Roles of the SST and Land-Sea Heat Contrast
in the Onset of the South Asian Summer Monsoon
Rosbintarti Kartika Lestari1) and Toshiki Iwasaki1)(1: Graduate School of Science, Tohoku University)
Department of Geophysics, Graduate School of Science, Tohoku University, Aramaki-Aoba, Sendai, Japan
980-5578.

Email: kartika@wind.geophys.tohoku.ac.jp
Abstract

1. Introduction
It has been recognized that the solar heating forms the land-sea heat contrast between the Eurasian continentand the Indian Ocean, and triggers of the Asian summer monsoon onset.

The land-sea heat contrast induces heat low

over the continent and westerly geostrophically in the surrounding coastal region (Yanai et al, 1992; Ueda and Yasunari,
1998; Minoura et al, 2003).
Except for the land-sea heat contrast, the other studies mentioned the influence of the SST on the Asiansummer monsoon.

Murakami and Wang (1992) showed that the high SST in the Indian Ocean enhances the Asiansummer monsoon westerly.

The similar conclusions were obtained by the some previous studies, e.g. Shukla (1975)

and Yamazaki (1988).
In the present study, we separately the influences of the SST and the land-sea heat contrast on the onset of theSouth Asian summer monsoon by using global model, GSM T63L40.
stOne experiment is the fixed SST run where themodel is run from April 1 until June 30 for five cases under the SST fixed on April 1st.
experiment as the SST fixed run.

thHereafter, we refer thisAnother is the solar fixed run, where the model is run from April 1st until June 30thfor five cases under the solar condition fixed on April 1st.

2. ModelWe use a T63L40 version of the Global Spectral Model which is in operation for long-range forescasts atJapan Meteorological Agency (JMA).

Spectral triangular 63 (T63) truncation is a roughly equivalent to the resolution

1.875 x 1.875 degrees latitude-longitude grids.
hPa.

There are 40 unevenly spaced hybrid levels from the surface to 1

The orography is truncated at the model's spectral T63 horizontal resolution.

Parameterization schemes forradiative processes are described by Sugi et al. (1990) and the cloud diagnostic scheme is modified by Iwasaki andKitagawa (1998).

An economical version is implemented (Arakawa - Schubert scheme, 1974; Moorthi and Suarez,1992; Randall and Pan, 1993).

3. Results
3.1. Performance of the control run
The model is run for five years from the initial condition of the global objective analysis of Japan
Meteorological Agency (JMA) on January 1st, 2000 with the climatological monthly mean SST data.

The results arereferred as the control run and compared with the NCEP/NCAR reanalysis data for the low-level wind field and theCMAP for the precipitation (not shown).

Both the NCEP (Fig.1 upper panel) and the GSM (Fig.1 lower panel) showthat the easterly in April (Fig.1 left panels) drastically changes to be westerly in June (Fig.1 right panels).
that the low-level wind of the GSM can describe the onset of the Asian summer monsoon well.

It indicates

On the other hand, incomparison with the CMAP data, the precipitation performance of the GSM has some climatic errors, e.g. GSM tends totake double ITCZ (not shown).

Fig.1. Wind fields at 850 hPa of the NCEP/NCAR reanalysis (upper panels) averaging over 10 years from 1990 to 1999
and the GSM (lower panels) averaging over five model years, in April (left panels) and June (right panels). Wind
speed is indicated by an arrow above the panel in the unit of m/sec.

3.2. Impact studies
Our experiment shows that the SST fixed run (the solar fixed run) decreases the surface air temperature over theocean (continent) in the northern hemisphere and increases it over the ocean (continent) in the southern hemisphere,respectively.

Both of the SST fixed run and the solar fixed run change the mean sea level pressure (MSLP), in which

the northern-hemispheric MSLP is increased and the southern-hemispheric MSLP is decreased (not shown).

Thechange in the MSLP trough of the SST fixed run weakens the southern-hemispheric trade wind, cross equatorial
northward flow in the east off the Africa (Somali jet) and Indian monsoonal westerly.

The reduction of the Indian

monsoonal westerly seems to suppress the South Asian summer monsoon, as depicted in Fig.2 (left panel).

The solar

fixed run reduces the land-sea thermal difference, weakens the low-level wind surrounding the Eurasian continent andsuppresses the Indian monsoonal westerly, as shown in Fig.2 (right panel).
Fig.3. shows the impacts on the precipitation.

