Evolution and Progression Characteristics of Indian Summer Monsoon

1V.S. Prasad 1and Taiichi Hayashi
Disaster Preventive Research Institute, Kyoto University, Gokasho, Uji, Kyoto,JapanOn Leave from National Center for Medium Range Weather Forecasting, A-50, Institutional Area, Sector62,New Delhi, NOIDA, India. e-mail: prasadvs@gmail.com

Abstract
Indian summer monsoon is the principal rainy season for India and thus the commencement and the cessation of thisseason bears a great significance. The initiation of the cross equatorial flow off the Somalia coast of Africa during May inresponse to the heating over the South Asian continent marks the beginning of the summer monsoon evolution process overthe Arabian sea. The onset of monsoon over the South Kerala coast is manifested as a consequence of significant changesof atmospheric circulation, cloudiness etc and evolves gradually over the Arabian Sea. Long term records of onset overKerala suggest that the event is more or less regular and its normal onset date of arrival over kerla is 31st May withstandard deviation of about eight days. The recent studies with the large scale analyses of the summer monsoon over theIndian region ( Ramesh et al. 1996, Swati et al. 1999, Fatsullo and Webster 2003, Prasad and Hayashi 2004) suggest that it
is possible to determine the onset of monsoon objectively by using potential indicators. In this study the onset, progression
and withdrawal of the Indian summer monsoon was retrospectively studied using ERA-40 data for the period 1958-2001.
In addition , the temporal evolution of OLR fields from NOAA long term OLR data sets to determine the association ofcloud cover with the onset of monsoon.
Key word : Monsoon, Onset, ERA-40, Withdrawal, objective method

1. Introduction.
The need for objective criteria for the determinationof onset/withdrawal dates for the Indian summer
monsoon (ISM) has been recognized for a long time.
Such a criteria is useful as a diagnostic tool for studying
the monsoon transitions as well as to evaluate theperformance of general circulation models in simulating
the monsoon. It can also aid objective assessment of thecapabilities of numerical models in reproducing
monsoon variability. The temporal and spatial structuresof the atmospheric circulation with the climatology andinter-annual variations of the ISM are one of important
indices to study. More over it can also help in
forecasting the monsoon onset/withdrawal dates using
numerical model out puts. The ability to accurate
prediction of an early or late monsoon onset andwithdrawal is also of substantial economic consequence,even if such forecast can apply to broad regions.
Various methods have been proposed to determine
the date of onset based local rainfall or convection
activity indicated by the satellite-observed data. (Tao andchen,1987;AnanthKrishnan and Soman,1988;, Murakamiand Matsumoto, 1994; Lau and Yang,1997;Wang andLinho,2002). Some other methods are based on thechange of prevailing winds (Holland,1986) or combined
wind-convection
criteria(WangandWu
1987;MasuMuto,1997, Xie et.al 1998). However avariety of dynamic and thermodynamic precursors are
known to exist ( AnanthKrishnan and Soman,1991;
Murkami et.al, 1986). Using such selected precursors
Ramesh et al 1996, Swati et al 1999, Fasulo andWebster,2003 and Prasad and Hayashi 2004 developed

some objective criteria for determining onset andwithdrawal dates of ISM.
The present study focuses specifically on theonset/withdrawal of the Indian summer monsoon systemespecially Arabian Sea branch and Bay of Bengal branch
by using European Center for Medium Range weather
forecasts (ECMWF) reanalysis (ERA 40) data sets.

2. Data & Objective Criteria.
The ECMWF Reanalysis (ERA) data sets withspatial resolution of 2.5 deg x 2.5 deg for the period
1957-2002 was used in this study (Gibson et al. 1997).
The re-analysis was carried out using three dimensionalvariational technique using T159L60 version of theIntegrated Forecasting system to produce the analyses
every six hours. As the reanalysis model also exerts aninfluence on the final output fields, outputs are
categorized by type. For type-A fields, such as therotational wind and upper-air temperatures, the output isstrongly influenced by assimilated data and considered
the most reliable. However, for type-B fields theinfluence of both observations and the model during theassimilation processes can be important. Humidity anddivergent wind fields are examples of type-B fields.
Fields, such as rainfall and evaporation, that are purely
model derived subject to the constraints imposed byassimilated observations, are categorized as type-C. In
the present study efforts were made using A and B typefields.
The studies of Prasad and Hayashi ,2004 shown
that, there is well-defined large-scale moisture variability
at the time of ASM onset, particularly over the ArabianSea. Further, Fasullo and Webter (2003) clearly shownthat hydrological processes such as Vertical Integrated
Moisture Transport (VIMT) are linked to the basic
monsoon forcing. They defined VIMT as:

