InteractionsInteractions between the Indonesian between the Indonesian Throughflow and circulations in the Throughflow and circulations in the

Indian and Pacific OceansIndian and Pacific OceansJay McCreary, Toru Miyama, Ryo Furue

Tommy Jensen, Hyoun-woo Kang,Bohyun Bang, Tangdong Qu

A short course on:Modeling IO processes and phenomena

University of TasmaniaHobart, TasmaniaMay 4–7, 2009

ReferencesReferences1) (PTNE) McCreary, J.P., T. Miyama, R. Furue, T. Jensen, H.-W. Kang,

B. Bang, and T. Qu, 2007: Interactions between the ITF and circulations in the Indian and Pacific Oceans. Prog. Oceanogr., 75(1), 70–114.

2) Godfrey, J.S., and A.J. Weaver, 1991: Is the LC driven by Pacific heating and winds? Prog. Oceanogr., 27, 225–272.

3) Hirst, A.C., and J.S. Godfrey, 1993: The role of the ITF in as global ocean GCM. J. Phys. Oceanogr., 23, 1057–1086.

4) Hirst, A.C., and J.S. Godfrey, 1994: The response to a sudden change in the ITF in a global ocean GCM. J. Phys. Oceanogr., 24, 1895–1910.

5) Wajsowicz, R., 1995: The response of the Indo-Pacific throughflow to interannual variations in the Pacific wind stress. Part I: Idealized geometry and variations. J. Phys. Oceanogr., 25, 1805–1826.

6) Godfrey, J.S., 1996. The effects of the Indonesian Throughflow on ocean circulation and heat exchange with the atmosphere: a review. J. Geophys. Res., 101, 12217–12237.

1) Why is most of the IT surface trapped in the upper 400 m, with indications of a second deep core?

2) Why does the near-surface flow in the IT come from the North Pacific, and the deep core from the South Pacific?

3) What processes account for remote impacts of the IT in the Indian and Pacific Oceans?

4) What processes transform the shallow, warm IT to deep, cool, South Pacific inflow, and vice versa?

5) What, then, is the impact of the IT on subthermocline circulations in the Pacific, particularly on the flow of thermostad water and AAIW?

Wajsowicz (1995) contributed to 1), contrasting surface trapping in solutions to an idealized OGCM with and without Indonesian sills.

Hirst and Godfrey (HG; 1993, 1994) addressed the near-surface flow in 2), and noted the propagation of baroclinic waves and their damping in 3).

ResultsResultsDynamics

Basic processes

Indonesian Seas IT vertical structure and source waters

Indian Ocean SICC

Pacific Ocean GBRUC, TJs, AAIW

SummarySummary

Model overviewModel overviewLCS model, LOM, COCO

Model overview:Model overview:A hierarchy of models

COCONon-eddy-resolving GCM with a horizontal resolution of 1°×1° and 40 levelsForced by HR winds in Indo-Pacific domainSolutions found with open and closed IT passagesIntegrations for 80 years

LCS modelLinear, continuously stratified system with diffusion by vertical mixingLinearized about a state of rest with background density ρb(z)Solutions are expansions in barotropic and baroclinic modes (N = 25)Forced by ERA15 and HR winds in Indo-Pacific domainSolutions found with open and closed IT passages, and their difference illustrates the IT-associated circulationIntegrations for 100 years

LOM 4½-layer model with diffusion by transfer between layers where they are thinLayers 1–4 are surface, thermocline, thermostad waters, and AAIW.Forced by ERA15 winds in Indo-Pacific domainSolutions found with open and closed IT passagesIntegrations for 100 years

The most important difference among the models is their parameterization of vertical mixing.

Solutions with open passages, without open passages, and their difference are indicated by Q, Q’, and ΔQ, respectively.

DynamicsDynamicsBasic processes

How does the IT impact circulations in the IO and PO?

