North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are two large-scale atmospheric oscillations in northern and southern hemisphere respectively. These two oscillations are known to affect Indian summer monsoon (June-September) rainfall (ISMR). It is logical to expect the combined effect of these two oscillations on monsoon rainfall over Indian subcontinent. In the present paper this combined effect of NAO and SO on Indian summer monsoon rainfall is studied. North Atlantic Oscillation (NAO) index in the month of April is showing an inverse and statistically significant relationship with Indian summer monsoon rainfall (ISMR). Southern oscillation (SO) indices from April through December show direct association with ISMR. The present study deals with the simultaneous effect of NAO and SO on monsoon rainfall over India. The effective strength index is newly defined to quantify the simultaneous effect of NAO and SO and its relationship with ISMR is investigated. For this purpose 40 years data (1951-90) has been used. The study emphasizes an important role of interaction between NAO and SO in the month of April. It also emphasizes that monsoon activity over India depends upon the strength of both the oscillations namely NAO and SO. The analysis suggests that the effective strength index can be used as a pre-cursor to understand the extreme monsoon events. The analysis also suggests an inverse association of ISMR with the effective strength index from April through December with statistically significant relationship in the months of April, October, November and December

Dugam, SS

Kakade, SB

Ministry of Earth Sciences, Government of India

GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 21, PAGES 3501-3504, NOVEMBER 1, 2000 
The simultaneous effect of NAO and SO 
on the monsoon activity over India 
S.B. Kakade and S.S. Dugam 
Indian Institute of Tropical Meteorology, Pune, India. 
Abstract. North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are 2. Data 
two large-scale atmospheric oscillations n orthern a d southern hemisphere (I) North Atlantic Oscillation (NAOI) 
respectively. These two oscillations are known to affect Indian summer monsoon The NAOI data is taken up from web site address 
(June-September) rainfall (ISMR). It is logical to expect the combined effect of "cpc.ncep.noaa.gov". 
these two oscillations onmonsoon rainfall over Indian subcontinent. I  the present (II) Southern Oscillation Index (SOD 
paper this combined effect of NAO and SO on Indian summer monsoon rainfall is The SOI data (the pressure difference b tween Tahiti and 
studied. North Atlantic Oscillation (NAO) index inthe month ofApril s showing Darwin) istaken from web site address "nic.fl•.noaa.gov". 
an inverse and statistically significant relationship with Indian summer monsoon 
rainfall (ISMR). Southern oscillation (SO) indices from April through December 
show direct association with ISMR. The present study deals with the simultaneous 
effect of NAO and SO on monsoon rainfall over India. The effective strength index 
is newly defined to quantify the simultaneous effect of NAO and SO and its 
relationship with ISMR is investigated. For this purpose 40 years data (1951-90) 
has been used. 
The study emphasizes an important role of intexaction between NAO 
(III) Indian Sunm•r Monsoon Rainfall (ISMR) 
The summer monsoon (June-September) ainfall data is 
obtained from Parthasarathy B. et al. (1994). 
3. Inter-relationships of NAO, SO and ISMR 
3.1 Relationship between SO and ISMR 
Figure-I gives the correlation coefficients of ISMR 
with monthly SO indices for 40 years (1951-90) data. It suggests that ISMR 
and SO in the month ofApril. Italso emphasizes that monsoon activity over India is inversely associated with SO from January to March but is directly 
depends upon the strength of both the oscillations namely NAO and SO. The associated with SO from April through December. The change in the 
analysis uggests that the effective strength index can be used as a pre-cursor to 
understand the extreme monsoon events. The analysis also suggests an inverse 
association f ISMR with the effective strength index from April through December 
with statistically significant relationship nthe months of April, October, November 
and December. 
1. Introduction 
Walker and Bliss believed that the teleconnection patterns in 
surface temperature and sea level pressure were manifestations of a single 
phenomenon which they termed as North Atlantic Oscillation (NAO). The 
pressure see-saw between equatorial Pacific and Indonesian regions is 
known as Southern Oscillation (SO). These two large-scale atmospheric 
oscillations are known to affect Indian summer monsoon (June-September) 
rainfall. Many earlier studies by Sikka (1980), Pant and Parthasarathy 
( 1981 ), Shukla and Paolino (1983), Rasmusson ( 1983 ), Verma t 1990) have 
shown the effect of Southern Oscillation on the monsoon activity over Ind•a. 
Recently, Dugam et al. (1997,1999) have studied the relationship between 
NAO and Indian summer monsoon rainfall. They suggested that the NAO 
index in the month of April is inversely associated with ISMR and th•s 
