Desai, B. N.

Rao, Y. P.

English

Publications of the IAS Fellows

LETTER TO THE EDITOR 
Cyclogenesis in the Bay of Bengal and Arabian Sea 
By Y. P. RAO, Meteorological Office, Poona 5 and B. N. DESAI, 173, Swami 
Vivekananda Road, Vile Parle (West) ,  Bombay-56 
(Manuscript received November 17, 1969) 
1. Introduction 
Mowla (1968) has stated that normal iso- 
therms of 200 mb level over the sea area bear 
some remarkable similarity with the isocyclo- 
genesis lines in the Bay of Bengal and the Arabian 
Sea pertaining to the same seasons. He has 
reproduced normal June and July 200 mb level 
isotherms in Figs. 1 b and 2 respectively of his 
paper from Ramakrishnan (1958) and also 
shown in Fig. 1 b the frequency of storms and 
depressions during the months June to Sep- 
tember from the paper of Ananthakrishnan 
and Bhatia (1958). He has mentioned that 
cyclogenesis tends to occur over both the seas as 
well as over the land area of the Indo-Pakistan 
subcontinent over the area where warm pools 
form a t  the 200 mb level, the same being 
generally bounded by 226"A isotherms and 
more. Cyclogenesis in eight cases (one in June 
in the Arabian Sea and seven in July, August 
and September) and 200 mb level warm pools 
during 1964 have been studied and details of 
case study of the depression of 22nd August 
have also been given. He has concluded that 
200 mb level warm pool which forms generally 
three to four days earlier can be considered as 
one of the causes of cyclogenesis of tropical 
storms and depressions. 
2. Discussion 
India Meteorological Department (I. Met. D.) 
has recently (1968) published normal upper air 
temperature charts during July for 850, 700, 
500 and 300 mb levels. It is seen from the 
isotherms for 300 mb that highest temperatures 
occur between about 25 and 35" N and 60 and 
100' E, the area having temperatures higher 
than 249"A. In Fig. 1 are given isotherms and 
winds a t  200 mb level, the data having been 
obtained from the I. Met. D. It will be seen 
from the figure that the region of highest 
temperatures-more than 230"A-extends be- 
tween about 28 and 35" N and 50 and 95" E, i.e. 
about the same area as a t  300 mb level. In  
Fig. 2 of Mowla giving normal July 200 mb 
level isotherms the region of highest tempera- 
ture-more than 226"A--occurs over the north 
Bay of Bengal, i.e. the highest temperatures 
are much further south of the areaof highest 
temperatures in our Fig. 1. This difference is 
probably due to the fact that Ramakrishnan 
(1958) did not consider effect of the Tibetan 
plateau on the temperature distribution and 
more observations have since become available 
for the entire area. In view of the isotherms 
in our Fig. 1,  it is not possible to accept the 
conclusion of Mowla that there is remarkable 
similarity between normal isotherms of 200 
mb level over the north Bay in July and fre- 
quency of storms and depressions there during 
the months June to September. 
In the case study of 22 August, 1964 depres- 
sion he has considered 00 GMT temperatures 
a t  200 mb level. Calcutta is the nearest radio- 
sonde-rawin station to the north Bay of Bengal 
and its temperatures and winds a t  200 mb level 
for 00 and 12 GMT for 18 to 24 August, 1964 
obtained from the I. Met. D. are given in Table 1. 
Although according to the temperatures given 
by Mowla (figures in bracket in Table 1 taken 
from Figs. 3a, b and c of his paper) for alternate 
days there is a rise in temperatures from the 
18th to 20th to 22nd, from the temperatures 
for 00 GMT for each day given in Table 1, it is 
seen that temperature rose from the 18th to 
20th, fell from the 20th t o  21st, rose from the 
21st to 22nd, was about the same on the 23rd 
and fell from the 23rd to  24th. 
Tellus XXII (1970), 4 
CYCLOGENESIS IN THE BAY OF BENGAL AND ARABIAN SEA 467 
Fig. 1. Mean temperatures and winds of July for 200 mb. 
At 12 GMT there was little change from the 
18th to 19th (Table 1); from the 21st to 22nd 
there was a rise, from the 22nd to 23rd a fall 
and from the 23rd to 24th a rise. 
If one considers changes from 00 to 12 GMT 
of any day to 00 GMT of the next day and so 
on, it will be seen from the Table that there is 
