Intra-Seasonal Oscillations of Indian Monsoon:
Dynamics through NCEP/NCAR Reanalysis Data
Ashwini Kulkarni1, RH Kripalani1,, SS Sabade1, M Rajeevan2
1

Indian Institute of Tropical Meteorology,Pune, India
2

India Meteorological Department, Pune, India

ashwini@tropmet.res.in
INTRODUCTION
The summer monsoon (June through September) rainfall variability over India is partly due to the externalsurface boundary forcing and partly due to its internal dynamics. Slowly varying surface boundary conditions
such as sea surface temperatures (SST), snow cover, soil moisture in the preceding winter and the pre-monsoon
season are believed to constitute a major forcing on the year-to-year variability of the Indian monsoon rainfall.
The variability due to internal dynamics is highly unpredictable. Several recent modeling studies have shownthat a significant fraction of inter-annual variability of the seasonal mean Indian summer monsoon is governedby internal chaotic dynamics. The intra-seasonal variability dominant during the monsoon season constitutes
this internal dynamics.
It is well known that the rainfall distribution over India during the summer monsoon season varies
considerably from day-to-day. Over major parts of the country rain occurs in spells under the influence offavorable circulation conditions. This intermittent behaviour of rainfall is associated with a hierarchy ofquasi-periods, namely 3-7 days , 10-20 days and 30-60 days. While 3-7 days periodicity is associated with theoscillation of monsoon trough over the Indo-Gangetic planes, the 10-20 days periodicity or the quasi-biweekly
oscillations are associated with the westward moving waves originating in the Pacific or the synoptic scaleconvective systems generated over the warm Bay of Bengal, propagating inland and contributing substantial
amount of rainfall. The 30-60 days periodicity is linked with the globally eastward moving wave numbers 1and 2 in the tropics in particular over the equatorial regions. This eastward moving mode is designated as
Madden-Julian oscillation (Madden and Julian 1971, Sikka and Gadgil 1980). These two periods 10-20 daysand 30-60 days have been related with the active and break cycles of the monsoon rainfall over Indian
subcontinent. The 30-60 days oscillations are also characterized by northward movement of weather anomalies
over Indian longitudes during summer monsoon season (Singh and Kripalani 1985, 1986, 1990, Kripalani et al
1991, 1992). Thus the inter-annual changes in the intra-seasonal variability are an important potential source ofinter-annual fluctuations of the strength of the summer monsoon rainfall over India (Kulkarni et al 2006)
Thus the monsoon strength may depend on the frequency and duration of the spells of active and breakperiods associated with the fluctuations of various intra-seasonal time scales. Thus the predictability of theseasonal mean Indian monsoon depends on the extent to which the intra-seasonal oscillations influence theseasonal mean relative to the externally forced component. Hence the primary objective of the present study isto bring out how and to what extent the intra-seasonal oscillations (ISOs) affect the seasonal mean and its
inter-annual variability.
DATA
The data sets used in this analysis are as follows :(i)Gridded daily rainfall for 52 blocks each of 2.5o lat x long uniformly spread over India for the period 1June

to 30 September for 1901-1989 were prepared earlier (Kripalani et al 1991; Singh et al 1992). This is based on365 stations well spread over the country. The original daily station rainfall data were acquired from the India
Meteorological Department (IMD). Average daily rainfall of the 52 blocks represents the intensity of rainfall(mm/day) over the country for that particular day.
(ii)Daily rainfall data for India as a whole for the period 1 June to 30 September for 1990-2007 have been
updated from the All India Weather Summary prepared by IMD. Day to day rainfall is based on approximately
120 to 150 stations. The daily rainfall anomalies for each of 1901-2007 based on the entire period constitutethe basic data for our analysis.
(iii)The time series of seasonal (June through September) Indian monsoon rainfall (IMR) for the above 107years (1901-2007) has been downloaded from the website (www.tropmet.res.in) of the Indian Institute ofTropical Meteorology. This series is the measure of the intensity of the whole monsoon over the Indian region
during summer monsoon season.
(iv)The NCEP/NCAR reanalysis data set (Kalnay et al 1996) for the 850 hPa vector winds for the period
1948-2007 is used to examine the dynamic circulation features associated with the extreme phases of the ISOs.
Large scale circulation features over the Indo-Pacific region are very well depicted by this data set .
(v)Data for the 10-parameters used by IMD in operational long-range forecasts for IMR for 1958-2003. Moredetails about these parameters are available in Rajeevan et al (2004).
INTENSITY OF THE INTRA-SEASONAL OSCILLATIONS
For examining the intensity of ISOs, Butterworth’s band-pass filter (Murakami 1979) with peak
response around the dominant periods are used. Thus half power points at 3 and 7 days for the 3-7 days
oscillations, at 10 and 20 days for quasi-biweekly oscillations and at 30 and 60 days for Madden-JulianOscillations is applied to the time series of daily rainfall anomalies. The strength of the ISOs are identified byfirst computing the unfiltered variance. The percentage of the original variance retained by the filtered timeseries is then determined. Thus the percent variance contained in the three periodicities is a measure of theintensity of these three oscillations. The standardized IMR series determines the seasonal monsoon strength.
The IMR series of 1901-2007 has long-term mean of 852mm with standard deviation of 83 mm. We define anexcess (deficient) monsoon when standardized IMR is greater (less) that +1.0 (-1.0).
To examine the relationship of the intensity of ISOs with the total seasonal monsoon rainfall, scatterplots and the correlation analysis between the percent variance in each band and the standardized IMR aredetermined.
INTRA-SEASONAL OSCILLATIONS AND SEASONAL MONSOON STRENGTH
333CC (103)= -0.05 CC (68)= 0.67

