The possible increase in the frequency of equatorial-origin cyclones due to the combination of global warming and the Pacific Decadal Oscillation (PDO) is a significant concern. This article delves into the phenomena of these cyclones, as well as explaining the role of the PDO and ENSO (El Nino Southern Oscillation) in their formation and impacts.
Understanding Equatorial-Origin or Low Latitude Cyclones
Equatorial Origin or Low Latitude Cyclones (LLCs) are cyclones that form between 5°N and 11°N. These cyclones, while smaller in size than those found at higher latitudes, intensify more rapidly. The formation of such cyclones near the equator is typically rare, but warm waters can fuel them with moisture, leading to increased intensity.
The majority of cyclones originate in the Western Pacific Ocean. The last major LLC in the Indian region was Cyclone Ockhi in 2017, which travelled over 2000 km and caused massive devastation in Kerala, Tamil Nadu, and Sri Lanka. The north Indian Ocean, particularly during the post-monsoon season (Oct-Nov-Dec), serves as a breeding ground for LLCs, which constitute approximately 60% of all Tropical Cyclones formed in the area since 1951.
The Role of Pacific Decadal Oscillation
The Pacific Decadal Oscillation (PDO) is a long-term ocean fluctuation in the Pacific Ocean. It is a cyclical event that repeats every 20-30 years and has both ‘cool’ and ‘warm’ phases. The PDO can drastically affect the global climate, including factors like Pacific and Atlantic hurricane activity, droughts and flooding around the Pacific basin, marine ecosystem productivity, and global land temperature patterns.
Notably, a warmer (‘positive-phased’) PDO correlates with fewer equatorial-origin cyclones. In 2019, the PDO shifted into a cooler (‘negative’) phase, which could result in more LLCs during the post-monsoon months if this cooling trend continues.
Interrelation between ENSO and PDO
The interplay between ENSO and PDO can significantly impact weather patterns. A positive PDO combined with ENSO can be troublesome; however, ENSO alongside a negative PDO tends to bring more rainfall to India. The impacts of El Niño or La Nina could potentially be magnified if both ENSO and the PDO are in the same phase.
Comparing PDO and ENSO
El Nino or La Nina events in the Pacific tend to repeat every 2-7 years. In contrast, PDO operates over much longer timescales, leaving its signature on decadal scales. The ‘positive’ or ‘warmer phase’ of a PDO can only be determined after several years of tracking ocean temperatures and their interaction with the atmosphere, whereas the stage of an ENSO event can be determined any year.
Questions from UPSC Civil Services Examination
Previous year questions from the UPSC Civil Services Examination have addressed these topics. One such question was about the difference between La Nina and El Nino and the effects of each on the south-west monsoon of India and the Australian floods.
In addition, Mains examination questions have asked candidates to explain unusual climatic happenings as outcomes of the El-Nino effect, and why tropical cyclones are mainly limited to the South China Sea, the Bay of Bengal, and the Gulf of Mexico. These questions highlight the importance of understanding these complex climatic phenomena for competitive exams.
