INDOEX IFP-99 was undertaken as part of the international experiment in the Indian Ocean to take observations pertaining to aerosols, radiation, cloud physics and other related meteorological parameters. The important-aim of the INDOEX was to quantify radiative forcing due to natural and anthropogenic aerosols and their feedback on regional and global climate systems. Since prevailing circulation features transports aerosols, it is essential that important synoptic patterns during the expedition phase, i.e. 20 January to 10 March 1999 be examined. Based on the synoptic features it was noticed that crossequatorial flow in lower levels from western Arabian Sea to southern Indian Ocean was significantly higher than the eastern Arabian Sea. Two cyclonic storms, one in the south Bay of Bengal during 1-3 February and another in the south Indian Ocean during 4-13 March were observed. Significant changes in the cross-equatorial flow in the lower/upper tropospheric levels and ITCZ locations were noticed

Iyenger, G.

Kalsi, S. R.

Madan, O. P.

Mohanty, U. C.

Paliwal, R. K.

Pareek, R. S.

Prakash, H.

Raju, P. V. S.

Ramesh, K. J.

Sikka, D. R.

English

Publications of the IAS Fellows

INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPLEMENT), VOL. 80, 10 APRIL 2001 30
Important synoptic features during INDOEX 
IFP -99 
O. P. Madan*, R. S. Pareek†, S. R. Kalsi†, U. C. Mohanty*,||, R. K. Paliwal‡,  
D. R. Sikka§, G. Iyenger‡, H. Prakash‡, K. J. Ramesh‡ and P. V. S. Raju* 
*Centre for Atmospheric Sciences, Indian Institute of Technology, New Delhi 1100 6, India 
†India Meteorological Department, Mausam Bhavan, Lodi Road, New Delhi 110 003, India 
‡National Centre for Medium Range Weather Forecasting, IMD Complex, Lodi Road, New Delhi 110 003, India 
§No. 40, Mausam Vihar, Delhi 110 052, India 
INDOEX IFP -99 was undertaken as part of the inter-
national experiment in the Indian Ocean to take  
observations pertaining to aerosols, radiation, cloud 
physics and other related meteorological parameters. 
The important-aim of the INDOEX was to quantify 
radiative forcing due to natural and anthropogenic 
aerosols and their feedback on regional and global 
climate systems. Since prevailing circulation features 
transports aerosols, it is essential that important  
synoptic patterns during the expedition phase, i.e.  
20 January to 10 March 1999 be examined. Based on 
the synoptic features it was noticed that cross-
equatorial flow in lower levels from western Arabian 
Sea to southern Indian Ocean was significantly higher 
than the eastern Arabian Sea. Two cyclonic storms, 
one in the south Bay of Bengal during 1–3 February 
and another in the south Indian Ocean during 4–13
March were observed. Significant changes in the 
cross-equatorial flow in the lower/upper tropospheric 
levels and ITCZ locations were noticed. 
INDIAN Ocean Experiment Intensive Field Phase-1999 
(INDOEX IFP-1999), and its Indian component over the 
Arabian Sea and the south Indian Ocean were undertaken 
by ORV Sagar Kanya from 20 January 1999 to 11 March 
1999, covering the longitudinal belt 50°E o 90°E. The 
international team deployed research aircraft equipped 
with highly specialized instruments, research vessel Ron 
Brown and European Space Agency satellite METEOSAT 
to take observations related to aerosols and cloud physics. 
The French Group took constant pressure balloon obser-
vations to study the air parcel trajectories emanating from 
the West Coast of India. The important aim of the 
INDOEX is to quantify the radiative forcing due to nat-
ral and anthropogenic aerosols and its feedback on  
regional and global climate. The region comprising  
Arabian Sea and Indian Ocean was selected for observa-
tional programme because the pollut d air from Indian 
Sub-continent, south-east Asia and southern China was 
expected to be carried equator-w rds by north-east trades 
of the Northern Hemisphere. Northeast trades while cross-
ing the equator in this longitudinal belt would meet the 
pristine air of the southeast trades of the Southern Hemi-
sphere. Since the prevailing wind circulation is expected 
to transport aerosols into the general area of ITCZ, that is 
located in the Southern Hemisphere during this time of 
the year, the radiative forcing due to clouds and aerosols  
og ther may undergo changes on a seasonal as well as 
climatic scale. 
 Godbole and Ghosh1 based on the analysis of aerologi-
cal data along meridionals 55°E and 65°E, from 23 May 
to 28 May, during MONEX-1973 showed the existence of 
broad zone of equatorial westerlies to the south of Inter-
Tropical Convergence Zone (ITCZ) in the Northern 
Hemisphere. Easterlies to the north of ITCZ were warm 
and dry while westerlies to its immediate south were cool 
and moist. They also found another trough in Southern 
Hemisphere, which extended from surface to 500 hPa and 
had poleward inclination. Asnani2 reviewed the work on 
ITCZ in various parts of the world, giving wind, tempera-
ture, moisture and distribution of some of the diagnostics 
such as divergence, vorticity, etc. in its neighbourhood. 
Objective of the study 
The objective of the present study is to study flow fea-
tures which are associated with the important synoptic 
systems that affected the south Bay of Bengal, south  
Arabian sea and Indian Ocean relevant to the interest of 
INDOEX IFP-99. 
Data 
The study is based on the daily analysis of Nat onal Cen-
tre for Medium Range Weather Forecasting (NCMRWF) 
charts of 925, 850, 500 and 200 hPa levels, zonal section 
of vertical circulation along the equator, forward and 
backward trajectories. India Meteorological Department 
(IMD) charts of Northern Hemisphere Analysis Centre 
(NHAC), INSAT visible and Infra red cloud imageries, 
daily INSAT OLR charts at 2.5° × 2.5°, associated rain *For correspondence. (e-mail: mohanty@cas.iitd.ernet.in) 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 31
 
