The Indian OceanFriedrich 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

•The shallow cross-equatorial exchange and upwelling

•Some regional results NW Arabian Sea: Red Sea outflow, monsoon response

Indonesian Throughflow and pathways in the Indian OceanSW 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

deep

NL, 2000

November - AprilMay - October

not covered

Moorings

not deployed

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

post WOCE

Mid-depth (1000m) float vectors mapped

•228 floats/ 1100 float yrs

•southw. flow in Mozambique Channel

•strong flow on equator

•see poster!!

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

Surface drifter currents for both monsoon

seasons

•180 WOCE drifters 1992-95

•350 TOGA/ENSO drifters since 1995

shows seasonal reversal of meridional Ekman transports

Courtesy P. Niiler

SEC

SW Monsoon

NE monsoon

SECC

shallow cross-equatorial cellshallow cross-equatorial cell

and upwelling budget problemand upwelling budget problem

deep overturningdeep overturning

ONR/JGOFsONR/JGOFsstudiesstudies

GWGWresponseresponse

MOC MOC variabilityvariability

Red Sea outflow

Moz. Channel flow and eddies

ThroughflowThroughflowstudiesstudies

..and Throughflow pathways

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 transport and 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

Schott, Schoenefeldt, Dengler, 2002

Subduction from Karstensen, Quadfasel, 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?

Upwelling comes from up to 400m depth

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

left: horizontal

right: vertical-meridional

Time backwards from upwelling 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

Miyama et al. (DSR II, 2002)

Particles up to 400m deep underway

(21.0 total)

Northern Indian Ocean upwellingin 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 net2.5 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)

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 cyclemax. 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

Flux buoy(Weller et al.,

2001)

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

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

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

Seasonal cycle in Arabian Sea by Rossby wave propagation

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

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

Seasonality of the meridionaloverturning circulation of the Arabian Sea

Stramma et al., 2002

Beal et al., 2002

seasonally reversing shallow overturning

cell

deep overturning

cell

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

Interannual variability of „Great Whirl“

Model study by Wirth et al. (2002):

interannual GW variability dominantlyinternally generated (instabilities)

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

Observed: ICM 7

large interannual differences of location and transport of GW.

1995

1996

40 Sv

„Great Whirl“ transports 1995, 1996

Depth of seasonal response in northern Somali Basin

Explained variance in %

8 month of max. amplitude

Amplitude of annual harmonic in cm s-1

Dengler et al., 2002

WOCE ICM-7

ThroughflowThroughflowstudiesstudies

..and Throughflow pathways

Indonesian Throughflow studies during WOCE

Ship sections, XBTs

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

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

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

Indonesian Throughflow studies during WOCE

Ship sections, XBTs

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

Inverse model results for ThroughflowGanachaud et al, 2000: 15 Sv Sloyan and Rintoul, 2001: 10 Sv

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

Moz. Channel flow and eddies

Dutch moored array in Mozambique Channel 2000-2001

Mean=-17 Sv

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

40

Sv

-40

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

Circulation of SW Ind. Ocean

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

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

ICM 3 array (see Bryden talk)

Float studies

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

Moz. Channel flow and eddies

Is there a route to the Mozambique Channel from south of Australia (lower thermocline)?

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

„Tasmanleakage“

Mean current field at 310m of global ECCO model

Courtesy D. Stammer

Mozambique Channel eddies (1996-2001)

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 overturningdeep overturning

2.3Sv

6.7Sv

Warren and Johnson, 2002

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

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

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

Sloyan and Rintoul (2001)

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

Ganachaud et al. (2000)

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

11

8

Inverse analysis results for the deep Indian Ocean

Estimates on meridional overturning at 32S

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...)

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

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

Inferred Eddy Diffusivities

Dengler and Quadfasel 2001

1 m2s-1 1 10

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

Ferron and Marotzke, DSR II (in press)

18

10

04

Deep overturning in assimilation of WOCE Ind. Ocean lines

•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

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

•upwelling at 5-10S

Ferron and Marotzke (DSR II, in press)

ECCO, 1 degree

21

0

3

18

10

04

Note different sign convention!

Comparison of regional and global assimialtion results forIndian Ocean overurning transports

Stammer et al., JGR 2002)

Much weaker deep cell in ECCO model

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)

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)

-20

Wm-2

-60

150

W

m-2

-150

Meridionalheat transport (PW)

SOC fluxes courtesy S. Josey

Role of intraseasonal variability

~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

~15 day fluctuations

40-60 daysNew 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 (cm2 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

Indian Ocean „Dipole mode“ or „Zonal Mode“

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

After Saji et al. (1999)

Indian Ocean Dipole mode ensemble patternWind stress and SST

But, is it really a dipole or zonal mode?

Courtesy H. Annamalai

Peak (SON)

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

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

SOI Index (inverted)and windstress anomaly

Indian Ocean „Zonal Mode“ parameters

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 (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

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

Xie et al., 2002)

And beyond...

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 measurementsADCP moorings 2003-2006 in the important passages!!

(AU/FR/INDON/NL/US)

A. Gordon, J. Sprintall,pers. communication

And beyond...

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

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

surface chlorphyll (mg/m3) averaged over 10S-12S as measured by SeaWiFS during 1998,shows Rossby wave propagation(Kawamiya and Oschlies, 2002)

Drinks!!