LCS: Δd, n = 1 mode

The other baroclinic modes respond similarly, but are increasingly weakened by

damping, which strengthens with n.

Courtesy of Toru Miyama

n = 0 n = 1

n = 3n = 2

LCS: Δd, n = 0–3 modes

Barotropic flow confined to the perimeter of the southern IO and western boundaries of Australia and New Guinea.

Damping of baroclinic Rossby waves by diffusion allows interior flows in both basins,

with flows becoming weaker and more coastally confined as damping increases.

LCS: Δd, all modes

The IT has a basin-wide impact on circulations in both oceans. It is largest in the southern Indian Ocean and in the tropical Pacific.

There are surface geostrophic currents flowing along lines of constant Δd. Because the baroclinic modes have no net transport, surface currents in the interior ocean are balanced by subsurface currents in the opposite direction.

LCS: Δd, all modes, IO

There is a southeastward surface (northwestward subsurface) geostrophic flow across the interior of the South IO. They act to deepen the ITF transport,

around the perimeter of the IO.There is a circulation near the west coast

of Australia similar to the Leeuwin Current system, with surface

(subsurface) poleward (equatorward) flow? It is too weak, however, because,

for realistic damping , the coastal signal is carried too far offshore by RWs

(McCreary and Kundu, 1987).

There is anomalous downwelling (damping) caused by the ITF in the South IO wherever Δd is positive. This process

is not physically correct.

There is southeastward geostropic flow across the SIO, whereas the observed

flow is eastward or northeastward.

LCS: IO transport profiles

The baroclinic interior flow acts to deepen the circulation around the perimeter of the southern IO

LCS: Δd, all modes, PO

There is a westward and equatorward surface (eastward and poleward) geostrophic flow across PO interior. They act to

shallow the ITF transport as it flows northward along the east

coast of Australia.

There is anomalous upwelling caused by the ITF in the Pacific wherever Δd is negative. This

process is not physically correct.

These general structures of circulation, upwelling and downwelling are similar among all the models. But details of the

baroclinic currents differ markedly because of different diffusion

parameterizations.

LCS: PO transport profiles

The interior flow in the South Pacific shallows the northward western boundary currents along the east coasts of Australia and New Guinea, eventually transforming them to the surface-trapped IT profile

Indonesian SeasIndonesian SeasVertical structure

How does the PO impact the IT?

LCS: ITF profiles

Velocity profiles are sensitive to the strength of vertical diffusion

(damping of baroclinic waves) and the background stratification.

When diffusion is sufficiently weak, the ITF is surface trapped because the low-order baroclinic modes tend to cancel the barotropic response at

depth.

All of the profiles have a subsurface velocity minimum or reversal. As a result, the ITF has two cores. Why? Because of subsurface currents in the

interior Pacific.

Mean Nb, strong mixingEq. Nb, strong mixingEq. Nb, weak mixingEq. Nb, 4th-order mixing

LCS: u section, far-western Pacific

u(x,160˚E,z)

Eastward currents north of about 2˚N (Halmahera) and centered near 200 m drain water from western Pacific, thereby

weakening the IT at those depths. Draining by the shallower, stronger,

eastward current (NECC) is balanced by flow from the NGCC or NEC, and

so does not weaken the IT.

Indonesian SeasIndonesian SeasSource waters

How does the PO impact the IT?

Almost all of ψ comes from the northern hemisphere. Why?

The two parts sum almost to cancel the flow in the southern part of the Sulawesi Sea, ensuring that the total

transport comes from the north.

LCS: Hellerman winds

Ψ, open

Streamfunction ψ' is obtained with closed passages. It is the part of ψ that is driven by winds in the interior of the Pacific. Streamfunction Δψ is the

part of ψ caused by the ITF.

ΔΨ

Ψ', closed

The anticlockwise circulation within the Sulawesi Sea is part of the NP Tropical Gyre. It is present there

because the latitude, y0, which divides the NP and SP Tropical Gyres (ψ' = 0), is near 2°N, south of the entrance to the

Sulawesi Sea.

