Onset of the Bay of Bengal summer monsoon and the seasonal timing of ENSO's decay phase

Abstract

Based on multiple sources of atmospheric and oceanic data, this study demonstrates a close relationship between the onset of the Bay of Bengal (BOB) summer monsoon (BOBSM) and the seasonal timing of El Niño-Southern Oscillation (ENSO)'s decay phase. Through distinguishing 'later-decay' and 'normal-decay' ENSO events, it is found that a later/earlier onset of the BOBSM following El Niño/La Niña is mainly caused by later-decay ENSO events, while no significant changes in BOBSM onset can be identified between normal-decay El Niño and normal-decay La Niña events. Diagnosis of the related dynamic and thermodynamic processes further confirms that, for later-decay ENSO events that remain active until mid-April, persistent ENSO-induced 'atmospheric-bridge' processes can yield a stronger vertical coupling of circulation between the upper and lower troposphere, and thus an anomalously earlier (later) BOBSM onset following a later-decay La Niña (El Niño). In the lower troposphere, the persisting zonal sea surface temperature (SST) gradient between the Indian Ocean and the western Pacific following later-decay ENSO events can significantly modulate the lower tropospheric barotropic instability over the northern BOB, which in turn overwhelms the local SST conditions to dominate the development of monsoon convections. In the upper troposphere, later-decay ENSO alters the position of the South Asian high and the upper atmospheric divergence-pumping through the anomalous Walker circulation. In contrast, due to the earlier damping of normal-decay ENSO, the BOBSM onset processes are barely modulated.KEY WORDS Bay of Bengal; summer monsoon onset; seasonal timing of ENSO's decay phase

Interannual variability of monsoon onset and withdrawal in Bangladesh

“…Notwithstanding, the negative IVT anomalies and southward wind might prevent the monsoonal circulation. Sun et al (2017) state that persistent El Niño events (warm phase) can induce later monsoon onset over the Bay of Bengal, adjacent to Bangladesh, and conversely for La Niña. A similar pattern of anomalies is observed for the positive and negative phases of ENSO during the withdrawal of the monsoon (Figures 4c,d).…”

“…The latter include circulation anomalies induced by intraseasonal oscillations (Karmakar and Misra, 2019), the passage of deep convection over the adjacent ocean (e.g., the Bay of Bengal) propagating into the continent, and the influence of the Tibetan plateau on circulation (Fasullo & Webster 2003;Yanai et al 1992). Furthermore, large-scale earlier or advanced monsoon onset have been associated with sea surface temperatures (SSTs) anomalies over the Indian and Pacific oceans (Lau & Yang 1997;Sun et al 2017). Likewise, El Niño (La Niña) years have been reported as associated with later (earlier) than normal onset of the monsoon season in India (Adamson & Nash 2014;Joseph et al 1994;Xavier et al 2007) and farther East (Wang et al 2013).…”

Association between regional summer monsoon onset in South Asia and Tibetan Plateau thermal forcing

“…The onset of the SASM shows significant interannual variability, and it is modulated by many factors. For instance, following warm El Niño-Southern Oscillation (ENSO) events (El Niño), the breakout of the summer monsoon over the South Asian regions is usually postponed, while following cold ENSO events (La Niña), the SASM onset tends to advance (Liu et al 2014b;Neena et al 2020;Sun et al 2017;Xavier et al 2007). ENSO can efficiently modulate the onset dates of the SASM through regulating the sea surface temperature (SST) anomalies in tropical oceans (Liu et al 2014b), the Walker circulation (Mao and Wu 2006;Meng and Wu 2000;Webster and Yang 1992), and the ENSOinduced 'atmospheric-bridge' (Sun et al 2017).…”

“…For instance, following warm El Niño-Southern Oscillation (ENSO) events (El Niño), the breakout of the summer monsoon over the South Asian regions is usually postponed, while following cold ENSO events (La Niña), the SASM onset tends to advance (Liu et al 2014b;Neena et al 2020;Sun et al 2017;Xavier et al 2007). ENSO can efficiently modulate the onset dates of the SASM through regulating the sea surface temperature (SST) anomalies in tropical oceans (Liu et al 2014b), the Walker circulation (Mao and Wu 2006;Meng and Wu 2000;Webster and Yang 1992), and the ENSOinduced 'atmospheric-bridge' (Sun et al 2017). Neena et al (2020) demonstrated that the El Niño in boreal spring can support two distinct eastward waveguides from the central and eastern Pacific to the SASM region, thus inducing Rossby waves to modulate the thermal structure over the SASM region in May and lead to a delayed monsoon onset.…”

Interannual Variability and Seasonality of Precipitation in the Indus River Basin

“…Previous precipitation trend analyses made conclusions dependent on specific intervals (Archer and Fowler 2004;Palazzi et al 2013;Singh et al 2014) that have limited applicability to regional and global change. Moreover, synoptic-scale teleconnections with phenomena such as El Niño-Southern Oscillation (ENSO) have traditionally focused on the larger East and South Asia region (Annamalai et al 2007;Goswami and Xavier 2005;Li et al 2018;Stolbova et al 2016;Sun et al 2017;Xavier et al 2007), obscuring the vital details of its effects on SAM arrival, duration, and withdrawal in the Indus basin. The anomalous behavior of hydrometeorological characteristics due to ENSO events exhibit regional differences (Byshev et al 2012), further emphasizing the need to consider the behavior of monsoon in specific relation to the Indus basin.…”