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

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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.
Spatial behaviour: percentage variance of
Lanczos filtered u850 anomalies
• Percentage of total intraseasonal variance explained in
each band in reasonable spatial agreement with
reanalysis (850hPa zonal winds).
• Similar pattern for OLR.
ERA-40
HadCM3
HadCM3FA
10-20day
30-60day
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
• The Vecchi-Harrison OLR index is used to composite
events on the condition they last at least five days.
OBSERVED
ACTIVE
BREAK
HadCM3
ACTIVE
BREAK
HadCM3FA
ACTIVE
BREAK
20 day
15 day
10 day
5 day
onset
• 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
ACTIVE
HadCM3
BREAK
ACTIVE
BREAK
10 day
5 day
3 day
1 day
onset
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
HadCM3
ACTIVE
BREAK
4
3
2
1
0
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 Ocean
Annual mean at 200m from Levitus
& Boyer (1994)
• The
low
salinity
tongueagreement
near the Indonesian
throughflow
and
There
exists
a general
on the pattern
of salinity
the
low salinity
in theobserved
head Bayand
of Bengal
similarly located.
distribution
between
model are
simulations.
JJAS (HadCM3)
Stddev of JJAS SSS (HadCM3)
• 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
5m
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
Han et al (2001) derived from Levitus & Boyer (1994); Levitus et al (1994)
• 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).
30-60 day filtered composite anomalies of MLD prior to
event onset
Active
HadCM3
Break
20 days
15 days
10 days
5 days
onset
• 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 10S).
• 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.
Thermocline depth (20 C isotherm) and
active/break composites
Mean JJAS(HadCM3)
Active (HadCM3)
• BoB thermocline
deeper
• Presence of equatorial westerlies
in the centralisand
(shallower)
to the
north
eastern basin, sloping the thermocline
to the
east.
(south) during the active
• Model thermocline dome is misplaced
eastward
too
phase whilst
deeperand
over
deep relative to SODA.
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 10 S, 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?
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