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
What is the Ocean Boundary Surface Layer? (Mixed Layer) Eastern-northern Pacific along 14 o W Ferrari and Rudnick, 2 2
What is the Ocean Boundary Surface Layer? (Mixed Layer) Wind Buoyancy flux: Net Heat Evaporation/ Percipitation 3
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
What is the Ocean Boundary Surface Layer? (Mixed Layer) Wind Buoyancy flux: Heat Fresh water 3D turbulence 1 m from Rafael Ferrari 5
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
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
BUT there is spatial variability 8 NASA Ocean Color image gallery
Baroclinic instability r h b Warm Eddy overturning stream function Cold H N = b z deformation radius L d = NH f 9
BUT there is spatial variability 1 NASA Ocean Color image gallery
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
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
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
Wind Driven Flux L.N. Thomas et al. / Deep-Sea Research II 91 (213) 96 11 11 5 4 y(m) 3 2 1 z (m) -4-8 -12 Down-front wind Ekman Thermal Wind Surface heat flux r h b -16 h (m) 4 6 8-2 Thomas et al. (213) 1 2 3 4 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) 4 6 8 w f k r h b Q Ek < Q Ek > destratifying (cooling) stratifying (heating)
Total Equivalent Heat Flux Q tot = Q heat + Q fresh + Q BCI + Q Ek Surface (One dimensional) Horizontal gradients 15
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
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
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
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
Inertial/Symmetric/Gravitational instability Ri < 1 (Ri < f/ g ) Thomas et al. (213) Ri = f 2 N 2 r h b 2 2
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
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
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
Example Data Salinity Buoyancy [m s-2] Depth (m) 2 4 3 35.7 x 1 3 35.65 2 35.6 1 35.55 35.5 6 1 35.45 2 35.4 8 3 35.35 85 9 95 1 Depth (m) 2 4 35.3 85 4 9 95 1 35.7 35.65 2 35.6 1 35.55 35.5 6 1 35.45 2 35.4 8 3 35.35 85 9 95 Time (days) Time (days after 1/9/212) 1 3 x 1 3 35.3 85 4 9 95 Time (days) Time (days after 1/9/212) 1
Buoyancy - time series Autumn b [ms 2 ] x 1 3 5 1 pressure 2 3 4 5 2 3 4 5 6 7 8 9 1 11 12 time since 1/9/212 Winter x 1 3 5 1 pressure 2 3 4 5 13 14 15 16 17 18 19 2 21 22 time since 1/9/212 Spring-Summer x 1 3 5 1 pressure 2 3 4 24 25 26 27 28 29 3 31 32 33 34 5 time since 1/9/212
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
Symmetric Instability Example 23/12/212 SST and glider Temperature Salinity 12 12.2 12.4 12.6 12.8 13 13 35.7 Latitude 48.9 48.85 48.8 48.75 48.7 48.65 48.6 48.55 48.5 16.3 16.2 16.1 16 Longitude Depth (m) 1 2 3 4 5 6 7 8 9 1 16.3 16.2 16.1 16.25 16.2 16.15 Longitude Longtiude 12.5 12 11.5 11 1.5 1 9.5 9 8.5 8 1 2 3 4 5 6 7 8 9 1 16.3 16.2 16.1 16.25 16.2 16.15 Longitude Longitude 35.65 35.6 35.55 35.5 35.45 35.4 35.35 35.3 x 1 3 2.5 x 1 1 2 2 1 2 1 15 1 1 Depth (m) 2 3 4 16.24 16.22 16.2 16.18 16.16 16.14 Longitude 1.5 1.5 2 3 4 16.24 16.22 16.2 16.18 16.16 16.14 Latitude b [ms 2 ] q [1 9 s 3 ] log 1 (Ri 1 ) 1 5 2 3 4 2 16.24 16.22 16.2 16.18 16.16 16.14 Latitude 1
Potential Vorticity Autumn q [1 9 s 3 ] Winter Spring-Summer
Lateral Buoyancy gradient Autumn Winter Spring-Summer
Instabilities throughout the year Autumn Winter Spring-Summer Autumn - mostly gravitational instability Winter - symmetric and mixed instability Late Spring/Summer- Stable
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) 96 11 5 4 y(m) 3 2 1 z (m) -4-8 -12 Thermal Wind ML Baroclinic Instability -16-2 1 2 3 x (km) Ekman Buoyancy Flux Fig. 4. Schematic of the LES configuration. 31 4
Winter - Restratification processes Q BCI =.6 b2 xh 2 f C p g Q Ek = b x y f C p g
Winter - Restratification processes Strong positive buoyancy forcing (BCI) > Stable stratification. Persistent negative forcing > Gravitational instability.
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.
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.
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.
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.
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
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
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