Now, almost all of ψ comes from the southern hemisphere. Why?

LCS: ERA15 winds

ΔΨ

Ψ′, closed

Streamfunction ψ′ no longer has a strong anticlockwise circulation in the Sulawesi Sea. So, it cannot cancel the westward

current of Δψ north of Sulawesi. Why not?

Ψ, open

Latitude y0 is now near 3.7°N, so that part of the SP Tropical Gyre is present

in the Sulawesi Sea.

ERA15 and QSCAT winds

ERA15

There is a region of strong negative curl south of the Philippines. It generates a

shallow clockwise circulation in the Sulawesi Sea strong enough to eliminate

the anticlockwise flow driven by winds in the interior Pacific.

The region is absent in the QSCAT winds. It is also absent in the Hellerman

winds.

QSCAT

v(x,y,0),open

The surface flow all comes from the northern hemisphere, because

transport of ψ′ is concentrated near the surface and so dominates Δψ .

v(x,y,-300 m),open

Subsurface flow comes from the southern hemisphere, because

transport ψ′ is weak at depth so that it is dominated by Δψ.

LCS: Hellerman winds

ΨCOCO: HR winds

The total transport comes entirely from the north because the NGCC

retroflects before it can penetrate into the Indonesian Seas, apparently because horizontal viscosity is so large and the

inertial overshoot of the MC.

As in the LCS and LOM solutions, the shallow IT comes from the north and the deep flow comes from the south.

hv1+2, open

hv4, open

Indian OceanIndian OceanSouth Indian Countercurrent

How does the IT impact the IO?

LCS: Δd

There is no SICC jet-like flow, but rather a broad, southeastward flow across the basin.

LOM: Δd (CI = 0.05 cm)

Surface geostrophic (baroclinic) currents extend southeastward across

the SIO, with a richer meridional structure than in the LCS solution.

The curved bands in the southern oceans result from a southward (northward)

shift of the ACC in the IO (PO).

Δhv1+2

LOM: SICCThe circulation is more complex in

LOM because vertical diffusion (across-layer transfer) occurs

primarily where layer thicknesses become too thin (upwelling) or too

thick (subduction).

The South Indian Countercurrent extends across the basin along 25°S and

flows to the southeast corner of the basin.

There is flow from the IO to the PO driven by eastward subsurface currents

(Tsuchiya Jets) in the PO.

Subduction in the southeast IO and south of Australia drives the surface South Indian

Countercurrent and a compensating subsurface westward flow along 25°S.

Δhv3

LOM: SICC

COCO: Δd

Surface geostrophic (baroclinic) currents extend southeastward across

the southern IO, with richer meridional structure than in the LCS solution.

Subduction and convective overturning in the southeast IO drives a surface South

Indian Countercurrent along 25°S.

u, T,closedTu, T,

open

COCO: SICC

There is an eastward countercurrent overlying westward flow from 20−25°S. It extends from Madagascar to a region of convective overturning off Southwest

Australia. It is much stronger when the Indonesian passages are open.

Pacific OceanPacific OceanGreat Barrier Reef UndercurrentHow does the IT impact the South Pacific?

COCO: Δψ

Barotropic flow field circulates around the southern IO and flows northward along the east Australian coast, with no significant

currents in the interior of the Pacific

COCO: SEC bifurcation latitude

The observed SEC bifurcation shifts poleward with depth from about 16°S to

22°S. The deep northward flow within this latitude range is known as the Great

Barrier Reef Undercurrent (GBRUC).

A similar shift occurs in COCO but not in COCO´, a consequence of the strong

northward flow along the Australian coast associated with the IT. This result suggests

that the IT is the cause of the GBRUC.

v, open

v, closed

v, obs.

