Mount Everest Winds- The Jet Stream

By Michael FaginEverest Basecamp, Mt. Everest Weather

Mount Everest winds- The Jet Stream. What happens high on the slopes of Mount Everest is largely dependent on jet streams high up in the troposphere. The container of the Earth’s weather, the troposphere is four kilometers high at the poles and three times as high at the equator due to variations in barometric pressure. The energy it contains doesn’t come directly from the sun. It’s conducted and then convected upward from the surface. A global, three-celled separation of average motions results as the energies perform a balancing act. The cold, high-pressured air from the poles conflicts with the Ferrel cell of the mid-latitudes, which also conflicts with the Hadley cell of the tropics. All three are capped by a strong temperature inversion at the bottom of the stratosphere, called the troposphere.

Five to nine miles up in the troposphere, strong temperature gradients no more than three miles thick play the role of atmospheric highways. These jet streams were discovered by meteorologist Wasburo Ooishi in the 1920s during a study of high elevation wind patterns over Japan. It wasn’t until later when the Japanese utilized the jet stream when attacking the American mainland with fire balloons that the term became widely circulated. Due to the ongoing rotation of the Earth, jet streams typically travel west to east in both hemispheres between 80-140 mph but can accelerate to 275 MPH

The separations between the polar and Ferrel cells and also between the Ferrel and Hadley cells, when mixed with the necessity of the conservation of angular momentum, give birth to the northern (polar) and subtropical branches, respectively. The jets that most affect Mt. Everest are subtropical and are fueled by a high-altitude subtropical front. Subtropical jet streams are stronger in the western Pacific than anywhere else due to the high elevations near the Himalayas disrupting the normal upper tropospheric westerly flow, which leads to a confluence zone near Japan. They remain around 30 degrees latitude and, unlike polar jets, aren’t dependent on temperature. Their roots are capable of extending down to the 400 MB pressure level.

The Indian monsoon season is most dependent on subtropical jet streams and countering, seasonal, easterly jet streams, although northward bound tropical air delivers substantial moisture and heat. When the warm moisture meets and climbs the southern Himalayas, it eventually reaches its saturation vapor point as the air is cooled and compressed, which leads to heavy precipitation. Summer monsoon cells can develop when the subtropical jet is torn apart by the tall mountains of Central Asia. The jet naturally inhibits the formation of the monsoon but it can reappear and intensify very quickly once it’s no longer acting on the area. Typically there is a month long delay for the monsoon to affect India relative to the remainder of Asia.

The Indian monsoon changes direction twice annually. The heating of continental Asia during the summer causes a reversal of the jet stream, which contributes to the monsoon season in tropical high altitudes. This happens because the typical north-south temperature gradient is reversed when 30 degrees north becomes warmer than at the equator. Around May, the two forces tend to balance each other and nearly stagnate winds at Mt. Everest. Later in the season when cooling occurs, the subtropical jet reappears, winds begin to pick up, air subsides over India and then heads out to sea. The subtropical jet is stronger in the winter than in the summer and interactions between it and polar jets can lead to strong winter storms.

Winter jet stream at 200 mb winds about 140 knots. Photo: National Centers for Environmental Prediction

Winter jet stream at 200 mb winds about 140 knots. Photo: National Centers for Environmental Prediction

An additional facet of the reversing weather comes from a phenomenon called the Indian Ocean Dipole. Its positive phase leads to stronger trade winds and cooler than average sea surface temperatures near Indonesia and Australia. To the west, the waters around Madagascar remain warmer, intensifying convection and the monsoon. In its negative phase, the patterns are reversed, which helps the strengthening subtropical jet stream weaken the monsoon and rejuvenate winds over Mt. Everest.

Article written by Meteorologist Geoff Linsley