The Indian summer monsoon rainfall (ISMR) accounts for nearly 70–90% of the annual precipitation of India, which is highly interlinked with the socio-economic life of Indian people in many ways (Webster et al. 1998; Zimmermann 1987, Shukla and Huang, 2016). The winds from Arabian Sea bring great amount of moisture and rainfall over the Indian region during June-September (JJAS) months. Due to vagaries of the monsoon, the variations in the intensity and amount of rainfall may cause droughts or sometimes instant excessive floods in different regions of India; consequently it may cause major socio-economic impacts (Mohapatra et al. 2021). The Asian summer monsoon circulation during JJAS is prominently controlled by the two components, the monsoon low-level jet (MLLJ) and the tropical easterly jet (TEJ), which govern the large-scale spatio-temporal distribution and variability of the ISMR. A strong cross equatorial low level jet stream governs the variability in ISMR over India, which prevails over the Indian Ocean of South Asia with its core close to 850 hPa. This cross-equatorial flow brings the evaporative flux from the Arabian Sea and the moisture generated by the trade winds over the south Indian Ocean and adjoining south Asia, known as MLLJ or Somali Jet or Findlater jet (Findlater, 1969, Naidu et al. 2011). It develops during the onset phase of the summer monsoon and its strength and position can provide a crucial lead in the intensity and variation of the ISMR. The MLLJ is instigated by differential heating between the latitudes 20N and 20S, which induces pressure gradient forces between the heat low areas over the Indian subcontinent and high pressure zone Mascarene High (Krishnamurti and Bhalme, 1976). Emerging in April above the Somali coast region, the MLLJ turns westerlies in the northern hemisphere, and then drifts over the Indian region during JJAS (Boos and Emanuel, 2009). These MLLJ winds are responsible for transport of moisture from Southern Hemisphere to Northern Hemisphere, nurturing the formation of monsoon inversion layers over the western Arabian Sea and therefore, modulating the amount of rainfall in the Indian sub-continent (Sathiyamoorthy et al., 2013; Dwivedi et al., 2016; Roxy et al. 2017). Joseph and Raman (1966) provided observational evidence for the existence of cross equatorial flow or MLLJ over the peninsular India.
The MLLJ flows eastward over peninsular India at latitude 15°N during active monsoon; whereas it flows southeastward from the central Arabian Sea and moves eastward near to Sri Lanka at latitude between equator and 10°N (Joseph and Sijikumar, 2004). The formation of vertical upward air motion and monsoon depressions over the North Bay of Bengal (BOB) is primarily aided by cyclonic vorticity (rotation of air) north of MLLJ axis over South Asia (Joseph and Simon, 2005). Generally during monsoon season (JJAS), the monsoon depressions originate over the North BOB and moves in northwesterly direction over the Ganges valley up to the central parts of the country before incapacitating. However, the existence of a strong MLLJ over peninsular region of India favors the generation of these monsoon depressions in the North BOB (Sikka, 1978). Halpern and Woiceshyn (1999) observed that the easterly flow of MLLJ strengthens the surface wind convergence and therefore, increases the amount of integrated cloud liquid water along eastern Arabian Sea, which leads to increase the rainfall over the west coast of India.
The diurnal, intra-seasonal and inter-annual variation of MLLJ directly influences the Indian summer monsoon rainfall (Wang et al. 2003; Joseph and Sijikumar 2004; Wilson et al. 2018). Wang et al. 2003 showed that the variations in the intensity of MLLJ will directly affect the amount of water vapor through cross equator flow transported to South Asian and East Asian monsoon regions, which modulates precipitation. This moisture contribution along the MLLJ is mainly due to the northwestwards propagation of the monsoon depressions in JJAS (Sandeep and Ajayamohan, 2015). MLLJ shows distinct diurnal variability with respect to both intensity and height of jet, which is maximum during 0000 UTC and 0600 UTC with maximum wind speed (> 24 m/s) over the western Arabian Sea, vertically ranging from 950 to 850hPa (Nair et al. 2015; Viswanadhapalli et al 2020).
Halpern and Woiceshyn, 2001 found that the intensity of MLLJ influences the magnitude of the rainfall along the Indian west coast on monthly scale. Viswanadhapalli et al. 2020 have studied MLLJ by using WRF model initialized by ERA-Interim data and showed that MLLJ attains its maximum strength and spatial extent in July and August, this might be due to the merging of orographically driven winds from the Red Sea in MLLJ. Joseph and Sijikumar, 2004 found that there is high correlation with a lag of 2–3 days between the strength of the convective heating over the BOB and the strengthening of the MLLJ through peninsular India. On both seasonal and sub-seasonal scales, the variability of the MLLJ is highly correlated with average ISMR (> 95% confidence) except over the regions of extreme north, northeastern India and Tamilnadu (Viswanadhapalli et al 2020).
On inter-annual timescale, it is reported that the MLLJ strength over Indian subcontinents depends on the convective heating of the atmosphere rather than the strength of the south Indian Ocean trade winds (Wilson et al. 2018). A significant increasing trend (0.08 mm day per decade) in the monsoon rainfall over Northern Hemisphere has been observed as a result of the increased meridional pressure gradients, which leads to increased moisture convergence and enhanced the cross-equatorial flow from Southern to Northern Hemisphere along with intense rainfall over the Asian monsoon regions (Wang et al., 2013). MLLJ seemed to shift pole ward due to the increased land-sea thermal contrast because of global warming which subsequently shift associated rainfall over the Indian region (Sandeep and Ajayamohan, 2015). Krishnamurthy and Ajayamohan (2010) showed that the low-level southwesterly winds and monsoon trough strengthen the moisture transport from Arabian Sea towards the Indian subcontinent, which provides evidence for the intensification of rainfall over central India. Wilson et al. 2018 reported that there is a statistically significant decreasing linear trend in monsoon mean zonal wind through Peninsular India, which is associated with a decrease in the frequency of monsoon depressions and an increase in the number of break monsoon days. Aneesh and Sijikumar (2016), using different reanalysis data sets (ERAI, NCEP2, and MERRA), showed an increasing trend in the MLLJ during July and September. Viswanadhapalli et al. 2020 used WRF model simulations to show that the strength of the MLLJ has a significant decreasing trend in August, which combined with the decrease in the number of depressions in BOB cut off the ISMR during August and this happens as a result of reduced moisture transport from the BOB and Arabian Sea, which further leads to an increase in the number of break conditions over India. A threefold increase in the number of extreme rainfall events over central India has reported, which is significantly associated with the variations in MLLJ over the Arabian Sea (Roxy et al. 2017)
Though earlier studies focused on influence of MLLJ variability on ISMR, this study focuses on the relationship of inter-annual and intra-seasonal variability of the MLLJ (Monsoon Low Level Jet) strength and height with ISMR. We examined variation in MLLJ strength and height in the Arabian Sea during monsoon months and studied its impact on monthly monsoon rainfall. The study is structured as follows. Section 2 describes the data used for the study. Section 3 discusses the inter-annual variability of MLLJ and its impact in ISMR with lag0 days, lag 14 days and lag 1 month. Section 4 illustrates the key findings of this study and concludes the remarks.