Variability of Extreme precipitation Events in the Core of the North

American Monsoon

Fourth Symposium on Southwest HydrometeorologyTucson, AZ, 20-21 September 2007

Tereza Cavazos1

Cuauhtémoc Turrent1

Dennis P. Lettenmaier2

1CICESE, 2University 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, 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)

300

400

500

600

700

1950 1960 1970 1980 1990 2000

Prec

ipita

tion

(mm

)

CoreUW

Results

There is not a long term significant linear trend

Seasonal rainfall Index (JJAS )

JJAS P61-90 = 490 mmJJAS P77-03 = 481 mm

-5

0

5

10

1950 1960 1970 1980 1990 2000

Mea

n P9

5 C

hang

e (m

m) CMI

Trend: 0.63 mm/dec

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 (%)

0

10

20

30

40

50

1950 1960 1970 1980 1990 2000

P95

Con

tribu

tion

(%) CMI

TCTrend: 1.5 %/dec

In 1980-2000 there were 16 TCs that affected the core monsoon and 5 made landfall:

Hurricane Paul in 1982and Hurricanes Lydia, Ismael, Fausto, and Isis in the 1990s

Mtn P95 Contribution (%)

0

10

20

30

40

50

1950 1960 1970 1980 1990 2000

P95

Con

tribu

tion

(%) MTN: Non TC

MTN: TC

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 rainfallJJAS: 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)

- +

(A) TC: surface Tan (oC) (B) non TC: surface Tan (oC) One week before onset of extreme events in the core monsoon

Thermal gradient (> 1oC)

WHWP, SST > 28.5 oC (Wang et al. 2006)

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

Forcing mechanisms

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

Large land-sea thermal gradient (> 1oC)

Onset

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