Stratification of the Ocean Boundary Surface Layer - year-long observations with gliders

Transcription

1 Stratification of the Ocean Boundary Surface Layer - year-long observations with gliders Ayah Lazar 1,2 Andrew Thompson 2 Gillian Damerell 3 Karen Heywood 3 Christian Buckingham 4 Alberto Naveira Garabato 4 Liam Brannigan 5 1 National Institute of Oceanography, IOLR 2 California Institute of Technology 3 University of East Anglia 4 National Oceanography Centre, Southampton 5 Oxford University

2 What is the Ocean Boundary Surface Layer? (Mixed Layer) Eastern-northern Pacific along 14 o W Ferrari and Rudnick, 2 2

3 What is the Ocean Boundary Surface Layer? (Mixed Layer) Wind Buoyancy flux: Net Heat Evaporation/ Percipitation 3

4 What is the Ocean Boundary Surface Layer? (Mixed Layer) Wind Buoyancy flux: Net Heat Evaporation/ Percipitation B surf = g Q C p + g (E P )S Q surf = Q + Q fresh 4

5 What is the Ocean Boundary Surface Layer? (Mixed Layer) Wind Buoyancy flux: Heat Fresh water 3D turbulence 1 m from Rafael Ferrari 5

6 Why do we care about the upper ocean? Heat/gas/momentum fluxes Air/Sea fluxes (heat, gas) - fast Mixed Layer/Deep Ocean fluxes (heat, gas) - slow This is the part of the ocean that the atmosphere sees 6

7 Why do we care about the upper ocean? Biology - Phytoplankton Winter low light & heat low phyt. high nutrient Spring increase light & heat increase phyt. decrease nutrient Summer high light & heat decrease phyt. low nutrient Autumn decrease light & heat low phyt. increase nutrient light Phytoplankton (Dead) Nutrients 7

8 BUT there is spatial variability 8 NASA Ocean Color image gallery

9 Baroclinic instability r h b Warm Eddy overturning stream function Cold H N = b z deformation radius L d = NH f 9

10 BUT there is spatial variability 1 NASA Ocean Color image gallery

11 Submesocale turbulence Turbulence emerges at smaller scales Sea Surface Temperature 6 km resolution.75 km resolution [km] [km] Capet et al., 28 [km] [km] 11

12 Mixed layer baroclinic instability Ri & 1 b x Hot x Cold h x Fox-Kemper et al., 28 deformation radius N = b z L d = NH f 12

13 Mixed layer baroclinic instability Ri & 1 b x Hot x Cold h = Ch 2 rb k f µ(z) µ(z) 1 C.6 =.6 b xh 2 f x Fox-Kemper et al., 28 B BCI =.6 b xh 2 f b x Q BCI =.6 b2 xh 2 f C p g Always stratifying 13

14 Wind Driven Flux L.N. Thomas et al. / Deep-Sea Research II 91 (213) y(m) z (m) Down-front wind Ekman Thermal Wind Surface heat flux r h b -16 h (m) Thomas et al. (213) x (km) Fig. 4. Schematic of the LES configuration. Ekman buoyancy flux B Ek = 2 2 Q Ek = b x y f C p g h (m) w f k r h b Q Ek < Q Ek > destratifying (cooling) stratifying (heating)

15 Total Equivalent Heat Flux Q tot = Q heat + Q fresh + Q BCI + Q Ek Surface (One dimensional) Horizontal gradients 15

16 Inertial/Symmetric/Gravitational instability Ri < 1 Instability occurs when: fq < q =(fk + r u) rb Ertel potential vorticity q =(f + )N 2 +(w y v z )b x +(u z w x )b y q vert q hor Assuming thermal wind balance (u z,v z ) ( b y,b x )/f q hor 1/f r h b 2 q =(f + )N 2 1/f r h b 2 16

17 Inertial/Symmetric/Gravitational instability Ri < 1 Instability occurs when: fq < q =(fk + r u) rb Ertel potential vorticity q =(f + )N 2 1/f r h b 2 q vert q hor N 2 < Gravitational instability 17

18 Inertial/Symmetric/Gravitational instability Ri < 1 Instability occurs when: fq < q =(fk + r u) rb Ertel potential vorticity q =(f + )N 2 1/f r h b 2 q vert q hor q ver > q hor /f < 1 Inertial instability 18

19 Inertial/Symmetric/Gravitational instability Ri < 1 Instability occurs when: fq < q =(fk + r u) rb Ertel potential vorticity q =(f + )N 2 1/f r h b 2 q vert q hor q hor > q ver Symmetric instability 19

