Abyssal Ocean Circulation. Raffaele Ferrari Earth, Atmospheric and Planetary Sciences, MIT Les Houches, August 2017

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1 Abyssal Ocean Circulation Raffaele Ferrari Earth, Atmospheric and Planetary Sciences, MIT Les Houches, August 2017

2 Outline The deep ocean The deep circulation The sinking branch: deep convection The upwelling branch: Stommel-Arons theory The upwelling branch: Munk s Abyssal Recipes The deep and abyssal ocean circulations

3 The deep ocean

4 Ocean temperature Potential temperature WOCE section P15, 165 W

5 Ocean dissolved carbon Dissolved inorganic carbon WOCE section P15, 165 W

6 The deep circulation

7 Deep circulation Henri Ellis (1751) The cold increased regularly, in proportion to the depths, till it descended to 3900 feet: from whence the mercury in the thermometer came up at 53 degrees Fahrenheit (11 degrees Celsius); and tho' I afterwards sunk it to the depth of 5346 feet, that is a mile and 66 feet, it came up no lower. This experiment, which seemed at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate.

8 Deep circulation Benjamin Thomson Count Rumford of the Holy Roman Empire ( ) On the Propagation of Heat in Fluids Cooling Heating Cooling Lenz, 1845

9 Deep circulation Potential temperature WOCE section P15, 165 W

10 Pacific deep circulation AABW Salinity WOCE section P15, 165 W

11 Atlantic deep circulation NADW AABW Salinity WOCE section A16, 25 W

12 Ocean Heat & Carbon Uptake Observed temperature trends Durack and Wijffels (2010) Anthropogenic carbon Sabine et al. (2004)

13 Where does water sink?

14 Deep convection Deep convection in the ocean occurs when the air-sea fluxes act to increase surface density when the ocean stratification is weak These conditions are realized in sub polar gyres Nordic Seas of the North Atlantic Weddell and Ross Sea around Antarctica Air-sea fluxes and thermohaline circulation Figure 4. Annual air-sea density fluxes, contour interval kg m -2 s -1.

15 Deep convection Deep convection in the ocean occurs when the air-sea fluxes act to increase surface density when the ocean stratification is weak These conditions are realized in sub polar gyres Nordic Seas of the North Atlantic Weddell and Ross Sea around Antarctica

16 Deep convection Deep convection in the ocean occurs when the air-sea fluxes act to increase surface density when the ocean stratification is weak These conditions are realized in sub polar gyres Greenland and Labrador Seas in the North Atlantic Weddell and Ross Sea around Antarctica Air-sea fluxes and thermohaline circulation Figure 4. Annual air-sea density fluxes, contour interval kg m -2 s -1.

17 Where does water rise? Stommel-Arons theory

18 Uniform upwelling Ocean surface Warm upper ocean Cold polar waters w = w 0 Thermocline Uniform upwelling H Equator Cold abyss w=0 Convection. Mass source for lower layer. Pole Fig (Vallis, 2017) Upwelling in Pacific: w 0 = S 0 A = m 3 s m 2 ' 10 7 ms 1 ' 3m year 1

19 Vorticity equation Planetary geostrophic equations - geostrophic balance fv = - mass conservation Vorticity x p +fu y x u x v z w =0 v + f(@ x u y v)=0 v = f@ z w Taking the vertical integral with f = f 0 + y vh = fw 0 =) v = f w 0 H ' ms 1

20 Vertically integrated circulation Fig (Vallis, 2017) Meridional transport Upwelling T I vhl x = f w 0 L x Upwelling T U = w 0 L x (L y y) Latitude Upwelling Western boundary current T W = S 0 + T I T U Longitude T W y>0 = S 0 + T I T U T I v W v L x L W ' ms 1

21 Vertically integrated circulation Fig (Vallis, 2017)

22 Global deep circulation Stommel Arons ( )

23 Western Boundary Current found below Gulf Stream!

24 Western Boundary Current found below Gulf Stream! North Atlantic WOCE section N (Schott et al. 2004)

25 Deep WBC Atlantic Pacific Hogg (2001)

26 Where does water rise? Munk s Abyssal Recipes

27 Abyssal Recipes u r z (apple z ) Depth [m] Latitude Neutral density [kg/m 3 ]

28 Abyssal Recipes da @ apple T Depth [m] Latitude Neutral density [kg/m 3 ]

29 Abyssal Recipes Munk found that vertical profiles of density and 14 C in abyssal Pacific are consistent with 1-D w = 2 2 w0 (z) / exp z apple T Least-square curve field yield w 0 apple T ' 1.2 km 1 Upwelling in Pacific w 0 = S 0 A = m 3 s m 2 ' 10 7 ms 1

30 Abyssal Recipes Munk found that vertical profiles of density and 14 C in abyssal Pacific are consistent with 1-D w = 2 2 w0 (z) / exp z apple T Least-square curve field yield w 0 apple T ' 1.2 km 1 Turbulent diffusivity apple T ' 10 7 ms m 1 ' 10 4 m 2 s 1

31 Turbulent mixing Mixing in the deep ocean below1000 m is turbulent apple T 10 4 m 2 s 1 molecular di usion Turbulent mixing is typically associated with breaking internal waves Ri N 2 large for geostrophic z u 2 = O(1) for internal waves Smyth et al. (2001)

32 Abyssal circulation and κt

33 Observations of abyssal circulation: WOCE era

34 Abyssal overturning circulation Abyssal cell Deep cell Lumpkin and Speer (2007)

35 Abyssal overturning circulation Lumpkin and Speer (2007)

36 Abyssal overturning circulation Lumpkin and Speer (2007)

37 Conclusions The deep ocean circulation is fed by deep convection - in the North Atlantic - around Antarctica The deep waters return to the surface - pulled by winds above 2000m the Southern Ocean - through mixing in the abyssal Atlantic, Indian and Pacific Oceans Ferrari (2014)