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

Core Monsoon 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 Introduction

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 Objectives

Data (JJAS, ) Daily precipitation from 39 stations from SMN/IMTA (Eric III) Daily gridded precipitation (1/8 o 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 There is not a long term significant linear trend Seasonal rainfall Index (JJAS ) JJAS P = 490 mm JJAS P = 481 mm

Change in the intensity of P95 Significant increase in the intensity of extreme events, but not in the frequency Based period:

P95 Seasonal Contribution (%) In 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 (%) 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 Mtn: > 500 m ASL

TC-derived extreme rainfall JJAS: UW gridded precipitation (mm d - 1) Between 1980 and 2003, Sinaloa was the second most affected state, after Baja California, by TCs (CNA, 2004)

Forcings: surface Tan ( o C) - + (A) TC: surface Tan ( o C) (B) non TC: surface Tan ( o C) One week before onset of extreme events in the core monsoon Thermal gradient (> 1 o C) WHWP, SST > 28.5 o C (Wang et al. 2006) TC highest frequency: September non TC highest frequency: Jul-Aug

Forcing mechanisms (C) TC: air Tan ( o C), t=-5d (D) TC: V850an (m s -1 ), t=1d Large land-sea thermal gradient (> 1 o C)

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

August 2007

PRECIPITATION: 12Z 05 Sep - 12Z 06 Sep 2007 (CPC-NOAA) Hurricane Henriette (Cat 1) 5 Sep 2007 Hurricane Felix (Cat 5) (mm d -1 )

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)