Recent two decades witness an uptick in monsoon depressions over the northern Arabian Sea

The Indian summer monsoon (ISM) critically underpins India’s economy and agriculture, with even small deviations impacting livelihoods. Recent decades show a dipole in Indian rainfall: increases over northwestern India (NWI) and decreases over the Indo-Gangetic Plains (IGP). Prior studies link this to rising pressure over the Tibetan Plateau and a phase shift in the Silk Road Pattern (SRP), altering midlatitude circulation and influencing ISM rainfall over NWI. Ocean–atmosphere processes also modulate ISM rainfall regionally. A key component of ISM dynamics is the ITCZ position and the role of synoptic-scale systems—monsoon depressions (MDs)—which contribute substantially to monsoon rainfall. While MD frequency over the Bay of Bengal (BoB) has declined in recent decades, short-lived lows and dry days increased, consistent with negative rainfall trends over central north India/IGP. Weakening barotropic instability and reduced mid-tropospheric humidity over the BoB have been implicated. Conversely, cyclonic activity over the Arabian Sea has increased with warming SSTs, including during monsoon onset. Studies also report a poleward shift in the low-level jet (LLJ) and enhanced convective characteristics along India’s west coast. Despite MDs’ importance for rainfall, their dynamics over the Arabian Sea, particularly the northern sector, remain underexplored. This study investigates causes for the rising MD frequency over the northern Arabian Sea during JJAS, contrasts with BoB, and examines mechanisms driving genesis and their implications for ISM rainfall under climate change.

Background literature indicates: (1) A robust rainfall dipole over India with increased NWI rainfall and reduced IGP rainfall in recent decades; (2) Links between ISM variability and midlatitude teleconnections such as the SRP and changes over the Tibetan Plateau; (3) Documented decline in BoB MD frequency, attributed to weakened barotropic instability and lower mid-tropospheric humidity; (4) Rising Arabian Sea cyclonic activity associated with SST increases, including during the monsoon onset phase; (5) A poleward shift of the monsoon LLJ and a tendency toward more convective rainfall along India’s west coast; (6) Classic understanding of MD structure and dynamics, including cold-core lower troposphere/warm-core aloft and the roles of barotropic/baroclinic instability and CISK. These strands frame the hypothesis that evolving dynamics and thermodynamics over the northern Arabian Sea—driven by regional warming and remote teleconnections—could favor MD genesis there while BoB MDs decline.

Data: HadISST SST (1870–2022); IMD gridded land rainfall over India (1951–2022); ERA5 reanalysis (0.5°×0.5°) meteorological fields (winds, vertical velocity, humidity, potential vorticity, temperature, etc.) for 1981–2022. Monsoon depression track and genesis information from IMD cyclone e-atlas. Analysis focuses on JJAS season and the northern Arabian Sea domain (60°E–78°E, 10°N–26°N), with comparisons to the Bay of Bengal. Trends and significance: Seasonal means were computed; trends assessed with Student’s t-test for significance; Mann–Kendall test used to estimate slopes and p-values of time series. Diagnostics: Genesis Potential Parameter (GPP) computed using 850 hPa relative vorticity, mid-tropospheric RH, mid-tropospheric instability (T850–T500), and vertical wind shear (200–850 hPa). Vertically Integrated Moisture Flux Transport (VIMFT) and Convergence (VIMFC) were calculated from 1000–300 hPa using winds (u, v) and specific humidity (q). Low-level jet (LLJ) shifts quantified via regional averages of 850 hPa winds and a wind-direction method. Vertical structure evaluated via latitude–height and longitude–height cross-sections of potential temperature, potential vorticity, and winds. Midlatitude circulation examined using 200 hPa meridional/zonal wind trends, East Asian jet (EAJ) index variability, and SRP-related EOF/PC analyses. Model assessment: AMIP ECHAM5 simulations (with observed boundary conditions and ERA5 SST) were examined for their ability to reproduce circulation and rainfall changes between 2001–2020 and 1981–2000.