 
 
Figure 1. NCMRWF Wind analysis at 0000 UTC. a, 25 January 1999; b, 29 January 1999. 
 
b 
a 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPLEMENT), VOL. 80, 10 APRIL 2001 32
fall analysis, satellite-derived cloud motion vectors and 
Regional Specialised Meteorological Centre (RSMC), 
New Delhi analysis and forecast charts at various levels 
have also been used. 
Synoptic features – Leg A 
ORV Sagar Kanya sailed from Goa on 20 January 1999, 
and returned on 11 March 1999.The ship track and its 
pproximate locations on different days is as shown in 
Figure 1 of the Introductory Note. The following impor-
tant synoptic features were observed on Leg A (Goa–Port 
Louis). 
ITCZ 
From 20 to 29 January 1999, Northern Hemisphere Equa-
torial Trough (NHET) or ITCZ (N) as well as Southern 
Hemisphere Equatorial Trough (SHET) or ITCZ (S) 
existed simultaneously. The SHET was continuously  
active and the belt between equator to 10.0°S had con-
tinuous convective activity. A tropical disturbance, 
probably a depression was located in the Indian Ocean 
close to 12°S/107°E on 22 January 1999. The depression 
moved west- outh-westwards and on 30 January it was 
close to 19.5°S/83.0°E. Thereafter it weakened gradually 
and became unimportant. The ship was close to ITCZ (N) 
on 25 January 1999 and passed through ITCZ (S) on 
28/29 January 1999. The flow pattern at 925, 850, 700 
and 200 hPa on 25 January and 29 January is given in 
Figure 1. The visible INSAT pictures at 0600 UTC of 
25 January and 29 January are given in Figure 2. 
 Based on the above examination of weather charts and 
other meteorological parameters between 20 and 29 Janu-
ary 1999 the following conclusions can be drawn: 
 
(i) ITCZ (N) and ITCZ (S) were more active east of 
80°E, i.e. South Bay of Bengal and eastern part of south 
Indian Ocean. 
(ii) Low level flow pattern (925 hPa) indicated that cross-
equatorial flow from western Arabian Sea (West of 60°E) 
Figure 2. INSAT picture (VIS) at 0600 UTC. a, 25 January 1999; 
b, 29 January 1999.  
 
Figure 3. Bay of Bengal cyclonic storm, 1–3 February 1999. 
 
