The Indian economy and agricultural production are heavily reliant on the Indian Summer Monsoon (ISM) rainfall. Even minor variations in monsoon intensity have significant consequences. In recent decades, a dipole pattern in rainfall distribution has emerged, with excess rainfall in northwestern India (NWI) and a deficit in the Indo-Gangetic Plains (IGP). This dipole pattern, exacerbated by global warming and internal variability, significantly impacts the lives and livelihoods of millions. While the rising pressure over the Tibetan Plateau and alterations in mid-latitude circulation patterns (Silk Road Pattern - SRP) have been linked to increased NWI rainfall, the causes remain an area of active research. Understanding the dynamics of the Intertropical Convergence Zone (ITCZ), and the formation of synoptic-scale disturbances like MDs, is crucial. Recent studies have noted a decline in MD frequency over the Bay of Bengal (BoB), contrasting with an increase in cyclonic activity over the Arabian Sea, attributed to rising sea surface temperatures (SSTs). This research investigates the mechanisms driving the increased frequency of MDs in the Arabian Sea, particularly in the northern region, offering a contrasting perspective to the BoB trend. This understanding is vital for improving ISM rainfall forecasting under climate change.

Previous research highlights the strong interconnection between the Indian economy, agricultural production, and ISM rainfall. Studies have shown increasing trends in extreme rain events over India, along with spatial rainfall variations exhibiting a dipole pattern: excess rainfall in NWI and deficit in IGP. The role of rising pressure over the Tibetan Plateau and changes in mid-latitude circulation (SRP) in influencing NWI rainfall has been established. Numerous studies have explored the links between regional and country-wide ISM rainfall variations and ocean-atmosphere processes. However, the causes of the increase in rainfall over NWI and the debate surrounding the ISM rainfall dipole pattern remain unresolved. The movement of the ITCZ and the formation of synoptic-scale disturbances, such as MDs, are key components in understanding ISM dynamics. While the role of MDs in generating monsoon rains is known, recent research points to a decline in MD frequency over the BoB, aligning with negative rainfall trends in northern central India/IGP. Conversely, increased cyclonic activity in the Arabian Sea, driven by rising SSTs, has been observed, altering traditional rainfall patterns.

The study uses monthly Sea Surface Temperature (SST) data from the Hadley Center (HadISST), gridded rainfall data from the India Meteorological Department (IMD), and high-resolution meteorological parameters (winds, humidity, potential vorticity, temperature) from the ECMWF ERA5 dataset (1981-2022). The analysis focuses on the June-September (JJAS) monsoon season and the Arabian Sea region (60°E-78°E, 10°N-26°N). Statistical significance of trends is assessed using Student's t-test and Mann-Kendall test. The Genesis Potential Parameter (GPP), calculated using 850 hPa relative vorticity, mid-tropospheric relative humidity, mid-tropospheric instability, and vertical wind shear, is used to evaluate MD formation potential. Vertically integrated moisture flux transport and convergence (VIMFT & VIMFC) are also computed. Mid-latitude circulation changes are examined, specifically the Silk Road Pattern (SRP), using relevant indices. The study compares findings with those for the Bay of Bengal region. An Atmospheric General Circulation Model (AGCM) simulation using the ECHAM5 model is also analyzed, comparing its output with observed data to validate findings.

The study found a significant increase in the number of MDs over the northern Arabian Sea during JJAS, nearly doubling between 1981-2000 and 2001-2022. This increase correlates with a significant upward trend in NWI rainfall. Analysis revealed a positive trend in SST across the northern Arabian Sea (~0.3 °C per decade), along with increased 850 hPa relative vorticity, indicating enhanced cyclogenesis potential. Mid-tropospheric relative humidity and total column cloud liquid water content also exhibited significant positive trends. The Genesis Potential Parameter (GPP) showed a pronounced increase over the northern Arabian Sea, signifying enhanced cyclogenesis potential. A positive trend in the potential temperature difference between 500 and 850 hPa levels indicated increasing dynamical instability. VIMFC and VIMFT trends demonstrated increased moisture convergence over the northern Arabian Sea. Analysis of vertical wind shear and upper-level divergence confirmed conditions favorable for MD formation. A poleward shift of the monsoon low-level jet (LLJ) by approximately 1.13° was observed, potentially redirecting moisture towards NWI. Analysis of mid-latitude teleconnections revealed a phase shift in the SRP pattern, characterized by an anomalous anticyclone over the Caspian Sea and Korean Peninsula, inducing prevailing easterly wind anomalies and a poleward shift in the LLJ. The ECHAM5 AGCM simulation, however, failed to accurately reproduce the observed changes in atmospheric dynamics and rainfall patterns, highlighting the complexity of modeling these processes.

The findings demonstrate a clear link between increased MD frequency in the northern Arabian Sea and heightened rainfall in NWI. The multifaceted nature of the driving mechanisms – increased SSTs, enhanced moisture convergence, strengthening vertical wind shear, and changes in mid-latitude circulation – underscore the complex interplay of factors influencing monsoon dynamics. The poleward shift of the LLJ, particularly significant, redirects moisture towards NWI. The study highlights the limitations of current AGCMs in capturing these complex interactions, suggesting further refinement is needed in model development. The significant increase in MDs in the Arabian Sea, contrasting with the declining trend in the BoB, points to a potential shift in monsoon rainfall patterns, with implications for regional water resource management and agricultural planning.

This study provides compelling evidence of a significant increase in monsoon depressions over the northern Arabian Sea in recent decades, linked to increased rainfall in northwestern India. The observed changes are driven by a complex interplay of factors including rising SSTs, enhanced moisture convergence, altered LLJ dynamics, and shifts in mid-latitude circulation patterns. The limitations of current climate models in capturing these intricate processes highlight the need for further research and model improvements to accurately predict future monsoon rainfall patterns and associated extreme events.

The study primarily focuses on the northern Arabian Sea region, limiting the broader generalizability of findings to other regions. The use of reanalysis data, while extensive, has inherent limitations due to data assimilation processes. The complex interplay of factors influencing monsoon dynamics makes it challenging to isolate the precise contribution of each element.