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
P re c ip ita tio n (m m )
Seasonal rainfall Index (JJAS )
C o re
700
UW
600
500
400
300
1950
1960
1970
1980
1990
2000
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
M e a n P 9 5 C h a n g e (m m )
10
CM I
T r e n d : 0 .6 3 m m /d e c
5
0
-5
1950
1960
1970
1980
1990
2000
Significant increase in the intensity of extreme
events, but not in the frequency
Based period: 1961-1990
P95 Seasonal Contribution (%)
P 9 5 C o n trib u tio n (% )
50
CM I
40
30
TC
T r e n d : 1 .5 % /d e c
20
10
0
1950
1960
1970
1980
1990
2000
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
P 9 5 C o n trib u tio n (% )
50
40
M T N: No n T C
M T N: T C
30
20
10
0
1950
1960
1970
1980
1990
2000
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)
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)
+
WHWP, SST > 28.5 oC
(Wang et al. 2006)
Thermal gradient (> 1oC)
TC highest frequency: September
non TC highest frequency: Jul-Aug
Forcing mechanisms
(C) TC: air Tan (oC),
Large land-sea thermal
gradient (> 1oC)
t=-5d
(D) TC: V850an (m s-1),
t=1d
Onset
(E) TC: OLRan (Watts m-2), t=1d
(F) TC: air Tan (oC),
t=1d
August 2007
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)
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Clase: Teoria del Clima