Pacific OceanPacific OceanTsuchiya Jets (layer 3)

How does the IT impact the tropical Pacific?

hv3, open

LOM: Tsuchiya Jets

With open IT passages, eastward currents in layer 3 extend from the western boundary to off-equatorial upwelling regions in the eastern ocean, the model’s

Tsuchiya Jets (TJs). The IT drains enough water from layers 1 and 2 that upwelling extends into layer 3.

hv3, closed

With closed passages, the TJs are essentially eliminated. In this case, layers 1 and 2 are thicker, preventing much of the upwelling from extending to layer 3. In

addition, the NGCUC reverses to flow southeastward to about 9˚S, so that southern-hemisphere water never reaches the equator.

LOM: Tsuchiya Jets

Based on a similar solution to a 4½-layer model, McCreary et al. (2002) concluded that the IT was necessary for the existence of the TJs, but …

COCO: Tsuchiya Jets

With closed passages, the TJ is somewhat shallower with its core 1°C warmer, since less upper-ocean water is drained from the basin. Its strength is only slightly

weakened, suggesting that the TJs are supplied primarily by an overturning cell within the Pacific basin.

u, T,open

u, T,closed

1oC warmer

Pacific OceanPacific OceanAAIW (layer 4)

How does the IT impact the North Pacific?

Δhv4

The transport of the NGCUC in layer 4 (AAIW) is 3.5 SV. Of this amount, 2.7 Sv flows directly out of the Pacific into the Indian Ocean. The remaining 0.8 Sv flows into the far North Pacific. Its path through the tropics is not clear. It enters the subpolar

ocean through a “baroclinic window” (Pedlosky, 1984), and upwells into the shallower layers there via Ekman suction.

Δhv4 hv4, open

The difference field misrepresents pathways by which water actually flows into the North Pacific. It flows through the tropics by a circuitous route, due to deep, eddy-

driven circulations associated with the Hawaii Lee Countercurrent. The window circulation is reversed by the Pacific’s wind-driven, double-gyre (STC and SPG). So,

water enters the subpolar ocean along the boundary between the gyres.

LOM: Flow of AAIW into NP

With closed passages, no AAIW flows into the northern hemisphere, indicating that the IT is the cause of this

flow.

COCO: Flow of AAIW into NP

The NGCUC transport in layer 4 (AAIW) is 2.5 SV. Of this amount, 1.3 Sv flows directly into the Indian Ocean. The remaining 1.2 Sv flows into the North Pacific. It flows northward in a western-boundary current, and circulates about a deep part of the

NP Subtropical Gyre. As in Solution ΔLOM, it then enters the subpolar ocean through a “baroclinic window,” eventually upwelling into the shallower layers there.

Δhv4 hv4, open

The difference field misrepresents pathways by which water actually flows into the North Pacific. It again flows through the tropics by a circuitous route, first flowing

eastward near 7°N and then westward in the STG. The window circulation is reversed by the Pacific’s wind-driven, double-gyre (STC and SPG). So, water enters the

subpolar ocean along the gyre boundaries or near the eastern boundary.

As for LOM, with closed passages no AAIW flows into the northern hemisphere, indicating that the IT is the

cause of this flow.

SummarySummary

1) In the interiors of the Pacific and Indian Oceans, circulations associated with the IT are generated by the radiation and decay of baroclinic waves. As a consequence, subsurface currents are directed opposite to their surface counterparts. Details of the circulations are sensitive to the nature of diapycnal mixing.

2) The IT is split into near-surface and deep cores by baroclinic currents generated in the interior Pacific (EUC and TJs). ITF source waters come from the north (south) in the shallow (deep) core, the former due to the near-surface circulation driven by Pacific winds or to inertial overshoot.

3) The SICC is generated by subduction and/or convection near Southwest Australia. The ITF strengthens the SICC considerably.

4) The IT enhances northward flow along the east coast of Australia, generating the GBRUC.

