The Indian Ocean
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The Indian Ocean Friedrich Schott Institut für Meereskunde an der Universität Kiel, Germany with input from lots of people (much of it on posters here) Introduction: The monsoon circulation and Indian Ocean specifics WOCE plans vs what was accomplished: overview

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  • The Indian Ocean

  • Friedrich SchottInstitut für Meereskunde an der Universität Kiel, Germany

  • with input from lots of people (much of it on posters here)

  • Introduction: The monsoon circulation and Indian Ocean specifics

  • WOCE plans vs what was accomplished: overview

  • The shallow cross-equatorial exchange and upwelling

  • Some regional resultsNW Arabian Sea: Red Sea outflow, monsoon response Indonesian Throughflow and pathways in the Indian Ocean SW Indian Ocean and Mozambique Channel flow

  • Deep circulation and overturning

  • Climate modes

  • Remaining challenges and outlook


Circulation schematic for Southwest Monsoon

  • „Great Whirl“ and „Southern Gyre“

  • Upwelling concentrated in wedges and filaments

  • cyclonic domes both sides of Sri Lanka: upwelling?

  • Indonesian Throughflow max.

  • Ekman transport southward in both hemispheres, but northward winds on equ.

  • Subduction in southern subtropics


Circulation schematic for Northeast Monsoon

  • Somali Current and East African Coast Current merge into SECC

  • Doming NE of Madagascar open-ocean uwelling?

  • Indonesian Throughflow min.

  • some subduction in northern Arabian Sea

  • Ekman transport northward in both hemispheres

In between monsoon seasons: eastward wind stress causes„Wyrtki Jets“ along equator, that send out coastal Kelvin waves


WOCE sections, arrays(plan: do sections north of 10°S in each monsoon season)

deep

NL, 2000

Moorings

November - April

May - October

not deployed

not covered

post WOCE


Mid-depth (1000m) float vectors mapped

R. Davis: objectives of Ind. OceanWOCE float program accomplished !

  • 228 floats/ 1100 float yrs

  • southw. flow in Mozambique Channel

  • strong flow on equator

  • see poster!!


SW Monsoon

SEC

NE monsoon

SECC

Courtesy P. Niiler


ONR/JGOFsstudies

Red Sea outflow

GWresponse

MOC

variability

shallow cross-equatorial cell

and upwelling budget problem

..and Throughflow pathways

Throughflow

studies

deep overturning

Moz. Channel flow and eddies


Meridional streamfunction for Indian Ocean north of 8S(JAMSTEC model: MOM-2, 55 levels, 1/4 deg. resolution)

  • Shallow cross-equatorial cell of about 6 Sv transportand upwelling at 5-15 N

  • Equatorial roll (~10Sv), northward at surface southward underneath (Ekman transports)first discussed by Wacongne & Pacanowski (1996)

  • Upwelling at 5-10S

Miyama et al. (DSR II, in press)


Schematic diagram of shallow cross-equatorial circulation

  • Cross-equ. transport ~6.5 Sv by--thermocline Somali C.--Sverdrup/Ekman tr.

  • upwelling in models~ 1/3 in domes around India

Subduction from Karstensen, Quadfasel, 2002

Schott, Schoenefeldt, Dengler, 2002


  • Northern Somali upwelling

  • outflow from Great Whirl

  • which fraction of the 10 Sv of shallow offshore flow is transformed into surface water?

  • which fraction slumps back into thermocline?


Particle trajectoriesin JAMSTEC model(backtracking from upwelling regions: time in years)

left: horizontal

right: vertical-meridional

Time backwards from upwelling

Upwelling comes from up to 400m depth

Miyama et al (2002)


Where does upwelling come from (Jamstec model)?

  • Upwelling sources Southern subduction region Indonesian Throughflow

  • Northern upwelling off Somalia, Arabia,in domes around India / Sri Lanka

Particles up to 400m deep underway

Miyama et al. (DSR II, 2002)


Northern Indian Ocean upwelling

in different models

(in Sv)

Model

Somalia

Oman

Indian NH total

domes

Miyama et al.

