Monsoon in Nepal: Winds Shift from West to East, Is the Monsoon Withdrawing?
Following three consecutive days of rainfall and a week-long period, authorities have confirmed that several indicators for the monsoon to become inactive in Nepal have been met. On average, the monsoon withdraws around October 2 in Nepal. Last year, it lasted until October 10, but the official duration for this year’s monsoon activity is yet to be declared. Vibhuti Pokharel, spokesperson for the Department of Hydrology and Meteorology, stated that continuous monitoring and analysis of the seasonal system, atmospheric moisture, and the influence of western winds are underway, and a formal announcement about the monsoon’s end will be made in the upcoming week. She noted, “There is still a slight amount of moisture remaining in our atmosphere, so we are keeping it under constant observation.”
The Indian Meteorological Department has issued a preliminary forecast indicating that the monsoon will become inactive in Nepal’s eastern regions starting October 10. Additionally, early forecasts suggest the monsoon will remain over southern and eastern India until about October 15. Madan Sigdel, Associate Professor at Tribhuvan University’s Department of Hydrology and Meteorology, mentioned that the influence of western winds on Nepal’s monsoon is gradually increasing. He explained, “The monsoon has already withdrawn up to Delhi in India. The winds coming from the east to west have weakened, while winds moving from west to east are gaining strength. There is no indication of new seasonal systems developing in the eastern sector.”
What are the signs of monsoon withdrawal? A reduction in rainfall is the primary indicator that the monsoon is ending. However, meteorologist Ashok Bakharel from the Department of Hydrology and Meteorology explained that atmospheric humidity, wind direction, and the temperature difference between day and night are also related to the monsoon’s status. He said, “Humidity in our region diminishes and gradually shifts eastward. Additionally, the wind direction changes from prevailing easterly during the monsoon to westerly winds starting to return.” The monsoon in Nepal weakens first in the western region and subsequently recedes from the eastern part.
The moisture-laden winds originating from the Bay of Bengal bring heavy rainfall to nearby Nepal during the monsoon season. Yet, during winter, the process of moisture formation over the Bay of Bengal weakens, while winds from the Mediterranean Sea, Caspian Sea, and Arabian Sea have more influence, leading to drier conditions in Nepal, meteorologists say. Associate Professor Sigdel added, “The sun is the primary driver of any climatic system. Fluctuations in solar temperature and the length of days affect evaporation.”
The end of the monsoon is not caused solely by the push of western winds; the moisture from the Bay of Bengal must also weaken. However, once the monsoon withdraws from Nepal, western winds have limited influence as they push the monsoon out, resulting in a decline. Western winds become active in Nepal only from December during winter, which is locally known as the winter season. October and November are referred to as the ‘post-monsoon’ period. During this time, although the moisture-rich winds from the Bay of Bengal reduce, the western winds are not yet fully active. According to meteorologist Bakharel, the temperature difference between day and night also begins to decrease as the monsoon ends.
By the time winds arrive from western oceans to Nepal, they have cooled down and carry less moisture. “Dry air comes towards the end of the monsoon. Since it travels a long distance, it carries minimal moisture upon reaching here. Rainfall decreases only when other seasonal systems shut down as well,” explained meteorologist Ashok Bakharel. “Being close to the Bay of Bengal is why we receive rainfall during the monsoon. Had we been closer to the Arabian Sea, we would receive more rain during the cold season.”
However, western ocean winds gradually accumulate moisture and develop seasonal systems during winter, which can also cause rainfall in Nepal. Meteorologists note that Nepal, which was directly affected by India during last winter, is likely to celebrate a bright Dashain festival with sunlight this year. This year’s monsoon arrived late, on June 19, compared to the past six years. The El Niño system developing in the Pacific Ocean is expected to cause below-average rainfall during the monsoon, though the department has yet to measure the total rainfall amount. Typically, monsoon rainfall ranges between 1,500 to 1,600 millimeters while active. The timing of monsoon onset and withdrawal in Nepal is not always consistent. Some years have shorter monsoon durations. For example, in 1979, the monsoon began on June 24 and withdrew after 73 days on September 4, whereas in 2022, it started on June 5 and lasted 134 days before ending on October 16. This year, a weather system developing in the Bay of Bengal caused some rainfall across the country even as the monsoon was ending.
