Context:

• Cyclones are powerful, rotating storm systems that develop over warm tropical oceans, significantly impacting the climate and geography of a region. The Arabian Sea, a crucial part of the north Indian Ocean, experiences a distinct cyclone activity pattern, with two cyclone seasons unlike most other regions of the world. The Arabian Sea’s unique cyclonic patterns, compounded by climate change, has created interesting phenomena such as Cyclone Asna in August 2024, the first of its kind since 1981.

Cyclone Seasons in the North Indian Ocean:

• Unlike other cyclonic regions, the north Indian Ocean, which includes the Bay of Bengal and the Arabian Sea, experiences two distinct cyclone seasons:
• Pre-Monsoon Season (May-June): During this period, the Arabian Sea warms rapidly due to the sun’s movement across the equator into the northern hemisphere, setting the stage for cyclogenesis (cyclone formation).
• Post-Monsoon Season (October-November): After the monsoon withdrawal, the warm ocean waters support the formation of cyclones in the north Indian Ocean, especially in the Bay of Bengal, which remains warmer than the Arabian Sea.

Geographical Factors Influencing Cyclogenesis:

The unique cyclonic behavior in the north Indian Ocean is influenced by several geographical factors:

1. Oceanic Tunnels: The Indian Ocean is connected to the Pacific Ocean and the Southern Ocean via oceanic tunnels. Warm waters from the Pacific and cool waters from the Southern Ocean influence the temperature and moisture dynamics of the Arabian Sea, creating a contrasting environment for cyclones.
2. Monsoonal Circulation: The Indian Ocean experiences a dramatic shift in wind patterns due to the monsoonal circulation, which reverses seasonally. This change directly affects the sea surface temperatures, aiding or suppressing cyclone formation.
3. Bay of Bengal vs. Arabian Sea: The Bay of Bengal is generally warmer than the Arabian Sea, making it more conducive to cyclones. On the other hand, the Arabian Sea is relatively cooler, with less cyclonic activity, particularly during the monsoon months, due to the vertical wind shear (difference in wind speed at different altitudes) and dry air intrusion.

Climate Change and its Impact:

• Climate change has intensified the uniqueness of the Indian Ocean by altering ocean temperatures and atmospheric conditions, thus influencing cyclone patterns. Factors include:
• Increased Heat Input: The Arabian Sea is now receiving more heat from the Pacific Ocean, leading to the rapid warming of its waters. This, combined with the warming Southern Ocean, contributes to increased moisture availability for cyclones.
• Higher Evaporation Rates: As the sea surface temperatures rise, evaporation increases, enhancing the moisture content in the atmosphere, which can fuel the development of cyclones.
• Northward Shift of the Low-Level Jet: The warming of West Asia has shifted the low-level jet, affecting the circulation patterns over the Arabian Sea, which impacts cyclone intensity and frequency.

Cyclone Asna:

Cyclone Asna, which developed in August 2024, is notable for its rarity. August cyclones are uncommon in the Arabian Sea, and the last such occurrence was in 1981. Cyclone Asna’s formation was due to the following unusual events:

• Land-Born Depression: A powerful land-born low-pressure system, which originated over western India, transitioned onto the Arabian Sea. Typically, low-pressure systems over land weaken; however, Asna intensified upon encountering the warm sea surface.
• Oceanic Heat Contribution: The Arabian Sea, warmed significantly, provided the required energy for Cyclone Asna to strengthen, transforming it into a full-fledged cyclone.
• Climate Change Influence: The unusually high temperatures in the Arabian Sea, exacerbated by climate change, likely played a crucial role in the cyclone’s formation and strength. The rapid warming caused by El Niño and other global warming factors helped sustain the system.

Cyclone Asna caused significant damage, including 50 deaths and widespread destruction of crops and properties, before dissipating over the Arabian Sea due to dry desert air.

Geographical Terms Explained:

• Cyclogenesis: The process of cyclone formation and development, usually requiring warm ocean temperatures, sufficient moisture, and specific wind conditions.
• Vertical Wind Shear: The difference in wind speed and direction between the lower and upper levels of the atmosphere. High wind shear disrupts the formation of cyclones by tearing apart the developing system.
• Low-Level Jet: A strong, narrow band of wind located near the earth’s surface that contributes to the movement of air masses and the development of cyclones.
• Oceanic Tunnels: Pathways through which waters from other oceanic regions, such as the Pacific and Southern Oceans, enter the Indian Ocean, affecting its temperature dynamics.
• Monsoonal Circulation: Seasonal reversal of winds due to the differential heating of land and sea, a prominent feature over the Indian subcontinent, driving the Indian monsoons.
• El Niño: A climate phenomenon that causes periodic warming of sea surface temperatures in the Pacific Ocean, impacting weather patterns globally, including cyclogenesis in the Indian Ocean.