Case Studies of the Monsoon Onset Vortex and Early Season Monsoon Features Using the UFS Coupled Model

Abstract 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.