Influence of Intraseasonal Variability on the Development of Monsoon Depressions

Abstract

Intraseasonal oscillations (ISOs) in the Indian summer monsoon represent dominant modes of variability of that circulation (Sikka & Gadgil, 1980; Yasunari, 1979). One of the most prominent manifestations of these ISOs is characterized by northward propagation of rain bands from the equatorial Indian Ocean to the foothills of the Himalayas with a periodicity of 20-60 days (Ajaya Mohan & Goswami, 2003; Karmakar et al., 2017a; Krishnamurthy & Shukla, 2007). This ISO is often associated with an eastward-propagating planetary-scale circulation traveling at a speed of almost 8° longitude/day (Krishnamurti et al., 1985) that is commonly referred to as the Madden-Julian oscillation (Karmakar & Krishnamurti, 2019; Madden & Julian, 1972; Pai et al., 2011; Singh et al., 1992). Active-break cycles of rainfall over India during the monsoon season are often associated with the passage of this ISO (Karmakar et al., 2017a; Rajeevan et al., 2010). Here, we examine the association of this ISO with synoptic-scale precipitating vortices, also known as low-pressure systems (LPS). In South Asia, weaker LPS are typically called monsoon lows and stronger ones are called monsoon depressions. These lows and depressions together account for around half of the

Classification of Middle Tropospheric Systems over the Arabian Sea and Western India

“…These differences in the nature of propagation result in a northwest to southeast tilt, which has been observed in prior studies (Karmakar and Misra, 2020). The northward movement is seen up to Day 0 over both basins; after which the Bay of Bengal branch shows a west-northwest extension from Day 0 to 3 (Figure 1e-f) that is maybe a reflection of relatively high-frequency variability associated with the westward motion of monsoon lows and Rossby waves (Goswami, 1987;Sobel and Horinouchi, 2000;Karmakar and Misra, 2020;Karmakar et al, 2021). Moreover, all the way through Days 0 to 9 (Figure 1e-i), while regions of positive correlation gradually weaken, they are seen to remain quasi-stationary, with one maximum over the northeast Arabian Sea and the other over the east coast of India and the Bay of Bengal, indicating the co-existence of convective activity over both these regions.…”

“…This is not surprising because, during these phases, convection and cyclonic vorticity are enhanced over the north Bay of Bengal, which supports the formation of cyclonic systems (Karmakar and Misra, 2020;Diaz and Boos, 2021). Since Type 1 systems over the Arabian Sea are a result of downstream development of Bay of Bengal lows, their frequency is expected to be directly modulated by cyclonic activity over East India and Bay of Bengal, thereby dependent on ISO phases which control low-pressure center formation over the Bay of Bengal (Deoras et al, 2021;Karmakar et al, 2021). Type 2a systems genesis also occurs during all phases of the ISO but shows peak activity during phase 5, followed by phase 4.…”

“…During Phases 6 through 8, convection weakens, representing the dry or break phase over the Indian subcontinent. Several authors have suggested that the BSISO phase and amplitude modulate various meteorological phenomena including the onset of monsoon, formation of cyclones, and monsoon lows (Wheeler and Hendon, 2004;Karmakar et al, 2021;Hunt et al, 2021;Deoras et al, 2021). Thus, it is worth exploring whether the BSISO also influences the formation of synoptic systems in the Arabian Sea and western India.…”

“…Hence, the active phase of BSISO, and to some extent, the QBWO, and their location are essential for the Arabian Sea and western India system formation and evolution. This sensitivity of system formation on ISO phases is expected to some extent because positive phases of ISOs enhance large-scale moisture, vorticity, and barotropic instability, which are conditions that are required for the formation of synoptic monsoon systems (Karmakar et al, 2021).…”

“…These systems show vorticity and wind maxima in the lower troposphere with closed isobars around the center, usually have a vertically upright structure and primarily move west or northwest from the Bay of Bengal to the Indian landmass (Godbole, 1977;Krishnamurthy and Ajayamohan, 2010;Boos et al, 2015). The genesis of these low-pressure systems (LPSs) has received enormous attention in the last few decades, and various mechanisms for their formation have been proposed (Krishnamurti et al, 1977;Shukla, 1978;Goswami et al, 1980;Adames and Ming, 2018;Meera et al, 2019;Diaz and Boos, 2019a,b); further, recent work has brought forth the role of Intraseasonal Oscillations (ISOs) in creating a favorable environment for the genesis of these systems (Karmakar et al, 2021;Deoras et al, 2021). In fact, during the summer monsoon, LPSs over the Bay of Bengal have been classified as being born in-situ or associated with downstream development of South China Sea disturbances (Krishnamurti et al, 1977;Chen and Weng, 1999;Meera et al, 2019).…”