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Contrasting Summer Monsoon Cold Pools South of Indian Peninsula
Presented at
ROMS/TOMS Asia-Pacific Workshop-2009, Sydney Institute of Marine Sciences, Sydney, Australia
on 31st March 2009
By
Francis P. A. and M. Ravichandran
MOG, INCOIS, Hyderabad
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
What is Summer Monsoon Cold Pool?
Average TMI SST for July-August 2002
Summer Monsoon Cold Pool
The mechanism proposed for the formation of the SMC is the coastal upwelling due to strong monsoon wind.
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
TMI SST Patterns for July/August 2002 and 2003
Intensity of SMC varies both in interannual and intraseasonal time scales
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
TMI SST Difference Patterns (2002-2003)
JULY AUGUST
The variation in the intensity of SMC is attributed to the variation in the strength of upwelling due to the variation of forcing
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Difference in the wind filed (July 2002-July2003)
This difference is too small to produce such a difference in the cooling
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
OLR Anomaly patterns for July 2002 and July 2003
July 2002 July 2003
The convection pattern in July 2002 and July 2003 are opposite over the equatorial Indian Ocean
Such a scenario was present in may occasions, and it is known as Equatorial Indian Ocean Oscillation (EQUINOO) [S. Gadgil, P. N. Vinayachandran, P. A. Francis and S. Gadgil, GRL, 2004]
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Hypothesis
The signature of such an oscillation is seen in many atmospheric and oceanic parameters
When it is coupled with the ocean, then it is referred as Indian Ocean Dipole mode [Saji et al 1999, Webster et al 1999]
The differential heating of the atmospheric column due to anomalous convection associated with the EQUINOO leads to anomalous easterlies/westerlies over the equator
Difference in the wind filed over the tropical Indian Ocean
(July 2003-July2002)
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Hypothesis
•The zonal wind along the equatorial Indian Ocean was dramatically different in the boreal summer months of 2002 and 2003
Zonal wind averaged over 70-90E, 2.5oS-2.5oN
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Strong easterlies along the equatorial Indian Ocean trigger upwelling Kelvin waves, which propagate to the east and downwelling Rossby waves which propagate to the west
The off-equatorial maxima of downwelling (upwelling) can deepen (shoalen) the isothermal layers and decrease (increase) the efficiency of surface cooling
EQ upwelling
Downwelling
Downwelling
upwelling
Downwelling
upwelling
EQ
easterlies
westerlies
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
We show, with the help of ROMS simulations, that it is this downwelling that prevented the summer monsoon cooling (south of peninsnular India) in July 2003.
Further, by August 2003, the zonal wind along the equator became westerly.
This lead to the triggering of downwelling Kelvin waves and upwelling Rossby waves in the equatorial Indian Ocean which in turn lead to intensification of SMC
Hence, in addition to the local forcing, the interannual and intraseasonal variation in the intensity of summer monsoon cold pool is related to the processes over the equatorial Indian Ocean.
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
A supporting evidence of this hypothesis has come from the difference in SSH patterns between July 2002 and July 2003
Difference in SSH anomaly (TOPEX) patterns (July2002- July2003)
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
This hypothesis needs to be tested with a “GOOD” Ocean General Circulation Model, which can simulate not only the coastal features, but also the basin scale features
It is essential to make sure that the equatorial wind forcing influence the subsurface thermal structure in the SMC region
We use ROMS for confirming our hypothesis
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Model set up
Major CPP options chosen:
1. Bulk parameterization2. EMINUSP computed3. Prescribed net long wave radiation4. Eastern and southern boundaries are relaxed to climatology5. Mellor-Yamada mixing scheme6. Salinity correction with a time scale of 15 days
Domain: 30-160E, 30S-30N
Horizontal resolution:0.25 x 0.25 degree
Vertical levels :40Theta_s:10Theta_b :0Hc :100m
Spin-up : 10yrs
Interannual : from 2000 onwards
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Forced with
Wind fields : QSCAT
Air temperature : NCEP-Reanalysis
Relative humidity : NCEP-Reanalysis
Precipitation : CMAP
Net longwave Radiation : NCEP-Reanalysis
Net shortwave Radiation : NCEP-Reanalysis
Sea level pressure : NCEP-Reanalysis
All forcing fields except precipitation are daily mean
precipitation field is monthly mean.
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Validation
Temperature profiles at 90oE, 1.5oS compared with PMEL-RAMA buoy profiles
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Timeseries of Temperature and current at 1.5m depth at 90oE, 1.5oS
Validation
Temperature
Zonal Current
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Simulated summer monsoon cold pools in July/August 2002/2003
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Difference Patterns of surface temperature (2002-2003) for July/August
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Difference in the dynamic height (zeta) anomaly between Jul2002 and Jul2003
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Difference in vertical velocity (2002-2003) at 65m in July and August
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Evolution of the difference in the vertical structure of temperaturein the cold pool region (4-8oN, 75-83oE)
July August
8
20
30
43
60
90
150
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Conclusions:
The observed intraseasonal/interannual variation in the intensity of the summer monsoon cold pool is closely linked to the positive/negative phase of the equatorial Indian Ocean oscillation (EQUINOO)
The easterlies along the central equatorial Indian Ocean (CEIO) during a positive EQUINOO trigger upwelling Kelvin waves to the east and downwelling Rossby waves to the west. The off-equatorial maxima of downwelling prohibit effective surface cooling
It is essential to consider the basin wide processes in order understand the coastal processes in the Indian Ocean domain. Hence any GCM for the coastal forecasting system in the Indian Ocean should have a domain, at least covering the entire basin.
ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009
Important References:
1. Rao et al 2006, 2007(GRL)
2. Shankar et al 2008, (J. Earth Sys. Sci.)
3. Gadgil et al 2003, 2007 (Cur. Sci.)
4. Gadgil et al 2004 (GRL)
5. Saji et al 1999, (Nature)
6. Webster et al 1999, (Nature)