relationship is statistically significant at 5% level. The interactive mechanism 
of NAO and SO is discussed by Dugam et al (1999). 
Copyright 2000 by the American Geophysical Union. 
Paper number 1999GL011201. 
0094-8276/00/1999GL011201 $05.00 
relationship of ISMR and SO from April is important from the next 
discussion point of view. As the positive phase of SO during this time is 
intensifying east-west Walker circulation, it results in more rainfall activity 
over Indian subcontinent. 
3.2 Relationship between NAO and ISMR 
Figure-2 gives the correlation coefficients of ISMR 
with monthly NAO indices for 40 years (1951-90) data. It suggests 
statistically significant inverse relationship between ISMR and NAO index 
during the month of April. The probable physical explanation for this may 
cc 
0.6 
0.4 
0.2 
00 
-0.2 
-0.4 
-0.6 
I ß I , I , I , I , I , I , I ß I 
Jan Feb Mir Apr Mi I Jun Jul Aug Scp 
Month 
I I 1 
ß 
Figure l - Relationship between Indian summer monsoon rainfall 
an SO indices on monthly scale [ 1951-90]. 
3501 
3 502 KAKADE AND DUGAM: EFFECT OF NAO AND SO ON MONSOON ACTIVITY OVER INDIA 
0.2 
00 
-02 
CC. 
Month 
Figure2 - Relationship between Indian summer monsoon rainfall 
an NAO indices on monthly scale [ 195 !-90]. 
APRIL NAO APRIL SO 
0.4 
0.3 
0.2 
0.0 
-0.1 
-0.2 
-0.3 
-0.4 
ß -- ß -- SOla 
0.2 
"•"/' % '%. 
ß .0 I 
. -fl.2 
ß . 
, I , I , I , ! . I , I I I , ! . I , I • I i I ß 
j,m Itek Mar Apr May Jul Jul Aug .•p Oct Nev De• Jen 
MONTH 
FigureA - Relationships of ApriI-NAO with monthly SO indices 
and April-SO with NAO indices [1951-90]. 
be as: during negative phase of NAO, the Icelandic low and Azores high 
shifts poleward which decreases the Eurasian snow cover (Gutzler and 
Roses, 1992) which has been linked to an earlier and strong warming of the 
Eurasian continent. This will create large ocean-land temperature contrast 
which results in earlier and stronger Asian summer monsoon (Walker, 1910: 
Hahm and Shukla, 1976; Sankar Rao et al, 1996). 
3.3 Relationship between NAO and SO 
Figure-3 gives the association between NAO and SO 
on a monthly scale. It suggests an inverse association between NAO and SO 
during April, October, November and December only The reverse 
association between NAO and SO in April becomes direct ass(xzlation from 
May through September which again becomes inverse association fi-om 
October through December. 
An inverse association of NAO with SO (though it is 
not statistically significant) in the month of April, the statistically significant 
inverse association of ISMR with April-NAO and change in relationship of 
ISMR and SO from April onwards force us to look into lag-lead relationship 
between NAO and SO in the month of April. Figure-4 gives the relationships 
of ApriI-NAO with monthly SO indices from January through December 
0.2 
0.1 
O 0.0 
o 
-o 1 
-0.2 
-0.3 
MONTH 
Figure3 - Association between NAO and SO on monthly scale 
and April-SO with monthly NAO indices from January through December. 
It suggests that ApriI-NAO is showing an inverse association with SO 
indices from April through December whereas April-SO is showing direct 
association with NAO indices from May through December, with the only 
exception in the month of June. 
4. Combined effect of NAO and SO on ISMR: 
While studying the relationship of ISMR with combined effect of 
NAO and SO, it is necessary to understand the behavior of these two 
1 
_._ 
%()1 
' , ':, ß , 
'..., ,, , 
,. 
4 '1' I'l'l' 1'1'11'' '' '' I 
Nov Dec Jan Feb Oar An, •av Jun J,,I A4,.J(} Sep 
1986-87 
• ---- N^O SOl 
, 
, 
ß 
, , 
".,, ,, ,, 
, 
4 
--.-- N^O 
3 ',()1 
Figure5 -Observed NAO and SO indices for contrasting summer 
[ 1951-90]. monsoon years. 
KAKADE AND DUGAM: EFFECT OF NAO AND SO ON MONSOON ACTIVITY OVER INDIA 3 503 
oscillations in contrasting monsoon years. For this purpose we have plotted Is unfavorable. In 1987, the NAO phase is favorable (-ve phase) but SOl 
observed NAOI and SOl for every extreme (excess or deficient) monsoon phase is unfavorable (-ve phase). In 1988 and 1998, both the oscillations are 
year, starting from the October of the previous year through September of favorable for monsoon activity over India. Thus if both the oscillations are 
the extreme year. Consider a sample case, as depicted in Figure-5, of 19'72, favorable for monsoon activity over India then the summer monsoon rainfall 
1987 - the two major drought years; 1988 - an excess monsoon year and over India is increased and if any one of these two oscillations i  unfavorable 
1998 - a normal monsoon year (The NAO and SO data for this year has then the summer monsoon rainfall over India is reduced. Therelbre the 
been taken up from monthly climate diagnostic bulletin, October-1998). The monsoon activity over India depends on the strength of both the indices 
year 1998 is considered inthe sample case so as to understand the behavior (NAOI and SOl) and the strength of the single oscillation of NAO and SO is 
of these oscillations for the year outside the test-sample p riod (1951-90). In not sufficient to understand ISMR. This suggests he need to understand the 
1972, there is a positive phase of SO from May through September which is simultancous effect of NAO and SO on the monsoon activity over India. 
