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Tsuchiya Jets: A tropical arrested front?

Tsuchiya Jets: A tropical arrested front?. Jay McCreary. A short course on: Modeling IO processes and phenomena. University of Tasmania Hobart, Tasmania May 4–7, 2009. References.

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Tsuchiya Jets: A tropical arrested front?

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  1. Tsuchiya Jets: A tropical arrested front? Jay McCreary A short course on: Modeling IO processes and phenomena University of Tasmania Hobart, Tasmania May 4–7, 2009

  2. References • McCreary, J.P., P. Lu, and Z. Yu, 2002: Dynamics of the Pacific subsurface countercurrents. J. Phys. Oceanogr., 32, 2379–2404. • Furue, R., J.P. McCreary, Z. Yu, and D. Wang, 2007: Dynamics of the Southern Tsuchiya Jet. J. Phys. Oceanogr., 37 (3), 531–553. • Furue, R., J.P. McCreary, and Z. Yu, 2009: Dynamics of the northern Tsuchiya Jet. J. Phys. Oceanogr., in press.

  3. Introduction • Background • Layer ocean model (LOM) solutions • OGCM solutions • Summary

  4. Background

  5. Observed Tsuchiya Jets TJs 165ºE (west) 155ºW (center) 110ºW (east) u T 8S 8N Eq. Johnson et al. (2002) Thermostad The TJs and the EUC both riseand lighten to the east, and the TJs diverge from the equator There often appear to be two southern TJs

  6. Theories • Remote forcing • Linear wave dynamics (McPhaden 1984) • Inertial jet (Johnson & Moore 1997) • Arrested front(McCreary et al. 2002). TJ? TJ? • Local (y-z) forcing • Conservation of angular momentum (Marin et al. 2000, 2003; Hua et al. 2003) • Eddy forcing (Jochum & Malanotte-Rizzoli 2004; Ishida et al. 2005)

  7. LOM solutions

  8. 2½-layer model Equations:Equations of motion for the analytic model are: (1) where the pressure gradients are and If h1 become less than he1, w1 instantly adjusts it back to he1. The interesting features of the solutions discussed below all happen because of the nonlinear terms. For example, n = 2 Rossby waves no longer propagate only westward.

  9. 2½-layer model h equation:Neglecting horizontal mixing and in regions where w'1 = 0, (1) can be solved for a single equation in h, yielding (2) ht + where is the speed of the n = 2 Rossby waves, In a 2-layer model, one can retain ht, modifying (2) as indicated. In this form, it is clear that baroclinicRossby waves don’t propagate due westward. An additional component of their propagation velocity is the geostrophic current of the background Sverdrup flow. are the depth-averaged geostrophic currents associated with the Sverdrup circulation, and is the Sverdrup transport streamfunction.

  10. 2½-layer model Characteristics: Equation (2) can be solved by finding the characteristic curves, xc and yc, which are integrations of (3) where s is a time-like parameter. The solution follows from the property that h is constant along characteristics. Boundary conditions: The solution requires knowledge of h and h1 at some boundary of the domain. Interior h1:To evaluate cr, h1 must also be known in the interior ocean. It is determined from the constraint where H1 and H are values of h1 and h at the eastern boundary, xe.

  11. 2½-layer model Geostrophic streamlines:The characteristic curves are parallel to h isolines. Thus, in regions where h varies on the domain boundary, h contours are geostrophic streamlines for the layer-2 flow. Arrested fronts and shocks:Stationary fronts (shocks) form wherever characteristics converge and intersect in the interior ocean (Dewar, 1991, 1992). Rossby waves: In transient situations, n = 1 Rossby waves propagate westward, as expected from linear theory. In contrast, n = 2 Rossby waves propagate along characteristics.

  12. 2½-layer model H = 1000 m τy H = 300 m T1 = 29ºC Characteristics change markedly depending on model parameters. As the nonlinearities strengthen, they bend more equatorward, eventually intersecting to form a shock.

  13. 2½-layer model H = 1000 m Numerical analogs of the analytic solutions are driven by wind stress and inflow into layer 2 and outflow from layer 1. τy The outflow drains water from layer 1 until h1 becomes less than he1 somewhere in the domain. Eventually, mass balance is attained in which inflow = upwelling = outflow. H = 300 m As nonlinear terms strengthen, the model TJ becomes narrower and hence swifter, but its transport is unchanged. T1 = 29ºC

  14. 2½-layer model The TJ is not visible in the total transport field (h1v1 + h2v2), which is in Sverdrup balance, with boundary layers along the equator and 30ºS. There is strengthened westward flow in layer 1 that compensates for the eastward TJ, an indication that it is generated by a signal like the second baroclinic mode. There appears to be an analog to this situation in the subtropical SIO. There, subduction (the reverse of upwelling) generates a stronger than expected (i.e., stronger than Sverdrup) subsurface westward flow, which is compensated for by near-surface eastward flow.

  15. 2½-layer model H = 1000 m τxe H = 300 m T1 = 29ºC Characteristics change markedly depending on model parameters. As the nonlinearities strengthen, they bend more equatorward, eventually intersecting to form a shock.

  16. 2½-layer model H = 1000 m Numerical analogs of the analytic solutions are driven by wind stress and inflow into layer 2 and outflow from layer 1. τxe The outflow drains water from layer 1 until h1 becomes less than he1 somewhere in the domain. Eventually, mass balance is attained in which inflow = upwelling = outflow. H = 300 m As nonlinear terms strengthen, the model TJ becomes narrower and hence swifter, but its transport is unchanged. T1 = 29ºC

  17. 4½-layer model Meridional section at 140ºW from the 4½-layer solution. Each layer represents a specific water-mass type. The solution is forced by Hellerman and Rosenstein winds and by an ITF transport of 10 Sv. thermostad The model develops a realistic equatorial thermal structure, including thermal fronts at the edge a thermostad (layer 3).

