What is a Monsoon?

by Mike Godsey, mike@iwindsurf.com

Several times each mid summer Southern California coast winds fade away as mid and upper level clouds, humidity and even showers or thunderstorms come over the region. In this situation you will see the media and our forecast use the term “Monsoonal” in the forecast. Most of you think of Monsoons as having something to do heavy rains in SE Asia but there are actually much weaker and more intermittent Monsoons Mexico, and the southwest USA. So they only bring scattered showers to most of the desert SW but their impact on Southern California coast winds is often dramatic.

The cause of our Monsoons is the same as in SE Asia: intense dry inland heating pulling moisture and clouds from warm ocean areas. Our monsoon begins in early summer in southern Mexico periodically expands to Southern California in July. Let’s take a look at the factors producing today’s Southern California monsoonal conditions.

Looking at the graphic to the right 
The monsoon is triggered when an upper level high pressure area, also called the monsoonal or subtropical ridge, develops in Four Corners region (where all the square states meet). This in turns creates a deep low pressure or thermal trough in the 4 corners which expands into Southern California. Normally this means a heat wave for Southern California and strong pressure gradient and solid coast winds.

However sometimes a clockwise flow of wind and clouds develops around this upper level high pressure area. As you can see in the graphic this circulation creates SE flow above Southern California but it also brings in moisture from  the waters east and west of Baja. The Sea of Cortez, the narrow warm body of water between Baja and mainland Mexico, is especially important in pumping low-level moisture into the Southern California atmosphere via this SE flow. In addition upper level moisture transported into the region from the Gulf of Mexico  by easterly winds aloft.

So how does this all impact Southern California coast winds. The key to the strongest mid summer coast winds is heating in the inland valleys. This heating creates a pressure gradient from the cool dense ocean air so once the marine layer clouds burn off local sea breezes ramp up and the same pressure gradient sucks the North Pacific High’s surface NW winds to curve into the beaches as WSW flow augmenting the local sea breezes.

In monsoonal conditions all the clouds limit inland valley cooling so the pressure gradient is weaker while the unstable air makes any local sea breezes unreliable. And without much of a pressure gradient the North Pacific High’s surface NW winds do not curve into the beach. Typically after a day or two the upper level high pressure moves away from Southern California and the winds pick up.