M RoxyIndian Institute of Tropical Meteorology, Pune, India
1. SST - Precip. relationship2. Spatial variability of SST - Precip.
relationship3. Linearity: Threshold, Quantification
Spatial and temporal dimensions of SST-precipitation relationship during the Asian summer monsoon
Waliser, D. E., Graham, N. E. & Gautier, C. Comparison of the Highly Reflective Cloud and Outgoing Longwave Radiation Datasets for Use in Estimating Tropical Deep Convection. Journal of Climate 6, 331-353 (1993).
Gadgil, S., Joshi, N. V. & Joseph, P. V. Ocean-atmosphere coupling over monsoon regions. Nature 312, 141-143 (1984).
SST-precipitation relationship over tropics- the upper threshold and
CAPE
Upper threshold of 28.5 - 29.5℃
Explanation given:Precipitation tends to occur where positive convective available potential energy (CAPE) exists
-> the occurrence of deep convection will tend to squelch CAPE?
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SST-precipitation relationship over Indian Ocean
- the upper threshold and CAPE
Sabin, T., Babu, C. & Joseph, P. SST–convection relation over tropical oceans. International Journal of Climatology (2012).
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SST-precipitation relationship over West Pacific- negative relationship at temperatures > 29℃?
Rajendran, K., Nanjundiah, R. S., Gadgil, S. & Srinivasan, J. How good are the simulations of tropical SST–rainfall relationship by IPCC AR4 atmospheric and coupled models? J. Earth System Science, 1-16 (2012).
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Atmosphere: NCEP Global Forecast System (GFS)
horizontal: spectral T126, ~90 kmvertical: 64 sigma-pressure hybrid levels
Ocean: GFDL Modular Ocean Model v4 (MOM4p0)40 levels in the vertical, 0.25-0.5°horizontal.
Sea Ice: GFDL Sea Ice Simulator (SIS)an interactive, 2 layer sea-ice model
Land: NOAH, an interactive land surface model with 4 soil levels
Model: NCEP CFSv2
SST, Precipitation: TMI/TRMM
Air Temp, Sp. Humidity, Divergence: ERA-Interim
Reanalysis
Satellite Data/Observations/Reanalysis
Data, Model and Methods
1998-2011 (14 years)
~ 100 years simulation with mixing ratios of time varying forcing agents set for the current decade
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SST-precipitation relationship over monsoon basins
- SST mostly above the upper threshold
So does the high SSTs over the monsoon basins play an active role on the Asian monsoon?
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Intraseasonal SST influence the atmospheric variability, eg: Precipitation (Vecchi and Harrison 2002, Fu et al. 2008).
VH 2002: Negative SST lead monsoon break by 10 days (r = 0.67). Step by step process on the SST- precipitation relationship?
OLR Index
ISV of Bay of Bengal SST (TMI)
SST-precipitation relationship- brief history
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Easterly Wind Anomalies
Increased Evaporation
moist stat. anomalies
Convective Activity
Ocean
AtmosphereKemball-Cook & Wang, 2001
(a) SAT & SST (b) ∆θe & SST (c) CLW & SST(d) Precip. & SST
positive SST anomalies => destabilize lower atmos. column => convective activity (Roxy and Tanimoto 2007)
Easterly Wind Anomalies (reduced total winds)
Reduced Evaporation
Increased SST
∆θe anomaliesunstable
conditions
Convective Activity
Ocean
Atmosphere
Latent heat flux (-ve upward) anomalies enhance precipitation by enhancing the moist static energy (Kemball-Cook and Wang 2001)
Mean monsoon winds are westerly !
Roxy & Tanimoto, 2007
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Holt and Raman, 1987.Mean profile of virtual potential temperature θv, for 0600 GMT on 2 June (pre-monsoon) and 11-14 June (onset) from MONEX 79 ship data over Arabian Sea.
SST influence on the destabilization of lower atmospheric column:Virtual potential temperature (θv) over Arabian Sea during pre-active phase
Pres
sure
θ v (oC)
June
Atmospheric soundings between June 2-14
June 11: pre-active phase
(a) θe1000 anomalies (b) θe700 anomalies
Roxy and Tanimoto 2007
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TMI
Precip. lags SST days Precip. leads SST
The SST-precipitation relationship have different lead-lags over the Arabian Sea and the Bay of Bengal/South China Sea
Spatial variability of SST – Precipitation relationshipresponse time difference of 1 week!
5 days12 days
Ocean -> Atmosphere | Atmosphere -> Ocean
The magnitude of the correlation refers to the intensity of the driving force, and the corresponding lag (lead) time denotes how quickly the atmosphere responds to the SST anomalies and vice versa.
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Role of surface convergence on the response time
Relatively stronger surface convergence over the Arabian Sea accelerates the uplift of the moist air, resulting in a relatively faster response in the local precipitation anomalies
TMI
Observations Model
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SST – Precipitation relationship with/without lags
In observations
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1℃ rise in SST -> 2 mm/day in rainfall
SST – Precipitation relationship with/without lags
In model simulations
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1℃ rise in SST -> 2 mm/day in rainfall
Comparative roles of Eq. potential temperature and upper level divergence
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Large scale circulation features has a role in modulating the SST-precipitation relationship
SST – Precipitation relationshipIn a changing
climate
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1℃ rise in SST -> 2 mm/day in rainfall
Increase in Monsoon Precipitation with increasing SST
Regions of maximum precipitation tends to be located at the ascending branches of the tropical circulation (Lau et al. 1997)
Conclusion1. SST – precipitation relation has a spatial variability over the monsoon basins, with a lag of:
~ 5 days over the Arabian Sea~ 12 days over the Bay of Bengal and
South China Sea
2. Considering this lag, a quantifiable relationship can be deduced between SST (above 26℃) and precipitation:
1℃ rise in SST -> 2 mm/day in rainfall
3. Relationship holds for a changing climate.Warming environment weakens the
monsoon circulationBUT increasing SST balances the amount of
precipitation
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