Dynamics of Breaks in the Indian Summer Monsoon

Abstract

In this paper the authors present results of diagnostic analysis of observations and complementary experiments with a simple numerical model that enable them to synthesize the morphology and dynamics of ''breaks'' in the Indian summer monsoon (ISM). Almost one week ahead of the onset of a break spell over India, a monotonically decreasing trend in convective activity is found to occur over the Bay of Bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial Indian Ocean. A major intensification of the convectively stable anomalies over the Bay of Bengal is seen about 2-3 days prior to commencement of a monsoon break. Both observations and modeling experiments reveal that rapid northwest propagating Rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies. It is shown that an abrupt movement of anomalous Rossby waves from the Bay of Bengal into northwest and central India marks the initiation of a break monsoon spell. Typically the Rossby waves are found to traverse from the central Bay of Bengal to northwest India in about 2-3 days' time. With the establishment of a break phase, the eastward spreading low-latitude anomaly decouples from the rapid northwest propagating anomaly. This decoupling effect paves the way for the emergence of a convectively unstable anomaly over the equatorial Indian Ocean. It is proposed that the dynamics of the rapid northwest propagating anomalous Rossby waves from the central Bay of Bengal toward northwest India and decoupling of the eastward propagating anomaly are two extremely vital elements that determine the transition from an above normal phase to a break phase of the ISM and also help maintain the mutual competition between convection over the Indian subcontinent and that over the equatorial Indian Ocean. Through modeling experiments it is demonstrated that low-latitude Rossby wave dynamics in the presence of a monsoon basic flow, which is driven by a steady north-south differential heating, is a primary physical mechanism that controls the so-called monsoon breaks.

Relationship between Indian and East Asian summer rainfall variations

“…The propagation of intraseasonal signals in the tropics may play a role in the relationship between Indian and western North Pacific-East Asia intraseasonal rainfall variations (e.g., Lau and Chan, 1986;Kripalani et al, 1991). The midlatitude Asian wave pattern on intraseasonal timescales and its relationship with the intraseasonal variability of the Indian monsoon has been noted in previous studies (e.g., Krishnan et al, 2000). But does this wave pattern play a role in linking the active and break phases of the Indian summer monsoon to intraseasonal fluctuations in northern China rainfall?…”

“…Correlation analysis carried out by also indicates the presence of such a wave pattern on intraseasonal time scales. The composite 200-hPa wind anomalies during the Indian monsoon break phase display a similar wave pattern (Krishnan et al, 2000).…”

Active and break spells of the Indian summer monsoon

“…Since the study of Ramamurthy (1969), active spells and weak spells/breaks of the Indian summer monsoon have been extensively studied, particularly in the last decade (e.g., Sikka and Gadgil 1980;Magana and Webster 1996;Rodwell 1997;Webster et al 1998;Krishnan et al 2000;Krishnamurthy and Shukla 2000, 2008; Annamalai and Slingo 2001;Goswami and Ajayamohan 2001;Lawrence and Webster 2001;De and Mukhopadhyay 2002;Gadgil and Joseph 2003;Goswami et al 2003;Waliser et al 2003;Kripalani et al 2004;Wang et al 2005;Mandke et al 2007 and the recent reviews by Goswami 2005 andWaliser 2006). However, different scientists have used the same term 'break', to denote different features of convection and/or circulation over different regions.…”

Intra-seasonal variability of cloud amount over the Indian subcontinent during the monsoon season as observed by TRMM precipitation radar

“…The active phase is characterized by above normal rainfall over central India and below normal over northern India (foothills of the Himalaya) and southern India and vice-versa during the break phase (Krishnamurthy and Shukla, 2000; Krishnan et al, 2000). An extensive study of Ramamurthy (1969) using Indian rainfall data reveals that during "break" phase, the Continental Tropical Convergence Zone (CTCZ) is shifted northward from its normal position to the foothills of the Himalayas, and the rainfall activity increases over south-eastern peninsular India during the same period.…”