The Southern Ocean and Climate: What did we learn during WOCE?

1. The Southern Ocean and Climate:What did we learn during WOCE? Steve Rintoul CSIRO Marine Research and Antarctic CRC Australia

2. Pre-WOCE view of the ACC/SO • 2 circumpolar fronts • wind-driven, in (flat-bottom) Sverdrup balance • bottom form stress balances wind? • Drake Passage transport = 134±13 Sv • transport variability is barotropic • no net meridional flow through Drake Passage gap • poleward eddy heat flux in Drake Passage, SE NZ • zonal circulation independent of meridional circulation • water masses exported to lower latitudes, but rates and mechanisms unknown

3. Progress in the “WOCE era” • remote sensing (SST, SSH) • new instruments (e.g. ALACE floats) • observations outside of Drake Passage • improved model realism/resolution/diagnostics • air-sea flux estimates from reanalyses • advances in dynamical understanding

4. 10,000 stations south of 25S since 1990 Orsi, 2002

7. Oxygen on 27.4

9. 4-year mean SST gradient from ATSR reveals multiple filaments and branches, which merge and split. Rintoul, Hughes and Olbers 2001

10. Tracking ACC fronts using satellite altimetry Careful comparison of hydrography and absolute sea surface height maps shows each frontal branch corresponds to a particular SSH contour. We can use altimetry to track fronts, every 10 days since 1992. Sokolov and Rintoul, JMS, 2002

11. SAF: 3 branches, merge near 140E, eddy-rich downstream of change in orientation of SEIR. • PF: 2 branches, separated by >500 km at SR3, merge after crossing ridge crest. • PF, SACCF: strong equatorward deflection over ridge. • Narrow meander envelopes near ridge.

12. ACC Transport Repeat sections show heat transport south of Australia varies by 0.6 x 1015 W (relative to 0C). Variability is large (e.g. relative to north-south heat flux in Indian and Pacific.) Climate impact? Rintoul and Sokolov, JGR, 2001

13. Drake Passage transport: 1368.5 Sv Cunningham et al., JGR, 2002

15. ACC transport  500 billion Lone Stars/sec www.mylifeisbeer.com

16. ACC transport in neutral density layers Australia (SR3) color; Drake Passage (SR1) black Rintoul and Sokolov, 2001; Cunningham et al., JGR, 2002

17. The tight relationship between temperature at 650 m and the baroclinic transport streamfunction can be used to determine transport (above 2500 m) from temperature msmts. alone. Rintoul, Sokolov and Church, JGR, 2002

18. Net baroclinic transport time series from XBT data (squares) and CTD data (diamonds)

19. Net baroclinic transport south of Australia (1993-2000) Empirical relationship between surface height and transport fn used to estimate transport. Continuous record from altimeter shows XBT time series is aliased. Transport estimated from altimeter (thin line), low-passed (thick blue line). Rintoul, Sokolov, Church, 2002

20. “Streamwise” average of absolute velocity of Subantarctic Front: Total transport = 116 Sv; barotropic = 16 Sv. Phillips and Rintoul, JPO, 2002

21. Eddy heat flux Poleward eddy heat flux across SAF south of Australia is larger than previously measured elsewhere in the Southern Ocean. Phillips and Rintoul, JPO, 2000

22. Is the ACC in Sverdup balance? ßx = pb  H +  + F Bottom pressure torque (color); barotropic streamfn (black) Rintoul, Hughes and Olbers 2001

23. Steady, zonally-integrated momentum balance: 1 -fV1 = - '1p'1x + o - R1 Surface (includes Ekman) 2 -fV2 = '1p'1x -'2p'2x - R2 “unblocked” layer 3 -fV3 = '2p'2x - hpbx - R3 “blocked” layer V = net meridional volume flux o = wind stress  = layer thickness p = pressure R = Reynolds stress divergence pb = bottom pressure

24. No interfacial form stress: V1 = - o/f Ekman transport in surface layer V2 = 0 No transport in “unblocked” layer V3 = hpbx /f = o/f Deep geostrophic flow balances Ekman  Overall balance of zonal momentum is between wind stress and bottom form stress.

