On uncommon cyclones in the Arabian Sea

On uncommon cyclones in the Arabian Sea

Sub: Geo

Sec: Climatology

Cyclone Asna:

• Asna is a rare cyclone that formed in August 2023, making it the first north Indian Ocean cyclone in August since 1981. What made Asna unique was its origin as a land-born depression that transitioned into a cyclone after moving over the warm Arabian Sea.
• Formation of Cyclone Asna:
  • Typically, low-pressure systems form over the Bay of Bengal, delivering around 60% of the monsoon’s rainfall as they cross India. However, Asna started as a powerful land depression over western India, fueled by excess soil moisture from earlier heavy rains. As it neared the Arabian Sea, warm ocean waters gave it additional energy, transforming it into a full-fledged cyclone.
  • The development of Asna was rare, with the warming Arabian Sea, influenced by shifts in wind patterns and rapid warming in West Asia, playing a crucial role. Eventually, dry desert air caused the cyclone to weaken and dissipate over the ocean.
• Asna was part of a series of unusual weather patterns during 2023-2024, linked to global warming, El Niño, and possibly underwater volcanic eruptions.

Cyclones in the North Indian Ocean:

• The north Indian Ocean plays a vital role in supporting the summer monsoon by being a major moisture source, requiring significant evaporation from the Arabian Sea and the Bay of Bengal.
• Warm sea temperatures drive this evaporation. Despite being warm tropical waters, the north Indian Ocean has the lowest cyclone activity among the world’s oceans.
• A unique combination of factors in this region either promotes or suppresses cyclone formation, affecting the timing, number, and intensity of cyclones, and how they respond to global warming.

What Makes the Indian Ocean Unique?

1. Monsoonal Circulation: North of the equator, seasonal winds reverse dramatically, driving unique weather patterns.
2. Oceanic Tunnels:
   • Pacific Tunnel: Brings warm water in the upper 500 meters of the Indian Ocean.
   • Southern Ocean Tunnel: Transfers cooler water below 1 km depth from the Southern Ocean.
3. Seasonal Characteristics:
   • Pre-monsoon: The Arabian Sea warms rapidly, while the Bay of Bengal stays relatively warmer and continues to heat up.
   • Monsoon Onset: By mid-May, a low-pressure trough forms over the Bay of Bengal, signalling the onset of the monsoon in Kerala.
   • Post-monsoon: The northeast monsoon brings significant rainfall to several Indian states.
4. Cyclogenesis (Cyclone Formation): Wind patterns and sea surface temperatures influence cyclone development, creating a stark contrast between the Arabian Sea and the Bay of Bengal. These factors affect cyclone formation year-round.

Role of Climate Change:

• Climate change is amplifying the warming of the Indian Ocean, driven by heat influx from the Pacific and Southern Oceans, and changes in winds and humidity. This rapid warming impacts global ocean processes, influencing the frequency of cyclones and their intensity. The Indian Ocean acts as a hub for warming effects, leading to increased cyclone formation and stronger responses to global warming.

Cyclones in the north Indian Ocean, especially in the Arabian Sea, are unique for several reasons:

1. Monsoonal Effects: During the monsoon, strong southwesterly winds mix cold subsurface waters with warmer surface waters, thus cooling the Arabian Sea. In contrast, the Bay of Bengal sees strong convective activity, which generates low-pressure systems, though these rarely develop into full cyclones.
2. Vertical Wind Shear: Monsoon winds vary in strength and direction between the surface and mid-atmosphere. This variation, called vertical shear, disrupts the formation of cyclones by stripping away their energy.
   • As a result, the cyclone season over the north Indian Ocean is split into two periods: pre-monsoon and post-monsoon. In contrast, other regions typically have one cyclone season each year.
3. Cyclogenesis: While the warm ocean, heat content, and atmospheric conditions generally support cyclone formation, the Arabian Sea experiences less convective activity before the monsoon, making cyclogenesis less likely. After the monsoon, the northeast winds and dry continental air cool the Arabian Sea, further reducing cyclone activity.
4. Cyclone Distribution: Cyclones are less frequent in the Arabian Sea, with roughly half the number of cyclones compared to the Bay of Bengal during both pre- and post-monsoon seasons. The cooler temperatures, strong wind shear, and reduced convective activity make the Arabian Sea less favourable for cyclone formation.
5. Recent Trends: Despite an increase in cyclone activity around 2010, the Arabian Sea has been unusually calm in recent years.

Source: TH