What is a Monsoon?
Even in the tropics, where the weather is warm year round, the march of the seasons has a profound influence on the rhythm of life. Rainy seasons, usually referred to as monsoons, alternate with dry seasons and each has its own distinct pattern of prevailing winds.
The term “monsoon” is derived from the Arabic word “mausim” which means season. Ancient traders sailing the Indian Ocean and Arabian Sea used it to describe a system of alternating winds that blow persistently from the northeast during the winter and from the opposite direction, the southwest, during the summer.
It is now well understood that seasonal reversals of wind direction occur because of temperature differences between the land and sea across all of the Tropics. There are many features in common to these summer monsoon circulations, but the most recognizable are the seasonal changes in rainfall patterns, both increases and decreases.
Why Predict a Monsoon?
Billions of people in the tropics and subtropics rely on the summer sun to draw rain clouds off the oceans and onto the continents. For centuries, people have sown and harvested crops, bred livestock, and planned outdoor activities such as construction projects or hunting expeditions according to a relatively well-defined set of calendar dates based on the arrival and departure of the monsoon rains.
In many countries the arrival of the summer monsoon rainfall is good news since it replenishes the waterways and provides a critical supply of water for agriculture and other economic concerns. However, occasionally the rains are excessive and can cause serious and life-threatening floods. At other times a weak monsoon can cause a drought to develop, leaving fields and waterways parched and dry. Since extreme weather conditions associated with monsoons, like floods and drought, occasionally wreak havoc on a region’s economy and people, monsoons throughout the world need to be accurately understood and predicted by weather and climate models, so that scientists can anticipate upswings and downswings in the monsoon and help societies plan accordingly.
Is there a North American Monsoon?
The circulation and rainfall patterns over the tropical and subtropical Americas and the adjacent oceans are dominated by seasonal monsoonal circulations. However, seasonal reversals of the wind are less pronounced over the Americas than in other parts of the world like Australia, India and Southeast Asia. The North American monsoonal circulation is characterized by distinct rainfall maxima over western Mexico and the Southwestern United States (Fig. 2) and by an accompanying upper-level anticyclone (indicated by “A”) over the higher terrain of northwestern Mexico. Heating over the mountains of Mexico and the western United States plays a major role in the development and evolution of the monsoon, in a manner similar to what is observed over the Tibetan Plateau and the Bolivian Altiplano. Rivers of air in the lower-troposphere, referred to as low-level jets, from the Gulf of Mexico and the Gulf of California bring moisture to the continent and play an important role in the daily cycle of precipitation (Fig. 3).
The North American Monsoon System is perhaps the least-understood of all of large-scale circulations patterns that affect the United States. What’s more, the regions that are most affected by the monsoon are among the most rapidly growing parts of both the United States and Mexico. The thunderstorms that are generated by the monsoon system can bring life-giving and beneficial rains, but can also be life-taking as they unleash violent flash floods, thousands of lightning strikes, crop-damaging hail, and walls of damaging winds and blowing dust.