Abstract
The role of intraseasonal oscillations (ISOs) in modulating synoptic and interannual variations of surface winds over the Indian monsoon region is studied using daily averaged National Centers for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalyses for the period 1987–1996. Two dominant ISOs are found in all years, with a period between 30–60 days and 10–20 days respectively. Although the ISOs themselves explain only about 10–25% of the daily variance, the spatial structure of variance of the ISOs is found to be nearly identical to that of high frequency activity (synoptic disturbances), indicating a significant control by the ISOs in determining the synoptic variations. Zonal and meridional propagation characteristics of the two modes and their interannual variability are studied in detail.
The synoptic structure of the 30–60 day mode is similar in all years and is shown to be intimately related to the strong (‘active’) or weak (‘break’) phases of the Indian summer monsoon circulation. The peak (trough) phase of the mode in the north Bay of Bengal corresponds to the ‘active’ (‘break’) phase of monsoon strengthening (weakening) the entire large scale monsoon circulation. The ISOs modulate synoptic activity through the intensification or weakening of the large scale monsoon flow (monsoon trough). The peak wind anomalies associated with these ISOs could be as large as 30% of the seasonal mean winds in many regions. The vorticity pattern associated with the 30–60 day mode has a bi-modal meridional structure similar to the one associated with the seasonal mean winds but with a smaller meridional scale. The spatial structure of the 30–60 day mode is consistent with fluctuations of the tropical convergence zone (TCZ) between one continental and an equatorial Indian Ocean position. The 10–20 day mode has maximum amplitude in the north Bay of Bengal, where it is comparable to that of the 30–60 day mode. Elsewhere in the Indian Ocean, this mode is almost always weaker than the 30–60 day mode. In the Bay of Bengal region, the wind curl anomalies associated with the peak phases of the ISOs could be as large as 50% of the seasonal mean wind curl. Hence, ISOs in this region could drive significant ISOs in the ocean and might influence the seasonal mean currents in the Bay.
On the interannual time scale, the NCEP/NCAR reanalysed wind stress is compared with the Florida State University monthly mean stress. The seasonal mean stress as well as interannual standard deviation of monthly stress from the two analyses agree well, indicating absence of any serious systematic bias in the NCEP/NCAR reanalysed winds. It is also found that the composite structure of the 30–60 day mode is strikingly similar to the dominant mode of interannual variability of the seasonal mean winds indicating a strong link between the ISOs and the seasonal mean. The ISO influences the seasonal mean and its interannual variability either through increased/decreased residence time of the TCZ in the continental position or through occurrence of stronger/weaker active/break spells. Thus, the ISOs seem to modulate all variability in this region from synoptic to interannual scales.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Ahlquist J, Devaras A and Carlo T 1990 Intraseasonal monsoon fluctuations seen through 25 years of Indian radiosonde observations;Mausam 41 273–278
Dakshinamurti J and Keshavamurthy R N 1976 On oscillations of period around one month in the Indian summer monsoon;Indian J. Meteorol. Geophys. 27 201–203
Ferranti L, Slingo J M, Palmer T N and Hoskins B J 1997 Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP integration;Q. J. R. Meteorol. Soc. 123 1323–1357
Gadgil S 1988 Recent advances in Indian monsoon research with particular reference to Indian monsoon;Australia Meteorol. Mag. 36 193–205
Goswami B N 1994 Dynamical predictability of seasonal monsoon rainfall. Problems and prospects;Proc. Indian Natl. Sci. Acad. 60A 101–120
Hartman D and Michelson M I 1989 Intraseasonal periodicities in the Indian rainfall;J. Atmos. Sci. 46 2838–2862
Kalnay Eet al 1996 The NCEP/NCAR 40 years reanalysis project;Bull. Am. Meteorol. Soc. 77 437–471
Keshavamurthy R N 1973 Power spectra of large scale disturbances of Indian southwest monsoon;Indian J. Meteorol. Geophys. 24 117–124
Krishnamurti T N and Bhalme H N 1976 Oscillation of monsoon system. Part I: observational aspects;J. Atmos. Sci. 45 1937–1954
Krishnamurti T N and Ardanuy P 1980 The 10 to 20 day westward propagating mode and ‘Breaks in the Monsoon’;Tellus 32 15–26
Krishnamurti T N and Subramanian D 1982 The 30–50 day mode at 850 mb during MONEX;J. Atmos. Sci. 39 2088–2095
Legier D M, Navon I M and O’Brein J J 1989 Objective analysis of pseudostress over the Indian Ocean using a direct minimization approach;Mon. Weather Rev. 117 709–720
Mehta V M and Krishnamurti T N 1988 Interannual variability of 30–50 day wave motions;J. Meteorol. Soc. Japan. 66 535–548
Murakami M 1979 Large scale aspects of deep convective activity over the GATE area;Mon. Weather Rev. 107 994–1013
Murakami T, Nakazawa T and He J 1984 On the 40–50 day oscillation during 1979 northern hemispheric summer Part I: Phase propagation;J. Meteorol. Soc. Japan 62 440–468
Murakami T and Nakazawa T 1985 Tropical 45 day oscillation during the 1979 northern hemisphere summer;J. Atmos. Sci. 42 1107–1122
Parthasarathy B, Munot A A and Kothawale D R 1995 Monthly and seasonal rainfall series for all India, homogeneous regions and meteorological subdivisions: 1871–1994;Indian Institute of Tropical Meteorology Research Report No. RR065, ISSN 0252-1075, IITM, Homi Bhabha Road, Pune 411008, India
Schott F, Reppin J, Fischer J and Quadfasel D 1994 Currents and transports of the monsoon current south of Sri Lanka;J. Geophys. Res. 99 25127–25141
Sikka D R and Gadgil S 1980 On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon;Mon. Weather Rev. 108 1840–1853
Singh S V and Kripalani R H 1990 Low frequency intraseasonal oscillations in Indian rainfall and outgoing longwave radiation;Mausam 41 217–222
Singh S V, Kripalani R H and Sikka D R 1992 Interannual variability of the Madden-Julian oscillation in Indian summer monsoon rainfall;J. Climate 5 973–978
Trenberth K E and Olson J G 1988 An evaluation and intercomparison of global analysis from the National Meteorological Centre and Europian Centre for Medium Range Forecasts;Bull. Am. Meteorol. Soc. 69 1047–1058
Yasunari T 1979 Cloudiness fluctuations associated with the northern hemisphere summer monsoon;J. Meteorol. Soc. Japan 57 227–242
Yasunari T 1980 A quasi stationary appearance of 30–40 day period in the cloudiness fluctuations during the summer monsoon over India;J. Meteorol. Soc. Japan 58 225–229
Yasunari T 1981 Structure of an Indian summer monsoon system with around 40–day period;J. Meteorol. Soc. Japan 59 336–354
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Goswami, B.N., Sengupta, D. & Kumar, G.S. Intraseasonal oscillations and interannual variability of surface winds over the Indian monsoon region. Proc. Indian Acad. Sci. (Earth Planet Sci.) 107, 45–64 (1998). https://doi.org/10.1007/BF02842260
Issue Date:
DOI: https://doi.org/10.1007/BF02842260