CC(103)=-0.15 CC(85)= -0.53-1-22Standaedised IMR

10-3CC (103) = 0.26 CC (80) = 0.602Standardised IMRStandardised IMR

210-1-20102030

Variance Explained

4050-310-1-201020

Variance Explained

3040-30102030

40

Varience Explained(a)(b)(c)
Fig 1 : Scatter plots of variance explained by (a) 3-7(b)10-20 and (c)30-60 days mode on X-axis andStandardised Indian monsoon rainfall on Y-axis
Fig 1 (a) shows the scatter between the standardised IMR and the intensity of the 3-7 days mode. The scatterplot suggests the positive relationship (+0.26). However if some contradictory points (solid dots) are removed,

the correlation enhances to +0.60 based on 80 values which is highly significant. Similar scatter plots for 10-20
days and 30-60 days mode are shown in Fig 1(b) and Fig1 (c) . The relationship of IMR is positive (negative)
with 10-20 (30-60) days mode. Again if some points contradicting the relationship are deleted then thecorrelation shoots up to +0.67 from -0.05 and = -.53 from -0.15 for 10-20 and 30-60 days mode respectively. In
summary it can be inferred that the positive relationship between the intensity of 3-7 and 10-20 days modewith IMR suggests that the dominance of these modes during the monsoon season will favor monsoon activity
over Indian region. On the other hand the negative relationship between the intensity of 30-60 days mode with
IMR suggests that the monsoon activity will not be favored when this particular mode prevails, indicating asignificant control by the ISOs in determining the seasonal rainfall. Kripalani et al (2004) have shown thesimilar results for contrasting monsoons of 2002 and 2003. Whether ISOs modulate the seasonal monsoon
strength through the intensification or weakening of the large-scale monsoon flow is examined in the next
section.
CIRCULATION PATTERNS ASSOCIATED WITH THE EXTREME PHASES OF THE MODES
To study the associated circulation with these two modes we first identify the 5 most intense and mostweak phases of 30-60 and 10-20 days oscillations. These years and the associated standardized IMR for 30-60
days mode are tabulated in Table 1 while those for 10-20 days are tabulated in Table 2.
Table 1 : 5 strongest and weakest episodes of 10-20 days mode. Values in the brackets are associatedstandardized IMR.
EXTREME INTENSE AND WEAK 10-20 MODE
MAXIMUM 10-20
YEARS(Std IMR)VE %MINIMUM

10-20

YEARS
(Std IMR)

VE %1956 (+1.8)

21.4

1982 (-1.4)

7.2

1988 (+1.5)

19.9

1972 (-2.4)

7.3

1953 (+1.0)

18.9

1986 (-1.3)

7.3

1957 (-0.7)

18.5

2002 (-1.9)

7.3

1971 (+0.5)

18.5

1981 (+0.1)

7.4

Table 2 : 5 strongest and weakest episodes of 30-60 days mode. Values in the brackets are associatedstandardized IMR.
EXTREME INTENSE AND WEAK 30-60 MODEMAXIMUM 30-60

MINIMUM 30-60

YEARS(Std IMR)

VE %

YEARS
(Std IMR)

VE %1972 (-2.4)

41.1

1993 (+0.7)

2.3

1979 (-1.7)

28.9

1949 (+0.8)

5.4

1986 (-1.3)

20.9

1988 (1.5)

5.7

1998 (+0.1)

20.1

1970 (+1.2)

5.8

1966 (-1.3)

19.2

1984 (-0.1)

5.9

10-20 days mode
The vector wind composites based on the years in Table 1 are prepared. Fig 2 upper (center) panel