b 
a 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 33
to south Indian Ocean was significantly higher than over 
the eastern Arabian Sea. Similarly significant cross-
equatorial flow was observed in the western parts of Bay 
of Bengal. 
(iii) Low level winds to the south of ITCZ (N) were us-
ally westerlies and appear to be of northern hemispheric 
origin. The ITCZ (S) appears to be between monsoonal 
westerlies and south-east trades of the southern hemi-
sphere. From the wind fields, both the ITCZ (N) and 
ITCZ (S) appear to be nearly vertical. 
(iv) In the upper levels at 200 hPa, cross-equatorial flow 
from south Indian Ocean to Bay of Bengal was signifi-
b 
a 
Figure 4 a, NCMRWF Wind analysis at 0000 UTC on 3 February 1999; b, RSMC (IMD) Wind analysis at 0000 UTC on 3 February 1999. 
 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPLEMENT), VOL. 80, 10 APRIL 2001 34
cantly higher, while in the Arabian Sea side it was rather 
weak.  
Cyclonic storms 
(i) A broad low pressure prevailed over southern parts 
of Bay of Bengal on 31 January in which 1008 hPa isobar 
covered most of the area south of 15°N latitude. Within 
this broad low there was a relatively well-marked low 
pressure area with central pressure of 1006 hPa over 
southeast Bay of Bengal and neighbourhood. The surface 
winds were of the order of 10–15 knots from the south-
west. However the persistent convection which was asso-
ciated with this configuration gave rise to a depression on 
2 February. This system eventually intensified further into 
a cyclonic storm at 0300 UTC of 3 February when it was 
located around 11.5°N/86.0°E. Hereafter this vortex came 
under the increasing influence of stronger middl  and  
upper tropospheric westerly flow and got sheared off. 
Since 1891 this has been the third significant vortex seen 
in Bay of Bengal during 1st week of February. Though it 
was seen in the Limited Area Model (LAM) analysis, it 
was not very well captured in the NCMRWF analysis 
charts. 
(ii) A depression on 1 February 1999, lay centered close 
to 12.5°S/70.5°E in the south Indian Ocean. Moving west-
south-westwards on 3 February its position was 19.0°S/ 
65.0°E and on 5 February it was located close to 22.0°S/ 
50.5°E. It had become unimportant after crossing south 
Mozambique on 8 February 1999. The positions are  
extracted from the INSAT imagery and did not tally  
exactly with the NCMRWF analysis. 
(iii) Another tropical disturbance was located close to 
8.0°S/67.0°E on 3 February 1999. On 5 February, the 
tropical disturbance lay close to 10.0°S and 60.0°E and 
was possibly a tropical depression. On 8 February it lay in 
the Mozambique Channel close to 15.0°S and 47.0°E. 
Simultaneously in the north Arabian Sea a trough at 
200 hPa lay close to 75°E. 
 
 The track of the above systems along with correspond-
ing INSAT pictures and flow patterns are given in Figures 
3 to 8. Composite OLR chart and rainfall analysis for 4 
February 1999, are shown in Figures 9 and 10, indicating 
the presence of highly organized vortex in the south Bay 
of Bengal. The large-scale precipitation estimate as per 
Arakin3 technique indicates a broad region of convective 
precipitation of 1 to 2 cm, which includes a maxima  
of 2.5 cm. 
 The following important observations are made: 
 
· In the Bay of Bengal, particularly in the western parts, 
significant flow from Southern Hemisphere across the 
equator is taking place in the lower troposphere on 3 
February 1999 while on 5 February the direction is  
reversed. 
· The monsoonal westerlies in the south Indian Ocean 
are not noticed on 3 or 5 February 1999 and both the 
ITCZ (N) and ITCZ (S) are suppressed. 
· At 200 hPa, in the Arabian Sea the cross-equatorial 
flow depends upon the location and the amplitude of 
the mid-latitude westerly trough of Northern Hemi-
sphere over India. 
Figure 5. INSAT picture (VIS). a, 2 February 1999 at 0600 UTC; 
b, 3 February 1999 at 0900 UTC. 
 
b 
a 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 35
 
Figure 6. Track of tropical depressions and a tropical storm DAVINA in the south Indian Ocean. 
 
Figure 7. NCMRWF Wind analysis at 0000 UTC on 5 February 1999. 
 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPLEMENT), VOL. 80, 10 APRIL 2001 36
Synoptic situation on Leg B 
Cyclonic circulation in the Bay of Bengal and  
ITCZ (S) 
On 17 February 1999, a low-level cyclonic circulation 
was noticed at 850 hPa in the south Bay of Bengal. It 
moved across Sri Lanka and south Tamil Nadu on  
19 February and finally emerged in the Arabian Sea on  
22 February and became unimportant on 24 February.  
On the same day ITCZ (S) was observed close to 7.5°S.  
The INSAT picture for 22 February 1999 is shown in 
Figure 11. 
Tropical storm in the Indian Ocean 
A tropical disturbance was noticed on 1 March 1999 close 
to 8.0°S/92.0°E. It became a depression and lay close to 
9.5°S/89.0°E. It became a cyclonic storm on 4 March 
1999, and lay centered at 1200 UTC close to 12.5°S/ 
87.5°E with intensity T3.0. The storm was named 
‘DAVINA’. It moved west-southwestwards and intensfied 
gradually to the stage of T5.0 on 8 March when it was close 
to 17.0°S/67.5°S. Thereafter it showed signs of weakening. 
On 10 March 1999, it was located close to 22.0°S/56.
It became unimportant on 14 March. NCMRWF analysis 
at 0000 UTC and satellite picture at 0600 UTC for 
8 March 1999 are presented in Figures 12 and 13. 
Figure 8. INSAT picture (VIS) at 0600 UTC on 5 February 1999. 
 
