A distant Pacific phenomenon may be drying India’s north, says study

Rising temperatures and falling rainfall driven by a lesser-known oceanic phenomenon are causing climatic shifts in several parts of India, says a research paper by scientists at the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), published this month.

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For the analysis, the team sorted India into climate types based on temperature and rainfall thresholds.

The study found that in north-west India, the hot, semi-arid steppe zone grew from 25.7% of the region’s area in 1951-80 to 33.1% in 1991-2020, while the milder temperate zone shrank from 25.8% to 18.7%. The steppe zone’s eastern edge moved from west Uttar Pradesh to eastern part of the State.

In the north-east, tropical savannah (warm) zone fell from 18.4% to 11.8%, while ‘temperate’ rose from 74.8% to 82.5%. Gangetic West Bengal also moved from savannah to temperate.

The authors attribute the changes to a phenomenon in the Pacific Ocean, called the ‘Victoria Mode’ (VM). The VM is a recurring pattern of warm and cool surface water in the North Pacific, separate from the better-known Pacific Decadal Oscillation.

Future concerns

The drying in northern India, covering west and east Uttar Pradesh, Bihar and Gangetic West Bengal, is attributed to the negative phase of the VM, which they say reduced the moisture flowing in. The continuing shrinkage and eventual disappearance of the tropical savannah and temperate zones “will cause numerous problems related to food safety and local population displacement,” they note in the paper.

In peninsular India, the hot semi-arid steppe zone fell from 30.3% of the region’s area in 1951-80 to 16.4% in 1991-2020. Tropical savannah rose from 61.6% to 75.7%. Konkan and Goa, coastal Karnataka, and Kerala and Mahe saw their tropical monsoon zones expand, according to the paper published in the latest edition of MAUSAM, the in-house journal of the IMD.

While the Indian subcontinent is used to the effects of El Niño — a warming of the Central Pacific and linked to the shrinking of the southwest monsoon — previous studies have found that most VM events were followed by El Niño events. The MAUSAM study, however, doesn’t explicitly link the VM to El Niño.

The Pacific Decadal Oscillation (PDO) correlates negatively with Indian monsoon rainfall, like the El Nino-La Nina cycle. Studies have shown that when the El Niño Southern Oscillation and the PDO are in phase, they reinforce the usual El Niño-monsoon link, and when out of phase, they offset each other. The globe is currently in the grip of one of the strongest El Niños in history and it has shrunk India’s monsoon rainfall this year by 12%.

Steppe zone is dry — but not desert — with limited rainfall and an average annual temperature of 18°C or more. Besides Uttar Pradesh, the paper places West Vidarbha and Marathwada in this class in 1991-2020, when they had earlier been mostly savannah. Western Saurashtra and Kutch, by contrast, moved from fully arid to semi-arid. ‘Temperate’ is a milder zone, where the coldest month averages between 0°C and 18°C. Tropical savannah is a warm zone, with the coldest month at 18°C or above and a clear dry season, with rain falling mainly in the monsoon months.

For the study, the scientists first sorted India into climate types, applied them to the IMD’s gridded records on 0.25-degree squares, roughly 25 km across. They repeated this for five 30-year windows, each starting a decade after the last, and tracked how the map changed.

To look for a cause, they compared North Pacific sea-surface temperatures with rainfall over the northern plains from 1981 to 2020 and examined how moisture moves across the region.