(DSR II 2002)

McCreary

2.7

1.9

1.8

5.8

net

2.5 layer

(1993)

(5.4)

(2.3)

(2.5) (14.0 total)

JAMSTEC

3.1

1.0

1.2

5.2 net

(10.1)

(1.5)

(2.4) (22.8 total)

(our study,

Progr. Oceanogr.

2002)

Obs. Estim. 4.2

1.0

??

SODA

0.9

1.0

1.0

6.2

net

(5.8)

(1.3)

(3.0)

(21.0 total)

Ekman div.

6.5

1.0

4.0

Northern Indian Ocean upwellingannual basis (Sv)


Red Sea outflowMurray &Johns, 1997and 2002:seas. outflow cycle

Red Sea outflow cycle max. in Feb.


Spreading of Red Sea WaterBower et al. (GRL, in press): energetic eddies in Gulf of Aden (from Somali Current?)

Beal, Ffield, Gordon (2000)Bower, Hunt, Price (2000):

spreading along East Africa to Mozambique Channel monsoonal RSW variability in

Gulf of Aden, off northern Somalia


Oman upwelling wedge observations (A.Fischer, K.Brink et al., (2002)...and JGOFs

Flux buoy(Weller et al., 2001)

8N MOC/heat/FW transport studiesBeal et al, 2002: 1995 SW monsoon developmentStramma et al, 2002: SW vs. NE monsoon


Seasonal cycle in Arabian Sea al., (2002)...and JGOFsby Rossby wave propagation

Remote forcing from Laccadive High or around India (McCreary et al., 1993)?

Amplitude of annual harmonic of SSH from TOPEX/POSEIDON altimetry and ship section

Phase of annual harmonic

Brandt et al., 2002Stramma et al.,2002:

annual Rossby wave explains density/geostrophy difference in seasonal ship sections at 8N


Seasonality of the meridional al., (2002)...and JGOFsoverturning circulation of the Arabian Sea

seasonally reversing shallow overturning cell

deep overturning cell

Stramma et al., 2002

Beal et al., 2002

Heat transport: -0.6 PW in Aug. 93, 0.2 PW in Jan.98


„Great Whirl“ transports 1995, 1996 al., (2002)...and JGOFs

40 Sv

1995

1996

Interannual variability of „Great Whirl“

Observed: ICM 7

large interannual differences of location and transport of GW.

Model study byWirth et al. (2002):

interannual GW variability dominantlyinternally generated (instabilities)

rather than externally (i.e. wind stress variability)


Depth of seasonal response in northern somali basin

8 al., (2002)...and JGOFs

month of max. amplitude

Depth of seasonal response in northern Somali Basin

WOCE ICM-7

Amplitude of annual harmonic in cm s-1

Explained variance in %

Dengler et al., 2002


..and Throughflow pathways al., (2002)...and JGOFs

Throughflow

studies


From individual moorings in different years: al., (2002)...and JGOFsabout 10 SvGordon et al (1999,2002)

Ship sections, XBTs

Fieux et al, 1994; 1996Sprintall et al., 2002Feng & Meyers (inpress)

Cresswell et. al., 1993Molcard et al., 1996, 2001

Indonesian Throughflow studies during WOCE


Inverse model results for Throughflow al., (2002)...and JGOFsGanachaud et al, 2000: 15 Sv Sloyan and Rintoul, 2001: 10 Sv

15 Sv Throughflow14 Sv Mozambique Channel(Ganachaud, Wunsch, Marotzke 2000)

Ship sections, XBTs

Fieux et al, 1994; 1996Sprintall et al., 2002Feng & Meyers (inpress)

Indonesian Throughflow studies during WOCE


Dutch moored array in Mozambique Channel 2000-2001 al., (2002)...and JGOFs

Moz. Channel flow and eddies

Mean=-17 Sv

Ridderinkhof, de Ruijter DSR II (in press) see poster #118

40

Sv

-40


ICM 3 array al., (2002)...and JGOFs(see Bryden talk)

70Sv from ICM 1 array Bryden and Beal (2001)

Circulation of SW Ind. Ocean

derived by Donohue and Toole (DSR II, in press) and poster!

