Monsoons are massive, seasonally changing sea breeze circulations that form due to temperature differences between land and ocean.
Why do I care? Monsoon variability is connected with ENSO variability and can have an impact on weather in the United States through atmospheric chain reactions called global teleconnections.
Figure A. Monsoon circulations.
A monsoon is not just any heavy rain that lasts for a long time. Rather, a monsoon is like a giant sea-breeze. A normal sea breeze changes daily, with the day-to-night change of land temperature as the sun rises and sets; monsoons change seasonally. Figure A compares seasonal shifts in the monsoon circulation. Most of the time during the summer, the land is warmer than the ocean. This causes air to rise over the land and air to blow in from the ocean to fill the void left by the air that rose. As you know, rising air leads to cloud formation and precipitation. These monsoon clouds that form are full of moisture from the ocean, so they can produce heavy rainfall for long periods of time. Because the land stays warmer than the ocean for most of the summer and the ocean is a constant source of moisture, these heavy rains last for months at a time. A monsoon climate is marked by dry winters and wet summers. About a quarter of the globe experiences a monsoon climate.
Cherrapunji, India, affected by the Indian summer monsoon, is one of the rainiest places on earth with an annual rainfall average of 34 feet. Back in 1861, Cherrapunji got over 87 feet of rainfall during the year with 30 feet of it falling in July alone. The southeast United States is not affected by monsoon rainfall, and only averages about 4 feet of rain per year, distributed fairly evenly across all seasons.
There are several monsoon systems throughout the globe, far too many to be discussed in depth individually. Some of the most famous ones are the:
Indian summer monsoon – affects all of India. In the summer, the wind blows north from the Indian Ocean south of the Indian subcontinent and dumps heavy rain on the area from roughly April to October.
Figure B. Asian-Australian Monsoon. (From NOAA's Climate Prediction Center).
Asian-Australian monsoon – affects southeastern Asia, the Australasian islands, and northern Australia. This monsoon happens from December to March because these areas are located slightly east of the opposite end of the Indian summer monsoon circulation. Since these areas are south of the equator, December through March is their summer, making this a summer monsoon, too.
North American Monsoon – affects the southwestern United States. It is also a summer monsoon. This doesn’t really have a seasonally reversing wind pattern, but heavy rain and storms occur more frequently from July to September over the desert Southwest. This monsoon is fueled by both the Gulf of Mexico and the Pacific Ocean.
West African Monsoon – affects western Africa. It is fueled by the Atlantic Ocean to its south and produces clusters of storms that can become Atlantic hurricanes from June to September.
Even if your weather isn’t directly affected by a monsoon circulation, your climate is. Monsoons are very large-scale circulations that can simultaneously affect and be affected by global climate. For instance, if El Niño is weak, the Indian summer monsoon is heavier than normal. This could start a chain reaction throughout the atmosphere, causing stronger lows and heavier rain to form downstream, or toward the east. These chain reactions that affect weather patterns throughout the world are called global teleconnections.