M. R. RAMESH KUMAR, Y. SADHURAM, G. S. MICHAEL and L. V. GANGADHARA RAO. Physical Oceanography Division, National Institute of Oceanography, ...
S T R U C T U R E OF T H E M A R I N E B O U N D A R Y L A Y E R O V E R N O R T H W E S T E R N I N D I A N O C E A N D U R I N G 1983 S U M M E R MONSOON
(Research Note)
M. R. R A M E S H K U M A R , Y. S A D H U R A M , G. S. M I C H A E L and L. V. G A N G A D H A R A RAO Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa - 403 004, India
(Received 7 August, 1989) Abstract. The spatial variability of the structure of the lower troposphere over the northwestern Indian
Ocean for the period 12th July to 2nd September, 1983 has been studied using upper air data collected during the first scientific cruise of ORV 'Sagar Kanya'. An analysis of thermodynamic structure and kinematics of the marine boundary layer for different zonal and meridional sections revealed the following features: (a) Temperature and humidity inversions were generally absent over the study area except over a few locations in the western region', (b) Largescale subsidence was found over the central equatorial Indian Ocean; (c) The convective activity over the western Indian Ocean was found to be moderately suppressed as compared to the eastern region; (d) The zonal and meridional components of winds along the equator and 10° N zonal section exhibited a mirror-image-like distribution.
I. Introduction
It is well known that the 'southwest monsoon' plays a key role in India's agricultural economy as about 60-90% of the annual rainfall is received during this period (June to September). The monsoon circulation has also been found to be an important component of the global general circulation. Several investigators (Pisharoty, 1965; Sikka and Mathur, 1965; Desai, 1975; Findlater, 1969; Saha, 1970; Saha and Bavadekar, 1973; Ramamurthy et al., 1976; Pant 1976 & 1978; Ghosh et al., 1978; Rao and Van De Boogaard, 1981; Howland and Sikdar, 1983; Ray and Bedi, 1985; Ramesh Kumar et al., 1986; Sadhuram et al., 1987, Holt and Sethuraman, 1987; Cadet and Greco, 1987; Sadhuram and Ramesh Kumar, 1988) have looked into various aspects of the monsoon such as structure of the atmospheric boundary layer, cross-equatorial flux, the role of the Somali jet, the role of the southern hemispheric equatorial trough, heat and momentum fluxes across the air-sea interface and sea surface temperature anomalies over the Arabian Sea. Pisharoty's study revealed that the air over the equator was quite dry and studies of Bunker (1965), Sikka and Mathur (1965) and Desai (1975) have shown the existence of an inversion layer between the 900 and 750 mb levels over the western and central parts of the Arabian Sea. Pant (1976) studied the structure of the Boundary-Layer Meteorology 52: 177-191, 1990. © 1990 Kluwer Academic Publishers. Printed in the Netherlands.
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southwest monsoon over the equator using the data collected during ISMEX-73. This study revealed that the equatorial Indian Ocean region can be broadly divided into three regions namely (a) the western region (from east Africa up to 56 ° E); (b) the central region (from 56 ° E to about 65 ° E) and (c) the eastern region (east of 65 ° E). The western and eastern regions were characterised by strong convective activity and the central region by large-scale subsidence. Desai (1975) found a southerly wind component, its depth increasing to the west of 60 ° E. He also observed low level jet streams over the western Indian Ocean region between 850 and 700 mb and high humidity values up to 500 mb, indicating considerable crossequatorial flow. Most of the above observations were based upon the data collected during IIOE, ISMEX-73, MONSOON-77 and MONEX-79. Surprisingly, in spite of all these experiments, very little is known about the structure of the lower troposphere (up to 3 km) over the north Indian Ocean during the active monsoon conditions
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(i.e., July and August). In the present article, we attempt to present the structure of the lower troposphere (up to 700 mb) at different locations during the active period of the 1983 summer m o n s o o n . The summer m o n s o o n of 1983 was exceptionally good with almost no breaks in m o n s o o n conditions. The rainfall was well distributed and 95% of the country received an excess to normal rainfall. The season's rainfall was high in 15, normal in 17 and deficient in 3 meteorological subdivisions, respectively. The m o n s o o n advanced over A n d a m a n and Nicobar islands on 27 May and over Kerala Coast on 12 June (the onset was delayed by about 11 days, the normal date being 1 June). The m o n s o o n advanced towards the north and covered the entire country by 18 July. The m o n s o o n started withdrawing from west Rajasthan on 13 September, two w e e k s later than normal and the withdrawal phase was complete over the entire subcontinent by 19 October.