shows the vector wind composite anomalies associated with the intense (weak) phase of 10-20 days oscillation.
These two patterns are remarkably contrasting over Indian as well as Pacific ocean. An anomalous anticyclonic
circulation centered around 10N,70E over the Arabian sea is evident during the intense phase of this mode.
This pattern suggests a transfer of moisture from the equatorial Indian ocean region and the Arabian sea
towards the Indian sub-continent enhancing rainfall activity. Also the establishment of monsoon trough is seenaround 20N. On the other hand the anomalous cyclonic circulation around 5N, 80E prevails during weak phaseof this oscillation. This pattern clearly inhibits moisture supply towards Indian mainland. Over the Pacificeasterly (westerly) flow is evident in the intense (weak) phase of this oscillation which resembles La Nina (ElNino) phase of the ENSO phenomenon over the Pacific. It is interesting to note that the easterly flow during
the intense phase may transport moisture from the west Pacific through Southeast asia and Bay of Bengal. The
difference between these two composites are tested using t test and are shown in Fig2 (bottom panel). The
significant difference is seen over equatorial Pacific region and not over Indian ocean suggesting that theremote Pacific ocean is modulating the behaviour of this mode more than neighbouring Indian ocean.
30-60 days mode
Similar composites for extreme phases of 30-60 days mode (Table 2) are also prepared and presented
in Fig 3. It is seen that anomalous anticyclonic flow prevails over Indian ocean in the itense phase of this mode
which inhibits the moisture supply from the Indian ocean/Arabian sea towards Indian landmass. A similar
reverse circulation pattern is also visible over Pacific ocean.While the westerly flow dominates the equatorial
Pacific for the intense phase of this mode, easterly flow dominates for the weak phase. Again the composites
show significant difference over the Pacific (Fig 2, bottom panel).In summary the intense phase of the 10-20days mode and the weak phase of 30-60 days mode is associated with anti-cyclonic circulation over the Indianocean, easterly flow over the equatorial Pacific ocean resembling the cold phase of ENSO and the weakeningof North pacific Subtropical High. On the other hand the weak phase of 10-20 days mode and the intense phaseof 30-60 days mode shows remarkable opposite flow patterns. Development of both these modes appears to bein remote Pacific Ocean rather than the neighbouring Indian Ocean. These results suggest some influence ofthe boundary forcing, here ENSO , on the intra-seasonal behaviour of the monsoon.
PREDICTABILITY OF THE MODES
To foreshadow the behaviour of these modes the circulation features during preceeding winter andspring have been examined. For the 10-20 days mode the vector wind patterns during May give clear picture
while for 30-60 days mode the April winds are more clear. It is observed that there is tendency for the flow
patterns observed in pre-monsoon months to persist during monsoon months. Some indications of thebehaviour of these modes during the monsoon season could be foreshadowed from the spring season
circulation patterns.
RELATIONSHIP BETWEEN INTENSITY OF MODES AND LRF PARAMETERS
All of these studies indicate that the intra-seasonal monsoon fluctuations represent internal modes ofthe system and are therefore not necessarily related to the slowly varying surface boundary forcings such as
SST anomalies, hence unpredictable (Goswami and Shukla 1984; Gadgil and Srinivasan 1990). In this section
we examine the relationship between the intensity of the modes and the long-range forecasting (LRF)
parameters used operationally by the IMD to see whether the predictors of IMR contain any information
about the intermittent behavior of rainfall. The number of parameters have been used to predict seasonal IMR
such as Arabian sea SST, Eurasian snow cover, NW Europe temperature, NINo3 SST anomalies, East Asia
Pressure, NH 50 hPa wind pattern, Europe pressure gradient etc. We compute the correlations between theFig 2: Composite vector wind anomalies (JJA) based on
extreme phases of 10-20 days oscillations
and the t value of difference. The significant
Values at 10%, 5% and 1 are shaded

Fig 3: Same as Fig 2 but for 30-60 days
oscillations

variance explained by the 3 modes of intra-seasonal variability and these parameters based on 1958-2003. The
east asia pressure in prceeding February+march shows the strongest relationship with these modes. The CC is-0.44 (0.55) with the intensity of 30-60 (10-20)days oscillations based on selected 40 (28) years (selected inFig 1) which are highly significant at 1% level. Hence the predictors for IMR do contain some skill inforeshadowing the behavior and nature of ISOs.
CONCLUSIONS
From the above study we conclude that dominance of 10-20 (30-60) days mode in monsoon season
favors (do not favor) the monsoon activity over India. The intense phase of 10-20 days oscillation and a weakphase of 30-60 days oscillation is associated with anti-cyclonic circulation over Indian Ocean, easterlies over

Pacific ocean and weakening of NPSH leading to intensification of Indian monsoon and weakening of East
Asian monsoon. The flow patterns during premonsoon months April and May do persist during monsoon
season hence spring season patterns help to foreshadow the behaviour of ISOs. East Asia pressure (Feb+Mar)
is the most important predictors for intra-seasonal behaviour of monsoon.
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Kripalani RH, A Kulkarni , SS Sabade, JV Revadekar, SK Patwardhan , JR Kulkarni 2004. Intra-seaonal
oscillations during monsoon 2002 and 2003. Current Science 87 325-334
Kripalani RH, A Kulkarni, SS Sabade 2005 Are intra-seasonal oscillations “speed-breakers” to seasonal
prediction? Clivar Exchanges, 10 17-21
Kulkarni A, SS Sabade, RH Kripalani 2006 Intra-seasonal vagaries of Indian summer monsoon rainfall IITM
Research Report RR-114, 51 pp
Madden RA , PR Julian 1971 Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific .
Journal of Atmospheric Science 28 702-708
Sikka DR , S Gadgil 1980 On the maximum cloud zone and the ITCZ over Indian longitudes during the West
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Singh SV , RH Kripalani 1985 The south to north progression of rainfall anomalies across India during the summer
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Singh SV, RH Kripalani , DR Sikka 1992 Inter-annual variability of the Madden-Julian oscillations in Indiansummer monsoon rainfall Journal of Climate 5 973-978.