The impact of the SST fixed run on the precipitation over theocean is very different from that of the solar fixed run, although the two similarly reduce the precipitation around themonsoon Asia.

The SST fixed run considerably suppresses the ITCZ jump from the southern hemisphere to thenorthern hemisphere in June.

On the other hand, the solar fixed run increases the precipitation around the equatorial

Indian Ocean and the western maritime continent regions, but decreases it in the northern hemisphere (Fig.3 rightpanel).

Fig.2. Impacts on the 850 hPa wind field in June. The left panel is the difference between the SST fixed run and thecontrol run, and the right panel is the one between the solar fixed run and the control run, averaging over five cases.
Wind field is indicated by an arrow above the panel in the unit of m/sec.

Fig.3. Impacts on the precipitation in June. The left panel is the differences between the SST fixed run and the control
run, and the right panel is the one between the solar fixed run and the control run. The data is averaged over five cases.
The contour intervals are 3 mm/day and the regions of positive difference are shaded.

4. Discussions
As shown in the left panel of Fig.3, in the SST fixed run, the ITCZ tends to stay in the southern hemisphere in June.
It probably suppresses the Hadley circulation that exerts eastward acceleration in the summer hemisphere and enhances
westerly in the South Asian summer monsoon.

In order to observe this mechanism, we consider the zonal momentum

equation for the low-level wind in an equatorial β -plane,∂u∂u≈ f−v − ku ,
∂t
∂y(1)

where u, v, y, f (= β y) and k represent zonal wind speed, meridional wind speed, latitudinal distance, Coriolis andfriction, respectively.
Derivation of Eq.(1) under the assumptions of steady states∂u= 0 and constant meridional velocity v ,∂tresults in:u≈βyv,kforkyv>> 1 ,(2)

where k is constant.
The Eq.(2) indicates that the frictional forcing zonal mean low-level westerly almost balances with the Coriolis force
due to the mean-meridional wind away from the equator ( y >> vk).

This solution explains that the low level windbecomes easterly in the winter hemisphere and westerly in the summer hemisphere.
Return to the model result, it is confirmed that the SST fixed run suppresses the Hadley circulation (Fig.4 leftpanel.) because of the decrease of the northern-hemispheric cumulus convection, ITCZ (Fig.3. left panel.).

Theweakening of the Hadley circulation corresponds to the weakening of southerly.

Then, according to the Eq.(2), theweakening of southerly ( ∆ v <0) induces easterly anomaly ( ∆ u <0) in the low latitude of the northern hemisphere (Fig.4.
right panel).
westerly.

The easterly anomaly in the northern hemisphere indicates the weakening of the Indian monsoonal

It indicates the suppression of the South Asian summer monsoon.

Fig.4. Meridional wind circulation (left) and time-averaged zonal wind speed (right) differences between the SST fixed
run and the control run (SST fixed – control) in June of GSM T63L40. The data are averaged over five cases. Theregions with the negative difference are shaded.
The solar fixed run directly weakens the South Asian summer monsoon.

The Asian monsoon westerly is strong in

June when the land-sea heat contrast between the Eurasian continent and the Indian Ocean is large (not shown).

Thesolar fixed run reduces the land-sea heat contrast and suppresses the heat low over the Eurasian continent. Therefore,
the low-level wind surrounding the Eurasian continent is geostrophically weakened (not shown).

5. Conclusions
We confirmed that the seasonal marches of the SST and land-sea heat contrast contribute to the Asian summermonsoon onset by the same order of magnitude.
Asian summer monsoon.

The SST has an indirect impact on the strengthening of the SouthThe SST controls the seasonal march of the ITCZ jump between the southern and northernhemispheres, strengthens the Hadley circulation, and then enhances the monsoon westerly.
has a direct impact on the activation of the South Asian summer monsoon.
June induces the low level wind surrounding the Eurasian continent.

References
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Murakami, T. and Bin Wang, JC, 6, 932-952, 1992
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Keywords: monsoon, SST, solar, Hadley

The strong land-sea thermal difference in

Therefore, the Asian monsoon westerly isstrengthened.

Moorthi, S. and M.J. Suarez, MWR, 120, 978-1002, 1992

The land-sea heat contrast