STRUCTURE OF THE MBL OVER NORTH WESTERN INDIAN OCEAN
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2. Data and Methodology During the first scientific cruise of ' O R V S A G A R K A N Y A ' , the ship occupied several stations along four zonal and four meridional sections in the northwestern Indian Ocean and collected vast amount of data on hydrography, surface and upper air meteorological parameters over the area '4 ° S-10 ° N; 50°-75 ° E' during the period 12th July to 2nd September, 1983 (see figure 1). The potential temperature (0), virtual potential temperature (0v) and equivalent potential temperature (0E) are computed following the well known empirical equations (Ramesh Kumar and Sadhuram, 1985), Variability of the cumulus convection is studied following the method proposed by Aspliden (1976). The vertical sections of temperature (T), specific humidity (q), 0, 0v and 0E are prepared for the zonal sections along 4 ° S, equator, 5° N and 10° N and the meridional sections along 52° E, 57 ° E, 63°E and 68 ° E.
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The quality of the upper air data was good in general. The spatial variability of the thermodynamic and kinematic structure of the lower troposphere (up to 700 mb) was evaluated assuming quasi-stationarity. We are justified in making such an assumption since 1983 was an exceptionally good monsoon year with almost no break in monsoon conditions. In other words, there were no abrupt changes in the synoptic situation which minimises the errors in the analysis by assuming quasi-stationary conditions for a longer period (Cadet, 1985). 3. Results and Discussion
(a) Zonal section along 4° S Figure 2 depicts the profiles of various upper air parameters along 4° S. There were no large-scale variations in the temperature and humidity profiles along this section, indicating the presence of a similar type of airmass in the lower layers. The 0E minimum in this section was observed at about 800 mb (approximately 2 km) showing that the convective activity was moderately suppressed. The zonal and meridional components of winds (figure not shown) were weak easterlies and northerlies on the eastern side and strong on the western side at 63° E. (b) Zonal section along equator Figure 3 shows the zonal variation of different parameters along the equator. The humidity values were quite high on the western side of this section i.e., about 49° E, showing the presence of strong cross-equatorial flow. These results agree with those of Desai (1975). The humidity values dropped sharply with height at the stations at 55° E and 57 ° E showing the presence of relatively dry air over this area. The 0, 0v and 0E curves have more or less similar profiles. The minimum 0E occurred at still higher levels over the other areas. The occurrence of a 0E minimum inhibits convective activity and hence large-scale subsidence takes place. Pant (1976) found such an area in the central equatorial Indian Ocean. The zonal and meridional components of winds along this section (figure not shown) exhibited a mirror-image-type distribution. This type of wind distribution was earlier reported by Sadhuram and Sastry (1987). The winds in general were strong southerlies and weak easterlies.
(c) Zonal section along 5° N The humidity decreased sharply with height over the central Indian Ocean region (52 ° E to 62° E) and the maximum drop occurred around 58° E (Figure 4). The 0E minimum also occurred at lower altitudes over this region, indicating moderate to extreme suppressed conditions. The 0E minimum occurred at pressure levels (850 to 800 mb) on the western side as compared to the eastern side (750 to 700 mb). The intensity of the westerlies (figure not shown) increased with height and was a maximum around 800 mb. The strength of the southerlies decreased with height; these winds turned to northerlies around 750 mb over the western side.
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(d) Zonal section along 10° N The temperature and humidity values dropped gradually over the eastern side as compared to the western side (Figure 5). The humidity values dropped sharply at 57°E and 59.5°E showing the presence of relatively dry air above 900 mb. The 0E minimum occurred at higher altitudes on the western side and at lower altitudes on the eastern side, indicating convective suppression over the western region. The westerlies (figure not shown) strengthened with height and reached the speed of a low level jet stream at 61°E and 64°E about 850 mb and thereafter their intensity gradually decreased. Strong northerlies were present over the eastern side and their intensity increased with height, reaching a minimum at 850 mb. On the western side, weak to moderate westerlies occurred.