no regular rise from day to day till the 22nd. 
Temperatures a t  12 GMT were generally 
Table 1. Calcutta Temperatures and Winds at 
200 nt6 kvel in Aug. 1964. 
00 GMT 12 GMT 
\ -  
Temp. Wind Temp. Wind 
D V  D V  
("C) (") (m.p.5.) ("C) ("1 (m.p.4 
- -
18 -53 ( -53)  065 6.5 -40 060 16.5 
19 -48 095 17.0 -41 065 16.0 
20 -44 ( -43)  070 18.0 - - - 
- 4 8  070 5.0 21 -48 _ _  
22 -39 (-39)  045 13.5 -44 360 9.5 
23 -39 085 4.0 -45 050 6.0 
24 -45 090 12.0 -41 145 3.0 
higher (equal on the 21st) than those a t  00 GMT 
on all the days except on the 22nd and 23rd. 
This perhaps represents effect of radiation a t  
that level except on the 22nd and 23rd when the 
radiation effect would appear suppressed and 
there was an actual fall in temperature from 
00 to 12 GMT. There is no regular fall in 
temperature from 12 GMT of any day to 00 
GMT of the next day; temperatures rose from 
12 GMT of the 21st and 22nd to 00 GMT of the 
22nd and 23rd respectively and was the same 
a t  12 GMT of the 23rd and 00 GMT of the 24th. 
From the data in Table 1 i t  can be stated 
that there was no regular and persistent 
development of a warm pool at 200 mb three 
to four days before the formation of the depres- 
sion on the 22nd August, 1964. 
In  Table 2 of his paper Mowla (1968) has 
given the values of isotherms (Row No. 1) which 
surrounded the warm pool a t  200 mb level 
associated with the formation of depressions and 
also indicated (Row No. 2) the intensity of the 
warm pools one day before the formation of the 
depression. For the depression of 22nd August 
233'8 is given in both the Rows 1 and 2. Cal- 
cutta temperatures a t  00 GMT on the 22nd was 
Tellus XXII (1970), 4 
468 Y. P. RAO AND B. N. DESAI 
- 39"C, but - 48°C on the 21st although it  was 
- 39°C on the 23rd as seen from Table 1 above. 
As the temperature on the 21st was -48OC 
(Table 1) it is not clear how Mowla has given 
temperature of -40°C (233"A) for that day. 
It is seen from Mowla's Table 2 that while 
the warm pool temperature was lowec on the 
day of formation of the depression than on the 
previous day in the cases of 9th June and 4th 
and 10th August, it was higher on 3rd July 
and 7th and 22nd September, being the same 
on the 15th and 22nd August. It is considered 
that it is not safe to draw conclusions from such 
variations in temperatures one day before and 
on the day of formation of the depression. 
From a study of the synoptic charts it is 
seen that between the 17th and 18th, a low 
pressure wave was moving westwards through 
upper Burma, and the Bay monsoon began to 
strengthen under its influence; Akyab had 11 
cm rain and Chittagong 6 between the 18th 
and 19th. Heavy rain again fell on the Chitta- 
gong-Arakan coast during the next 24 hrs. By 
the 21st morning, weather was unsettled over 
the northeast Bay off Arakan-Chittagong coast. 
The region of unsettled weather moved west- 
wards and a depression formed near 21.5"N, 
88.5OE at 03 GMT of the 22nd, the cyclonic 
circulation extending to 6.0 km a t  least. 
From the normal distribution of tempera- 
tures at 200 mb given in Fig. 1 one will be 
inclined to  think that air with southerly 
component over Calcutta will be colder than 
that with northerly component. On examining 
temperatures and winds over Calcutta given in 
Table 1 it will, however, appear that no such 
regular relation is observed except a t  00 GMT 
from the 19th to 20th. Similarly there is no 
regular relation between temperature and wind 
changes from 00 to 12 GMT of the day or from 
12 GMT of any day to 00 GMT of the next day. 
It would thus appear that interpretation of 
day to day changes in temperature with re- 
ference to wind a t  200 mb is not easy. 
It is not possible to state that the passage 
of a low pressure wave or of a ridge a t  200 mb 
level was responsible for the formation of the 
depression on the 22nd. One of the authors 
(Desai, 1968), has discussed the conditions 
which favour formation of depressions in the 
north Bay during the months June to Septem- 
ber. Movement of pressure systems a t  200 mb 
level would not appear to have cause-effect 
relation with developments a t  the surface or 
