Antarctic Circumpolar Current:

Transcription

1 Taking the Circumpolar out of the Antarctic Circumpolar Current: The ACC and the OC University of Washington, Program on Climate Change 2016 Summer Friday Harbor Marine Lab Andrew Thompson, Caltech collaborators: A. Stewart, T. Bischoff, A. C. Naveira Garabato, J. Sprintall, J. Adkins, R. Ferrari, J.-B. Sallée, K. Heywood, G. Viglione, Z. Erickson, X. Ruan, M. Youngs, A. Lazar, S. Bishop September 13, 2016

2 Let s not completely forget the circumpolar part...

3 An ACC overturning primer S N Surface Ekman transport Key components: Westerly winds create regions of convergent and divergent Ekman transport. Titled isopycnals support the grown of baroclinic mesoscale eddies. Mesoscale eddies stir along density surface and transport mass poleward. Density surfaces may outcrop at the surface and be subject to buoyancy forcing.

4 An ACC overturning primer S N Surface Ekman transport isopycnals Key components: Westerly winds create regions of convergent and divergent Ekman transport. Titled isopycnals support the grown of baroclinic mesoscale eddies. Mesoscale eddies stir along density surface and transport mass poleward. Density surfaces may outcrop at the surface and be subject to buoyancy forcing.

5 An ACC overturning primer S N Surface Ekman transport Poleward eddy transport Key components: Westerly winds create regions of convergent and divergent Ekman transport. Titled isopycnals support the grown of baroclinic mesoscale eddies. Mesoscale eddies stir along density surface and transport mass poleward. Density surfaces may outcrop at the surface and be subject to buoyancy forcing.

6 An ACC overturning primer S N Surface Ekman transport Poleward eddy transport Key components: Westerly winds create regions of convergent and divergent Ekman transport. Titled isopycnals support the grown of baroclinic mesoscale eddies. Mesoscale eddies stir along density surface and transport mass poleward. Density surfaces may outcrop at the surface and be subject to buoyancy forcing.

7 The gospel according to Marshall and Radko (2003) The residual overturning arises from a near-cancellation of wind (mean) and eddy overturning circulations The Southern Ocean is adiabatic outside of the surface mixed layer where buoyancy forcing sets the residual overturning.

8 Antarctica arises under permanent sea ice, where the heat fluxes are weakwater and salinity fluxes areand strong due to brine rejection. The mass modification the overturning circulation solid white line in Fig. 3 confirms that the transition between negative and positive buoyancy flux closely coincides with the In steady state, the S. Ocean surface buoyancy forcing constrains the geometry of the overturning circulation. Surface buoyancy flux mm2s From SOSE Longitude Fig. 3. Annual mean buoyancy flux from a state estimate that combines 3 y of available observations with an ocean model (26). The black line denotes The buoyancy flux separatrix north of the Antarctic continent is roughly co-located the 70%mean quantile ofsea annual mean sea ice coverage. the 70% quantiles of with annual ice concentration, essentially the area of the ocean covered by ice 70% of the time. The change in sign of the buoyancy flux just north of the Antarctic continent is roughly colocated with

9 Antarctica arises under permanent sea ice, where the heat fluxes are weakwater and salinity fluxes areand strong due to brine rejection. The mass modification the overturning circulation solid white line in Fig. 3 confirms that the transition between negative and positive buoyancy flux closely coincides with the In steady state, the S. Ocean surface buoyancy forcing constrains the geometry of the overturning circulation. Surface buoyancy flux Upper cell 60 Upper cell 0 mm2s Lower cell 75 From SOSE Lower cell Longitude Fig. 3. Annual mean buoyancy flux from a state estimate that combines 3 y of available observations with an ocean model (26). The black line denotes The buoyancy flux separatrix north of the Antarctic continent is roughly co-located the 70%mean quantile ofsea annual mean sea ice coverage. the 70% quantiles of with annual ice concentration, essentially the area of the ocean covered by ice 70% of the time. The change in sign of the buoyancy flux just north of the Antarctic continent is roughly colocated with

10 The ACC in the global overturning circulation

11 M( n z)oc: The meridional (and zonal) overturning circulation It is a truth universally acknowledged... Adiabatic cell associated with NADW and eddy-driven upwelling in the Southern Ocean. B τ Diabatic cell associated with AABW and diffusive upwelling. (km) O2 (µmol l 1 ) S 60 S 30 S 0 30 N 60 N 80 N Marshall & Speer (2012)

12 Our modern figure-eight overturning The modern global overturning circulation consists of a single figure-eight cell. see Lumpkin & Speer (2007); Talley (2013) Indonesian Throughflow Pacific Longitude Atlantic Zonal ACC transport This structure is distinct from the traditional two-cell paradigm composed of adiabatic (surface-forced) and diabatic (diffusive) cells.

13 A non-zero overturning circulation requires a transformation of water to different density classes. Modification processes

14 Water mass modification and the overturning circulation Modification processes 1. Interior (diapycnal) mixing (internal wave breaking);

15 Water mass modification and the overturning circulation Modification processes 1. Interior (diapycnal) mixing (internal wave breaking); 2. High latitude formation processes (sea ice formation; ocean-ice shelf interactions; buoyancy forcing in polynyas);

16 Water mass modification and the overturning circulation Modification processes 1. Interior (diapycnal) mixing (internal wave breaking); 2. High latitude formation processes (sea ice formation; ocean-ice shelf interactions; buoyancy forcing in polynyas) 3. Buoyancy forcing at the surface of the Antarctic Circumpolar Current.

17 Our modern three-dimensional overturning circulation Ferrari et al. (2014)

18 A three-dimensional residual-mean overturning b =0 (adiabatic interior)

19 A three-dimensional residual-mean overturning b =0 (adiabatic interior) Three-dimensional b b =0 Transport streamfunctions: (x) i = i = Uz i Zonal (barotropic) ACC transport (y) i = +? = f + Wind Eddies (i = A, P )

20 A three-dimensional residual-mean overturning b =0 (adiabatic interior) Three-dimensional b b =0 Transport streamfunctions: (x) i = i = Uz i Zonal (barotropic) ACC transport (y) i = +? = f + Wind Eddies (i = A, P ) Average across a sector of the ACC: Atlantic Pacific (y) A (y) P

21 Two-layer example (b) 0 T ml A =3.2 Sv Atlantic 1 =3.8 Sv TA apple =5.1 Sv (a) 0 Symmetric (m) = 3.8 Sv T NADW = 12 Sv (m) T apple = T ml =4.9 Sv T NADW =0Sv ` 0 L b (c) (m) (b) T ml P = 11.3 Sv Pacific T apple P = 15.0 Sv 2000 (c) ` 0 L b Jones and Cessi (2016), Thompson et al. (2016)

22 Model-observation comparison (Pacific) y =0 Atlantic Pacific (Pacific) Atlantic Pacific (m) Model Observations 2000 (a) 2500 y = l (Atlantic) K =1500m 2 s 1, =0.12 N m 2, T =10SvandU =10cms 1, (b) (Atlantic) Thompson et al. (2016) Key points: Difference in isopycnal depth is concentrated to the north of the ACC. Isopycnals are deeper in the Pacific to support a transport from the Atlantic to the Pacific in deeper density classes and from the Pacific to the Atlantic in shallower density classes.

23 Flow-topography interactions & ACC hotspots