Nepal’s Monsoon: Why Is This Year’s Monsoon Delayed Compared to Last Year’s Early Arrival?
Image source: EPA
Last year, the monsoon arrived in Nepal two weeks earlier than average, on May 29, but this year, despite the season officially starting past June 13, meteorologists say the monsoon has yet to enter Nepal.
Experts indicate that the monsoon could still arrive in a few days. The Department of Hydrology and Meteorology is analyzing certain criteria it has established to determine monsoon onset.
Associate Professor Madan Sigdel of Tribhuvan University’s Central Department of Hydrology and Meteorology forecasts that this year’s monsoon will be “slightly delayed and somewhat weaker.”
“The western disturbance system remains active. Normally, such a system diminishes by late May,” says Associate Professor Sigdel.
“This system is preventing the monsoon flow from moving east to west. Although the monsoon has reached West Bengal and Assam, it cannot expand westward because of this.”
He notes that the monsoon has weakened while entering through the Bay of Bengal and though it tried to strengthen upon reaching West Bengal, it was suppressed again under the pressure of the active western disturbance.
Typically, the monsoon reaches India’s Kerala coast around June 1 and from there spreads westward.
Records over past decades show that only three times since 2012 has the monsoon entered Nepal after June 13; in other years, monsoon rains occurred earlier.
What are the criteria?
According to meteorologist Binu Maharjan from the Department of Hydrology and Meteorology, a primary criterion is the presence of prevailing westerlies in the upper atmosphere. At the same time, monsoonal winds from the east should be visible, and a high-pressure system must develop in that area.
“Additionally, the longwave radiation—reflected solar radiation measured per square meter—should be below 200 watts, whereas currently it is observed to be higher,” Maharjan explains. “This means that sometimes the criteria are met, and sometimes they are not.”
When radiation decreases, cloud density increases, leading to heavier rainfall.
Image source: EPA
Associate Professor Sigdel adds, “Although we observe monsoonal winds entering Nepal from the lower atmosphere, the upper atmosphere still blocks their movement, causing the monsoon to remain obstructed over Nepal.”
The monsoon is essentially wind carrying moisture from the Bay of Bengal, flowing from east to west.
“It appears it will take a few more days for all the designated criteria to be fulfilled,” explains Maharjan.
Meanwhile, the Indian Meteorological Department has reported that the monsoon has arrived in Bihar. South Bengal has experienced the earliest monsoon arrival in five years.
Last year, the monsoon entered Nepal on May 29 and exited on October 10, marking a record 135-day duration—the longest monsoon recorded in Nepal’s history.
Data since 2012 show that the latest monsoon arrivals occurred in 2014 and 2019, both entering after June 20.
What Is Natural and What Is Unusual?
Associate Professor Madan Sigdel considers this seasonal pattern “somewhat abnormal.”
He attributes the extended activity of the western disturbance in the atmosphere until mid-June as the primary cause.
Under usual conditions, the monsoon enters around June 13 and lasts until October 2, giving Nepal an average monsoon period of 112 days, although this varies year to year.
Image source: RSS
Meteorologists say the monsoon typically enters eastern Nepal and spreads westward within 7 to 10 days.
“Last year, the monsoon also took a long time to cover the entire territory. However, this year’s monsoon behavior will become clearer only after it fully arrives,” Maharjan explains.
“Currently, moist air from the Bay of Bengal is arriving in the lower atmosphere, but the upper atmosphere still holds a western low-pressure system causing rainfall.”
This year marks the fourth consecutive year in which Nepal is expected to experience above-average temperatures. Meteorologists observe that current conditions point toward higher than usual temperatures.
“For now, the monsoon appears less vigorous than average, but a clearer picture will emerge once it has arrived fully,” says Maharjan.
Meteorologists also note that the pre-monsoon season has already seen sufficient rainfall, which could influence monsoon activity.
“For the monsoon to arrive, the land must heat up significantly, drawing moisture-laden air from the ocean to trigger a strong monsoon system. If heavy cloud cover occurs during the pre-monsoon period, the monsoon can be delayed or advance unpredictably.”
Nepal’s shortest recorded monsoon was in 1979, lasting only 73 days: it started on June 24 and ended on September 4.
“Super El Niño”
Numerous factors influence the monsoon’s characteristics.
Meteorologists identify the El Niño and La Niña phenomena in the Pacific Ocean as major influences on the South Asian monsoon.
El Niño refers to an anomalous warming of ocean waters, whereas La Niña is characterized by cooler-than-average sea temperatures.
According to forecasts by many national meteorological agencies, this year’s El Niño could be the strongest recorded, often described as a “Super El Niño.”
Meteorologists explain that El Niño years tend to bring reduced rainfall to South Asian countries, while La Niña years usually result in increased rainfall.
El Niño events occur when sea surface temperatures rise in the central Pacific Ocean.