Vertical velocities associated with open-ocean deep convection Session OS2.2: B987 Advances in understanding of in the NW Mediterranean Sea as indirectly observed by gliders the multi-disciplinary dynamics of the Southern European Seas Anthony Bosse1 [[email protected]], P. Testor1, G. Legland2 L. Mortier2, L. Houpert1, L. Prieur3 (Mediterranean and Black Sea) Mediterranean Ocean Observing System for the Environment 1LOCEAN/IPSL, CNRS-Université Pierre et Marie Curie, Paris, France; 2 ENSTA-Paristech, Palaiseau, France; 3 LOV, CNRS-UPMC, Villefranche/mer, France. Context of the Study Extracting vertical velocities from glider data: Methodology Open-ocean deep convection is a key process of the thermohaline circulation, as it renews the deep For a set of ight parameters ( ), once Following Merckelbach et al [2010], we wrote the CD0 ; , Vg α waters of the oceans. This phenomenon is triggered by strong dry winds blowing in winter over a few is computed, one can get w Gliders are autonomous underwater vehicles pro- dierent forces working on the glider (F : buoyancy, U = mod=sin(θ + α) known places : the NW Mediterranean Sea, the Labrador Sea and the Greenland Sea. b from the force balance considering the glider to be pelled by buoyancy. They acquire a vertical veloc- Fg gravity, Fl lift and Fd drag) following: ity by changing their volume thanks to an oil pump. in quasi-steady ight. The open-ocean deep convection is classically described as: Large-scale cyclonic gyre (~100km) Their wings then convert it into a horizontal mo- Fb = gρ(Vg[1 − P + αT (T − T0)] +∆ Vg) As some parameters are not accurately known and tion. In this way, they glide along a saw-tooth trajec- occurring within the center of a large-scale cyclonic Fg = mgg might change during a deployment (biofouling, tory from the ocean surface (where they send their change in oil volume pumped, ...), an optimiza- gyre. The geostrophy implies a doming of the isopyc- 1 U2 real-time data) to a maximum depth of 1000m. A Fl = ρS aα tion of three critical parameters ( ) is per- nals resulting in a large-scale preconditioning to deep 2 CD0 ; , Vg mixing; dive/ascent cycle is performed in about 4h, while the 1 formed using the following cost function: glider horizontally moves at the about speed of 20- F = ρSU2(C + C α2) d 2 D0 D1 X 2 X 2 being modulated by the mesoscale eddy eld, which 30cm/s (, 4km/cycle). By inferring their the- 2 wmod w C + C α J(CD0 ; , Vg) = (dP=dt− ) = water can enhance or reduce the intensity of the mixing; α = D0 D1 oretical displacement through the water thanks to a a tan(θ + α) taking place in small convection plumes (L 1km), glider ight model in absence of mean vertical wa- The optimization is done within 1-day intervals ∼ Fig. 5: Schematic view of a glider moving in a with in green the parameters recorded by the glider Fig. 1: The dierent scales implicated where strong negative velocities ( 10cm/s) are pre- ter displacement, it is possible to extract the vertical allowing the parameters to slightly change during ∼ vertical plane [from Merckelbach et al., 2010]. and in red the unknown parameters to be optimized in open-ocean deep convection [from Mar- vailing (positive vertical velocities occur around these velocities of the water. the deployment and the water velocity to be about (the others are prescribed). shall and Schott, 1999]. plumes and are prescribed by mass conservation). zero in average. Being able to accurately observe these strong vertical velocities with the sucient time and spatial resolution remains a big challenge to characterize the convective plumes and get a deeper understanding Results from a glider deployment during winter 2013 of their role in deep open-ocean convection. 2 ! The glider experienced several severe winter storms Heat flux [W/m ] with heat ux peaking at <-1000W.m-2; 500 The deep convection event in the NW Mediterranean Sea in winter 2013 0 12.8 12.9 13 13.1 13.2 13.3 In the NW Mediterranean Sea, the deep convec- 500 tion takes place in the center of a cyclonic gyre 1000 ! Coastal turbid waters in the NC near the shelf + 44.5 whose northern part is the Northern Current (NC) 0 Enhanced turbidity signal during a strong con- 25 Feb 2013 Turbidity 44 and its southern part is associated with the North- vective event (right after the Feb. 25) in the mid- 0.1 dle of the deep water formation zone: could be Balearic Front (NBF). 