Monsoon Season and the Drought

Across the country we have been facing severe droughts, the severity of which have not been seen for over two decades.  Here in the Sonoran Desert we also have been experiencing a severe drought which has recently been relieved, at least temporarily, by the beginning of monsoon season rainfall.  Drought in the desert is sort of an odd thing.  In comparison with other regions drought is a constant pattern in deserts and actually makes a desert a desert.  What else would define a desert a part from lack of rainfall?   Every year most deserts will go through a period of time where no precipitation falls at all.  In the Sonoran Desert that almost always is late April through June.  The Mojave and Great Basin Deserts go almost all summer without any rain at all.  These differing rainfall patterns are actually part of the reason we have different types of desert.  While drought, or lack of moisture may constantly be a problem in the desert, some years are worse than others.  This year for example has been pretty extreme.  Late winter and early spring rains where next to nothing.  This meant higher temperatures due to the lower humidity causing things to dry out even more.  By the time early July hit the desert around Phoenix had only about half an inch of rain for the year!  This is way less than the typical three to four inches of rain received during the first six months of the year.  Fortunately, on July fourth we received a major rainfall which hopefully began to break this dry spell.  More rain fell on most of the desert than had fallen in the previous six months combined.

Normally, late winter and early spring rainfall helps sustain the desert until monsoon season.  This year however, the lack of rain has taken its toll on nature.  Normally, Palo Verde and Ironwood trees are able to maintain their leaves through June and July.  While it isn't that unusual for Palo Verdes to loose all their leaves it isn't typical in this part of the Sonoran Desert, but this year only a handful of trees held onto their leaves.  Ironwoods very rarely loose their leaves but this year the drought caused the majority of leaves to fall off of the trees.  This also resulted in extremely poor seed production.  The story is the same for nearly all the other plants in the desert also.  As a direct result, many desert birds and animals will also be harmed by the lack of food productivity as well as the lack of water.  Earlier this year I noted extremely poor Wolfberry production which likely had a negative effect on birds.

On July fourth however, three quarters to one inch of rain fell breaking this trend.  Since then we have had two other rainfall adding up to at most half an inch.  This immediately relieved drought conditions, causing a rapid bounce back of the plants.  Within a week the Ironwoods had greened up and added leaves.  Palo Verdes also have sprouted leaves along with Wolfberries.  Cacti also have plumped up as they soak in the extra moisture.  Everything seems to have awaken with the rain, clouds, and slightly cooler temperatures.  So hopefully this early monsoon rain continues.  At least part of the reason for the lack of rain earlier this year was the presence of La Nina conditions in the Pacific Ocean.  In June however La Nina broke and we seem to be trending towards an El Nino to start up in the next several months.  Often this means greater rainfall during the winter months, but only time will tell.

Sonoran Desert Rain: El Nino and La Nina

If anything is consistent in the desert, it is its consistently variable rainfall and drought.  Though still a desert, portions of the Sonoran Desert are considered some of the wettest deserts in the world.  Rainfall ranges between three and twelve inches annually, depending on location, and is split roughly between summer monsoon season and winter.  Between these "rainy" seasons there are two annual droughts, one in spring and one in fall.  Resulting from "abundant" rainfall compared to other deserts, and both warm and cool rainy seasons, the Sonoran Desert is likely the botanically richest desert in the world.  Other deserts receive less rain and only have one rainy season, if any at all.  Despite this so called abundance, rain is still sparse and variable.  I have seen years with three inches of rain and others with 20 inches.  Trace rain showers are the most common type of shower, but I have seen storms that drop eight inches, an entire years worth, in one storm over a few days.  Months often pass without even a trace of rain, then drought is broken by a storm dropping two inches in a single day, followed by more months without rain.  Variability is the rule, and much of what controls this variability is connected to ocean temperatures thousands of miles away.

Amazingly, the Pacific Ocean off the coast of South America can have huge effects on weather in the Sonoran Desert.  Within the Pacific Ocean, along the equator, water temperatures vary from season to season and year to year.  These temperatures cycle above and below the average temperatures on a seemingly an unpredictable basis.  This cycling is called the El Nino Southern Oscillation, or ENSO.  The weatherman is still trying to figure all this out but does know that both above and below ocean temperatures produce predictable weather patterns.  When equatorial ocean temperatures off the coast of South America are below average it is called a La Nina event.  When ocean temperatures are above average it is an El Nino event.  Weathermen watch for these and make long-term predictions based off of them.

El Nino, top map, with red and orange representing warmer ocean temperatures along the equator off the coast of South America.  La Nina, bottom map, with blue representing cooler ocean temperatures. 

Check out this link with lots of ENSO information, including an animation that tracks Pacific Ocean temperature change over the past three months.

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

Link to current National Weather Service ENSO observations: ENSO Diagnostic Discussion 

La Nina's lower ocean temperatures results in less evaporation from the ocean and therefore a weak pacific jet stream bringing moisture to the Sonoran Desert.  Typically this means winter rainfall is lower than normal.  El Nino's higher ocean temperatures results in greater evaporation from the ocean and therefore a stronger Pacific Jet Stream bringing in moisture to the Sonoran Desert.  So typically La Nina means less winter rainfall, while El Nino means more winter rainfall.  Summer rainfall patterns seem to be less controlled by ENSO.  However, La Nina does seem to occasionally increase summer rainfall but El Nino seems to have the opposite.  

Even with this information, predicting rain in the desert just about drives me crazy.  As always, even when all forecasts align, the exact opposite often happens.  Though El Nino and La Nina often do accurately predict rainfall, they also are often wrong.  The driest years I remember where always strong La Nina's.  The wettest, El Nino's.  But I have also seen wet La Nina's and below average rainfall during El Nino's.  

This winter we currently have strengthening La Nina conditions, which typically would mean less rainfall.  However, we have already had about a half inch of rain for November in the Phoenix area.  More rain is predicted in the next week or so.  This is unusual, but for the desert, the unusual is expected.  As a result of this rainfall, temperatures have been slightly below what is typical.  If even a little more rain does come in the short term, and temperatures do stay cooler, the soil will stay moist and cause an abundance of dormant wildflower seeds to germinate.  Then, if rainfall continues, say one decent rainfall every month through March, massive blooms of wildflowers will display themselves in the spring.  We will all have to wait and see, but predicting good spring desert wildflower displays is even more difficult than predicting desert rain.  

A spring desert wildflower display after a winter of above average rain.  Picture from Arizona-Sonoran Desert Museum.