Do Wet Monsoons Follow Dry Winters?
What will this summer’s monsoon season be like? We ended up with an above-average monsoon season in Tucson last year, thanks to a late season storm, but before that, the last year with above-average monsoon rainfall was 2007, according to the National Weather Service for the Tucson International Airport. The monsoon is notoriously difficult to forecast, and the current forecast from NOAA’s Climate Prediction Center for June-July-August precipitation for the Southwest calls for equal chances for above, below, or average conditions. Not much guidance there.
Over the last half of the 20th century, a tendency for wet winters to be followed by dry monsoons, and vice versa, has been noted in Arizona. This apparent relationship, if robust, could lend some predictability to the monsoon. Several avenues of research have explored the link between winter and monsoon rainfall. One hypothesis has been that winters with heavy snow cover over the US Southwest, including Utah and Colorado, could delay the onset of the monsoon (Zhu et al., 2005). This could occur if snow cover kept land surfaces cool later in the spring, inhibiting the warming that leads to convection and the start of the monsoon. A later start would result in a shorter monsoon season and less precipitation. Another body of research has linked the relationship between winter rainfall and the monsoon to the state of the El Niño/Southern Oscillation (ENSO) (Castro et al.,2007). During El Niño events, winters in the Southwest tend to be wet, but monsoons during El Niño can be quite dry, with the opposite conditions during La Niña events. For example, Tucson had a dry winter and the wettest monsoon on record in 1964 during a La Niña event, while in 1994, Tucson experienced a wetter-than-average winter and the seventh driest monsoon during El Niño conditions. However, research and experience indicate that while both land surface feedbacks and ENSO could be influential to monsoon conditions (and these may not be independent of each other), these influences are quite variable over space and time.
If the records of cool season (November-April) and monsoon (June-July-August) rainfall totals are examined from 1896-2008, the tendency for wet winters to be followed by dry summers, and dry winters to be followed by wet summers, is most apparent since 1950. Here, “wet” and “dry” are defined by dividing the record into thirds; the “wet” years are the 37 wettest years and the “dry” years are the 37 driest years. In the North American Monsoon Region 2, which covers most of central, southern, and eastern Arizona as well as parts of far western New Mexico, twice the number of years with opposite conditions have occurred, compared to years when both seasons are wet or dry (20 years compared to 10 years). However, in the first half of the record (prior to 1950), the numbers are quite even. So, what should we expect in the future, based on this record? Unfortunately, the gage record is too short to evaluate the most typical set of conditions.
Fortunately, new research using tree-ring data from the US Southwest has allowed reconstructions of cool and monsoon season precipitation going back to the 16th century (see previous blog). By measuring the width of the light-colored wood (called earlywood) and the width of the dark part of the ring (called latewood) separately, it is possible to reconstruct past records of both cool season and monsoon precipitation. Using these extended records, we can now assess how often cool and monsoon season conditions are similar in a given year, and how often they are the opposite.
The figure to the right shows 50-year running sums of numbers of years with shared (both seasons wet or dry) conditions in the red line, and opposite (wet winter followed by dry summer and vice versa) conditions in the black line. What is very obvious is that while there is a great deal of variability, the record of opposite conditions shows a notable peak at the end of the 20th century, with highest values showing 16 years in a 50-year span with opposite conditions. This result, in agreement with what the rain gage record shows, suggests the opposite conditions we have experienced in the past half century are relatively unusual in the context of past centuries. Possible causes for the dominance of these conditions over this time period have not yet been investigated. Also notable are the low number of years with shared winter and monsoon moisture conditions in the past half century (dropping to a minimum of five years in a 50-yr span). However, over the full record, it is much more often the case that winter and monsoon rainfall conditions are the same in a given year. This is particularly apparent during most of the 18th century and the second half of the 19th century. Overall, about a dozen years, on average, have wet or dry conditions over both seasons during any 50-year period. In contrast, the average is about 10 years with opposite conditions, for a 50-year period. This is not a great difference, but the variability over the centuries is worth noting.
So, what does this suggest for this year’s monsoon? Tree-rings from the past cannot predict future climate, but if the past is a guide for the future, we might expect dry winters to be followed by dry summers and wet winters to be followed by wet summers more often than opposite conditions in the two seasons. Over the last 10 years, Tucson has experienced six years with dry winters followed by dry summers, and four years with dry winters followed by wet summers. What would be your bet for this year?