What is the reason for the high moisture content pf southwest monsoon winds
Answers
Until this year, the National Weather Service (NWS) declared that the monsoon season began on the first of three consecutive days when the average dew point temperature was greater than 54 degrees Fahrenheit in Tucson and 55 F degrees in Phoenix. The average monsoon start date in Tucson was July 3, according to statistics compiled by the NWS for 1949 to 2007. The earliest onset occurred in 2000 on June 17. In Phoenix, the average start date was July 7 and the earliest onset similarly occurred on June 17, 2000. Unlike Arizona, New Mexico has not quantitatively defined the onset of the monsoon.
The dew point temperature, however, is just one of several indicators of the monsoon and it is typically the last index to suggest that the monsoon has arrived, said Eric Pytlak, science and operations manager at the NWS in Tucson.
In June, for example, numerous monsoon storms occurred around Tucson while the dew point remained below 54 degrees F. For this reason, and to allow the media to more effectively communicate to the public when the monsoon storms are likely to form, the NWS in Arizona has designated June 15 as the official monsoon start date, starting this year.
Arizona and New Mexico receive up to half of their annual precipitation during the monsoon season. The monsoon’s wild winds and driving rains are most dramatic in Tucson and in western New Mexico, tapering off in Phoenix and Yuma (Figure 1). On average, the monsoon delivers approximately 6.1 inches of rain a year to Tucson compared to 2.8 inches to Phoenix.
But the monsoon is variable. Everything from the timing of storms to their intensities and frequencies to the production of lightning, changes from region to region and from year to year. If the amount of rainfall is the basis for judging a summer, then it is common to have good summers and bad summers within the same city.
The character of the monsoon has not changed in the past 100 years, said Pytlak. The monsoon in the 1930s and 1940s is just as variable as it is today, he added.
Similarly, the Las Cruces region “has not seen any trends in the precipitation of the North American Monsoon,” said Dave Novlan, meteorologist for the NWS in El Paso.
A close look at the monsoon precipitation in Tucson and Phoenix supports Pytlak’s assertions (Figure 2). Since 1895, precipitation has neither generally increased nor decreased.
Despite no clear-cut trends in precipitation, people often claim that the monsoon is either weakening or strengthening. People still cite the 2006 monsoon season, in which rain fell in near records amounts, as evidence that the monsoon rains are intensifying. But people tend to remember more vividly extreme years, said Pytlak.
How is the monsoon shaping up this summer? In June, rains were below average in both Tucson and Phoenix. It is too early to judge the monsoon season, however, and predicting it is difficult. This year may be especially tricky because two monsoon-enhancing and two monsoon-damping forces are at work.
La Niña, although weakening, is still at hand, said Tom Evans, Warning Coordination Meteorologist at the NWS in Tucson. A La Niña event increases easterly air flow, which tends to bring more moisture from the Gulf of Mexico, Evans said. In addition, the tropical Pacific Ocean has heightened convection, known in the meteorology world as an active Madden-Julian Oscillation (MJO). This year’s energetic MJO can help push more moisture into the Southwest from the Gulf of California.
However, the Midwest has not been hot and the Rocky Mountains have seen an above-average snowpack this past winter; both factors act to decrease monsoon activity. Which of these forces will ultimately win out has yet to be seen.
Answer:
because they come over the bay of bengal and indian ocean from the south and carry lots of moisture in them.