Progress of the North American Monsoon as Indexed Vapor Influxes across the Arizona/New Mexico Border with Mexico

Michael Dettinger, US Geological Survey, San Diego, CA,

The movie below shows lag correlations between a daily time series of northward water-vapor transport rates across the US/Mexico border (along 32.5N between 115W and 105W) and daily station precipitation rates from 7 days earlier to 5 days later, for Augusts from 1948-2000. Blues indicate positive correlations (wet when the northward vapor transport is high); reds indicate negative correlations. Transport rates are vertical (mass-weighted) integrals (through the whole troposphere) of the products of NCAR/NCEP Reanalysis values of specific humidity (q) and northward wind speed (v); precipitation rates are from Eischeid et al. (2000) serially-complete station records.

Notice the gradual build up and decline of precipitation in the interior West (beginning perhaps 4days and ending about 7 days after in these correlations). Perhaps more interestingly, notice the pronounced reductions in precipitation across Texas and the southeastern corner of the precipitation data that is associated with the day-0 influxes.

Click on figure to see at full scale.

The still figure below shows correlations between June-September totals of the water-vapor influx series and June-September precipitation totals in the western United States. Clearly, the relation between vapor influx and precipitation at the daily level (above) is carried over and imprinted strongly on the influx-vs-precipitation relations at the seasonal timescale (below).

The seasonal (June-September totals) time series of water-vapor influxes northward across the Mexican border with Arizona and New Mexico used to derive these figure can be obtained by clicking here.