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Meteorology and Atmospheric Physics

, Volume 35, Issue 3, pp 166–176 | Cite as

Interannual variability of the 30–50 day activity in the Indian summer monsoon

  • V. M. Mehta
  • J. E. Ahlquist
Article

Summary

In order to study the interannual variability of the 30–50 day activity in the southwest monsoon, time series at seven levels over India were formed using the NMC global analysis for four summers (1977–1980). After 30–50 day bandpass filtering, graphs of zonal wind, relative vorticity and divergence were plotted. Vertical structure of the activity is complex between 100 kPa and 10 kPa. Activity at 85 kPa moved northward, steadily in the summer of 1979 but less so in the other three summers. At 20 kPa, propagation was northward in the summer of 1979 but southward in the summers of 1977 and 1980. Meridional propagation was not clear at 20 kPa during the summer of 1978.

Monsoon onset and breaks are highly correlated with 30–50 day cyclonic and anticyclonic activity, respectively. In addition, about 90% of the depressions in the summers of 1977 and 1979, and about 70% of the depressions in the summers of 1978 and 1980 formed within regions of cyclonic vorticity on the 30–50 day time scale. Rotational circulations are much stronger than divergent circulations on this time scale in the NMC global analyses.

Keywords

Depression Vorticity Summer Monsoon Interannual Variability Bandpass Filter 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Jahr-zu-Jahr-Schwankung in der 30–50tägigen Aktivphase des indischen Sommermonsuns

Zusammenfassung

Zum Studium der Schwankung der 30–50tägigen Aktivphase des indischen Sommermonsuns wurden Zeitreihen von sieben Niveaus aus vier Sommern (1977–1980) der globalen NMC-Analyse herangezogen. Nach einer dem Zeitintervall von 30–50 Tagen entspiechenden Filterung wurden der zonale Wind, die relative Vorticity und die Divergenz dargestellt. Die Vertikalstruktur ist zwischen 100 kPa und 10 kPa komplex. Die aktive Zone in 85 kPa bewegte sich im Sommer 1979 stetig nach Norden, weniger in den anderen Jahren. in 20 kPa 1979 nordwärts, dagegen südwärts 1977 und 1980. 1978 war die Bewegung nicht eindeutig.

Beginn und Ende des Monsuns sind stark mit 30–50 tägigen zyklonalen bzw. antizyklonalen Aktivitäten verbunden. Außerdem bildeten sich etwa 90% aller Depressionen der Sommer 1977 und 1979, sowie 70% der Sommer 1978 und 1980 in Regionen mit zyklonaler Vorticity im Zeitscale von 30–50 Tagen. Rotationszirkulationen sind in diesem Zeitscale in der NMC-Analyse viel stärker als divergente Zirkulationen.

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Copyright information

© Springer-Verlag 1986

Authors and Affiliations

  • V. M. Mehta
    • 1
  • J. E. Ahlquist
    • 1
  1. 1.Department of MeteorologyFlorida State UniversityTallahasseeUSA

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