Jay McCreary

1. Dynamics of Indian-Ocean shallow overturning circulations Jay McCreary A short course on: Modeling IO processes and phenomena University of Tasmania Hobart, Tasmania May 4–7, 2009

2. References • Miyama, T., J. P. McCreary, T.G. Jensen, S. Godfrey, and A. Ishida, 2003: Structure and dynamics of the Indian-Ocean Cross-Equaotorial Cell. Deep-Sea Res., 50, 2023–2048. • (MKM93)McCreary, J.P., P.K. Kundu, and R. Molinari, 1993: A numerical investigation of dynamics, thermodynamics and mixed-layer processes in the Indian Ocean. Prog. Oceanogr., 31, 181–244. • (SM04) Schott, F., J.P. McCreary, and G.C. Johnson, 2004: Shallow overturning circulations of the tropical-subtropical oceans. In: Earth Climate: The Ocean-Atmosphere Interaction, C. Wang, S.-P. Xie and J.A. Carton (eds.), AGU Geophys. Monograph Ser., 147, 261–304.

3. Questions • What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation? • What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell? • What are their fundamental dynamics? • What is their impact on the Indian-Ocean heat budget?

4. 2d structure in an idealized GCM solution SPC STC AMOC Bryan (1991) What are the 3-d structures of these cells? How do they vary on climatic time scales?

5. Subtropical Cells (STCs) in the Pacific Ocean Subtropics Tropics Subtropics Lu et al. (1998) The STCs carry cool subtropical thermocline water into the tropics. The two cells account for almost 30 Sv of overturning.

6. 3d structure in a GCM solution surface upwelling thermocline subduction subduction Rothstein et al. (1998)

7. Questions • What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation? • What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell? • What are their fundamental dynamics? • What is their impact on the Indian-Ocean heat budget?

8. Upwelling, subduction, and inflow/outflow regions in Indian Ocean Indian upwelling Somali/Omani upwelling Sumatra/Java upwelling 5-10°S upwelling Indonesian Throughflow Subduction Agulhas Current Southern Ocean

9. Meridional streamfunction from an IO GCM Equatorial roll Shallow cells Deep cell Equator C.I. = 1 Sv Garternicht and Schott (1997) from global GCM (Semtner)

10. Models used in Miyama et al. (2002) • MKM • 2½-layer model (0.5°) • 2) TOMS • 4½-layer model (0.33°) • 3) JAMSTEC • GCM (55 levels, 0.25°) • 4) SODA reanalysis • GCM + data • 5) LCS model

11. Annual-mean, layer-2 circulation in MKM model Layer 1 Layer 2 Subtropical Cell

12. Subsurface circulation of CEC (backward tracking from upwelling regions) MKM TOMS Subsurface water crosses the equator in a western boundary, a consequence of PV conservation

13. Subsurface circulation of CEC (backward tracking from upwelling regions) JAMSTEC Subsurface water crosses equator in a western boundary current, a consequence of PV conservation.

14. Surface circulation of CEC (forward tracking from upwelling regions) MKM TOMS Surface water crosses equator in interior ocean, increasingly to the east for Somali, Omani, and Indian upwellings

15. Surface circulation of CEC (forward tracking from upwelling regions) JAMSTEC In GCMs, surface water tends to flow across the basin in the interior ocean and only crosses the equator in the eastern basin. Particle trajectories show equatorial rolls.

16. Equatorial roll in JAMSTEC model Equator

17. Surface (10 m) trajectories in JAMSTEC model January July Surface trajectories cross equator in the eastern ocean because of equatorial roll, consistent with observed drifters.

18. Annual-mean, surface (0–75 m) circulation in SODA reanalysis Near-surface currents cross equator in the eastern ocean because of equatorial roll, consistent with observed drifters.

19. 3d structure of CEC in JAMSTEC model

20. Questions • What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation? • What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell? • What are their fundamental dynamics? • What is their impact on the Indian-Ocean heat budget?

21. STC dynamics

22. Wind forcing for STC Wind curl along the northern edge of Southeast Trades

23. Basic processes for STC Finally, the subsurface flow also includes the circulation of the Subtropical Gyre. As a result of all of these contributions, layer-2 STC water enters the upwelling region from the north. The water that upwells first flows northward along the western boundary and then eastward across the basin, a remotely forced response due to the radiation of Rossby waves from the upwelling region. There is an additional recirculation, the so-called “β plume.” Consider the response in layer 2 of a 2½-layer model forced by a mass sink(upwelling into layer 1) south of the equator. Eq.

24. Basic processes for STC Consider the response in layer 2 of a 2½-layer model forced by a mass sink(upwelling into layer 1) south of the equator.

