Internal tide energetics in the Sicilian Strait and Adjacent areas Jihène Abdennadher and Moncef Boukthir UR1304, Institut Préparatoire aux Etudes d’Ingénieur de Tunis, Tunisia ROMS_Workshop 2008

Objectives Characterize the internal tides generation and propagation in the Sicilian Strait and in the adjacent areas Characterize the internal tides generation and propagation in the Sicilian Strait and in the adjacent areas Estimate M 2 and K 1 internal tide energetics Estimate M 2 and K 1 internal tide energetics Evaluate the contribution of the internal tide into the mixing Evaluate the contribution of the internal tide into the mixing

Model domain and configuration (ROMS) 1/32° x 1/32° (gebco 1min resolution) 1/32° x 1/32° (gebco 1min resolution) 30 vertical levels 30 vertical levels Realistic summer stratification (MEDAR) Realistic summer stratification (MEDAR) Forcing at the open boundaries by tides elevation of M 2 S 2 K 1 and O 1 (Mog2d) Forcing at the open boundaries by tides elevation of M 2 S 2 K 1 and O 1 (Mog2d) Mellor turbulent closure schema Mellor turbulent closure schema 0.5° sponge layers 0.5° sponge layers Bathymetry of the model domain deduced from Gebco 1 minute resolution. Depths are in m. Tunisia Sicily Adventure Bank Messina strait

Internal tide budget is the background basic density stratification, is the reference water density, is the water density perturbation, is the pressure perturbation, is the vertical velocity induced by the barotropic flow, is the horizontal internal velocity, The governing equation for the baroclinic energy is given by Assuming that the advection of the baroclinic energy is negligible, the dissipation of the baroclinic energy averaged over a tidal period (denoted by an overbar) can be evaluated by

Spatial distribution of: a) Depth integrated Conversion rate of energy (W.m -2 ) ; b) Depth integrated energy flux divergence (W.m -2 ) ; c) Depth integrated dissipation rate (W.m -2 ). (a) (c) The highest values of conversion occur at the western edge of the Adventure Bank, the western Sicilian Shelf and in the NW of Pantelleria isle. Strong local dissipation near the generation sites. M2M2 M 2 internal tide in the Sicilian Strait (b)

(a) The Messina strait is a potential region of M2 internal tide generation. M 2 internal tide in the Messina Strait

Model-predicted distribution of the depth- integrated conversion rate from the M 2 surface to internal tide. Integrated M 2 baroclinic energy flux across the bounding transects is given in MW. Conversion(CRE), dissipation (DIS) and flux divergence (DIV) are given in MW. M 2 internal tide Energy budget The M2 conversion from surface to internal tide in the model domain amounts to 68 MW, 70% of which are in the Sicilian strait and 18 % in the Messina strait. The Messina strait seems to be more dissipative (92 % of the available energy is dissipated) than the Sicilian strait (83 %).

The M2 mode conversion integrated over these prominent topographic features sums up to 35.6 MW, which is 75% of that integrated over the Sicilian strait 42 % of the M2 baroclinic energy generated in the Sicilian strait is dissipated in close proximity to the baroclinic M2 generation sites. M2M2 Model-predicted distribution of the depth-integrated conversion rate from the M 2 surface to internal tide. Integrated M 2 baroclinic energy flux across the bounding transects is given in MW. Conversion(CRE), dissipation (DIS) and flux divergence (DIV) are given in MW

M 2 Depth integrated internal energy flux (W.m -1 ) Horizontal section at 100 m depth of the reconstructed vertical velocity at the M 2 frequency(m/s) (Wavelet decomposition).

Vertical section at the west shelf break of the Adventure Bank This transect corresponds to the prominent direction of propagation of the M2 internal tides from the most efficient generation site

Strong conversion occurs over the steepest parts of the continental slope (shelf edge of the AB) which is characterized by a supercritical slope as reveals the plot of the internal generation criteria parameter. M 2 Internal conversion rate of energy (10 -6 W.m -3 ) M 2 Internal vertical displacement amplitude (m) (IDA) M 2 internal energy density (J.m -3 ) M 2 depth integrated internal energy flux along (Fxbar) and across (Fybar) component (W.m -1 ) The cross and along shelf components of the M2 depth integrated energy flux reveal a seaward propagation as suggested by Sherwin (1991) and the ray theory of Baines (1982). Internal generation criteria parameter  M2 internal vertical displacement amplitudes (IDA) in excess of 24 m are reached near the seabed. The largest value of internal energy density (7 J.m-3) is also located at the shelf break. Elsewhere, the internal energy is concentrated at the surface layers and is almost negligible in the bottom layers.

