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Variability of Extreme precipitation Events in the Core of the North American Monsoon. Tereza Cavazos 1 Cuauhtémoc Turrent 1 Dennis P. Lettenmaier 2 1 CICESE , 2 University of Washington. Fourth Symposium on Southwest Hydrometeorology Tucson, AZ, 20-21 September 2007. 2007 .

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Variability of Extreme precipitation Events in the Core of the North American Monsoon

Tereza Cavazos1

Cuauhtémoc Turrent1

Dennis P. Lettenmaier2

1CICESE, 2University of Washington

Fourth Symposium on Southwest Hydrometeorology

Tucson, AZ, 20-21 September 2007




  • Semiarid climate
  • Annual P < 700 mm y-1
  • Important producers of grains,
  • vegetables, grasses, and cattle
  • Maximum precip variability
  • (Gutzler 2004)
  • Persistent droughts and heavy
  • rains  impact in agriculture
  • and water availability

Core Monsoon


Core Monsoon

  • Last two decades: strong floods
  •  Severe damage (Bitrán, 2001), but also
  • benefits recharge of major dams (CNA, 2004)
  • Annual increase in heavy rains (P95, P99)
  • (Groisman et al. 2005, Alexander et al. 2006)
  • Future projections:
    • Increase in aridity and much less water
    • availability (IPCC, 2007; Seager et al. 2007)
    • Changes of extremes will be more important
    • than changes in mean precipitation


  • To investigate trends in extreme
  • precipitation events (P95) in the
  • core monsoon
  •  Monsoon derived extremes (non TC)
  •  Tropical cyclone derived extremes (TC)
  • To examine the role of the
  • land-sea thermal contrast in the
  • initiation of extreme events

Data (JJAS, 1950-2003)

  • Daily precipitation from 39 stations from SMN/IMTA (Eric III)
  • Daily gridded precipitation (1/8o resolution) from the UW
  • Eastern Pacific hurricane tracks – 550 Km from the monsoon (Unisys)
  • Daily composites of atmospheric variables from NCEP Reanalysis (NOAA/CDC composites web page)
  • Weekly SSTs from in situ and satellite data (OISSTV2, Reynolds et al. 2002)


  • Quality control of 39 station data from ERIC III
  • Daily and seasonal index of the core monsoon (CMI)
  • Daily precipitation extremes (top 95% of wet days) Thresholds of P95:
        • CMI: 14.5 mm d-1
        • Coastal stations (0 – 500 m ASL): 50 mm d-1
        • Mountain stations (>500 m ASL): 42 mm d-1
  • Extremes derived from TC rainfall and from monsoon rainfall (non TC)
  • Trends, statistical significant changes (p < 0.05): Mann-Kendall test (Frequency, intensity, and seasonal contribution of extreme events)


Seasonal rainfall Index (JJAS )

There is not a long term significant linear trend

JJAS P61-90 = 490 mm

JJAS P77-03 = 481 mm


Change in the intensity of P95

Significant increase in the intensity of extreme events, but not in the frequency

Based period: 1961-1990


P95 Seasonal Contribution (%)

In 1980-2000 there were 16 TCs that affected the core monsoon and

5 made landfall:

Hurricane Paul in 1982

and Hurricanes Lydia, Ismael, Fausto, and Isis in the 1990s


Mtn P95 Contribution (%)

Mtn: > 500 m ASL

The total seasonal % contribution of P95 in mountain sites shows a significant increase of 1.5% per decade

Coastal stations did not show any significant changes


TC-derived extreme rainfall

JJAS: 1981-2003

UW gridded precipitation (mm d-1)

Between 1980 and 2003, Sinaloa was the second most affected state, after Baja California, by TCs (CNA, 2004)


WHWP, SST > 28.5 oC

(Wang et al. 2006)

Forcings: surface Tan (oC)

One week before onset of extreme events in the core monsoon

(A) TC: surface Tan (oC) (B) non TC: surface Tan (oC)



Thermal gradient (> 1oC)

TC highest frequency: September non TC highest frequency: Jul-Aug


(C) TC: air Tan (oC), t=-5d (D) TC: V850an (m s-1), t=1d

Forcing mechanisms

Large land-sea thermal gradient (> 1oC)



(E) TC: OLRan (Watts m-2), t=1d (F) TC: air Tan (oC), t=1d


Hurricane Henriette (Cat 1) 5 Sep 2007

Hurricane Felix (Cat 5)

(mm d-1)


12Z 05 Sep - 12Z 06 Sep 2007




  • Intensity and seasonal contribution of extreme events in the core monsoon have increased significantly
  • Especially TC-derived extremes and in mountain sites
  • Frequency of extremes and seasonal rainfall do not show a significant linear trends
  • Extreme events in coastal stations do not show significant changes
  • TC-derived extreme events are characterized by
    • La Niña-like conditions
    • Strong land-sea thermal contrast near the study area, and
    • Large Western Hemisphere warm pool

Ongoing Work

  • Numerical investigation of the land-sea thermal contrast (MM5)
    • Indices to characterize the intensity of monsoon onset (C. Turrent)
    • Seasonal predictability of the monsoon based on soil moisture (Zhu et al.)
    • Sensitivity analyses of different monsoon years (C. Turrent, Zhu et al.)
  • Climate projections of extremes for the 21st Century (S. Arriaga)