The study of the instability of the meridional flow: Part 1

1981

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SummaryThe object of the present paper is to study the stability of a monsoon depression which is a low level wave disturbance over the Indian region during summer. The stability of a wave disturbance depends intimately on the phase velocity. Again, the phase velocity of Rossby-gravity wave does not explain the phase velocity of a monsoon depression. Thus, knowing that the convective activity plays an important role in the development of a monsoon depression, one modifies the phase velocity of zonally propagating Rossbygravity waves, i. e. one obtains the modified dispersion relation considering the effect of convective heating in the equivalent barotropic model which is derived for the low levels in tropics. The modified dispersion relation explains the phase velocity of monsoon depression. Beyond the limiting stage, there is no further depression of the central pressure. So, it is essential to study the stability of a monsoon depression at the limiting stage. The stability analysis shows that the monsoon depression is neutral (i. e.; does not break down) as long as there is convective activity. But the monsoon depression becomes unstable (i. e.; breaks down) if the convective activity ceases. In the latter case, one obtains a possible stream of zonal wind. Zusammenfassung Die Stabilitiit von MonsundepressionenIn der vodiegenden Arbeit wird die Stabilitiit yon Monsundepressionen untersucht, die durch Wellenst6Iungen in der unteren Atmosph~ire Indiens im Sommer dargestellt werden k6nnen. Die Stabilit~t dieser Wellenst6mngen h~ingt in grot~em Ma~e yon deren Phasengeschwindigkeit ab. Diese jedoch ist nicht mit der Phasengeschwindigkeit yon RossbySchwerewellen identisch. Im Hinblick auf die Tatsache, daft die Konvektion eine bedeutende Rolle in der Entwicklung einer Monsundepression spielt, kann die Phasengeschwindigkeit yon zonal sich fortpflanzenden Rossby-Schwerewellen dahingehend modifiziert werden, daf~ man eine ge~inderte Dispersionsbeziehung ethiitt, welche den Einflut~ konvektiver Erw~irmung in das fiir die unteren Atmosphiirenschichten entwickelte ~iquivaient-barotrope Model1 einbezieht.

“…(t4)) is similar to that, obtained by Mahanti [9] for the upper troposphere in tropics during summer; but, the first equation is special for the lower troposphere. H is always positive.…”

Section: The Equivalent Barotropic Model For Low Levels In Tropicssupporting

confidence: 63%

Section: The Equivalent Barotropic Model For Low Levels In Tropicsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The stability of monsoon depression

1981

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“…For the upper atmosphere in tropics during summer, the equivalent barotropic model which is derived by Mahanti [ 11 ] may be written as…”

Section: A Brief Discussion On the Equivalent Barotropic Models For Tmentioning

confidence: 99%

“…However, previously, there were no such efficient models which would satisfy the above requirements. Recently, Mahanti [11,12] has derived a pair of equivalent barotropic models for tropics. One model is for the upper troposphere while the other model is for the lower troposphere.…”

Section: Introductionmentioning

confidence: 99%

Die Entstehung tropischer Störungen

1981

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SummaryThe mutually interacting pressure wave and diabatic heating wave originally cause the observed tropical disturbances. The equivalent barotropic models relate the tendency of vorticity with the convective heating effect directly, and so these models elucidate the roles of these mutually interacting waves on the development of observed tropical disturbances readily. The exact solution for the complex frequency adequately explains the growth and movement of tropical disturbances during summer. The results are divided into two parts 1) for the upper atmosphere 2) for the lower atmosphere. For the upper atmosphere during summer, the standing and amplifying natures of wave numbers 1 and 2 are satisfactorily interpreted. So the standing natures and the intensity variations of Atlantic trough, African high, Tibetan high, Pacific trough, and Mexican high etc. are also explained thereby. Also, the amplification and westward movement of short wave number 8 are interpreted, For the lower atmosphere during summer, the amplifications and westward movements of Carribean easterly wave, African monsoon cyclone, Indian monsoon depression, west Pacific ITCZ wave etc. are explained. Zusammenfassung Die Entstehung tropischer St6rungen

A study of the instability of the meridional flow: Part 2

1981

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In an earlier study [6] the author has shown that the time-dependent meridional flow can be expressed as superimposition of a number of easterly Rossby-gravity waves of infinite lateral extent. Using an equivalent barotropic model, it has been further shown that the short waves of the time-dependent meridional flow are unstable, releasing their kinetic energy to the tropical zonal flow. This is favourable for the generation of the tropical easterly jet. There are, however, some points which have not been fully discussed in the earlier study [6]. This paper deals with the following points: (1) the equivalent barotropic model has been fully derived; (2) it has been shown that short waves have greater contribution to the meridional wind than long waves; (3) the time-dependent meridional wind is quite strong during the disturbed condition of the upper atmosphere in the tropics during summer; (4) the amplitude of wave number 8 is calculated depending on observational facts; (5) the vanishing of modified beta is further discussed. ZusammenfassungUntersuchung der Instabilitfit der Meridionalstr6mung: Tell 2In einer bereits ver6ffentlichten Studie [6] zeigte der Verfasser, daft die zeitabh~ngige Meridionalstr6mung als eine Ubedagerung einer Anzahl yon Rossby-Schwerwellen mit unendlicher lateraler Ausdehnung ausgedriickt werden kann. Unter Verwendung eines ~quivalent-barotropen Modells konnte ferner gezeigt werden, daft die kurzen Wellen der zeitabh~ingigen Meridionalstr6mung instabil sind und ihre kinetische Energie an die tropische Zonalstr6mung abgeben. Es verbleiben jedoch efliche Punkte, die in der ursprtinglichen Studie [6] nicht yoU diskutiert wurden. Die vorliegende Arbeit betrachtet folgende Gesichtspunkte: 1. Das ~iquivalent-ba~otrope Modell wurde vollstiindig abgeleitet; 2. es wird gezeigt, dal~ kurze Wellen einen gr6t~eren Beitrag zur meridionalen Windkomponente liefern als lange Wellen; 3. die zeitabMngige Meridionalstr6mung ist w~ihrend St6rungsperioden in der h6heren tropischen Atmosph~ire im Sommer verh~ilt-nism~igig stark; 4. die Amplitude der Wellenzahl 8 wurde auf Grund von Beobachtungsdaten ermittelt; 5. die Tatsache, dag der modifizierte Beta-Weft (der meridionale Gradient der potentiellen Vorticity) die Gr6ge Null erreicht, wird welter diskutiert.