in the lower troposphere although in some 
cases i t  might appear so (Desai, 1969). The 
pressure pattern developing in the lower tropo- 
sphere represents integrated effect of various 
factors and not of any individual factor. 
The hitherto prevailing idea that tempera- 
tures fall in the rear of the troughs moving in 
the westerlies in the middle and upper tropo- 
sphere during the months June to September 
would require modification in view of the upper 
winds and temperature charts for 500 and 
300 mb given in the I. Met. D. charts (1968) 
and a t  200 mb given in Fig. 1. As temperatures 
are high over the Tibetan plateau and further 
westwards over the highlands of Afghanistan 
and Iran, when the rear of the westerly troughs 
affects areas south of about 30" N, temperatures 
will rise unless there is a rush of cold air from 
latitudes further north of 35" N and it is able 
to move southwards across warmer tempera- 
tures between 35 and 30" N without any signi- 
ficant rise in its original temperatures. This 
point requires detailed examination as the effect 
of the rear of the westerly troughs in the middle 
and upper troposphere is believed to weaken or 
lead to disappearance of the monsoon trough 
at the surface and break in the monsoon rains 
(Ramaswamy, 1962, 1969). In  this connection, 
a reference is invited to papers of Changraney 
(1966) and Desai (1967) according to which 
the monsoon trough may even get accentuated 
by the movement of the troughs in the wester- 
lies. 
3. Concluding Remarks 
It would appear from the foregoing discussion 
that it is not possible to accept Mowla's con- 
clusions about development of warm pools 
at 200 mb level three to four days before the 
depression forms a t  sea level below them. It is 
difficult to interpret temperature wind changes 
a t  200 mb level due to area of high temperatures 
over Tibet and westwards; Mowla's statement 
about movement of cold trough in the westerlies 
across north India extending upto 28' N on the 
18th and 19th August and of a cold trough in 
the easterlies which extended upto 26' N on the 
19th has, therefore, to be considered with utmost 
caution from the point of day to day forecasting. 
Tellus XXII (1970), 4 
CYCLOOENESIS IN THE BAY OF BENOAL AND ARABIAN SEA 469 
REFERENCES 
Ananthakrishnan, R. & Bhatia, K. L. 1958. Tracks Rao, Y. P. & Ramamurti, K. S. 1968. Part 1. 
of Monsoon Depressions and their recurvature Climatology of India and Neighbourhood, 2. 
towards Kaahmir. Proc. Symp. on Monsoons of Climate of India. Forecasting Manual, India Met. 
the World (New Delhi), 157-172. Dept., 1-17. 
Changraney, T. G. 1966. The role of Westerly Waves Mowla, K. G. 1968. Cyclogenesis in the Bay of Ben- 
in causing flood producing storms over northwest gal and Arabian Sea. Tellus 20, 151-162. 
India (excluding Rajasthan and Gujarat) during Ramakrishnan, K. P., Sreenivasaiah, B. N. & 
southwest monsoon period. Indian J. Met. Beo- Venkiteshwaran, S. P. 1958. Upper Air Climato- 
phya. 17 (Spl. No. April), 119-126. logy of India and Neighbourhood in the monsoon 
Desai, B. N. 1967. Trough of low pressure over the seasons. Proc. Symp. on Monaoons of the World 
Gangetic Valley during the southwest monsoon (New Delhi), 3-34. 
season and its implications-A suggested new Ramaswamy, C. 1962. Breaks in the Indian Summer 
approach. Indian J. Met Beophya. 18, 473-476. Monsoon as a phenomenon of interaction between 
Desai, B. N. 1968. Influence of the topographical the easterly and the sub-tropical westerly jet 
features of the Indian subcontinent on its weather streams. Tellus 14, 337-349. 
and climate. Beogr. Rev. India 30 (4), 33-44. Ramaswamy, C. 1969. The basic causes of the large 
Desai, B. N. 1969. Causes of large scale variations scale deficiency in the southwest monsoon rainfall 
in the southwest monsoon rainfall over the over India in 1965 and 1966. Curr. Sci. 38, 
Indian subcontinent. Curr. Sci. 38, 313. 53-56. 
Tellua:XXII (1970), 4 


Cyclogenesis in the Bay of Bengal and Arabian sea

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Cyclogenesis in the Bay of Bengal and Arabian sea

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