  18. 4½-layer model The TJs still exist in layer 3 of a more realisticmodel forced by realistic winds, and with a realistic upper-ocean circulation. There are NP and SP STCs in layers 1 and 2, with subduction in the subtropics and upwelling in the eastern, tropics. The outflow drains water from layers 1 and 2 water until h1 and h2 become less than he1and he2, in which case water upwells from layer 3 into layer 2.

  19. OGCM solutions

  20. OGCM solutions • Forcing • Idealizedτx,τy • Inflowof cool water (7.5 Sv;6oC–14oC) thru s.b. • Outflow of warm water from 2oN–6oN thru w.b. • Relax SST toT*(y) = 15oC–25oC. 10N Eq 40S 100o • Mixing • To minimize diffusion, we set KV0 = 0 in the P-P vertical diffusion, and … • … only allow isopycnal diffusion (107 cm2/s) when |dz/dx| > a critical slope • Third-order upstream advection scheme is weakly diffusive. • Laplacian horizontal viscosity (108 cm2/s) with 20×108 cm2/s in the WBL. Configuration • COCO 3.4(Hasumi at CCSR, U Tokyo): level model; primitive equations on spherical coordinates. • 2o×1o×36 levels → no eddies • Constant salinity • Box ocean: 100o×(40oS–10oN) × 4000 m for southern TJ

  21. Hierarchy of solutions 14oC–6oC, yr 120 Layer 2 is defined by the integral No wind Without wind, there is no interior Sverdrup flow. As a result, water flows directly from the inflow to the outflow port.

  22. Hierarchy of solutions ty without curl (zonally uniform) Because of ty,upwelling shifts to the eastern boundary. Because ty has no curl, there is still no interior Sverdrup flow and hence no vg. So, layer-2 water flows zonally across the basin to supply water for the upwelling. 14oC–6oC, yr 120 ty= t0Y(y) t0 = 1dyn/cm2

  23. Hierarchy of solutions vg u, T 14oC–6oC, year 120 tywith curl Because ty has curl, there is an interior Sverdrup flow with a northward vg. Now, layer-2 water bends equatorward to the westto form an interior jet,the model TJ. 0 dyn/cm2 14oC–6oC, yr 120

  24. Hierarchy of solutions u,T 14oC–6oC, yr 120 tx+ ty(control run) Because of the additional zonal wind, vg increases. As a result, the model TJ bends more equatorward, narrows, and strengthens. tx= t0X(x)Y(y) t0 = 0.5 dyn/cm2

  25. TJ pathways (control run) 12oC–11oC The shallow part of the TJat 50ºE, shifts southward to join the top of the TJat 80ºE The deep part of the TJat 50ºE, shifts southward and weakens to the east. By 80ºE, it lies outside the main jet. 10oC–9oC Due to these processes, both the TJ and EUC rise and warm to the east, consistent with the observed TJ.

  26. TJ pathways (higher resolution run) PV & (uh, vh) PV, hv • 1º×¼ºnear the equator, and low viscosity • Now, EUC water first reverses to flow westward before joining the TJ What happens as resolution is increased further, and the system enters an eddy-resolving regime?

  27. No inflow/outflow No I/O Control The TJ weakens, and its core temperature rises by 2.5°C.

  28. Southern TJ in a global model These properties suggest that the model TJ is supplied primarily by an overturning cell internal to the Pacific, one that is somewhat broader and deeper than the STCs. • A southern TJ exists in a global GCM(1°×1°×40 levels; Nakano, 2000) with both open and closed IT passages • With closed passages, the TJhas almost the same strengthbut its core is 1°C warmer Open Closed 130°W 130°W

  29. Summary

  30. The TJs are driven by off-equatorial upwellingin the tropics. The upwelling for the southern TJ occurs along the South American coast. The upwelling for the northern TJ occurs in the in the eastern ocean in the ITCZ band (6ºN to 12ºN), most notably in the Costa Rica dome. • Our solutions indicate that the TJs are geostrophic currents flowing along arrested fronts. Arrested fronts are generated when characteristics, associated with different h values, converge or intersect in the interior ocean. As a result, h jumps across the front. • The top of the TJs are fedin part by the bottom of the EUC. As a result, both the EUC and the TJs rise in the water column and shift to lighter densities toward the east. • The TJs weaken and warm when there is no IT. This happens because the IT drains water from the upper layers, intensifying the upwelling from the TJ layer.

  31. Arrested fronts in a 2½-layer model Eq. vg 30°S 0° 100° Forced by winds like those in the South Pacific, vg bends characteristics meridionally. Consistent with the observed TJ, an arrested front occurs where characteristics overlap and the front shifts southward to the east. A numerical solution illustrates the characteristic solution. It is forced by 1) winds and 2) by an inflow into layer 2 and an ouflow from layer 1. Layer-1 water is drained from the system, and hence there is eastern-boundary upwelling. An analogous solution exists for the northern TJ. In this case, there is upwelling in the Costa Rica dome. In steady state, the total thickness field, h = h1 + h2, satisfies where ugand vg aregeostrophic components of Sverdrup flowand cr is the speed on a non-dispersive, n = 2, Rossby wave.

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