The seasonality of Southwestern US climate: does a wet monsoon follow a dry winter?

This past winter was extremely dry across the US Southwest, and partly because of this, many forecasts predicted a wet monsoon.   Anecdotally, at least, we often hear that a wet monsoon tends to follow a dry winter and vice versa. However, this summer’s monsoon was quite spotty, with some areas (especially southern and eastern New Mexico) receiving above normal precipitation, while other areas were very dry. 

New research by members of the CLIMAS team on the climatology of the Southwest (Arizona and New Mexico) reveals a high degree of variability in precipitation across space and time. This is due to the widely variable topography and geography, as well as the variable influences of the winter storm track and the North American Monsoon (NAM) across the region.  In our study of 5 subregions, all but the eastern part of New Mexico experiences peaks in both winter and summer precipitation (a bimodal precipitation regime). Some areas get more monsoon precipitation (especially southwestern New Mexico and southwestern Arizona) and are more strongly influenced by the NAM.  Other areas have a stronger peak in winter precipitation because they are more often in the path of the jet stream that brings winter storms and their moisture (western and northeastern Arizona and northwestern New Mexico).  Springs are also variable, with little spring precipitation across Arizona and increasingly wetter springs in central and eastern New Mexico.

These differences result in highly variable patterns of seasonal precipitation over the Southwest, but these patterns are also quite variable over time. A deep dive into an analysis of these patterns reveals periods of time when dry winters are followed by wet monsoons and vice versa, but this seasonal sequence is not constant over time.  This “inverse season” pattern has been most prevalent since about 1980, particularly in southeastern Arizona, southwestern New Mexico, and the Rio Grande basin in central and southern New Mexico.  In contrast, in the early decades of the 20^th century, sequences with wet winters followed by wet summers, and dry winters followed by dry summers (“dual season” droughts and pluvials) were more common.  It is unclear what controls changes in these patterns over time, but ENSO (El Niño/Southern Oscillation) is likely to be partly responsible. 

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(Click the above figure to access the interactive version.)

Seasonal sequences of precipitation have also influenced the nature of droughts in the Southwest.  Two examples of this are the 1950s and 2010s droughts, which were both widespread across the Southwest.  The 1950s drought was a dual season drought, with dry conditions in both the cool season (fall and winter) and the monsoon in multiple years.  In contrast, the 2010s drought had dry winters and springs, but the monsoons were not particularly dry. However, the 1950s temperatures were relatively moderate, while much above average temperatures characterized the 2010s drought. Temperatures have a strong influence on drought, increasing evapotranspiration, resulting in reductions in surface water supplies and increased demand for water by crops and natural vegetation. So even though the 2010s was not characterized by dual season drought years, the consistently warm temperatures increased the impact of this drought. 

Getting back to the question, does a wet monsoon follow a dry winter? Yes, but not uniformly across the Southwest and not consistently over time. Because we do not fully understand what controls sequences in seasonal precipitation, we cannot rule out a return to a period of more dual season (both cool season and monsoon) pluvials or droughts, as in the early decades of the 20^th century. If the climate system returns to one characterized by dual season droughts, particularly under warmer conditions, impacts to a region with already limited surface water supplies and significant wildfire risks would be a challenge to resource management.