18 Sv through Mozambique Channel above 27.96 kg m-3(about 2000m)

19 Sv East Madagascar Current

4 Sv northward in DWBC east of Madagascar

76 Sv Agulhas


Float studies al., (2002)...and JGOFs

Moz. Channel flow and eddies

Chapman, di Marco,Davis, Coward DSR II (in press) see poster # 117


Is there a route to the Mozambique Channel al., (2002)...and JGOFsfrom south of Australia (lower thermocline)?

„Tasmanleakage“

ORCA model study of Speich et al. (2002) backtracking of particlesposter #24!


Mean current field at 310m of global ECCO model al., (2002)...and JGOFs

Courtesy D. Stammer


Mozambique Channel eddies (1996-2001) al., (2002)...and JGOFs

Schouten et al DSR II, in press

  • Eddies propagate through Moz. Channel toward Agulhas and encourage pinch-off of Agulhas Rings

  • are eddies triggered at northern Channel end by Rossby waves? And these in turn by equatorial/boundary Kelvin waves? („trans- Indian teleconection“)? De Ruiter et al (2002)


deep overturning al., (2002)...and JGOFs


Spreading of AABW into the Indian Ocean (bottom desity; Mantyla and Reid, 1995;

Tracers have arrived in DWBCs (Fine et al, 2002,poster!)

Deep inflow into the Australian Basin (and across the 90E Ridge)

2.3Sv

6.7Sv

Warren and Johnson, 2002


Inverse analysis results for the deep Indian Ocean Mantyla and Reid, 1995;

11

8

Ganachaud et al. (2000)

deep flow belowgn=27.09: 8 Sv at 32 S 10.5 Sv at 20S11 Sv at 10 S

Sloyan and Rintoul (2001)

23 Sv of deep overturning at 32S, only 4Sv at 18S


Estimates on meridional overturning at 32S Mantyla and Reid, 1995;

Toole and Warren (1993) 27 Sv

Robbins and Toole (1998) 12 Sv

Sloyan and Rintoul (2001) 23 Sv

Ganachaud et al. (2000) 8 Sv (obtain similar structure as T&W‘93 i.e. upwelling to shallower layers as S&R‘01)

Difference Ganachaud et al vs. Sloyan&Rintouldue to different diapycnal flux assumptions?(sections are partially the same...)


Dengler and Mantyla and Reid, 1995;Quadfasel 2001

Inferred Eddy Diffusivities

1 m2s-1 1 10

- even at topographic „hot spots“Kr lower by factor ~10

Is there enough deep mixing to a explain a ~10 Sv deep overturning cell?

Overturning and required eddy coeficients in Ganachaud et al. (2000) solution

but poster by Talley/Reid/Sprintall (#19)concludes that only 5x10-4 m2s-1 is needed


Deep overturning in assimilation of WOCE Ind. Ocean lines Mantyla and Reid, 1995;

  • Basin model with 10 Sv Indonesian Throughflow

  • 18 Sv of deep inflow below 3200m

  • 8 Sv of deep cell crosses equator

  • strength of deep cell independent of Throughflow

  • southward outflow at 1000-3200m

  • only small heat transport (0.1 PW) by deep cell

0

4

10

  • Equatorial roll present, southward near-surface (Ekman) transports

  • upwelling at 5-10S

18

Ferron and Marotzke, DSR II (in press)


ECCO, 1 degree Mantyla and Reid, 1995;

3

0

2

1

Comparison of regional and global assimialtion results forIndian Ocean overurning transports

Note different sign convention!

