Rain, Rain Go Away
The great monsoon system of the world is experienced around the globe. South America, Africa, South Asia, Southeast Asia, and the western Pacific Basin all have very clearly defined monsoon seasons. So, do you know how the monsoon system operates?
The monsoon is a complex mechanism that is driven by several things. The landmass of Siberia and Central Asia is an important player and so are the vast subtropical highs over the oceans. It would take too many pages to describe the monsoon system for the whole world, so let’s narrow it down to just one part, India. India is everyone’s textbook example of the monsoon climate.
The monsoon system is made up of two basic seasons, the northeast monsoon of the Northern Hemisphere cold months and the southwest monsoon of the warm months. They are called northeast and southwest monsoons for the prevailing wind directions during those seasons and the wind determines the weather. For India, the northeast monsoon is the dry season and the southwest monsoon is the wet season. It all begins far to the north on the other side of the Himalayan Mountains.
In the Northern Hemisphere cold half of the year, Central Asia gets cold and Siberia gets profoundly cold. This creates a massive, shallow, cold, high-pressure system, called the Asiatic high, over the northern two-thirds of Asia. This cold high pushes cold, dry air outward in all directions at the surface. It drives the near equatorial tradewind convergence (NETWC) southward and dry, stable air dominates north of it. You will hear a bit more about the NETWC shortly.
Meantime, the great subtropical oceanic highs over the Pacific and Indian Oceans weaken and shift south. As they retreat, they bring the NEWTC move farther and farther south even as a thermal low over Australia (don’t forget it’s summer down under) draws it southward.
Back to the NETWC, as promised. For the monsoon climate, the NETWC represents the boundary between the northeast monsoon and the southwest monsoon. It is a discontinuity zone between the massive, warm and moist air mass that dominates over the Indian and Pacific Oceans and the cold, dry air mass that dominates over Asia. You can tell where the NETWC is over India just by the rain. South of it, the rains continue. North of it, conditions go dry. When the NETWC goes south of a region for the last time of the season, the southwest monsoon (rainy season) is over and the northeast monsoon (dry season) has begun. Eventually, the NETWC will push far to the south of India.
In the warm half of the year, the situation reverses. It begins over Central Asia and Siberia with warming land. As this vast landmass warms, it heats the air over it and eventually breaks down the Asiatic high of winter. It keeps right on warming the air and gradually develops the equally massive Asiatic low. This serves to draw air into the low from all directions.
At the same time, the subtropical oceanic highs expand and shift northward with the sun. The Australian low of Southern Hemisphere summer (Northern Hemisphere winter) disappears and is gradually replaced by its counterpart, the Australian high. All these huge systems combine to push and pull the NETWC northward.
There are a number of indicators that the rainy season is imminent, and Indian meteorologists have it down as well as anyone in the world. The Somali jet that develops along the eastern African coast with the southwest monsoon is a big clue and there are other signs as well. The biggest hint that the NETWC is moving through the area on its way northward is the monsoon burst. The monsoon burst involves a line or mass of thunderstorms that arrive over an area all at once to begin the rainy season with a BOOM! Heavy rainshowers and thunderstorms accompany the NETWC northward and leave behind the moist, warm, and very unstable air mass of the southwest monsoon. Once the NETWC is north of an area, the rainy season is on with frequent rainshowers, some thunderstorms, high humidity, and lots of rain.
Although there are plenty of details left out, that is the bare bones description of the monsoon system in India. It works in basically the same manner everywhere it occurs, with the NETWC (whatever it is called locally) the boundary between wet and dry.