1.0aHOWI

0.5
0.0
-0.5
-1.0
1.0306090120150

306090120150180210240270300330360

180210240270300330360b0.5CWI

where q is specific humidity, U is wind vector. Above
300 hPa specific humidity in the tropics is at least twoorders of magnitude smaller than near the surface andmoisture transports are of therefore negligible influence
to the calculation of Total VIMT. Earlier studies ofWebster and Yang (1992) also clearly brought out thatthe variability of the summer monsoon reflects in timemean Zonal Wind (u) Shear between 850 and 200 hPa(ZWS). Parasd and Hayashi, 2004 also shown that at thetime of withdrawal, wind changes are more prominent
then the changes in the moisture regime. In that study
they developed and tested objective procedures todetermine the onset and withdrawal of ASM using
normalized time series of the above VIMT and ZWS.
The time series χ is normalized by transformation:⎯χ = 2 X {[χ - min(⎯X)]/[max(⎯X)- min(⎯X)]} -1
where, ⎯x is the mean annual cycle⎯χ is thenormalized time series such that the climatalogicalannual cycle ranges from -1 to +1.
Figure 1 depicts the annual cycle of normalized
time series of VIMT and ZWS for all the years (19572001) of the ERA 40 data averaged over the Arabian Sea
region (65-75E, 10-17.5N). Both the figures a and b
clearly shows a very distinctive seasonal cycle and they
are having maximum amplitude during monsoon andreducing before and after that. The onset of the monsoon
is defined as the times of year at which VIMT index
exceeds the zero and withdrawal is defined as the timesof year at which ZWS index falls below the zero.
Further studies shows that the above will be fulfilled forArabianSea branch only.
But With a slight
modification to the onset and withdrawal criteria thesame index can be applied to other regions of ISM also.
The onset criteria, is modified as the starting day of theperiod with more number of positive HOWI days thannegative HOWI days in a cycle. Then withdrawal is thefirst day of the period with more number of negative
ZWS days compared to positive ZWS days in a cycle
after onset.
In order to show the check the objective criteria bymeans of large-scale circulations associated withmonsoon onset, pentad means (by centering onset day) ofdaily fields has been composed. The onset day is denoted
as day 0 while the signs of ‘-‘ and ‘+’ denotes prior toand after the onset day, respectively. Then pentad mean
fields are obtained with pentad 0 representing 0 to +4and pentad –1 denoting day –1 to day –5.Similler
procedures are done for other successive pentads.

0.0
-0.5
-1.0
Julian Day

Fig 1: Seasonal evolution of daily a) HOWI and b)
CWI for Arabian Sea branch (50° -70°E and 10° to17.5 °N) along with climatological mean values aredenoted by thick line.
Figure 2 depicts the 850 hPa wind pentad
composite based on Onset date for the Bay of Bengal
branch. This clearly shows that onset criteria used is
successfully able to judge onset day with clear
observable change in zero pentad, with strong monsoon
circulation after the pentad and near absence of suchfeatures before that. It also shown that onset over Bay ofBengal occurs earlier than that of Arabian sea branch.
Similarly to show the broad scale circulation features at
the time of withdrawal pentad means centered onwithdrawal dates are composited The pentad means infigure 3 composited based on the date of withdrawal. Itclearly shows the absence of south-westerlies in theregion and by +1 and +2 they are replaced by midlatitude circulation. The above figures clearly indicatesthat the objective criteria for determining onset/withdrawal dates is comparatively judges better.
The onset and withdrawal dates for the period of
1957-2001 are computed for both Arabian Sea and Bayof Bengal braches of ISM using ERA-40 data and byapplying the above objective criteria. The results aretabulated in the Table-I along with the All India Rainfall
index ( Parthasarathy et al 1992; parthasarthy et al 1995;].
While the methods for determination of monsoon
withdrawal are very few there are number of objective
methods for determination of monsoon onset. Earlier
studies by Fasullo and Webster 2003 show that theassociation between
total ISM rainfall and bothmonsoon onset and withdrawal when onset dates fromthe traditional onset dates from the IMD are considered.