25. Interfacial form stress  0: Adding the three equations and using fact that mass is conserved ((Vi) = 0): o = hpbx  Again, overall balance of zonal momentum is between wind stress and bottom form stress.

26. Adiabatic flow (Vi = 0): o = 'ip'ix=hpbx  Wind stress = interfacial form stress = bottom form stress Note that both standing and transient eddies contribute to interfacial form stress.

27. Diabatic flow (Vi 0): Mixing and surface buoyancy fluxes drive mass exchange between layers, so Vi = net diapycnal exchange  0. z('ip'ix)  0  Divergence of interfacial form stress drives meridional flow in the unblocked layer.  Buoyancy forcing, eddy stresses, and meridional flow are intimately linked to the zonal momentum balance.

28. What controls the transport of the ACC? Observations and a variety of models suggest ACC transport is a function of: • n (n = 0-1?) •  x  • buoyancy flux • topographic interactions • baroclinic instability / eddy fluxes (Gent, Tansley, D. Marshall, J. Marshall, Karsten, Olbers, Rintoul, Sokolov, Gille, Gnanadesikan, Hallberg, …)

29. Schmitz (1996)

31. Orsi et al., 1999

32. CFC inventory: 8 Sv AABW; 21 Sv total input to deep ocean Orsi et al., JGR, 2002

34. SO Overturning • By including the water mass transformations driven by air-sea fluxes, we can quantify the overturning circulation for the first time. • vigorous deep cell • weak upwelling through the thermocline • NADW global cell closed by DW  IW conversion in SO

35. 34 4 2 eddy mass flux 52 46 Speer et al., 2000; Sloyan and Rintoul, JPO, 2001

36. Models also suggest the NADW overturning cell is closed by upwelling and water mass transformation in the SO. Döös and Coward (1997)

37. Formation, circulation and consumption of intermediate and thermocline waters. 11 4 10 13 30 25 8 8 Sloyan and Rintoul (2001)

38. Upper branch of the global OTC “cold” = 6.5 Sv “warm” = 5.3 Sv “cool” = 3.1 Sv Speich et al., GRL, 2001

39. Intermediate depth waters in both hemispheres have become fresher in recent decades. Wong et al., 1999

40. Climate models show similar response; suggest strongest ocean climate change signal in SO. Banks et al., GRL, 2000

41. Observations south of Australia show large variability in mode water properties from year-to-year, driven by changes in cross-frontal Ekman transport (not air-sea fluxes). Circles show T-S properties of SAMW south of Tasmania; size of dot is proportional to strength of mode. Triangles and squares are data from 1968 and 1978. Rintoul and England, JPO, 2002

43. Warming of the Southern Ocean Gille, Science, 2002

44. Warming of Weddell Sea Warm Deep Water Warm Deep Water flowing into and out of the Weddell Sea has warmed by about 0.3C since the mid-1970’s. (Robertson et al., 2002)

45. Climate models suggest SO overturning will slow down as a result of global warming. Warming and freshening increases the high latitude stratification, shutting down AABW formation. Is this result realistic? Can we observe the change in stratification? Hirst (1999)

46. The Southern Ocean is the largest zonally-integrated sink of anthropogenic CO2. Sabine et al., 2002

47. Massom et al., 2001

48. Southern Annular Mode/Antarctic Oscillation Thompson and Solomon, Science, 2002

49. Antarctic Circumpolar Wave White and Peterson, 1996

50. Air temperature Sea ice extent Antarctic Dipole Subtracting May composites for El Nino and La Nina events reveals the impact of ENSO on the Southern Ocean. Response consists of a dipole with centres in the Atlantic and Pacific sectors, driven by the PSA teleconnection. (Yuan, 2001). SLP: El Nino SLP: La Nina