, which is more intense in winter 50 (November–February) and minimum in August (Gasparini et al., 2007; Manzella, 1994; Rinaldi et al., 2014). 51 Recently, using 20 years of altimeter data and concurrent sea surface temperature images, Jebri et al. (2016) 52 showed that the impact of the AW flow seasonality on the surface circulation of the SC results mainly in the 53 strengthening of the ATC and ALC branches nearby and over the Tunisian shelf during winter. 54 Nevertheless, many questions still remain open about the interannual component of the SC surface circula- 55 tion. Numerical modeling studies have mostly focused on a restricted part of the Central Mediterranean 56 (e.g., Jouini et al., 2016; Pinardi et al., 1997), while sparse in situ based studies remain spatially limited to the 57 SC entrance (e.g., Ben Ismail et al., 2012, 2014; Sammari et al., 1995) or to the northwestern part of the chan- 58 nel (e.g., Bonanno et al., 2014). The previous works of Buongiorno Nardelli et al. (1999, 2006) have demon- 59 strated the potential of standard altimetry for the estimate of currents inside the SC. While long time series 60 of reprocessed altimetry data are now available over the entire Mediterranean Sea, no attempt was yet 61 made to study the interannual variability of the regional circulation in the SC from altimetry as it has been 62 the case for other areas with narrow coastal boundary currents, such as the Liguro-Provenc¸al Current in the 63 North Western Mediterranean (Birol et al., 2010; Bouffard, 2007) and the Navidad current along the northern 64 coast of Spain (Le Henaff et al., 2011).