25. CEC dynamics • Why does surface water cross the equator in the interior ocean? • What causes the equatorial roll?

26. Wind forcing for CEC The IO winds circulate clockwise (anticlockwise) about the equator during the summer (winter). The annual-mean winds have the summer pattern.

27. Basic processes for CEC Ekman Transport EQ Wind (boreal Winter) Ekman Transport EQ Wind (boreal Summer, annual mean) Ekman transport appears to be involved offthe equator. But, what dynamics are involved near the equator?

28. Analytic solution Consider forcing by τx that is antisymmetric about the equator The Sverdrup transport is but V can be rewritten Thus, for this special wind the Sverdrup and Ekman transports are equal. It follows that the concept of Ekman flow can be extended to the equator, since τxtends to zero as f does.

29. Consider the equations for a 1½-layer model, • Then, • For a τxthat is antisymmetric about the equator • and so h never changes in response to this wind! So, no geostrophic currents are ever generated, and the total flow field is entirely Ekman drift.

30. Linear, continuously stratified (LCS) model • Model equations of motion linearized about a state of rest and Nb(z) • Solutions expressed as sums of 50 vertical modes • Horizontal resolution is 0.25° • Realistic Indian-Ocean coastline • Forced by Hellerman and Rosenstein (1983) winds • Spun up for 10 years

31. Symmetric zonal wind meridional velocity

32. Antisymmetric zonal wind meridional velocity

33. CEC dynamics • Why does surface water cross the equator in the interior ocean? • What causes the equatorial roll?

34. Symmetric meridional wind Section at 70 E meridional velocity

35. Roles of zonal and meridional winds 2) Zonal wind 1) Total wind 3) Meridional wind Courtesy of Toru Miyama LCS solution forced by July HR winds. Cross-equatorial flow is driven by τx(middle), and equatorial roll is driven by τy.

36. Comparison of LCS and GCM solutions Courtesy of Toru Miyama Meridional velocityzonally averaged between 40–100ºE. The linear model reproduces the GCM solution very well!

37. Questions • What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation? • What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell? • What are their fundamental dynamics? • What is their impact on the Indian-Ocean heat budget?

38. So, the heat flux into the ocean is caused by oceanic upwelling. Advection then spreads cool SSTs away from the upwelling region, causing heating over a larger area.

39. … that vanishes when cooling due to upwelling is dropped from the model. In this model, then, the annual-mean heating happens entirely because of upwelling. There is a net annual-mean heat flux into the Indian Ocean, … How model dependent is this result? Perhaps in this model it is overemphasized because heating in the 5–10°S band is too strong.

41. Conclusions Subtropical Cell Driven by upwelling caused by Ekman pumping at the northern edge of the Southeast Trades (5–10ºS). Subsurface water for the upwelling comes from the north, due to the formation of a “β-plume.” Cross-equatorial Cell Driven by upwelling in the northern ocean. Its source waters are all from the southern hemisphere, requiring cross-equatorial flow. Subsurface flow crosses the equator only near the western boundarydue to PV conservation. Near-surface water crosses the equator in the interior ocean. It is driven by the antisymmetric component of the zonal wind, which drives a southward, annual-mean, cross-equatorial Ekman drift. Because of the equatorial roll, the CEC surface branch dives below the surface as it crosses the equator. Moreover, flow right at the surface (e.g., as measured by surface drifters) can cross only near the eastern boundary. Heat flux The observed annual-mean heat flux into the IO exists only because of upwelling associated with the STC and CEC.

44. Wind forcing for CEC Reversing cross-equatorial winds Upwelling-favorable annual-mean winds (dominated by July) As a result, the IO winds circulate clockwise (anticlockwise) about the equator during the summer (winter). The annual-mean winds have the summer pattern.

45. Linear, continuously stratified (LCS) model To model the mixed layer, wind stress enters the ocean as body force with structure Z(z). Equations: A useful set of simpler equations is a version of the GCM equations linearized about a stably stratified background state of no motion. The resulting equations are where Nb2= –gbz/ is assumed to be a function only of z. Vertical mixing is retained in the interior ocean.

46. Vertical modes: With the assumptions that ν = κ = A/Nb2(z), the ocean has a flat bottom, and convenient surface and bottom boundary conditions, solutions can be represented as expansions in the normal (barotropic and baroclinic) modes, ψn(z), of the system. Expansions for the u, v, and p fields are where the expansion coefficients are functions of only x, y, and t. The resulting equations for un, vn, and pn are Thus, the ocean’s response can be separated into a superposition of independent responses associated with each mode.