5) The IT provides all (LOM) or some (GCMs) of the thermostad water that flows across the Pacific in the TJs to upwell in the eastern ocean.

6) The IT is the reason why AAIW flows into the North Pacific, eventually to upwell in the Subpolar Gyre.

LOM: Δh1+2 (CI = 10 m)

The thermocline depth (h1 + h2) deepens by more than 100 m off

the west coast of Australia.

Δd

Surface geostrophic (baroclinic) currents extend westward and equatorward across the PO…

…particularly in the South Pacific from 20S o 5S.

Sea-surface temperature

Δhv4

The transport of the NGCUC in layer 4 (AAIW) is 3.5 SV. Of this amount, 2.7 Sv flows directly out of the Pacific into the Indian Ocean. The remaining 0.8 Sv flows into the far North Pacific. Its path through the tropics is not clear. It enters the subpolar

ocean through a “baroclinic window” (Pedlosky, 1984), and upwells into the shallower layers there via Ekman suction.

LOM: Flow of AAIW into NP

With closed passages, no AAIW flows into the northern hemisphere, indicating that the IT is the

cause of this flow.

COCO: Flow of AAIW into NP

The NGCUC transport in layer 4 (AAIW) is 2.5 SV. Of this amount, 1.3 Sv flows directly into the Indian Ocean. The remaining 1.2 Sv flows into the North Pacific. It flows northward in a western-boundary current, and circulates about a deep part

of the NP Subtropical Gyre. As in Solution ΔLOM, it then enters the subpolar ocean through a “baroclinic window,” eventually upwelling into the shallower

layers there.

Δhv4

As for LOM, with closed passages no AAIW flows into the northern hemisphere, indicating that the IT is

the cause of this flow.

DynamicsDynamicsBecause of the ITF, in the IO sea level rises, there are anomalous south-

westward surface (northwestward subsurface) baroclinic currents across the interior of the southern IO, and downwelling. Conversely, in the PO sea level lowers, there are anomalous westward and equatorward, surface (eastward and poleward, subsurface) interior currents, and upwelling. Details of the circulations depend on the nature of diapycnal mixing.

Indonesian SeasIndonesian SeasThe ITF is split into near-surface and deep cores by baroclinic currents

generated in the interior Pacific (EUC and TJs). ITF source waters come from the north (south) in the shallow (deep) core, the former due to the near-surface circulation driven by Pacific winds or to inertial overshoot.

Pacific OceanPacific OceanIn LOM, the IT generates TJs by thinning layers 1 & 2 until upwelling can

extend to layer 3; in COCO, the IT shallows the TJs.

1) Pacific circulations associated with the IT are generated by the radiation and decay of baroclinic waves. As a consequence, subsurface currents are directed opposite to their surface counterparts.

2) The structure of the interior flow field depends on the types of diffusion present in the model, namely, vertical mixing (LCS model, GCMs), upwelling and subduction (LOM, GCMs), and convection (GCMs).

3) The IT provides some (GCMs) or all (LOM) of the thermostad water that upwells off Peru and in the Costa Rica dome.

4) The IT is the reason that water in the density range of AAIW flows from the South Pacific to the North Pacific Subpolar Gyre.

1) What sets the strength of the IT?Godfrey’s Island Rule, and its modifications

2) Why is most of the IT surface trapped in the upper 400 m, with indications of a second deep core?

3) Why does the near-surface flow come from the North Pacific, and the deep flow from the South Pacific?

4) Why does the South-Pacific inflow extend to intermediate depths?

5) What processes account for remote impacts of the IT in the Indian and Pacific Oceans?

Wave propagation, and their damping due to diffusion, upwelling, or convection

6) In particular, what processes transform the warm, shallow IT to deep, cool SP inflow, and vice versa

7) What is the impact of the IT on subthermocline circulations in the Pacific, particularly on the flow of thermostad water and AAIW?