Fig 2. Pentad mean evolution of the composite 850
hPa wind vector with centering the onset dates for
Bay of Bengal region. Here 0 denotes the onset
pentad and “-“and “+” refer to prior to and after theonset pentad respectively. See the text for detaildefinition of onset pentad.
However the present criteria shows strong correlation
between total ISM rainfall both onset and withdrawal.
Thus the length of monsoon is shown to be strongly
correlated to its over all strength. Length of monsoon iscomputed using both Arabian Sea and Bay of Bengal
branches.
Using similar approach Prasad and Hayashi 2004
are able to determine multiple transitions of Asiansummer monsoon and thus they proved that the aboveobjective criteria is able to identify the onset/withdrawal
over wide regions. In the present study attempts are
made to use these two indexes for monitoring theprogress and withdrawal of ISM. For this purpose some46 points evenly spread over India as suggested byFig 3. Same as fig2 but for centered at date of thewithdrawal.
Ramesh et al are selected. Then daily variations of
the above indexes were computed for all 46 points of
total ISM period for monitoring progress and retreat of
ISM using ERA-40 data. The results thus obtained will
be presented in this paper. These aspects of the summer
monsoon over the India play crucial role in the initiating
the crop sowing process with the commencement ofrainy season and also for the planning of various otheragriculture operations over the different parts of Indian
sub-continent. In the present study these aspects werecomputed retrospectively. But it has lot of potential forusing in forecasting using global circulation model outputs.

Year

AIRI

Bay of Bengal
Arabian Sea
Onset
Withd. Onset
Withd.
1958
889
02Jun
29Oct
13Jun
18Oct
1959
944
23May 20Oct
22May 19Oct1960
839
16May 24Oct
04Jun
15Oct
1961
1020
18May 29Oct
02Jun
15Oct
1962
809
28May 16Oct
6Jun
11Oct
1963
857
24May 28Oct
2Jun
13Oct
1964
922
30May 13Nov 07Jun
18Oct
1965
709
23May 08Oct
04Jun
04Oct
1966
740
30May 14Oct
06Jun
14Oct
1967
860
13May 14Oct
10Jun
11Oct
1968
754
02Jun
25Oct
09Jun
07Oct
1969
831
13May 26Oct
27May 27Sep
1970
939
14May 01Nov 27May 21Oct
1971
886
28May 04Nov 24May 28Oct
1972
652
11May 16Oct
15Jun
26Sep
1973
913
03Jun
22Oct
03Jun
14Oct
1974
748
20May 09Nov 01Jun
24Oct
1975
962
23May 13Nov 31May 03Nov1976856
20May 04Oct
28May 01Oct
1977
883
11Jun
09Oct
04Jun05Oct
1978
909
22May 16Oct
18May 13Oct
1979
707
15Jun
07Oct
12Jun08Oct
1980
882
27May 08Oct
01Jun09Oct
1981
852
27May 21Oct
03Jun08Oct
1982
735
28May 04Oct
04Jun01Oct
1983
955
28May 28Oct
13Jun
19Oct
1984
836
24May 27Oct
28May 16Oct
1985
760
21May 21Oct
25May 20Oct
1986
74309Jun
22Oct
04Jun
12Oct
1987
697
31May 17Oct
01Jun
08Oct
1988
861
27May 26Oct
08Jun
09Oct
1989
866
17May 25Oct
27May 17Oct
1990
908
07May 19Oct
19May 13Oct
1991
784
31May 03Nov 05Jun
13Oct
1992784
10Jun
26Oct
12Jun
19Oct
1993
896
24May 21Oct
28May 15Oct
1994
938
29May 16Oct
01Jun
19Sep
1995
790
06May 20Oct
08Jun
19Oct
1996
852
11Jun
10Oct
01Jun
11Oct
1997
870
16May 04Oct
12Jun
29Sep
1998
851
14May 29Oct
06Jun23Oct
1999
820
18May 04Nov 18May 02Nov
2000
773
12May 28Oct
15May 27Oct
2001
804
20May 19Oct
21May 23Oct
Table :1 showing the Onset and witdarwal dates of ISM
alon with all India rainfall Index.
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