2021/05/27 Hokkaido University EOAS seminar

1. 2021/05/27 EOAS seminar ＠ Online
Behavior of ocean eddies in the Southern Ocean
南大洋における海洋渦の振る舞い
Kaihe Yamazaki
1. Large-scale circulation
2. Temporal variability

2. Talley et al. (2013)
Southern Ocean: the hub of meridional overturning
1. Large-scale circulation 1/21

3. https://www.youtube.com/watch?v=8VMSF28J9H4
Southern Ocean circulation
2/21

4. Meridional Overturning – volumetric perspective
Antarctic Circumpolar Current
~120 Sv
Antarctic Slope Current
~10 Sv
3/21

5. Meridional Overturning – dynamical perspective
Marshall, J., and T. Radko, 2003: Residual-mean solutions for the Antarctic Circumpolar Current and its associated overturning circulation. J. Phys. Oceanogr., 33, 2341–2354.
J. Marshall, K. Speer, Closure of the meridional overturning circulation through Southern Ocean upwelling. Nat. Geosci. 5, 171–180 (2012).
Zonal momentum balance:
(wind stress) ~ (topographic form stress)
Internal balance:
(Coriolis force) = (eddy form stress)
“Thickness diffusion” by baroclinic eddies
drives residual transport
Thinner poleward
Potential vorticity (thickness) gradient
4/21

6. https://youtu.be/t-FqF6PFDP0?t=10
Eddies and instabilities
5/21

7. Eddy “saturation” in the Southern Ocean
= the insensitivity of the circumpolar transport with respect to wind changes
Geostrophic transport is saturated due to prevailed baroclinic instability
Marshall, D. P., M. H. P. Ambaum, J. R. Maddison et al. (2017), Eddy saturation and frictional control of the Antarctic Circumpolar Current, Geophys. Res. Lett., 44, 286–292
Straub, D.N., 1993. On the transport and angular momentum balance of channel models of the Antarctic circumpolar current. J. Phys. Oceanogr. 23, 776–782.
insensitive...
first argued by Straub (1993)
6/21

8. Eddy “compensation” in the Southern Ocean
= the eddy transport counteracting to the wind-driven meridional overturning
Meredith, M. P., Naveira Garabato, A. C., Hogg, A. M., & Farneti, R. (2012). Sensitivity of the Overturning Circulation in the Southern Ocean to Decadal Changes in Wind Forcing, Journal of Climate, 25(1), 99-110.
Marshall, J., and T. Radko, 2003: Residual-mean solutions for the Antarctic Circumpolar Current and its associated overturning circulation. J. Phys. Oceanogr., 33, 2341–2354.
coarse CM
eddy-permitting
CM
stronger wind
case
Eddy-driven overturning vs wind forcing
conceptualized by Marshall and Radko (2003)
(Residual overturning) = (Wind-driven overturning) + (Eddy-driven overturning)
Westerlies
7/21

9. Suppression of eddy diffusivity across jets
Eddy transport is localized in the “leaky” jets
well predict!
Ferrari, R., & Nikurashin, M. (2010). Suppression of Eddy Diffusivity across Jets in the Southern Ocean, Journal of Physical Oceanography, 40(7), 1501-1519.
Naveira Garabato, A. C., R. Ferrari, and K. L. Polzin, 2011: Eddy stirring in the Southern Ocean. J. Geophys. Res., 116, C09019.
Klocker, A., & Abernathey, R. (2014). Global Patterns of Mesoscale Eddy Properties and Diffusivities, Journal of Physical Oceanography, 44(3), 1030-1046
Eddy diffusivity across a broad, parallel oceanic jet:
(Ferrari and Nikurashin, 2010)
MKE > EKE: MKE < EKE:
Mean flow suppression factor:
8/21

10. Poleward transport of Circumpolar Deep Water
Tamsitt, V., Drake, H.F., Morrison, A.K. et al. Spiraling pathways of global deep waters to the surface of the Southern Ocean. Nat Commun 8, 172 (2017).
Topographic control of the eddy transport
Source of heat and salt
9/21

11. How eddies facilitate the onshore CDW intrusion?
Hotly debated realm…
Thompson, A., Heywood, K., Schmidtko, S. et al. Eddy transport as a key component of the Antarctic overturning circulation. Nature Geosciences, 7, 879–884 (2014).
Stewart, A. L., Klocker, A., & Menemenlis, D. (2019). Acceleration and Overturning of the Antarctic Slope Current by Winds, Eddies, and Tides, Journal of Physical Oceanography, 49(8), 2043-2074.
Yamazaki, Mizobata, Aoki et al., in preparation
Glider observation Slope current dynamics Mixing length framework
Tidal advection?
Transient jets?
Along-slope variability?
Thickness driven?
10/21

