Bangladesh experienced a colder-than-usual winter in 2023-24, and forecasts indicate that the upcoming winter of 2024-25 could be even chillier. In light of this, here’s an overview of the oceanic and atmospheric factors that could explain why Bangladesh is expected to experience another colder-than-usual winter this year.
La Niña
La Niña is a climate phenomenon that occurs in the central and eastern Pacific Ocean. It is the opposite phase of El Niño and is part of the larger El Niño-Southern Oscillation (ENSO) cycle. During La Niña, the surface waters in the central and eastern Pacific Ocean become cooler than average, while the western Pacific and parts of the Indian Ocean experience warmer-than-usual temperatures. This shift in ocean temperatures affects atmospheric pressure patterns and global weather systems.
This La Niña climate phenomenon is a major factor contributing to the colder winter in Bangladesh. In South Asia, including Bangladesh, La Niña tends to bring cooler temperatures, making winters colder than usual. According to current projections, La Niña is expected to emerge from October to December 2024 and persist through January to March 2025.
Jet stream
This Jet stream is also a key factor contributing to this year's colder winter. While this factor is significant on its own, it is also influenced by La Niña. During La Niña years, the jet stream shifts southward, bringing cold air into the South Asian region, which makes the winter feel colder in Bangladesh. The combined impact of La Niña and the jet stream may result in a colder-than-usual winter in Bangladesh this year.
A jet stream is a fast-moving, narrow band of strong winds found in the upper levels of the atmosphere, typically around 10 to 15 kilometers (6 to 9 miles) above the Earth's surface. These winds generally flow from west to east and can reach speeds of over 200 miles per hour (320 kilometers per hour). Jet streams form where cold and warm air masses meet, creating a sharp temperature difference. This difference in temperature causes the air to move rapidly along the boundary between the two masses.
During La Niña years, the jet stream shifts southward, bringing cold air into the South Asian region
Indian Ocean dipole
The Indian Ocean dipole (IOD) is a climate phenomenon that refers to the difference in sea surface temperatures between the western and eastern parts of the Indian Ocean. It is similar to the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean but specifically affects the Indian Ocean region.
This IOD is another important factor for colder winters in Bangladesh. If IOD turns negative, it can bring cooler air to the region, including Bangladesh. As of the latest information, the IOD will transition to a negative phase shortly in November-January, which generally means cooler waters in the western Indian Ocean and warmer waters in the east, potentially affecting weather patterns in the region.
Cold air from the north
Another big factor is cold air moving south from the Himalayas. High-pressure systems over northern India and Nepal push this cold, dry air into Bangladesh, leading to a significant drop in temperatures, especially during the winter months.
Dr Md Rashed Chowdhury is a Climate Scientist currently affiliated with Arizona State University in the USA. Email: [email protected].