Much weaker deep cell in ECCO model

0

4

10

18

Ferron and Marotzke (DSR II, in press)

Stammer et al., JGR 2002)


-20 Mantyla and Reid, 1995;

Wm-2

-60

New corrections to arrive at global near-zero net fluxes(calculated to match oceanic heat transport divergences/ convergences;essentially reduced incoming SW radiation and increased latent heat loss)

Grist and Josey (2002)

SOC net heat fluxes (Wm-2)over Indian Ocan(Josey et al., 1999)

(global 30 Wm-2 ´ net heat gain!!As da Silva et al., 1994)

150

W

m-2

-150


Meridional Mantyla and Reid, 1995;heat transport (PW)

SOC fluxes courtesy S. Josey


Role of intraseasonal variability Mantyla and Reid, 1995;

~27 day fluctuations (near-equatorial)

40-60 days

In early WOCE: reproduced in seasonally driven models: i.e they were presumably due to instabilities(Kindle &Thompson, 1989; Woodbury et al., 1989)


Role of intraseasonal variability Mantyla and Reid, 1995;

~15 day fluctuations

40-60 days

New observations (ICM-9): shorter periods (~15 days) and meridional comp. well correlated with local equ. winds (Yanai waves):

East of Madagascar: 60 day variability: resonant Rossby waves (Warren et al., 2001)


Difference in variance (cm Mantyla and Reid, 1995;2 s-2) between the run driven by diurnal winds and the run driven by smooth climatology.

Intraseasonally forced energy in equatorial band and in the west

Sengupta et al. (2001)and pers. comm.


Role of interannual variability Mantyla and Reid, 1995;

Indian Ocean „Dipole mode“ or „Zonal Mode“

Saji et al. (1999), Webster et al. (1999)


Indian Ocean Dipole mode ensemble pattern Mantyla and Reid, 1995;Wind stress and SST

Peak (SON)

Courtesy H. Annamalai

But, is it really a dipole or zonal mode?

After Saji et al. (1999)


Indian Ocean „Zonal Mode“ parameters Mantyla and Reid, 1995;

SST (west) and (east) in equ. Indian Ocean

SST-Diff W-Eand zonal windstress anomaly in central equ. Indian Ocean

SOI Index (inverted)andwindstress anomaly

Bars=ENs

Feng and Meyers (2002)


T/P altimetry sea level anomalies during „dipole“ episodes of 1994 and 1997

  • Ekman convergence at 5-10 S


  • EOF-1 of SSH-Anomaly episodes of 1994 and 1997(38% of variance)

  • SSH (and SST) max northeast of Madagascar

  • SSH (and SST) min west of Sumatra

  • thermocline deepening in west

  • shallowing in the east

Feng and Meyers,

DSR II, in press


Tropical cyclone days for Dec-Apr episodes of 1994 and 1997

Xie et al., 2002)

Climatological mean of number of cyclone days (contours)difference between deep and shallow mixed layer (shading)


And beyond... episodes of 1994 and 1997

Remaining WOCE challenges

  • What happens to the net deep inflow (by 3 western boundary currents): Is there a deep meridional overturning cell in the Indian Ocean? If so, what is the physical mechanism of water mass transformation and deep upwelling

  • Where does upwelling occur: really so much in offshore domes as suggested by models?

  • Connection of Indian Ocean STC with Pacific and Atlantic and role in decadal variability

  • Deep response to the monsoon variability (not studied in WOCE)

  • Role of remote vs. local forcing in regional monsoon phenomena

  • and the flux problem ...

  • Indonesian Throughflow not directly measured in WOCE but our hopes are with the new program INSTANT (Sprintall et al.)


INSTANT: Indonesian Throughflow measurements episodes of 1994 and 1997ADCP moorings 2003-2006 in the important passages!!(AU/FR/INDON/NL/US)

A. Gordon, J. Sprintall,pers. communication


surface chlorphyll (mg/m3) averaged over episodes of 1994 and 199710S-12S as measured by SeaWiFS during 1998,shows Rossby wave propagation(Kawamiya and Oschlies, 2002)

And beyond...

  • Ocean‘s role in climate anomalies:the „Indian Ocean Zonal Mode“ use Rossby waves for predictability?

  • Intraseasonal variability:growing evidence relating coupled mode in Bay of Bengal to Indian monsoon breaks: is there predictability?

  • How to improve physical fields for improved ecosystem models: Can we better quantify upwelling by combining observations? or more general:benefit from increased WOCE/JGOFs type cooperation


Drinks!! episodes of 1994 and 1997


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