Active-break cycles of the Indian Summer Monsoon in a coupled GCM: response of

the ocean fields

Pratap Mohanty1

Andrew Turner2, Pete Inness2 & Julia Slingo2

1Department of Marine Sciences, Meteorology and Physical Oceanography Laboratory, Berhampur University, Orissa, India

2Walker Institute, University of Reading, UK

Properties of active-break cycles of ISM

• Active-break cycles of the ISM are associated with fluctuations of the tropical convergence zone (TCZ; Yasunari 1981; Sikka & Gadgil 1980), between two favoured locations: over the Indian subcontinent and equatorial Indian Ocean.

• Repeated northward propagation from its oceanic to continental regimes exists on a 30-60 day timescale (Sikka & Gadgil 1980; Krishnamurty & Shukla 2000; Goswami & Xavier 2003). Eastward propagation also occurs.

• This mode has a large zonal scale (70-80) and a first baroclinic vertical structure (Goswami & Mohan 2001).

• At Indian latitudes, westward propagation also occurs in the 10-20 day band.

• During active phase, the northern TCZ is stronger, featuring cyclonic vorticity and enhanced convection, while anticyclonic vorticity and decreased convection persist over the southern region. The reverse condition occurs during break phase (Goswami & Mohan, 2001).

Objectives

• Test the capability of the coupled model (HadCM3) in simulating active-break cycles.

• Examine the meridional see-saw of convective anomalies through compositing of active-break events.

• Address the origins of the convective anomalies associated with each phase of active-break events.

• Test the influence of model basic state error on intraseasonal monsoon variation.

• Examine the response of the ocean fields to the monsoon ISO.

• Percentage of total intraseasonal variance explained in each band in reasonable spatial agreement with reanalysis (850hPa zonal winds).

• Similar pattern for OLR.

10-20day30-60day

ERA-40 HadCM3 HadCM3FA

Spatial behaviour: percentage variance of Lanczos filtered u850 anomalies

Temporal behaviour: lag-regression of Lanczos filtered OLR anomalies

Observed HadCM3 HadCM3FA

• Strong spectral peak in model at 30-60 day band, much weaker over 10-20 day periods.

Active-break monsoon index

• OLR index chosen to measure convective dipole of active-break events.

• After Vecchi & Harrison (2002), difference between OLR anomalies over Indian subcontinent / Bay of Bengal (BOX-1: 10-30oN, 65-85oE) and east equatorial Indian Ocean (BOX-2: 10oS-5oN, 70-90oE).

• Daily OLR anomalies are boxcar-smoothed for 7 days and the 50-day centred mean is subtracted to removal seasonal mean effects.

• Active events where index is one stdev below the mean.• Break events where index is one stdev above the mean.

Space-time evolution of 30-60 day OLR anomalies

OBSERVED HadCM3 HadCM3FAACTIVE BREAK ACTIVE BREAK ACTIVE BREAK

onset

5 day

10 day

15 day

20 day

• The Vecchi-Harrison OLR index is used to composite events on the condition they last at least five days.

• Evolution of the active-break events are displayed at various lag times prior to the onset of each event.

Space-time evolution of 10-20 day OLR anomalies

OBSERVED HadCM3

ACTIVE BREAK ACTIVE BREAK

onset

1 day

3 day

5 day

10 day

Response of the ocean surface fields

• Dynamic Response

• Thermodynamic Response

• Dynamic-thermodynamic Feedback

Active minus break wind stress anomalies

HadCM3

• Wind stress provides the dynamical linkage between the atmospheric winds and oceanic currents.

• The model is able to depict the general features of monsoon flow and also its intraseasonal variability inherent in the wind field.

• The Somali Jet is apparently stronger during the active phase rather than the break phase.

30-60 day filtered wind stress curl anomaly composites prior to event onset

HadCM3ACTIVE BREAK

0

1

2

3

4

x10-7 Pa/m

• During active phase, wind stress curl is positive over north BOB, north AS and the equatorial Indian ocean and negative over southern BOB, southern AS and southern Indian Ocean.

