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The Indian Ocean Friedrich Schott Institut für Meereskunde an der Universität Kiel, Germany

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 Schott Institut für Meereskunde an der Universität Kiel, Germany

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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!!

  6. 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 SW Monsoon SEC NE monsoon SECC Courtesy P. Niiler

  7. 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

  8. 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)

  9. 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

  10. 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?

  11. 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)

  12. 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)

  13. 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)

  14. Red Sea outflowMurray &Johns, 1997and 2002:seas. outflow cycle Red Sea outflow cycle max. in Feb.

  15. 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

  16. 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

  17. Seasonal cycle in Arabian Seaby 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

  18. Seasonality of the meridionaloverturning 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

  19. „Great Whirl“ transports 1995, 1996 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)

  20. 8 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

  21. ..and Throughflow pathways Throughflow studies

  22. From individual moorings in different years: about 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

  23. 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) Ship sections, XBTs Fieux et al, 1994; 1996Sprintall et al., 2002Feng & Meyers (inpress) Indonesian Throughflow studies during WOCE

  24. Dutch moored array in Mozambique Channel 2000-2001 Moz. Channel flow and eddies Mean=-17 Sv Ridderinkhof, de Ruijter DSR II (in press) see poster #118 40 Sv -40

  25. ICM 3 array (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

  26. Float studies Moz. Channel flow and eddies Chapman, di Marco,Davis, Coward DSR II (in press) see poster # 117

  27. Is there a route to the Mozambique Channel from south of Australia (lower thermocline)? „Tasmanleakage“ ORCA model study of Speich et al. (2002) backtracking of particlesposter #24!

  28. Mean current field at 310m of global ECCO model Courtesy D. Stammer

  29. 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)

  30. deep overturning

  31. 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

  32. Inverse analysis results for the deep Indian Ocean 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

  33. 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...)

  34. Dengler and 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

  35. 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 0 4 10 • Equatorial roll present, southward near-surface (Ekman) transports • upwelling at 5-10S 18 Ferron and Marotzke, DSR II (in press)

  36. ECCO, 1 degree 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)

  37. -20 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

  38. Meridionalheat transport (PW) SOC fluxes courtesy S. Josey

  39. 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)

  40. Role of intraseasonal variability ~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)

  41. 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.

  42. Role of interannual variability Indian Ocean „Dipole mode“ or „Zonal Mode“ Saji et al. (1999), Webster et al. (1999)

  43. Indian Ocean Dipole mode ensemble patternWind stress and SST Peak (SON) Courtesy H. Annamalai But, is it really a dipole or zonal mode? After Saji et al. (1999)

  44. Indian Ocean „Zonal Mode“ parameters 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)

  45. T/P altimetry sea level anomalies during „dipole“ episodes of 1994 and 1997 • Ekman convergence at 5-10 S

  46. 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

  47. Tropical cyclone days for Dec-Apr Xie et al., 2002) Climatological mean of number of cyclone days (contours)difference between deep and shallow mixed layer (shading)

  48. 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.)

  49. INSTANT: Indonesian Throughflow measurementsADCP moorings 2003-2006 in the important passages!!(AU/FR/INDON/NL/US) A. Gordon, J. Sprintall,pers. communication

  50. surface chlorphyll (mg/m3) averaged over 10S-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

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