12. Multidecadal warming of the Southern Ocean
S. Schmidtko, K. J. Heywood, A. F. Thompson, S. Aoki, Multidecadal warming of Antarctic waters. Science. 346, 1227–1231 (2014).
M. Auger, et al., Southern Ocean in-situ temperature trends over 25 years emerge from interannual variability. Nat. Commun. 12, 514 (2021).
Thompson, A. L. Stewart, P. Spence, and K. J. Heywood, 2018: The Antarctic Slope Current in a changing climate. Rev. Geophys., 56, 741–770.
~0.1℃/decade off East Ant.
(1975-2012; Schmidtko+ 2014)
+0.06±0.02℃ /decade
↑ 38.5±0.02m /decade
Robust warming & shoaling in 140E
(Auger+ 2021)
Sallée, J.-B. 2018. Southern Ocean warming. Oceanography
31(2):52–62
Thompson+ (2018) adopted from Sallée+ (2013)
21 CMIP5 multimodel mean under RCP8.5
ΔΘ in CMIP5
(post-pre21C)
at 975m
→ 0.5-1.0℃
2. Temporal variability
Response to the ongoing/projected (anthropogenic) intensification of westerlies?
11/21

13. Four questions on CDW:
1. ACC’s shift and/or 2. eddy transport cause CDW warming?
Gille, S.T., D.C. McKee, and D.G. Martinson. 2016. Temporal changes in the Antarctic Circumpolar Current: Implications for the Antarctic continental shelves. Oceanography 29(4):96–105
12/21

14. but with a significant spatial variation!
1. ACC’s shift … ACC has not shifted on a circumpolar average
Gille, S.T., D.C. McKee, and D.G. Martinson. 2016. Temporal changes in the Antarctic Circumpolar Current: Implications for the Antarctic continental shelves. Oceanography 29(4):96–105
Y. S. Kim, A. H. Orsi, On the variability of antarctic circumpolar current fronts inferred from 1992-2011 altimetry. J. Phys. Oceanogr. 44, 3054–3071 (2014).
How influential to the continental margin?
Zonally averaged mean latitude of ACC transport
13/21

15. Relation to the meridional overturning?
2. eddy transport … eddy energy is well-correlated with winds
thin black:
SAM index
=SLP 1st EOF mode
stronger westerlies
Hogg, A. McC., M. P. Meredith, D. P. Chambers, E. P. Abrahamsen, C. W. Hughes, and A. K. Morrison (2015), Recent trends in the Southern Ocean eddy field, J. Geophys. Res. Oceans, 120, 257–267,
14/21

16. 2019 hydrography captured
the poleward shift of the Southern Boundary
of the ACC off East Antarctica
Yamazaki, K., Aoki, S., Katsumata, K., Hirano, D., Nakayama, Y., Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica, Science Advances, in printing
N. L. Bindoff, M. A. Rosenberg, M. J. Warner, On the circulation and water masses over the Antarctic continental slope and rise between 80 and 150°E. Deep. Res. Part II Top. Stud. Oceanogr. 47, 2299–2326 (2000).
A. H. Orsi, T. Whitworth, W. D. Nowlin, On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep. Res. Part I. 42, 641–673 (1995).
2019: solid
1996: dashed
SB SB
Isotherms and geostrophic streamlines shifted poleward
# SB is defined by the poleward limit of 1.5 degC
15/21

17. Snapshots vs Climatology
• Consistent SB’s shift (49±25 km, 0.1-0.5℃ /decade)
• Shift is most prominent in 120E
Yamazaki, K., Aoki, S., Katsumata, K., Hirano, D., Nakayama, Y., Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica, Science Advances, in printing
Create decadal climatology (OI)
Compile Θ-max dataset
CTD/XCTD, Argo float, Biologging
• 1dbar Akima-interpolated
• fetch Θ-max below ‘Θ-min above 500dbar’
16/21

18. Isopycnal change from 1996 to 2019
(Total warming in the CDW layer)
= (Warming due to frontal shift)
+ (along-frontal warming of upper 28.10 γn)
SB’s poleward shift is attributable to:
Shift of barotropic ACC fronts by ~ 70 %
Enhanced upper overturning by ~ 30 %
②along-frontal
total ①frontal shift
① ②
SB
upper 300dbar
total
Yamazaki, K., Aoki, S., Katsumata, K., Hirano, D., Nakayama, Y., Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica, Science Advances, in printing
17/21

19. Yamazaki, K., Aoki, S., Katsumata, K., Hirano, D., Nakayama, Y., Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica, Science Advances, in printing
(Poleward shift of southern boundary)
= (ACC’s shift) + (eddy overturning)
18/21

20. “Unsaturated” subpolar zone (southern mode)
Transport in the subpolar zone is more sensitive to winds than in the mainstream
Stronger westerlies
Stronger only in the subpolar zone
C. E. Langlais, S. R. Rintoul, J. D. Zika, Sensitivity of antarctic circumpolar current transport and eddy activity to wind patterns in the Southern Ocean. J. Phys. Oceanogr. 45, 1051–1067 (2015).
Logarithmic mean velocity
Discussion 1
19/21

21. Importance of recirculating gyres
L. P. Nadeau, R. Ferrari, The role of closed gyres in setting the zonal transport of the antarctic circumpolar current. J. Phys. Oceanogr. 45, 1491–1509 (2015).
Discussion 2
Even in a saturation regime, wind-driven gyre transport can increase
20/21

22. Take home messages
Thx! (^^) Today’s slide is accessible from my homepage (https://kaiheyamazaki.github.io/) and Twitter @KaiheZak
• Antarctic meridional overturning is driven by eddy transport
• Two features: eddy saturation & eddy compensation
• Localized eddy transport in the lee of topography
• Southern Ocean is warming (maybe due to stronger winds)
• Poleward expansion of ACC warms the continental margin
21/21