Context: A combination of global warming and a cyclical event called the Pacific Decadal Oscillation that repeats every 20-30 years, could make cyclones that originate near the Equator more frequent in the coming years.
What the study says?
- In recent times, equatorial cyclones originating near the Equator have exhibited an unexpected trend of moderation. The most significant recent occurrence of such a cyclone was the 2017 Cyclone Okchi, which brought widespread destruction to regions including Kerala, Tamil Nadu, and Sri Lanka.
- A convergence of factors, namely global warming and the cyclic phenomenon termed the Pacific Decadal Oscillation (PDO), which operates on a 20-30-year cycle, suggests a potential elevation in the frequency of these cyclones in the years ahead.
- Between 1981 and 2010, equatorial-origin cyclones occurred at a strikingly lower rate of 43% compared to the period spanning 1951 to 1980. This decline aligns with the PDO’s ‘warmer’ or positive phase.
- A warming of the Central Equatorial Pacific, known as El Nino, often corresponds to decreased rainfall over India.
- Conversely, cooler-than-normal temperatures, referred to as La Nina, are associated with heightened rainfall.
- The interplay of these phenomena, collectively known as the El Nino Southern Oscillation (ENSO), recurs in the Pacific every two to seven years.
- In contrast, the PDO follows a non-annual cycle and, over considerably longer timescales, results in warmer conditions in the Western Pacific Ocean and relatively cooler conditions in the Eastern Pacific.
- Identifying a ‘positive’ or ‘warmer phase’ of PDO necessitates years of observing ocean temperatures and their interaction with the atmosphere.
- Notably, in 2019, the PDO transitioned into a cooler, negative phase. If this state persists, it could potentially lead to an increase in the number of equatorial-origin tropical cyclones during the post-monsoon months.
- Currently, an emerging El Nino event is evident in the Pacific, with its effects already manifesting in central and southern India, where rainfall deficits of 7% and 17% have been observed.
- While a positive PDO combined with ENSO typically yields unfavourable outcomes, a negative PDO coupled with the same phenomenon results in increased rainfall for India.
- Pacific Decadal Oscillation (PDO): A recurring climatic cycle manifesting every 20-30 years in the Pacific Ocean. It results in fluctuations in sea surface temperatures. When in a positive phase, it is associated with warmer waters in the western Pacific and cooler waters in the eastern Pacific. This phenomenon significantly influences global climate dynamics and weather patterns.
- El Nino Southern Oscillation (ENSO): A recurrent climate phenomenon observed in the Pacific Ocean, characterized by two primary states—El Nino (warmer-than-normal sea surface temperatures) and La Nina (cooler-than-normal temperatures). ENSO holds considerable sway over worldwide weather patterns, with implications for temperature and precipitation distributions.
- Equatorial Cyclones: Cyclones originating in proximity to the Equator. Fuelled by warm waters and abundant moisture, these cyclones typically form at a distance from the Equator due to the Coriolis effect. Although infrequent, they can be exceptionally intense and cause significant damage.
What is Tropical Cyclone?
- A tropical cyclone is a weather phenomenon characterized by a region of low atmospheric pressure that originates over warm tropical waters and subsequently advances towards coastal regions, unleashing powerful winds, intense rainfall, and storm surges.
- The rotational direction of the winds within a tropical cyclone follows a distinctive pattern: counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. This rotation is a consequence of the Coriolis effect, which influences the motion of air masses in relation to the Earth’s rotation.
Critical conditions contribute to the formation of tropical cyclones:
- Sea Temperature: The presence of an expansive sea surface with a temperature exceeding 27°C serves as a conducive environment for the genesis of tropical cyclones. Warm ocean waters provide the necessary energy for these systems to gather strength.
- Geographical Location: Tropical cyclones tend to develop within specific geographical boundaries, specifically between the Tropic of Cancer and the Tropic of Capricorn. These areas offer optimal temperature conditions for their formation.
- Coriolis Force: A crucial requirement for tropical cyclone formation is the presence of the Coriolis force, a result of the Earth’s rotation. Consequently, these cyclones do not form near the equator but rather at latitudes where the Coriolis effect is sufficient to induce rotation.
- Vertical Wind Speed: The presence of relatively stable vertical wind speeds, with minimal fluctuations, aids in the organization of a tropical cyclone’s structure. This consistency facilitates the development and maintenance of the cyclonic circulation.
- Upper Divergence: Successful tropical cyclone development necessitates a well-established upper-level divergence within the atmosphere. This divergence ensures the continual removal of rising air currents from within the cyclone, maintaining low pressure at its centre.
- High Humidity: Adequate humidity levels, typically ranging between 50 to 60 percent, are essential in the mid-troposphere. This humidity is fundamental for the formation of towering cumulonimbus clouds, a significant component of tropical cyclones.
Influence of Climate Change on Tropical Cyclone Formation
- Temperature Shifts: The temperature of both the ocean and the atmosphere plays a crucial role in the creation of tropical cyclones. As the climate warms, these storms gather strength by harnessing the energy released when evaporated ocean water condenses into rainfall within the cyclone.
- Warmer Ocean Impact: The escalation in temperature leads to a warmer ocean, elevating evaporation rates. This, in turn, augments the moisture content in the atmosphere, intensifying the availability of water vapor.
- Enhanced Rainfall: The rising temperature enables the atmosphere to retain more moisture, thereby increasing rainfall. The amplified rainfall not only releases more heat but also contributes to bolstering wind intensity.
- Heightened Rainfall Rates: Recent investigations reveal a 7% surge in hurricane rainfall rates per degree of warming, underscoring the pronounced impact of rising temperatures on precipitation.
- Storm Intensification: The warming climate is poised to elevate wind velocities, leading to a higher frequency of powerful Category 4 and 5 storms, with their potential for devastating impact.
- Sea Level Rise Influence: Climatic warming elevates sea levels, augmenting the height of ocean water. This elevated water level permits storm surges to penetrate further inland.
- Impact of Storm Slowing: The velocity of a storm significantly influences the cumulative rainfall in a given area. Slower-moving storms grant additional time for rainfall accumulation, heightening the potential for flooding.
- Complex Storm Speed Changes: While studies indicate a reduction in storm speed, the underlying mechanisms behind this trend remain elusive.
- Confluence of Storms: Within an increasingly warmer world, the merging of two significant tropical storms over global oceans could lead to the formation of extremely powerful mega cyclones.