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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 2007
Introduction • 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
Objectives • 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)
Methodology • 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)
Results 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)
Onset (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) PRECIPITATION: 12Z 05 Sep - 12Z 06 Sep 2007 (CPC-NOAA)
Conclusions • 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)