20 Inertial/Symmetric/Gravitational instability Ri < 1 (Ri < f/ g ) Thomas et al. (213) Ri = f 2 N 2 r h b 2 2

21 OSMOSIS: Ocean Surface Mixing, Ocean Submesocale Interaction Study Year-long study of seasonal variations in upper ocean turbulence at high resolution SST log(eke) Sept. 212 Sept. 213

22 OSMOSIS: Ocean Surface Mixing, Ocean Submesocale Interaction Study Year-long study of seasonal variations in upper ocean turbulence at high resolution SST Depth 9 Moorings

23 OSMOSIS - Gliders Two gliders throughout the year 3 deployments moorings( SG566((Sep(to(Jan)( SG52( SG566((Apr(to(Sep)( White(blobs(are(GPS(posi<ons( every(<me(a(glider(surfaces.( sampling temperature, salinity, pressure, dissolved oxygen, dive-averaged currents, CDOM fluorescence, chlorophyll fluorescence, optical backscatter, photosynthetically available radiation

24 Example Data Salinity Buoyancy [m s-2] Depth (m) x Depth (m) Time (days) Time (days after 1/9/212) 1 3 x Time (days) Time (days after 1/9/212) 1

25 Buoyancy - time series Autumn b [ms 2 ] x pressure time since 1/9/212 Winter x pressure time since 1/9/212 Spring-Summer x pressure time since 1/9/212

26 Potential Vorticity from Glider Data Ri < 1 Instability occurs when: fq < q =(fk + r u) rb Ertel potential vorticity q =(f + )N 2 +(w y v z )b x +(u z w x )b y q vert q hor = v x u y Glider path v z = b x /f q =(f + v x )N 2 b 2 x/f 26

27 Symmetric Instability Example 23/12/212 SST and glider Temperature Salinity Latitude Longitude Depth (m) Longitude Longtiude Longitude Longitude x x Depth (m) Longitude Latitude b [ms 2 ] q [1 9 s 3 ] log 1 (Ri 1 ) Latitude 1

28 Potential Vorticity Autumn q [1 9 s 3 ] Winter Spring-Summer

29 Lateral Buoyancy gradient Autumn Winter Spring-Summer

30 Instabilities throughout the year Autumn Winter Spring-Summer Autumn - mostly gravitational instability Winter - symmetric and mixed instability Late Spring/Summer- Stable

31 Total Equivalent Heat Flux - Restratification Q tot = Q heat + Q fresh + Q BCI + Q Ek Surface (One dimensional) Horizontal gradients L.N. Thomas et al. / Deep-Sea Research II 91 (213) y(m) z (m) Thermal Wind ML Baroclinic Instability x (km) Ekman Buoyancy Flux Fig. 4. Schematic of the LES configuration. 31 4

32 Winter - Restratification processes Q BCI =.6 b2 xh 2 f C p g Q Ek = b x y f C p g

33 Winter - Restratification processes Strong positive buoyancy forcing (BCI) > Stable stratification. Persistent negative forcing > Gravitational instability.

34 Winter - Restratification processes Forcing due to submesoscale fronts can reverse the sign of the equivalent surface buoyancy forcing up to 25% of the time during the winter.

35 Winter - Restratification processes Mean mixed-layer depth is only weakly dependent on the surface heat flux (black squares). Shallow MLDs are associated with the strongest total fluxes (red circles), both positive and negative.

36 Winter - Restratification processes Mean mixed-layer depth is only weakly dependent on the surface heat flux (black squares). Shallow MLDs are associated with the strongest total fluxes (red circles), both positive and negative.

37 Winter - Restratification processes Q Ek << Mean mixed-layer depth is only weakly dependent on the surface heat flux (black squares). Shallow MLDs are associated with the strongest total fluxes (red circles), both positive and negative.

38 Winter - Restratification processes Q tot = Q surf + Q BCI + Q Ek Q BCI = Q Ek Q Ek = b x y f C p g Q BCI =.6 b2 xh 2 f C p g h max =4 r y b x h = 255 m

39 Winter - Restratification processes - Negative PV instabilities Significant SI events coincide with low values of h/h. The reduction in h/h is not caused by an increase in H >SI also has an active role in modifying the stratification of the mixed layer

40 Summary Submesoscale fronts have a significant impact on upper ocean stratification (Both BCI and SI likely contribute). Submesoscale motions, with horizontal scales of 5-1km are ubiquitous in the open-ocean! Throughout the year. Seasonal cycle in amplitude of mixed layer lateral buoyancy gradients: elevated in fall. Seasonal cycle of negative PV instabilities - 1. Gravitational Instability in the Autumn 2. Symmetric and mixed instability in Winter 3. Stable in Spring-Summer Intermittentally forcing is significantly larger than the surface forcing (In winter the total flux is positive 25% of the time). 4