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(e) Meridional section along 52 ° E This section can be thought as representing the mean picture of the equatorial Indian Ocean. The temperature and humidity values dropped gradually over the southern areas and the equator as compared to the northern region (Figure 6). A temperature inversion was observed at 5°N between 850 and 800 mb. The 0e minimum occurred at lower altitudes on the northern side as compared to the southern side. The meridional winds (figure not shown) were strong at 3 ° N, revealing the presence of strong cross-equatorial flow. In general, the zonal winds were weak. (f) Meridional section along 57 ° E This section represents central Indian Ocean conditions. The humidity values dropped rapidly from I ° N to about 5°N showing the presence of relatively dry air over this area (Figure 7). The 0E minimum occurred at lower altitudes showing
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large-scale subsidence. Thus our present results confirm the earlier study of Pant (1976) regarding the large-scale subsidence over the central Indian Ocean region. The zonal and meridional components of winds both increase with height in general (figure not shown). The level of maximum wind also increased with latitude. The maximum meridional winds were observed at about 850 mb at 10° N, the maximum zonal winds were also observed at the same place at 800 mb.
(g) Meridional section along 63° E Higher humidity values were observed over the southern side in comparison with the equatorial and northern sides (Figure 8). The 0e minimum occurred at lower levels at 1°S indicating that the convective activity was moderately suppressed in this region as compared with other areas in this section. Strong easterlies (figure not shown) were observed on the southern side; they weakened towards the equator and weak westerlies occurred in the northern regions. Strong meridional
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winds were observed at 4 ° S, which gives a good indication of strong cross-equatorial flow over this area. The meridional winds weakened towards the north.
(h) Meridional section along 68 ° E This section can be considered representing central Indian Ocean conditions. In general, higher humidity values were observed over the southern parts as compared to the northern sides (figure not presented). The 0E values also depicted a similar trend. The 0E minimum occurred at lower levels on the northern side as compared to the southern side, showing that convective activity is moderately suppressed on the northern side. The zonal and meridional wind components exhibited a mirror-image-like distribution along this section (figure not shown). Strong westerlies (about 30ms -1) were observed at 10°N at about 950 rob; the strength of the westerlies decreased as one moved south and they became easterlies on the southern side. Strong northerlies were present in the lower altitudes at
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10° N and they weakened with height. These northerlies became weak southerlies towards the south.
4. Conclusions (1) In general, temperature and humidity inversions were absent over the study area except over a few locations in the western Indian Ocean. (2) Large-scale subsidence was found over the central region of the study area. (3) The occurrence of a 0e minimum at lower levels over the western region as compared to the eastern region of the study area indicates that convective activity was moderately suppressed over the western region in comparison with the eastern section. (4) The zonal and meridional wind components exhibited a mirror-image-like distribution over the equator and 10°N zonal sections and also along the 68°E meridional section.
Acknowledgement The authors are grateful to Dr. B. N. Desai, Director, National Institute of Oceanography and to Dr. J. S. Sastry, Deputy Director and Head, Physical Oceanography Division, N.I.O., for their keen interest and encouragement. Thanks are due to the I.M.D. team, responsible for collecting the upper air data during the cruise.
