Bay of Bengal — Aero Light Nov Dec Jan 2025/2026
Short summary (one paragraph)
For Nov – Jan 2025–26 the Bay of Bengal (western sector) typically moves into its winter (northeast/winter) monsoon regime. Expect a general weakening of convective activity over the central Bay but enhanced coastal rainfall and spells of showers along the northeastern Indian coast, the Andaman coast and the west Thai/Malay peninsulas driven by northeasterly flow and synoptic disturbances. Cooling over the Himalayas and the Indian landmass strengthens the land-sea temperature gradient and helps maintain northeastwards offshore flow; winds from the Arabian region and episodic west-to-east maritime surges (including influences from East African coastal winds) can modulate moisture transport and the timing/intensity of coastal rain and sea-state changes.
Main weather drivers explained
1) Monsoon winds (Nov → Jan)
By November the large-scale circulation transitions from the inter-monsoon into the winter (northeast) monsoon. Winds over the Bay of Bengal tend to be northeasterly to east-northeasterly near the northern bay and more variable farther south. This winter flow is driven by cooling over the Asian landmass relative to the ocean (strongest as the Himalayan-Tibetan region cools), producing offshore-directed low-level flow that reduces deep convective activity over the open bay but favours coastal convergence zones and orographic enhancement of rainfall along the Indian east coast, northeastern India, the Andaman & Nicobar and the western Thai peninsula.
2) Cooling Himalayas & enhanced land-sea gradient
As the Himalayas and the Indo-Gangetic plains cool in boreal winter, the resulting stronger land-high pressure supports stronger offshore (northeasterly) low-level winds across the North Bay. The cooled land also helps anchor synoptic troughs and western disturbances which can push moisture-laden air over coastal regions, producing rainy spells even while the central Bay is comparatively less convective.
3) Arabian-region winds & cross-basin modulation
Winds originating over the Arabian Sea—particularly during episodes of enhanced northerly or northwesterly flow around the northern Arabian Sea—can alter the large-scale low-level wind patterns across the northern Indian Ocean. Stronger Arabian Sea northeasterlies or surges can deepen cross-equatorial flow or redirect moisture, occasionally feeding increased humidity into the eastern Arabian Sea and into trajectories that curve into the Bay of Bengal via the southern tip of India and the equatorial belt. This cross-basin exchange occasionally seeds convective bursts in the southern Bay and along the west Thai peninsula.
4) East African maritime winds & Indian Ocean connections
The East African coastal / western Indian Ocean winds (including the East African Coastal Current region and Somali Jet influences) can influence the Indian Ocean basin through modulation of sea-surface temperature, moisture flux and large-scale pressure anomalies. During some winter intervals, westerly or southwesterly anomalies emerging from the western Indian Ocean can travel eastwards along the equatorial belt and interact with Bay of Bengal systems — modifying low-level convergence, altering the timing of convection, and occasionally enhancing the development of coastal rainfall when these anomalies reach the Bay or interact with the northeast monsoon flow.
Month-by-month practical advice (Nov, Dec, Jan)
November 2025
- Typical pattern: Inter-monsoon to early northeast transition. Expect variable winds, frequent afternoon/evening coastal and island convection, and gradually strengthening northeasterly winds late in the month.
- Mariners: be prepared for variable winds and sudden squalls near convective clusters; monitor local warnings especially around the Andaman and Nicobar islands.
- Coastal communities: afternoon showers possible, with localized coastal flooding where drainage is poor.
December 2025
- Typical pattern: Northeast monsoon generally established. Offshore winds increase near the northern Bay producing cooler onshore surges, while synoptic disturbances and western disturbances can bring spells of coastal rain to eastern India and the Bay’s northern margins.
- Mariners: increased risk of choppy seas and northeasterly winds; small craft advisories may be needed during surges.
- Fisher operators: plan for reduced sea-window for small vessels during stormy spells and monsoon surges.
January 2026
- Typical pattern: Peak of winter monsoon influence. Continued offshore flow with episodic surges and enhanced coastal rainfall events when low-pressure systems or moisture plumes interact with the northeast flow.
- Infrastructure: monitor river and drainage levels during heavy coastal rain; expect lower SSTs in the northern Bay which can influence marine productivity.
How Arabian and East African winds may interact with Bay systems (clear language)
Think of the northern Indian Ocean as a connected basin: winds and pressure anomalies over the Arabian Sea and along East Africa do not stop at invisible borders. When winds over the Arabian Sea strengthen or change direction they can alter the path of moisture streams and low-level jets that cross southern India and the equatorial belt. If the Arabian side produces anomalous moist flow or a stronger lower-level jet, this can feed extra humidity into the southern Bay (after curving around Sri Lanka or the southern tip of India), raising the chance of convective clusters. Similarly, strong East African maritime anomalies (e.g., shifts in the Somali Jet or coastal wind bursts) can send eastward-propagating signals (wind and SST anomalies) that interact with the Bay’s normal northeast flow — sometimes amplifying coastal rainfall when timing and phasing align.
Practical safety guidance
- Expect increased rainfall along the east coast of India, Bangladesh coasts and Andaman/Nicobar during pulses — watch local forecasts for flood watches.
- Boating & fishing: avoid small-boat operations during sudden squalls, monsoon surges, or when advisories are in force.
- Coastal operations: secure equipment and schedule heavy operations for calmer windows (midday/afternoon convection can be strong, so early-morning departures often safer).
- Navigation hazards: reduced visibility, sudden wind shifts and thunderstorms can create hazardous conditions — always check the latest marine warnings before heading out.
Technical notes (sources & uncertainty)
This advisory synthesizes the typical physical mechanisms active during boreal winter across the northern Indian Ocean: the northeast (winter) monsoon, the role of Himalayan cooling strengthening land highs and offshore flow, cross-basin modulation from the Arabian Sea and western Indian Ocean, and influences from equatorial and coastal wind jets (e.g., Somali Jet/East African coastal winds). Seasonal to subseasonal variability (MJO, extratropical western disturbances, IOD/ENSO states) will modulate timing and intensity — these create uncertainty in the precise timing of surges and coastal heavy rainfall. For operational decisions consult national meteorological services (IMD, RSMC, Thai Meteorological Department, Bangladesh Met Office) and marine warnings for real-time watches and warnings.