ENSO and Monsoon

How does El Niño/Southern Oscillation (ENSO) affect the monsoon in the Southwest?

Key Takeaways

• ENSO strongly influences cool-season precipitation (Oct–Mar) in the Southwest U.S. (Arizona and New Mexico): El Niño years are generally wetter and La Niña years drier across most of the region and over much of the instrumental record (starting in 1896) .
• Monsoon precipitation (Jul–Sep) shows weak and inconsistent links to ENSO, with high variability across the Southwest and over time.
• A limited recent signal (during the period 1980–2022) suggests somewhat drier monsoons during El Niño years over parts of the Southwest, and especially the Rio Grande region of New Mexico.
• Statistically significant ENSO relationships are common in the cool season but rare for the monsoon, reflecting a clear seasonal contrast in ENSO influence.

How ENSO historically relates to Southwest Precipitation

Image

While an association between cool season precipitation and ENSO has been documented across the Southwest, similar associations are not readily evident for monsoon season precipitation. 

The graphic to the left shows the spread of cool season (October-March, set of bars on the left in each panel a-f) and monsoon season (July-September, set of bars on the right in each panel) precipitation coinciding with El Niño (warm ENSO, panels a-c) and La Niña (cool ENSO, panels d-f) years. The colored bars in each panel represent the five sub-regions in the Southwest (see map, below).  The three rows show result for three time periods (top: 1896–1937; middle, 1938–1979; bottom, 1980–2022). 

Strong ENSO Signal in the Cool Season

In El Niño years, and in the most recent period (panel c, 1980-2022), all regions show wet October-March conditions in most years.  Precipitation values are show as percentiles, with the 50^th percentile (0.5) indicated by a horizonal line. Values above the 50^th percentile are wetter than median, and values below are drier.  Each box shows the range of values for all El Niño years for a given time period and region.  For example, the dark blue box in first cluster of boxes in panel c shows that in the Rio Grande region, all years but one (the dot near the 0.5 line) for the years 1980-2022 were wet, and the median value for all of these years was a bit above the 80^th percentile.  El Niño years also tended to correspond to wet cool seasons in the earliest period (panel a, 1896-1937), but precipitation was more variable during El Niño years in the middle period (panel b,1938-1979). 

 In La Niña years, cool season precipitation has tended to be on the dry side, across all regions and time periods (panels d, e, and d), but this has been variable. Median precipitation values are below the 50^th percentile line in all cases, but the boxes also extend above the 50^th percentile line, indicating that a number of years in almost every region experience above median cool season precipitation during La Niña years.

Weak and Variable ENSO–Monsoon Relationship

The results for ENSO and monsoon season precipitation are much different. For the most part, during El Niño and La Niña years, precipitation is highly variable over these years and across the regions.  The strongest suggestion of a relationship between monsoon precipitation and ENSO is for the most recent period, for El Niño years (panel c).  During these years, median precipitation is below the 50^th percentile line in four of five regions (all except the Colorado Plateau), and most markedly so for the Rio Grande region. 

Seasonal Connections and Regional Differences

The red triangles at the bottom of the panels summarize the strongest relationships between ENSO and seasonal precipitation across the Southwest. They indicate regions and periods with statistically significant wetter or drier conditions during warm or cool ENSO years, highlighting how these can vary over space and time. Cool season conditions were significantly wetter in El Niño years and drier in La Niña years, across 3-5 region over all three periods, except for El Niño years in the middle period. In contrast, for the monsoon season, there is only one region, for one period (Rio Grande region in the most recent period) with statistically drier conditions during El Niño years.  There were no significant associations between monsoon precipitation and La Niña years.

It is important to note that these results are based on ENSO conditions in the equatorial Pacific during the cool season, as this is typically when ENSO is the strongest. These conditions often persist into the spring.  The state of ENSO during the monsoon season has been shown to influence aspects of the North American Monsoon system—for example, the strength and position of the monsoonal atmospheric ridge—but that influence is often overwhelmed by other factors that influence monsoon variability. Sea surface temperatures outside the ENSO region—in the Gulf of California, in the North Pacific, and Gulf of Mexico—influence the supply of atmospheric moisture into the Southwest during the monsoon.

This discussion is based on Woodhouse et al. (2025), which analyzes over a century (1895–2022) of precipitation data across five Southwest subregions to assess variability in cool-season and monsoon precipitation, how these seasonal patterns evolve over time, and their relationships with ENSO and drought. 

Woodhouse, C. A., M. A. Crimmins, and M. D. Meko. 2025. “The Role of Seasonal Precipitation Sequences in Shaping the Climate of the United States Southwest.” International Journal of Climatology 45, no. 15: e70138. https://doi.org/10.1002/joc.70138.

Also see the related Fact Sheet:

https://climas.arizona.edu/sites/default/files/2025-11/CLIMAS-SW-Climatology-Fact-Sheet-Final.pdf