Four Trends Redefining the North Indian Ocean’s Storm Cycle

Context:
Recent climatological analysis highlights four major long-term trends reshaping the cyclonic storm cycle in the North Indian Ocean (NIO)—covering the Bay of Bengal and the Arabian Sea. These trends have significant implications for coastal vulnerability, disaster preparedness, and climate adaptation strategies.

Key Highlights:

Trend 1: Declining Frequency of Cyclonic Disturbances (Inverted U-shape):

• The frequency of cyclonic disturbances in the NIO shows an inverted U-shaped trend (1900–2025).

• 1900–1920: 10-year rolling average remained below 10 disturbances.

• 1930s: Sharp rise to over 15 disturbances.

• 1980s–1990s: Significant decline.

• 2000s onwards: Reached historic lows, indicating fewer overall storms.

Trend 2: Bay of Bengal Decline vs Arabian Sea Surge:

• Overall decline in NIO disturbances is driven mainly by a sharp reduction in the Bay of Bengal.

• Despite the decline, the Bay of Bengal remains the primary source of cyclonic disturbances.

• In contrast, the Arabian Sea has witnessed a rise in the number of disturbances.

• Cyclones forming in the Arabian Sea are historically more likely to intensify into severe storms.

Trend 3: Intensification of Cyclonic Severity:

• A higher proportion of cyclonic disturbances are now reaching severe cyclonic storm status.

• Severe Cyclonic Storms have wind speeds of 89–117 kmph.

• Warmer sea surface temperatures, especially in the Arabian Sea, provide greater latent heat energy.

• Result: Stronger, faster-intensifying, and less predictable cyclones.

Trend 4: Shift in Seasonality of Storm Formation:

• Prior to the 1980s, Bay of Bengal cyclones were concentrated between July and September.

• Post-1980s, there is a marked shift towards October–December.

• This increases risks during the post-monsoon period, when preparedness levels are traditionally lower.

Drivers & Climatic Explanation:

• Rising ocean temperatures linked to climate change.

• Altered monsoon circulation patterns.

• Changes in vertical wind shear and ocean-atmosphere coupling.

Relevant Prelims Points:

• Issue: Changing cyclone frequency, intensity, and seasonality in the North Indian Ocean.

• Causes:

  • Ocean warming

  • Climate variability

  • Atmospheric circulation changes

• Key Regions:

  • Bay of Bengal – declining frequency

  • Arabian Sea – increasing intensity

• Impact:

  • Higher disaster risk despite fewer storms

  • Increased coastal vulnerability

• Terminology:

  • Cyclonic Disturbance: Low-pressure system that may intensify into a cyclone

  • Severe Cyclonic Storm: Wind speeds 89–117 kmph

Relevant Mains Points:

• Static Geography Linkages:

  • North Indian Ocean cyclone formation dynamics

  • Role of sea surface temperature (SST) and Coriolis force

• Keywords & Conceptual Clarity:

  • Climate Change and Extreme Events, Cyclone Intensification, Seasonal Shift, Disaster Risk Reduction

• Environmental & Policy Implications:

  • Fewer storms do not imply reduced risk

  • Intensification increases damage potential

• Way Forward:

  • Strengthen early warning systems

  • Update cyclone calendars and preparedness protocols

  • Climate-resilient coastal infrastructure

  • Integrate long-term climate trends into disaster planning

UPSC Relevance (GS-wise):

• GS 1: Physical geography, climatology, cyclone dynamics

• GS 3: Climate change, disaster management, environmental risks

• Prelims: Cyclone classification, ocean-atmosphere interactions