1. Monsoon Onset Dates on the Tohono O'odham Nation Josef Marlow Tohono O'odham Community College

2. Develop local (TON) monsoon onset dates for climatology

3. North American monsoon system ● regional atmospheric circulation over sw NAm in JJAS ● 500 mb subtropical ridge moves northward into TX & NM ● Winds shift from westerly to southerly & easterly, causing an influx of moisture ● Thermal low and “Gulf surges” also important sources of moisture ● Moisture influx leads to convection and precipitation ● Areas along the W coast of mainland Mx between SMO and coast receive 60-70+% of annual precip during monsoon, TON receives 50% ● SW AZ on northern fringe of NAMS → variability

4. Onset Criteria for NAMS ● Ellis et al (2004) – 3-day, area avg. dew point temp (50°F) and measureable precip (20% of reporting stations), 1 st day ● Xeng & Lu (2004) – 3-day, normalized precipitable H 2 O index > 0.618 in 7 of 9 centered 1°x1° grid cells, 1 st day ● Higgins et al (1997) - 3-day, area avg. precip.>0.5mm, 3 rd day ● NWS TFO- 3-day, 54°F avg. dew point temp., one station, 1 st day ● NWS PFO- 3-day, 55°F avg. dew point temp., one station, 1 st day

5. Data ● NCEP North American Regional Reanalysis (NARR) – daily dataset, 32km grid cells, 1979-2002, data from DVD ● Surface precipitation, temp. at 2m, rel. hum. at 2m ● NOAA CPC (Climate Prediction Center) - 0.25°x0.25° Daily U.S. Unified Precipitation Data, 1979-2002 ● Only U.S. coverage ● NOAA NCDC (National Climatic Data Center) - U.S. Summary of Day, Dew Point Temperature, 1994-2002 ● 1°x1° bins of surface meteorological station data

9. Method for determining monsoon onset date ● calculate precipitation index = daily area average precipitation ● Precipitation index threshold = 0.5 mm/d ● Monsoon onset date is the first date the threshold is met or exceeded for 3 consecutive days after June 1

12. TON Monsoon Onset Dates Calculated from NARR

14. Comparison of NARR and CPC Onset Dates for TON

15. Red = CPCGreen = NWSTFOBlue = NARR

17. Summary ● TON monsoon onset dates determined using precipitation index and 3-day threshold ● First day of 3-day period would be improvement ● Helpful to use dew point temperature to eliminate “early precipitation events” not related to monsoon, especially in larger area ● Would be useful to have dew point temp. in daily NARR dataset ● TON onset dates were used to prepare onset +/- 60 day datasets for climatology study