• The pattern is exactly opposite during the break phase.

• The time evolution of the wind stress curl shows that it has a half cycle of about 20 days and the change in the sign of the wind stress curl occurs before five days of the active/break events.

Salinity distribution in the Indian OceanAnnual mean at 200m from Levitus & Boyer (1994)

JJAS (HadCM3) Stddev of JJAS SSS (HadCM3)

• There exists a general agreement on the pattern of salinity distribution between observed and model simulations.• The low salinity tongue near the Indonesian throughflow and the low salinity in the head Bay of Bengal are similarly located.

• The strong boundary current in the eastern BOB carries low salinity water equatorward and joins with the Wurtki Jet to make central EIO fresher (Sharma et al. 2007).

Vertical salinity profile (HadCM3)

ACTIVE

5m

25m

47m

95m

203m

• Positive salinity anomaly in the EEIO and negative anomaly in the BOB during active phase.

• During break phase the anomalies over the EEIO are negative

• Monsoon ISO in salinity is visible up to a depth of about 100m.

Maps of precipitation from the Xie and Arkin (1996) dataset

• Distinct features of salinity and rainfall distribution in EIO imply that ocean dynamics are critically linked to salinity (in agreement with Masson et al. 2002).

• Monsoon ISO reach their maximum amplitude in EIO as it receives the largest mean precipitation in South Asia (Lawrence & Webster 2002).

Vertical Temperature profile (HadCM3)ACTIVE

5 m

25 m

47 m

95 m

203 m

• Temperature anomalies are distinctly negative in the WIO upwelling regions.

• Signals of monsoon ISO are visible up to a depth of about 200m.

Mixed layer depth

• Observed mean MLD is deep in the central AS & southern Indian ocean.

• Findlater Jet during JJAS south of the equator and eventually crossing the Arabian Sea causes strong mixing, deepening the mixed layer (Schiller & Godfrey 2003).

Han et al (2001) derived from Levitus & Boyer (1994); Levitus et al (1994)

30-60 day filtered composite anomalies of MLD prior to event onset

HadCM3Active Break

onset

5 days

10 days

15 days

20 days• Active phase MLD is

deeper in the AS, north BOB and southern Indian ocean due to strong southwesterly monsoon jet (or south-easterlies in the southern Indian Ocean south of 10S).

• MLD is shallow (deep) in the region of suppressed convection over EIO during active (break) phase in agreement with Schiller & Godfrey (2003).

• Time evolution of MLD also confirms largest amplitude of intraseasonal variability in central and eastern EIO.

Mean JJAS(HadCM3)

Thermocline depth (20C isotherm) and active/break composites

• Presence of equatorial westerlies in the central and eastern basin, sloping the thermocline to the east.• Model thermocline dome is misplaced eastward and too deep relative to SODA.

Active (HadCM3)

• BoB thermocline is deeper (shallower) to the north (south) during the active phase whilst deeper over the EEIO reflecting calm and clear conditions there.

Summary• HadCM3 is able to make some depiction of monsoon ISO signals

inherent in the ocean fields.• Basic state biases seem to make little difference to the quality of

monsoon ISO.• Evolution of monsoon ISO is not limited to the surface layer of the

ocean and extends beyond the depth of the thermocline.• Mixed layer depth anomalies are deeper in the Arabian Sea, north

Bay of Bengal and south Indian Ocean south of 10S, consistent with positive wind stress curl anomalies during the active phase.

• During break phase the pattern of anomalies are exactly opposite of the active phase in the northern Indian Ocean.

• Despite an overall deep bias in the thermocline, large variations in its depth occur according to the phase of ISO, modulated by wind induced upwelling.

• Associated with the strong variability in MLD and SSS anomalies, fluctuations in barrier layer thickness occur (not shown).

Questions still to be addressed

• What is the nature of SST-thermocline coupling and how it feeds back on monsoon ISOs?

• Is the intraseasonal monsoon variability is a surface intensified process?

• What is the role of dynamic/thermodynamic forcing on salinity variations associated with active and break phases?

• What is the response of the regional seas to the monsoon ISO?