a favorable condition for good monsoon activity over India but NAO phase 4.1 Effective strength index: 
The effective strength index (ESI) is defined as the 
1.5 
(a) 
0.õ 
-½.5 
-1 
Oct Nov DeC Jan Feb Mar ,&p•' May Jun Jul Aug Sop 
Month 
algebraic difference between the indices of NAO and SO. It measures the 
resultant signal between these oscillations. Positive value of ESI indicates 
that NAO is relatively stronger than SO and viz. It inhibits the net air-flow 
from southern hemisphere to northern hemisphere. 
Figure-6 depicts the composite picture of ES1 lbr 
excess, deficient and normal summer monsoon years. A particular mons(x)n 
year •s classified as deficient, excess and normal depending upon the 
normahsed rainfall value less than -I, greater than I or lying between I and - 
I respectively. During excess monsoon years (1956,59,61,70,75,83 and 88), 
the ESI is positive from October of the previous year through March of the 
same year and it is negative from April through September with a slight 
positive value in June. During deficient monsoon years 
0.5 
-0.5 
(b) 
c! Nov Dec Jan Feb Mar A,or May Jun Jul Aug Sep 
Month 
(1951,65,66,68,72,74,79,82,85,86 and 87), the ESI is positive from March 
through September. During normal monsoon years (neither excess nor 
deficient monsoon years), the ESI is positive in the month of April but 
negative from May through September. Thus if the ESI shows decreasing 
trend from January onwards and its value become negative t?om April 
onwards then we can expect good summer monsoon activity over India. 
Similarly, if opposite conditions are observed then the monsoon rainfall over 
India is reduced. 
Figure-7 gives the relationship of ISMR with the 
effective strength indices from January through Dex•mber. Indian summer 
monsoon rainfall is directly associated with ESI from January to March and 
0.6 
(c) 
0.4 
0.2 
-0.2 
0.4 
Oct Nov Dec Jan Feb Mar AOt' May Jun Jul Au0 Sop 
Month 
is inversely associated with ESI from April through December. In April, the 
relationship is statistically significant at 1% level (C.C. = -0.43). For the 
recent 40 year period (1951-90), the correlation coefficient (C.C.)of ISMR 
with April-NAO is -0.33 (statistically significant at 5% level) and with the 
April-efi•tive strength index of NAO and SO, it is -0.43 which is 
statistically significant at 1% level. 
The negative ESI indicates that SO is relatively 
stronger than NAO which may (1) intensify the east-west Walker 
circulation and (2) increase the moist air-flow from southern hemisphere to 
northern hemisphere which intern results in increased rainfall activity over 
Figure6 - Composite picture of effective strength index of NAO Indian sub-continent. Thus negative ESI from April through September 
and SO for (a) excess monsoon years, (b) deficient monsoon years and (c) indicates intensification of Walker circulation which is favorable condition 
normal monsoon years during 1951-90. tbr Indian summer monsoon rainfall. The statistically significant relationship 
3:504 KAKADE AND DUGAM' EFFECT OF NAO AND SO ON MONSOON ACTIVITY OVER INDIA 
C.C. 
0.6 l 0.4 
necess• facilities for carrying out this study. Thanks are also due to Dr. 
S.S. Singh, Head, FRD, for his encouragement. 
0.2 
0.0 
-0.2 
-0.4 
-0.6 
Jan Fell Mar Apr May Jun Jul Aug Sep Oct No• Dec 
Month 
Figure7 - Relationship between Indian summer monsoon rainfall 
and effective strength index of NAO and SO on monthly scale [1951-90]. 
of ISMR with ESI during October, November and December is suggesting 
some feext-back mechanism but it needs to be studied separately in details. 
5. Concltrsion 
From this study following conclusions can be drawn ' 
(a) Interaction between NAO and SO during the pre-monsoon 
month of April plays an important role in the monsoon rainfall over India. 
(b) Indian summer monsoon rainfall depends upon both the 
oscillatfong NAO and SO simultaneously. Hence the joint effect of these two 
oscdlat•ons plays a key role in the sutl•mcr monsoon circulation ovel Indian 
region 
(c) When the efl•tive stren•h index is positive from the month 
of April then the summer monsoon rainfall activity over India is reduced. 
Thus eft&rive strength index can be used as a pre-cursor to predict drought 
monsoon conditions over India. 
(d) The effective strength index in the month of April is inversely 
associated with Indian summer monsoon rainfall. This relationship is 
statistically significant a 1% level. Thus April-effecuve strength index can be 
used as one of the predictor for the long-range-forecast of Indian summer 
monsoon rainfall. 
Acknowledgments 
The authors would like to thank Dr. G.B. Pant, Director, Indian 
Institute of Tropical Meteorology, Pune, for his interest and providing 
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(Received :November 5, 1999; Revised: March 27, 2000 ;Accepted :May 
9, 2000) 


The simultaneous effect of NAO and SO on the monsoon activity over India

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The simultaneous effect of NAO and SO on the monsoon activity over India

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