Analysis of Observed and Projected Interannual Variability of North American Monsoon Precipitation

The North American Monsoon provides 40-70% of the annual precipitation in many regions of the southwest, making it critically important for ecology, water resources, infrastructure, economic well-being, and quality of life. Previous research has shown that the southwest is projected to become more arid throughout the 21st century. However projections of monsoon precipitation are highly uncertain, and in many projections the magnitude of future trends is small compared to interannual variability. We carry out an analysis of monsoon precipitation in the southwestern U.S. to assess mechanisms of variability on interannual and climate change time scales by examining the covariance of anomalies of summer precipitation with concurrent and colocated anomalies of near surface air temperature, 500 hPa height, and lower tropospheric specific humidity.

In both observations and CMIP5 models for the historical period, we confirm that monthly precipitation anomalies are significantly negatively correlated with temperature and 500 hPa height anomalies, and positively correlated with specific humidity anomalies. Specific humidity is more strongly correlated with precipitation than is 500 hPa height, suggesting that the strength of moisture convergence into the arid southwest (so-called local forcing) is more important for modulating precipitation than the remote forcing associated with large-scale subsidence over the region.

We then examine the same statistics in CMIP5 projections of 21st Century climate generated by multiple models using the RCP8.5 greenhouse gas scenario. Temperature, geopotential height, and specific humidity all increase with time. However, summer precipitation remains nearly unchanged in 30-year averages spanning the 21st Century. The local and remote mechanisms modulating interannual variability are preserved in each 30-year epoch, with the local mechanism being more dominant, coinciding with results derived from observations and historical simulations. Therefore, the local and remote mechanisms, as defined in our analysis, are preserved in interannual variability as the climate changes, but they do not modulate summer precipitation on long timescales.