Abyssal and Deep Circulation in the Ionian Sea: Role of Diffusion from CTD Analysis

1. "Abyssal and deep circulation in the Ionian Sea: role of diffusion from the analysis of the ctd cast in the Ionian Sea"Artale V., M. Bensi, F. Falcini, S. Marullo, A. Rubino and R. Santoleri.

2. Sandström Theorem Eddy diffusion is an essential aspect of thermohaline circulation, since it is required to return dense (cold) waters back to the light (warm) surface. Cartoon illustrating the role of vertical (diapycnal) diffusion in the meridional overturning circulation, replacing deep tropical warm source with diapycnal diffusion that reaches below the effect of high latitude cooling.

3. Thermohaline circulation and vertical diffusion (slow ventilation) Sincethere are no significantdeepheatsources in the world ocean, watersthatfill the deepocean can onlyreturn to the seasurfaceas a result of downward mixing of heat from the seasurface. Thisoccursthrougheddydiffusion, Munkassumedthatmost of the oceanisdominated by the balance verticaladvection = verticaldiffusion thatis, w ∂T/∂z = ∂/∂z(κV∂T/∂z)

4. The new vision of the abyssal mixing (Ferrari et al., 2016) Strong internal wave–driven mixing is typically bottom intensified and thus drives sinking, not rising, of waters through density surfaces (downwelling). The emerging evidence that mixing increases toward the ocean bottom, at least above rough topography where mixing is strong and contributes significantly to water mass transformations, ….these abyssal waters are transformed into lighter waters in the ocean interior as a result of internal wave–driven mixing (upwelling). In Ferrari et al., 2016, the conundrum is resolved by arguing that abyssal waters come to the surface along turbulent bottom boundary layers, rather than in the stratified ocean interior: this abyssal waters rise toward the surface as lighter waters in narrow turbulent boundary layers that develop along the continental margins and abyssal ridges.

5. CTDs Analysis in the Ionian Sea (6226 stations)

6. Temporal evolution of the deep columns Deep stations at 4080 m from 1999 to 2011 1999 1999

8. Temporal evolution of the deep columns Deep stations at 4080 m from 1999 to 2011 TRANSFERT OF HEAT BY DIFFUSION AND MIXING Most recent stations (2008, 2011) Mixing layer fra 3000 e 4000 m 3000 m 4000 m

9. HYSTERESIS DIAGRAM from Artale et Ritmare team, Scientific Report, under revision, 2017

10. The analysesheretreatedconsisting on yearly time seriescollectedbetween 2012 and 2013 in Western Ionian Sea at 2100 m depth by the muldisciplinaryseafloorobservatory NEMO SN1, the WesterIoniannode of EMSOERIC(European ResearchInfrastructureConsortium)

11. Modular acousticVelocity Sensor mean 3.5 cm/s max 12.8 cm/s Speed Res. 0.03 cm/s ADCP 600 kHz mean 5.9 cm/s max 18 cm/s Speed Res. 0.01 cm/s

12. ROTARY SPECTRA ANALYSIS Interesting signals rise from the rotary kinetic spectrum, but instead of a typical inertial peak expected at 19.7h (37.54765 N, 15.3975 E) a inertial band (yellow box) rises centred at f=17.7h. The 'effective inertial frequency’ corresponds to a shift (‘Blue shift’, Simmons 2012) of about the 10% of f as the result of the internal mechanisms (Fu, 1981). 99% North-Atlantic 261m depth This well represent the Near-Inertial Waves (Alford, 2016) has shown Alford 2016 Ionian sub-basin 2100 depth NIWs can be generated by severalmechanisms: wind, wave-wavenonlinearinteraction or wave-topography, geostrophicadjstment (Simmons 2012)

13. Near-inertialWaves(NIWs) In our case, the origin of the NIW could be ascribed to the currents flowing along the western Ionian slope. The water mass interaction with the local topography creates a perturbation of the local buoyancy so that chain of near-inertially pulsating cyclones/anticyclones structures, with frequency ranging 19.65h and 20.34 h, can be generated (Rubinoet al., 2012). Rubino et al. (2012)

14. Thank you!!! “Newton did not show the cause of the apple falling, but he showed a similitude between the apple and the stars” D’ArcyWentworth Thompson, 1942