Scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM) have identified a distant oceanic phenomenon, known as the 'Victoria Mode', as a significant factor affecting semi-arid areas in north-west India. This shift, detailed in a recent study, could impact food supply and lead to population displacement.
The research, published this month, highlights how the Victoria Mode, a recurring pattern of warm and cool surface water in the North Pacific, influences climatic conditions in India. Unlike the more familiar Pacific Decadal Oscillation, the Victoria Mode's negative phase has been linked to reduced moisture flow into northern India, affecting regions such as west and east Uttar Pradesh, Bihar, and Gangetic West Bengal.
Over the decades, the study found notable changes in India's climate zones. In north-west India, the hot, semi-arid steppe zone expanded from 25.7% of the region's area during 1951-80 to 33.1% in 1991-2020. Conversely, the milder temperate zone contracted from 25.8% to 18.7%. The eastern boundary of the steppe zone has shifted from west Uttar Pradesh to further east within the state.
In the north-east, the tropical savannah zone decreased from 18.4% to 11.8%, while the temperate zone increased from 74.8% to 82.5%. Gangetic West Bengal also transitioned from a savannah to a temperate climate. These shifts could lead to significant challenges, including threats to food security and potential displacement of local populations.
The study also observed changes in peninsular India, where the hot semi-arid steppe zone shrank from 30.3% to 16.4% between 1951-80 and 1991-2020. Meanwhile, the tropical savannah zone expanded from 61.6% to 75.7%. Regions such as Konkan and Goa, coastal Karnataka, and Kerala and Mahe experienced an increase in their tropical monsoon zones.
While the Indian subcontinent is accustomed to the effects of El Niño, which is associated with the warming of the Central Pacific and the reduction of the southwest monsoon, the study does not explicitly connect the Victoria Mode to El Niño. However, it notes that most Victoria Mode events have historically been followed by El Niño occurrences.
The Pacific Decadal Oscillation (PDO), another oceanic cycle, also negatively correlates with Indian monsoon rainfall, similar to the El Niño-La Niña cycle. When the El Niño Southern Oscillation and the PDO are in phase, they amplify the typical El Niño-monsoon relationship. Conversely, when they are out of phase, they counteract each other.
Currently, one of the strongest El Niños on record is affecting global weather patterns, reducing India's monsoon rainfall this year by 12%. This reduction has significant implications for agriculture and water resources across the country.
The study's methodology involved categorizing India into various climate types based on temperature and rainfall thresholds. Researchers applied these classifications to the IMD's gridded records, which are divided into 0.25-degree squares, approximately 25 km across. This analysis was repeated for five 30-year periods, each starting a decade after the previous one, to track changes in climate zones over time.
To explore the causes of these shifts, the scientists compared sea-surface temperatures in the North Pacific with rainfall patterns over the northern plains from 1981 to 2020. They also examined the movement of moisture across the region, providing insights into the underlying mechanisms driving these climatic changes.
The findings underscore the complex interplay between distant oceanic phenomena and local climate conditions in India. As the Victoria Mode continues to influence weather patterns, understanding its impact becomes crucial for developing strategies to mitigate its effects on agriculture and population dynamics.