- Monsoon depressions over the northern Arabian Sea nearly doubled: 6 MDs (1981–2000) to 11 MDs (2001–2022). MD tracks increasingly shift northwestward, some affecting NWI. - Rainfall trends show increases over NWI and decreases over parts of the IGP (JJAS 1981–2022), consistent with earlier reports. MDs contributed about 3–4% to JJAS mean rainfall over NWI in recent decades. - Low-level jet shift: A poleward (northward) shift of the monsoon LLJ of about 1.13° over 55°E–67°E, 6°N–16°N, with strengthened 850 hPa zonal and meridional winds (regional V850 trend p=0.007, slope=0.008 m s⁻1 per year). - SST warming: Northern Arabian Sea exhibits a strong warming trend of roughly 0.3 °C per decade, particularly in the northwest sector. - Dynamics and moisture: 850 hPa relative vorticity shows significant positive trends over the northern Arabian Sea; mid-tropospheric RH increases by about 3.5% per decade; total column cloud liquid water increases along NWI and the west coast, indicating heightened convection. - GPP increases significantly over the northern Arabian Sea (time series trend p=0.013, slope=0.0054 in the study’s units), indicating enhanced cyclogenesis potential. - Instability and ascent: Potential temperature difference (500–850 hPa) rises by ~0.5 K per decade (time series p=6.92e-07, slope=0.042 K per year). 500 hPa upward motion trend is positive (p=0.075, slope=0.021 m s⁻1 per year). These support increased dynamical instability and CISK-favorable conditions. - Moisture transport and convergence: VIMFC trends show increased moisture convergence over the northern Arabian Sea (time series p=0.017, slope=0.027 kg m⁻2 s⁻1 per year), consistent with enhanced VIMFT into the region. - Shear and divergence: Strengthening vertical wind shear (200–850 hPa) over the northern Arabian Sea and increased 200 hPa divergence (time series p=0.037, slope=0.02×10⁻5 s⁻1 per year) co-occur with upper-level easterly anomalies along the monsoon trough, favorable for MD growth. - Vertical structure: A cold core in the lower troposphere with a warm core aloft is evident; potential vorticity peaks near 700 hPa with a top-heavy column up to ~300 hPa; zonal and meridional wind trends depict cyclonic structures centered near 20–22°N with deep convection. - Teleconnections: A phase shift in the SRP and changes in the East Asian jet (post-2000 positive shift; enhanced SAH) are linked to anomalous anticyclonic conditions to the north and easterly anomalies along the monsoon trough, contributing to a poleward LLJ shift and altered moisture pathways. Regression analyses indicate combined SRP and northwestern Arabian Sea (NWAS) SST influences on monsoon circulation and NWI rainfall. - Modeling: AMIP ECHAM5 fails to reproduce key recent changes in circulation (including cross-equatorial flow) and rainfall patterns (NWI increase and monsoon core region), underscoring challenges in simulating observed dynamics.

The study addresses why MDs increased over the northern Arabian Sea despite their decline over the BoB. Warming SSTs, increased low-level vorticity, higher mid-tropospheric humidity, and enhanced cloud liquid water have fostered a more convectively active environment over the northern Arabian Sea. GPP, instability metrics (Theta500–850), and vertical motion trends indicate a greater propensity for cyclogenesis and MD growth, consistent with CISK operating in a convectively unstable tropical atmosphere. Concurrently, circulation changes—particularly a poleward LLJ shift and upper-level easterly anomalies/divergence—enhance moisture transport and convergence over the northern Arabian Sea and NWI, while reducing moisture supply to the BoB and monsoon core region. Midlatitude teleconnections (SRP phase shift and EAJ variability) appear to modulate these circulation changes, reinforcing easterlies along the monsoon trough and supporting the LLJ’s poleward displacement. The resulting dynamical (barotropic and baroclinic) and thermodynamical states favor MD genesis in the northern Arabian Sea and contribute to increased NWI rainfall, helping to explain the observed rainfall dipole.

This study shows that MD frequency over the northern Arabian Sea has nearly doubled in the past two decades, driven by combined thermodynamic and dynamic changes: SST warming, enhanced vorticity and moisture, increased instability and ascent, stronger upper-level divergence and vertical shear, and a poleward-shifted LLJ. Midlatitude circulation changes (SRP phase shift, EAJ variability, intensified and shifted SAH) likely facilitate these regional circulation anomalies. These factors collectively increase MD genesis potential and contribute to rising NWI rainfall. The inability of an AMIP ECHAM5 simulation to reproduce the observed changes highlights the need for improved modeling of monsoon dynamics and air–sea coupling to better predict MD activity and rainfall distribution under climate change. The findings have practical implications for forecasting, disaster risk management, and water resource planning in NWI and adjacent regions.

The analysis relies primarily on observational datasets and reanalysis (ERA5), with statistical trend assessments; causal attribution is largely diagnostic. Some circulation trends (e.g., regional mean vertical wind shear; W500) are marginal in statistical significance. The AGCM (AMIP ECHAM5) used for comparison fails to capture key recent changes in circulation and rainfall, limiting model-based validation. Fully coupled model experiments and targeted process studies are needed to better isolate mechanisms and improve predictive capability.