Hovmuller diagram related to the vertical velocity w at the surface. T M2 x Time elapsed in days Distance from the transect origin (km) Double periodicity of the internal tide, the temporal one is within M2 period and the spatial wavelength () is within the second barcolinic mode of propagation (68 km). Internal mode of the M 2 internal tide

K1 The depth-integrated K 1 baroclinic energy flux and the depth integrated conversion rate (W.m -2 ) K 1 Internal tide generation The K1 internal tide is generated over the Adventure Bank’s edge, the surrounding of Pantelleria isle and the south east of the Malta plateau.

Model-predicted distribution of the depth-integrated conversion rate from the K 1 surface to internal tide. Integrated K 1 baroclinic energy flux across the bounding transects is given in MW. Conversion (CRE), dissipation (DIS) and flux divergence (DIV) are given in MW. 65 % The total conversion over the model domain is 46.4 MW, 65 % of which is in the Sicilian strait and 19 % in the south east of Malta plateau. The energy converted over the model domain and in the subregions are totally lost which is coherent with the fact that K1 frequency is subinertial at these latitudes and so cannot propagate. K1K1 K 1 internal tide Energy budget

Sensibility to initial stratification

x Depth integrated energy flux and the depth integrated conversion (W.m -2 ) using initial a) Summer stratification b) Winter stratification c) Climatology stratification (a) (c) (b) Enhanced conversion in summer conditions in the Sicilian Strait. No change in the K1 direction of the internal energy fluxes. K1K1

x (a) (c) (b) Depth integrated energy flux and the depth integrated conversion (W.m -2 ) using initial a) Summer stratification b) Winter stratification c) Climatology stratification M2M2 Enhanced conversion in summer conditions in the all generation sites. Change in the orientation of energy fluxes which is coherent with the fact that energy ray slope is influenced by the stratification.

(a) (b) (c) Depth integrated energy flux and depth integrated flux divergence (10 -3 W.m -2 ) using initial : a) Summer stratification b) Winter stratification c) Climatology stratification

Sicilian straitReduction of the conversion with respect to that obtained in summer solution M2M2 K1K1 Winter18 %30 % Climatology11 %21 % Sicilian strait% of the dissipation to conversion M2M2 Summer84 Winter93 Climatology91

Internal tide Mixing qualitative approach

Bottom internal currents in the straits of Sicily (left) and Messina (Right) (cm.s -1 ) Surface internal currents in the straits of Sicily (left) and Messina (Right) (cm.s- 1 )  The strong gradient between the surface internal current and the bottom one essentially at the A.B and in the narrow passage of the Messina strait may lead to strong mixing at these areas.

Turbulent Kinetic energy (cm 2.s -2 ) at the bottom averaged over a tidal cycle in the straits of Sicily (left) and Messina (right).

Validation

M2M2 S2S2 K1K1 O1O1 53 m93 m285 m380 m C01

M2M2 S2S2 K1K1 O1O1 46 m 103 m289 m408 m C02

The M 2 internal tide prevails over K 1 at all depths The internal signal is more important at C01 station Our results are coherent with the measurements of Gasparini et al. (2004). Internal density energy (J.m -3 ) CO1CO2

Conclusions M2 internal tide is generated over the western shelf edge of the Adventure Bank, at the NW of Sicily and in the NW of the Pantelleria isle. From the most efficient site the internal energy propagates toward the north and to the Tunisian coasts. The propagation from the Pantelleria isle is mainly toward the Tunisian coasts. The M2 conversion of energy in the whole domain is 68 MW, 70 % of which are found in the Sicilian strait where strong dissipation occur in close proximity to the generation sites.

The K1 internal tide is less energetic than M2. Moreover, the K1 converted energy is totally dissipated in close proximity of the generation sites. The generation sites appear as independents from the initial stratification, but the M2 conversion of energy as well as the propagation direction are strongly influenced by the initial stratification. The NE of the Adventure Bank and the Messina strait are found to be strong mixing areas.

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