INTRASEASONAL VARIABILITY OF THE EQUATORIAL INDIAN OCEAN CIRCULATION

1. INTRASEASONAL VARIABILITY OF THE EQUATORIAL INDIAN OCEAN CIRCULATION RETISH SENAN Centre for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore

2. OBSERVATIONS OF INTRASEASONAL VARIABILTY Gan Island 73°E EQ South of Sri Lanka 80.5°E

3. OBJECTIVES • To use new observations and an OGCM to understand the dynamics of currents in the equatorial Indian Ocean on intraseasonal and longer timescales. • To explain the origin of intraseasonal zonal jets and the observed biweekly variability. • To understand coherent intraseasonal variability in the ocean and atmosphere in the Asian Monsoon region.

4. MODEL SETUP • Three dimensional primitive equation OGCM (MOM2). • Boussinesq, Hydrostatic, and Thin shell Approximations. • Domain: 30°S-30°N 30°E-110°E • Topography: 1/12° X 1/12° resolution • Rigid artificial walls at 30°S and 110°E, with a sponge layer along 30°S • Variable resolution in the horizontal and vertical • ~1/3° X 1/3° in the Northern Indian Ocean • 19 levels (7 levels in top 100m) • Surface Boundary Conditions • Forced by QSCAT/NCEP wind stress: • Rigid Lid Approximation: w=0 atz=0 • Relaxation to Observed Temperature and Salinity • Horizontal Mixing:Eddy coefficients Ah= Am= 2000 m2s-1 • Vertical Mixing Scheme: Pacanowski-Philander Mixing Scheme

5. MODEL DOMAIN AND GRID

6. MODEL EXPERIMENTS • NCEP Run • NCEP daily wind stress • Observed climatology of Levitus (1982) • Integration period: 15 years (from 1987) • Spin-up: 2 years • QSCAT Control Run • QuikSCAT daily wind stress • Observed climatology of Levitus (1982) • Integration period: July 1999 – Jun 2003 • Initial conditions from NCEP run • QSCAT Seasonal Run • QSCAT seasonal wind stress • Idealized wind experiments

7. VARIABILITY OF ZONAL FLOW

8. Intraseasonal variability

10. POWER SPECTRUM

13. OBSERVATION Masumoto et al., Presented at IOGOOS conference, Mauritius, 2002; GRL (submitted). MODEL Monsoon jet

14. 93°E, 0° T= 4 - 6C

18. MONSOON JETS

22. BIWEEKLY VARIABILITY

28. SPACE-TIME SPECTRUM v 50m

29. SPACE-TIME SPECTRUMy

30. COHERENT INTRASEASONAL VARIABILITY IN THE OCEAN AND MONSOON ATMOSPHERE

33. OLR-SST LEAD-LAG REGRESSION

34. CONCLUSIONS • High frequency accurate winds are required for accurate simulation of Equatorial Indian Ocean (EqIO) currents, which have strong variability on intraseasonal to interannual time scales. • The fall Wyrtki jet is longer lived than the spring Wyrtki jet; the fall jet is modulated on intraseasonal time scales. • Zonal pressure force exerts strong control on the evolution of zonal flow; the deceleration of the eastward jets, and the subsequent westward flow in the upper ocean in the presence of westerly wind stress is due to the zonal pressure force. • Westerly wind bursts drive strong intraseasonal equatorial jets in the eastern EqIO (“monsoon jets”) during early summer, but not in winter. • Neither the westward current in the upper ocean nor the subsurface eastward flow (the observed spring and summer “undercurrent” ) requires easterly winds; they can be generated by equatorial adjustment due to low baroclinic mode number Kelvin (Rossby) waves generated at the western (eastern) boundary. • Most of the intraseasonal variability in the equatorial waveguide is driven by variability of the winds; there is some intraseasonal variability near the western boundary and in the equatorial waveguide due to dynamic instability of seasonal “mean” flows.

35. CONCLUSIONS (Contd…) • The 10-20 day (biweekly) variability in the EqIO is associated with forced mixed Rossby-gravity (MRG) waves resonantly forced by quasi-biweekly oscillation in the atmosphere, mainly through variations in meridional wind stress. • The biweekly MRG wave has westward and upward phase propagation, zonal wavelength of 4000-5000 km and phase speed of 4 m s -1; it is associated with deep, strong off equatorial upwelling. • Intraseasonal SST anomalies are driven by net heat flux anomalies in the central and eastern EqIO. • Northward propagating SST anomalies in summer In the Bay are due to net heat flux anomalies associated with the monsoon active break cycles. • Coherent variability in the ocean and atmosphere in suggestive of air-sea interaction.

36. PUBLICATIONS Sengupta, D., R. Senan and B. N. Goswami (2001): Origin of Intraseasonal variability of circulation in the tropical central Indian Ocean. Geophysical Research Letters, 28, 7, 1267-1270. Sengupta, D., B. N. Goswami and R. Senan (2001): Coherent Intraseasonal oscillations of the ocean and atmosphere during the Asian summer monsoon. Geophysical Research Letters, 28, 21, 4127-4130. Senan, R., D. Sengupta and B. N. Goswami (2003): Intraseasonal "monsoon jets" in the equatorial Indian Ocean. Geophysical Research Letters, 30, 14, 1750, doi:10.1029/2003GL017583. Sengupta, D., R. Senan, V. S. N. Murty and V. Fernando (2004): A biweekly mode in the equatorial Indian Ocean. Journal ofGeophysical Research (under revision).Sengupta, D., R. Senan and B. N. Goswami (2004): Variability of equatorial Indian Ocean circulation. In preparation. 

37. Current spectra at 80°E From Reppin, Schott, Fischer and Quadfasel, JGR, 1999.

39. MODEL BASIC EQUATIONS

42. Generation of Equatorial Jets force

46. Why does the eastward jet decelerate in the presence of persistent westerly wind stress? Why are the jets followed by westward flow in the upper ocean? Why are the spring and fall jets followed, about three months later, by eastward “undercurrents”?