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Dynamics of the Tsuchiya Jets

Ryo Furue ( 古恵 亮 , IPRC, U of Hawaii)In collaboration with Jay McCreary, Zuojun Yu & Dailin Wang

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Introduction

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Observed Tsuchiya Jets

Johnson et al. (2002)ThermostadThermostad

Eq.Eq.8S8S 8N8N400m400m

0m0m

165165ooE (west)E (west) 155155ooW (center)W (center) 110110ooW (east)W (east)

uu

TT

TJsTJs

Eq.Eq.8S8S 8N8N Eq.Eq.8S8S 8N8N

TJs shift poleward and to higher temperatures as they flow eastward

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Fate of sTJ

The fate of the southern TJ is not clear. Recirculates to flow westward in EIC

(Rowe et al. 2000)? Upwells at the coast of Peru? The “primary” sTJ recirculates and

“secondary” sTJ upwells at Peru (Ishida et al. 2005)?

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nTJ and the Costa Rica Dome

Kessler (2002, 2006)

nTJ transport ≈ 6 SvCRD upwelling ≈ 3 Sv

nTJ is a beta plume driven by CRD upwelling?

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Theories Conservation of angular momentum

(Marin et al. 2000, 2003; Hua et al. 2003).

Eddy forcing (Jochum & Malanotte-Rizzoli 2004).

McPhaden-type linear dynamics (McPhaden 1984; McCreary 1981).

Inertial jet (Johnson & Moore 1997).

Arrested front (McCreary et al. 2002).

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Arrested fronts

McCreary et al.’s (2002) 2 ½-layer model: Layer 2 contains the TJ; Streamfunction h characteristics:

ug and vg are geostrophic components of Sverdrup flow;

vg bends characteristics meridionally; Arrested front occurs where

characteristics overlap.

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Northern TJ as arrested front

(McCreary et al. 2002)

vg

Recirculation around a patch of upwelling driven by wind curl forms a near-equatorial front due to the bending of characteristics.

Eq.Eq.

30N30N

100100°°00°°

AnalyticalAnalytical NumericalNumerical

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Objective

To determine if “arrested-front” TJs exist in an OGCM.

Arrested-front solutions are reproduced. Plus some new features.

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Ocean model

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Model & Configuration COCO 3.4 (Hasumi at CCSR, U Tokyo) 2o× 1o× 36 levels no eddies. Box ocean: 100o × [40oS–40oN] × 4000 m Constant salinity.

Eq.

40N

40S

wE

100o

Northern TJ

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Forcing Inflow of cool water (14oC–6oC)

thru the s.b. (7.5 Sv)

Outflow of warm water thru the w.b. at 2oN–6oN.

SST: relaxed to T*(y) = 15oC–25oC. Basin-wide x, representing trades. y coastal upwelling in the south Pac. e, “Costa Rica Dome” wind patch.

Must upwell!

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Mixing

P.-P. vertical diffusion with b = 0.

Isopycnal diffusion (107 cm2/s); KH = 0.

diffusive only when |dz/dx| > critical. Laplacian horizontal viscosity (108 cm2/s)

with 20×108 cm2/s in the WBL. Third-order upstream advection scheme

weakly diffusive.

To minimize diffusion…

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Northern TJ

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No wind

The inflow water flows along the southern and western boundaries and directly exists through the outflow port.

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CRD patch only14oC–6oC CRD

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CRD patch only14oC–6oC

CRDy

CRD

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Standard solution

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Sublayers (lower)

10oC–9oC

11oC–10oC

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Sublayers (upper)

12oC–11oC

14oC–12oC

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Summary of the nTJ solutions

The summary is almost the same as that of sTJ.

A nTJ and thick thermostad are reproduced.

The hierarchy of solutions agrees with 2½ -layer ones.

The nTJ becomes warmer to the east because it is supplied by water that diverges from the lower part of the EUC.

But, there is one more interesting thing….

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Beta plume and eddy form stress

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Vertical structure of the subsurface recirculation gyres

Why is the nTJ so deep? What drives the other, cyclonic

recirculation gyre?

Eddies (slow instability waves in the CRD region with a period ~1 yr).

An extended beta plume: V = fwe Ux + Vy = we

+ curlform stress

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Beta plume driven by we

Subsurface circulation

Us from OGCM and from diag. model

Subsurface recirculation gyre is a beta plume driven by we .

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Alternative formalisms of eddy flux

Eddy form stress (F*): vertical transfer of horizontal momentum.

Bolus transport (U*). Isopycnal PV flux.

Under geostrophy, these formalisms are largely equivalent (Greatbatch 1998).

curl F* ≈ f div U*, f u* = – hq u

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Alternative formalisms (cont’d)

In our e –only solution, curl F* ≈ f div U* holds, and a similar diagnostic model driven by the OGCM’s div U* reproduces the subsurface recirculation.

U* & div U*

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Subsurface circulation regimes

Strong forcing regime (Haidvogel & Rhines 1983; Berloff 2005) Under direct forcing; Upgradient (northward) PV flux; Southward bolus flux; Eastward acceleration in the middle.

eddy PV flux

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Circulation regimes (cont’d)

Weak forcing regime (Rhines & Holland 1979; Berloff 2005) Under indirect forcing; Downgradient (souththward) PV flux; Northward bolus flux; Westward acceleration in the middle.

eddy PV flux

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Eddy PV flux ~ U*

Baroclinic instability (mean PV gradient inversion)

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Conclusions

A non-eddy-permitting OGCM reproduces TJs with properties similar to those for arrested fronts.

The deep part of EUC leaves the equator to be the top part of the TJs.

eastward warming of the TJs.

Eddy form stress drives a deep, cyclonic and anticyclonic gyres.

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Conclusions (cont’d)

Diffusion of any sort acts to erode the thermostad and, hence, to weaken the TJs.