500 43.5 0.05 the sign of suspended particles due to bottom- N During winter 2013, a multi-platform experiment 0 Depth [m] reached vertical mixing (also strong horizontal 43 was conducted to thoroughly study the deep con- 1000 -1 O2 [ mol.L ] currents ∼50cm/s over the whole water column); 42.5 vection event and the development of the spring 200 LION bloom: two R/V cruises (∼100 CTD stations in ! Oxygenation of the mixed patch + Vertical gra- 42 winter and spring) + deployment of a eet of 500 180 160 dient; gliders and release of bio-Argo oats. Depth [m] Latitude 41.5 NC These data altogether represent an unprecedented 1000 Chl [mg.m-3] Vertical dilution of the phytoplankton in the 41 NBF ! S observational network and enable a good delimi- 0.5 mixed patch + only signicant signals are located 40.5 tation of the deep convection area only from in 500 in frontal areas; situ proles (see the blue area in gure 2). 0 40 Depth [m] 1000 -3 Density fronts at the northern (:NC) and southern 39.5 ϑ [kg.m ] ! ! The individual dots represent all the proles (:NBF) boundaries of the mixed patch + signals of 39 carried out by gliders, Argo oats and R/V CTD 500 29.12 mesoscale eddies detaching from the rim current + 2 3 4 5 6 7 8 9 29.1 Longitude casts in a ±25 days temporal windows. Ar- density increase of the mixed patch from the begin- Depth [m] Fig. 2: Potential temperature averaged from rows represent the mean circulation. Tempera- 1000 ning to the end of the deep convection; 400m to 600m. The coloured contours is the ob- ture <13°C can be associated with deep convec- S [‰] jectively analysed eld on February 25, 2013. ! tion (MLD>∼2000m, see gure 3). 500 38.55 ! Slight salinity (θ) increase (decrease) of the mixed 38.5 38.45 patch from the beginning to the end of the deep Depth [m] convection due to atmospheric forcing + evidence 1000 Observation at the LION mooring ϑ [°C] of submesoscale activity resulting in vertical ex- The LION mooring is a highly instrumented mooring line (11 Seabird microcats + 5 Aquadopp current- 13.5 changes at the fronts; 500 meters), which is maintained since 2008. Its location is perfectly suited to observe the deep convection. 13 The winter 2013 can be characterized as a strong convective year. The convection reached the Depth [m] 1000 bottom (∼2500m) at LION by about mid-February (also observed by CTD casts with MLD >2500m). w [cm/s] ! Vertical velocities ∼ 0 during period of weak at- The convection ceased on the 20th of March after a quick re-stratication followed by a last winter 0.1 mospheric forcing or in stratied conditions + storm that made the MLD reach ∼2000m. During the deep convection phase, ADCPs measured down- 500 0 vertical velocities > ±10cm/s when net heat loss -2 and upward vertical velocities of O(10cm/s) every 30 minutes. 0.1 <-500W.m ; Depth [m] -1 Deep convection phase Pot. Temperature [°C] Horizontal currents (cm.s ) 1000 0 13.3 40 01/26 01/31 02/05 02/10 02/15 02/20 02/25 03/02 30 Date ! Position of the glider (north/south sections) from 20 S 13.2 N 500 S N point N to S, see gure 2. 10 0 13.1 150m 250m 506m 1000m 2315m 1000 80 Distribution of vertical velocities Vertical velocities at the plume scale, a closer look on Feb. 24 Vertical currents (cm.s-1) 13 60 Surface waters are denser 1500 ! LION mooring LION mooring 40 0.25 0 than below due to the in- -3 2 ϑ [kg.m ] 12.9 20 0.1 0.2 N <0 tense surface heat loss (<- 2000 -2 2 Deep convection phase 500W.m ): N <0 0 0.15 29.126 12.8 500 gravitationally unstable ; 01 Nov 15 Nov 01 Dec 15 Dec 01 Jan 15 Jan 01 Feb 14 Feb 01 Mar 15 Mar 01 Apr 15 Apr 01 May 07/2012 09/2012 11/2012 01/2013 03/2013 05/2013 07/2013 0.05 0.1 29.124 Fig. 3: (left) Potential temperature observed at the LION mooring with the Mixed Layer Depth Depth [m] 0.05 29.122 ! θ=S/σθ exhibits small (MLD) in yellow; (left) horizontal and vertical velocities measured by the ve current meters of the Probabilty density function Probabilty density function inhomogeneities mooring line. 0 0 1000 −20 −15 −10 −5 0 5 10 15 20 −20 −15 −10 −5 0 5 10 15 20 S [‰] within the mixed layer: 0.1 0.5 38.5 O(0.01°C)/O(0.001 %) 500 glider glider -3 0.08 0.4 500 38.498 /O(0.001 kg.m ); old DW 38.496 ] 0.06 0.3 Depth [m] fresher/colder deep waters 2 0 ! are associated with the old 0.04 0.2 1000 ϑ [°C] −500 deep waters; 0.02 0.1 12.93 Probabilty density function Probabilty density function ! Downward convective latent 0 0 12.92 Heat fux [w/m −1000 sensible −20 −15 −10 −5 0 5 10 15 20 −20 −15 −10 −5 0 5 10 15 20 500 plumes are clearly identi- w [cm/s] w [cm/s] old DW 12.91 -1 net able: w -15/-10cm.s Depth [m] ∼ Fig.