On the 40-50 day Oscillations During the 1979 Northern Hemisphere Summer

Abstract

Based on FGGE Level IIIb data, the structural features of 40-50 day oscillations over ann extensive region (30*S-30*N, 30*E-150*W) during the 1979 summer are detailed.The analysis confirms earlier investigations that these low frequency modes are primarily associated with the zonal wind oscillations.These 40-50 day perturbations propagate northward and eastward, which is most clearly defined over the monsoon region north of the equator from 60* to 150*E. The monsoon region is characterized by prominent spectral peaks in the 850mb meridional winds with periods shorter than 10 days, probably reflecting the activities of monsoon disturbances.However, the local Hadley circulation, as defined by averaging the meridional component of the wind between 60* and 150*E, exhibits a distinct spectral peak in the period range of 40-50 days.Similarly, the square of the meridional winds, which is a measure of synoptic-scale disturbance activity, also changes with a period of 40-50 days. These features, which are similar to the group velocity phenomena, are pronounced only over the central monsoon region (10*-20*N, 60*-150*E). The low frequency modes propagate northward and become most intensified near 10*-20*N through mutual interaction between synoptic-scale disturbances, the local Hadley circulation, and the tonal mean flows over the monsoon region.At the equator, the 40-50 day zonal wind pertubations propagate systematically eastward (500km/day) and upward (0.7km/day). In the equatorial region, the low frequency oscillations owe their existence to a lateral geopotential wave-energy flux from the monsoon region, which represents the major energy source for 40-50 day perturbations via the conversion from potential to kinetic enegy.Compared to the equator, the phase propagation of zonal wind perturbations along 15*N, although moving eastward, is not as systematic.At this latitude, zonal wind perturbations are pronounced in the lower troposphere over the monsoon region, and also in the upper troposphere over the western Pacific. As an integral part of E-W interaction between these two regimes, there occurs downward progression of westerly (or easterly) perturbations over to the Arabian Sea region.The downward phase of westerly (easterly) modes corresponds to the commencement of "active" ("break") monsoons over South and Southeast Asia.

Large- and Meso-α-scale Characteristics of Meiyu/Baiu Front Associated with Intense Rainfalls in 1-10 July 1991

“…These studies paid special attentions on the variation of the precipitation over the whole monsoon region, influence of the heating over the Tibetan Plateau on the monsoon circulation, relation between the Indian and the East Asia monsoon, moisture transport from the southern hemisphere and the 30-50 days oscillation in the monsoon (e.g., Murakami et al 1984;Tao and Chen 1987;Yanai et al 1992;Nakazawa 1992;Lau et al 1988). However, such planetary-scale aspects are not the main subjects in the present study.…”

An Observational Study of Tropical Large-Scale Fields: Part I: Statistical Analyses of the Wind Components, Geopotential Height and Temperature

“…Its relative intensity is weak as compared to the three other dominant oscillations mentioned above, and it seems to be stronger in the lower than in the upper troposphere above 500 mb level. This oscillation is better detectable in the Western than in the Central Pacific for the zonal wind, and this underlines the strong association of the ISO and the monsoon activity (Murakami and Nakazawa, 1984). Its contribution to the total fluctuations of the geopotential height is nearly the same in both parts of the Pacific Ocean.…”

“…Although they did not confirm explicitly the existence of this oscillation in the * and T data, they gave, instead, some evidence of mass flow in and out of the equatorial belt with the oscillation, showing that the meridional wind should be involved. This oscillation has been since the object of intensive studies over the Pacific and the Indian Ocean, using the FGGE-IIIb wind data (Murakami and Nakazawa, 1984), the cloudiness or the cloud wind vector (Yasunari, 1980(Yasunari, , 1981Takeda and Ikeyama,1984) or OLR (Weickmann et al, 1985).…”

Water vapour source identification for daily rain events at Ahmedabad in semi‐arid western India: wind trajectory analyses

“…Saha and Bavadekar (1973) using additional upper air data concluded that 70% of moisture flux from the Indian Ocean in the south accounted for the bulk of moisture needed for summer monsoon rainfall. While the importance of evaporation over the AS was suggested by Murakami et al (1984) and Ghosh et al (1978), the role of cross-equatorial flux was also emphasized by others (Cadet and Reverdin, 1981;Howland and Sikdar, 1983;Sadhuram and Ramesh Kumar, 1988). Most of these studies suggest that the cross-equatorial moisture flux provides an important source of moisture for the Indian Summer Monsoon (ISM) rainfall, although the evaporation from the AS is quite significant.…”