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

Pre

ssure

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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