References Aspliden, C. I.: i976, 'A Classification of the Structure of the Tropical Atmosphere and Related Energy Fluxes', J. Appl. Meteorol. 15,287--308. Bunker, A. F.: 1965, 'Interaction of the Summer Monsoon Air with the Arabian Sea', Proc. Symp. on Met. Results, IIOE, Bombay, pp. 3-16. Cadet, D. L.: 1985, Personal communication. Cadet, D. L. and Greco, S.: 1987, 'Water Vapour Budget Over the Indian Ocean During the 1979 Summer Monsoon, Part I: Water Vapour Fluxes', Mon. Wea. Rev. 115, 3,653q563. Desai, B. N.: 1975, 'Air Mass Stratification Over the Arabian Sea During the Summer Monsoon and Their Modifications', Ind. J. Met. Hydrol. and Geophys. 26, 271-273. Findlater, J.: 1969, 'Interhemispheric Transport of Air in the Lower Troposphere Over the Western Indian Ocean', Quart. J, Roy. Meteorol. Soc. 95, 400-403. Ghosh, S. K., Pant, M. C. and Dewan B. N.: 1978, 'Influence of the Arabian Sea on the Indian Summer Monsoon', Tellus. 30, 117-125. Holt, T. and Sethuraman, S.: 1987, 'A Study of Mean Boundary Layer Structures Over the Arabian Sea and Bay of Bengal During Active and Break Monsoon Periods', Boundary-Layer MeteoroL 38, 73-94. Howland, M. R., and Sikdar B. N.: 1983, 'The Moisture Budget Over the Northeastern Arabian Sea During Premonsoon and Monsoon Onset, 1979', Mon. Wea. Rev. 111, 2255-2268. Pant, M. C.: 1976, 'Structure of the Southwest Monsoon Near the Equator During Monex-197Y, Ind. J. Met. Hydrol and Geophys. 27, 1-8. Pant, M. C.: 1978, 'Vertical Structure of the Planetary Boundary Layer in the Western Indian Ocean
S T R U C T U R E OF T H E MBL O V E R N O R T H WESTERN INDIAN O C E A N
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During the Indian Summer Monsoon as Revealed by ISMEX Data', Ind. J. Met. Hydrol and Geophys. 29, 88-98. Pisharoty, P. R.: 1965, 'Evaporation From the Arabian Sea and the Indian Southwest Monsoon', Proc. Syrup. on Met. Results, IIOE, Bombay, pp. 43-45. Ramamurthy, K. R., Jambunathan, R, and Sikka, D. R.: 1976, 'Moisture Distribution and Water Vapour Flux Over the Arabian Sea During an Active and Weak Spell of Southwest Monsoon, 1973', Ind. J. Met. Hydrol and Geophys. 27, 127-140. Ramesh Kumar, M. R. and Sadhuram, Y.: 1985, 'Vertical Structure of the Boundary Layer - A Comparison Between Land and Sea', Mahasagar. 18, 501-506. Ramesh Kumar, M. R., Sathyendranath, S., Viswambharan, N. K. and Gangadhara Rao, L. V.: 1986, 'Sea Surface Temperature Variability Over the North Indian Ocean - A Study of Two Contrasting Monsoon Seasons', Proc. Indian Acad. Sci. (Earth & Planetary Sciences). 95,435-446. Rao, G. V. and De Boogard, H. M. V.: 1981, 'Structure of the Somali Jet Deduced from Aerial Observations Taken During June-July, 1977', in J. Lighthill and R. P. Pearce (eds.), Monsoon Dynamics, Cambridge University Press, pp. 321-331. Ray, T. K. and Bedi, H. S.: 1985, 'The Thermodynamic and Kinematic Structure of the Troposphere Over the Arabian Sea and Bay of Bengal During 1979 Monsoon Season', Mausam. 36, 417-422. Sadhuram, Y. and Sastry, J. S.: 1987, 'Structure of the Marine Atmospheric Boundary Layer Over the Central Indian Ocean During Southwest Monsoon', in T. S. S. Rao, R. Natarajan, B. N. Desai, G. Narayanaswamy and S. R. Bhat (eds.), Contributions in Marine Sciences, Published by Dr. S. Z. Qasim Sastyabdapurti felicitation committee, N.I.O., Dona Paula, Goa, pp. 175-188. Sadhuram, Y., Gopalakrishna, V. V., Ramesh Babu, V. and Sastry, J. S.: 1987, 'The Heat and Moisture Budgets of the Atmosphere Over the Central Equatorial Indian Ocean During Summer Monsoon', Mausam. 38, 227-232. Sadhuram, Y. and Ramesh Kumar, M. R.: 1988, 'Does Evaporation Over the Arabian Sea Play a Vital Role in the Moisture Transport Across the West Coast of India?', Mon. Wea. Rev. 115, 307-312. Saha, K. R.: 1970, 'Air and Water Vapour Transport Across the Equator in the Western Indian Ocean During Northern Summer', Tellus. 22, 681--687. Saha, K. R. and Bavadekar, S. N.: 1973, 'Water Vapour Budget and Precipitation Over the Arabian Sea During the Northern Summer', Quart. J. Roy. Meteorol. Soc. 99, 273-278. Sikka, D. R. and Mathur, M. B.: 1965, 'Transport of Water Vapour Over the Arabian Sea and Adjoining Indian Ocean Region During an Active Monsoon Situation', Proc. Symp. Met. Results, IIOE, Bombay, pp. 55-67.