Figure 9. OLR analysi (24 h) on 4 February 1999. 
 
Figure 10. Rainfall analysis (24 h) at 0300 UTC on 4 February 1999. 
 
Figure 11. INSAT picture at 0600 UTC (VIS) on 22 February 1999. 
 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 37
 
 
Figure 12. NCMRWF analysis at 0000 UTC on 8 March 1999. 
 
Figure 13. INSAT picture (VIS) at 0600 UTC on 8 March 1999. 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPLEMENT), VOL. 80, 10 APRIL 2001 38
 
 The following observations were made: 
 
(i) The low-level cross-equatorial flow in the Arabian Sea 
is reduced compared to the earlier situations. 
(ii) ITCZ (S) during the intensification stage of cyclonic 
storm in the south Indian Ocean became diffused. 
(iii) At the upper levels, at 200 hPa the cross-equatorial 
flow, particularly in the Arabian Sea has increased. 
Conclusions 
ITCZ (N) and ITCZ (S) during the INDOEX IFP-99 were 
more active to the east of 80°E. From 20 January to 29 
January 1999, both the ITCZ (N) and ITCZ (S) were  
observed simultaneously. The cyclonic storm in the Bay 
of Bengal in February 1999 was a rare phenomenon and it 
was the third significant vortex since 1891. Cyclonic cir-
culation’s in the low levels were observed on many days 
in the south Bay and were associated with convective 
clouds and precipitation. In the Indian Ocean a number of 
vortices possibly depressions could be discerned from the 
INSAT cloud imagery but only the important ones in rela-
tion to ORV Sagar Kanya track have been described. The 
monsoonal westerlies were observed in the Indian Ocean, 
between the two ITCZ positions. However during the 
presence of strong vortices, the ITCZ positions became 
diffused. 
 The cross-equatorial flow from Arabian Sea into the 
Indian Ocean is significantly higher (say up to 850 hPa) in 
the western Arabian Sea off the coasts of Somalia and 
Kenya than in the eastern Arabian Sea off Kerala and 
Karnataka coast. 
 In the Bay of Bengal, low-level cross-equatorial flow 
from the Bay into the Indian Ocean is significant in the 
western Bay, particularly off Tamil Nadu and Sri Lanka 
coast. 
 In the presence of synoptic systems such as lows/  
depressions/cyclonic storms, either in the Bay, or Indian 
Ocean within the equatorial belt of 15°N to 15°S, signifi-
cant variations in the low-level cross-equatorial flows take 
place and the position of ITCZ becomes diffused.
 In the upper troposphere at 200 hPa, cross-equatorial 
flow from the Indian Ocean into the Bay is signifi- 
cantly higher than in the Arabian Sea. Under the influence 
of deep amplitude troughs in the mid latitude westerlies 
at 200 hPa over the Indian sub-continent, some interes- 
ting variations in the cross-equatorial flow amongst  
Arabian Sea, Bay of Bengal and Indian Ocean are  
observed. 
 With the availability of glass-sonde, aircraft and ship 
data collected during INDOEX IFP-99, the re-analysis of 
wind field and other meteorological fields by NCMRWF 
w uld better represent the synoptic features, particularly 
i the Indian Ocean. 
 
 
 
1. Godbole, R. V. and Ghosh, S. K., TELLUS, 1975, XXVI , 125–131. 
2. Asnani, G. C., Trop. Meteorol., 1993, I , 19–134. 
3. Arakin, P. A., Mon. Wea. Rev., 1979, 107, 1382–1387. 
 
ACKNOWLEDGEMENTS. We thank Director General of Meteorol-
ogy for providing the facilities to complete this study. We also place on 
record the satellite data and other products made available by IMD. We 
also thank NCMRWF for using their analysis charts and other produc s. 
 


Important synoptic features during INDOEX IFP-99

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Important synoptic features during INDOEX IFP-99

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