Generally, monsoon conditions are unfavorable for cyclogenesis in the Arabian Sea due to the strong vertical wind shear present over the basin which prevents the vertical development of cyclones. However, there are instances where tropical cyclones form over the basin during the onset phase of the Indian summer monsoon (ISM). The current study examines the evolution of the synoptic conditions in the years during 1979 – 2019 in which a cyclone forms during the onset phase of the ISM. The effect of various atmospheric and oceanic factors, the modulation of wind circulation over the Indian subcontinent and the north Indian Ocean region due to the possible influence of tropical and extra-tropical weather systems has been taken into consideration. Our analysis indicates that a weak mean monsoonal wind circulation caused by the southward intrusion of subtropical westerlies (at 200 hPa) is conducive for cyclogenesis over the Arabian Sea during the monsoon onset phase. Such a weak onset phase of the monsoon, coupled with the seasonal warm SSTs may lead to cyclone formation in the Arabian Sea.

During certain years, a synoptic scale vortex called the monsoon onset vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden‐Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This asymmetric relationship between the MJO and MOV can be partially explained by the modulation of the large‐scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period.

The monsoon onset vortex (MOV) forms in the Arabian Sea around the beginning of the Indian summer monsoon. Many MOVs intensify into tropical cyclones and can potentially impact the onset and advance of the monsoon. The prevailing theory for the genesis of the MOV invokes the adiabatic hydrodynamic instability of the lower-tropospheric Somali jet. However, the role of precipitating moist convection in MOV formation remains unexplored. This study examines the structure of a canonical MOV, defined as a composite of 23 past cases, using reanalysis data. The MOV structure indicates lower-tropospheric maxima (from ∼700 to 850 hPa) and midtropospheric maxima (∼550 hPa) in the potential vorticity. A potential vorticity budget shows that this observed structure of the MOV likely results from adjustment to diabatic heating and vertical advection to a leading order. Both the low- and midlevel vortices in the developing MOV are primarily attributed to moist convection. Similarities and differences between the composite structures of the MOV and Bay of Bengal monsoon depressions are discussed. Additional work is needed to clarify the role of the traditional hydrodynamic instability in MOV formation. Significance Statement: The monsoon onset vortex (MOV) is an important weather system that forms in about 60% of the years in the Arabian Sea during the onset of the Indian summer monsoon. The MOV can affect the northward advance of the monsoon and be a coastal and maritime hazard. Compared to monsoon depressions, much less is known about the role of moist convection in MOV formation and growth. In this paper, we examine a composite MOV based on 23 past cases. The MOV structure shows two potential vorticity maxima—one around 700–850 hPa and the other around 550 hPa. Through a potential vorticity budget, we show that the genesis of both these vortices is predominantly attributed to moist convection. Diabatic heating and vertical advection are the main contributors to the MOV growth in its early stages.

During some years, a synoptic scale vortex called the Monsoon Onset Vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden-Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This non-linear relationship between the MJO and MOV can be partially explained by the modulation of the large-scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period.

Accurate daily gridded precipitation estimates are needed to characterize spatial rainfall variability and support retrospective hydrological analysis in Southeast Asia, where complex rainfall regimes and sparse rain gauge coverage remain major challenges. This study develops an Extended Triple Collocation Analysis (ETCA)-based merged daily precipitation dataset for Southeast Asia using 15 multi-source gridded precipitation products for 2000–2014. The products include gauge-based, satellite-based, reanalysis-based, and merged datasets, all regridded to a common 10 km × 10 km grid. ETCA was applied to all 455 possible three-product combinations to estimate product-level reliability, expressed as the squared correlation coefficient and error variance at each grid cell. The results reveal substantial spatial variability in product reliability. The final ETCA-merged product was generated through pixel-wise reliability-based product selection, quantile-based distributional adjustment using the locally highest-ranked product as an internal reference, and equal-weight averaging of the selected adjusted products. Validation against GSOD daily observations shows that the ETCA-merged product achieves lower RMSD of 2.95 mm day⁻¹, compared with 2.96 mm day⁻¹ for the simple all-product ensemble and 3.01 mm day⁻¹ for the ensemble of the five most regionally reliable products. The corresponding temporal correlations are 0.79, 0.81, and 0.78, respectively. The merged product also improves the representation of high-percentile rainfall, although very intense rainfall remains underestimated. Spatial climatology and annual cycle analyses indicate that the ETCA-merged product preserves the main rainfall patterns and seasonal evolution of Southeast Asia while introducing local adjustments based on product reliability. These findings demonstrate that ETCA provides a useful framework for developing uncertainty-informed precipitation datasets in regions with sparse gauge observations and spatially heterogeneous product performance.

The monsoon onset vortex (MOV) forms in the Arabian Sea during the onset phase of the Indian summer monsoon. The track and intensity of the MOV affect the monsoon rainfall during its onset phase, mainly over the west coast of India. This paper evaluates the reforecasts of MOV cases in the years 2011, 2014, and 2015 and their impacts on the monsoon rainfall in the Unified Forecast System (UFS) subseasonal to seasonal (S2S) prototypes (P5, P6, and P8). Out of the three MOV cases, the 2015 MOV is forecasted better across the UFS S2S prototypes. The possible factors influencing MOV forecasts in the UFS are analyzed through feature-based diagnostics. Our findings show that predicting the low-level Somali jet as well as the mesoscale-to-synoptic-scale evolution of deep convection is likely to result in more accurate MOV forecasts. The implications of MOV forecasts on the monsoon rainfall during the onset phase are discussed through these MOV cases. The UFS also captures the relationship between the Somali jet index and monsoon rainfall over India for the early–midpart of the monsoon (mid-June–mid-July). However, it underestimates the anomalous increase in rainfall corresponding to a stronger Somali jet. Overall, the UFS S2S prototypes P6 and P8 better forecast the MOV cases compared to P5.