prolonged dry episodes over north america during the last 40...
TRANSCRIPT
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Prolonged Dry Episodes Over Northern Eurasia and North
America: New Tendencies Emerging during the Last 40 Years
Pavel Ya. GroismanUCAR at National Climatic Data Center, Asheville, NC, USA
Richard W. KnightSTG, Inc., Asheville, North Carolina, USA
and
Vyacheslav N. RazuvaevAll-Russian Institute for Hydrometeorological Information,
Obninsk, Kaluga Area, Russian Federation
NEESPI/LCLUC Science Team Meeting with focus on the dry land area of Northern Eurasia, Urumqi, China, 16-21 Sept., 2007
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Plan of the talk
1. Summer dryness, obsevational evidence2. Prolonged dry episodes across Northern
Eurasia and North America
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Summer Dryness. Theoretical expectations
• Manabe, S., R.T. Wetherald, and R.J. Stouffer, 1981: Summer dryness due to an increase of atmospheric CO2 concentration. Climatic Change, 3, 347-386
• Manabe, S., R. T. Wetherald, P. C. D. Milly, T. L. Delworth, R. J. Stouffer, 2004: Century-scale change in water availability: CO2 quadrupling experiment. Climatic Change, 64, 59-76.
Everything else will be empirical evidence
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Seasonal changes in zonal temperature differences between tropics and mid-latitudes
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Changes in glacier volume (km3) since 1960
-5000
-4000
-3000
-2000
-1000
0
1960 1970 1980 1990 2000 2010
N.HemisphereAlaskaHigh Mountain Asia
cum
ulat
ive
volu
me
chan
ge, k
m3
Source: http://www.nsidc.org
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Spring snow cover over Eurasia
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Frequency of days with thaw over Alaska, Canada, and the fUSSR north of 50°N
a 20% (winter) to 40% (autumn) increase during the second half of the 20th century
Thaw: daily T >-2°C; Snow depth > 0)
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Warm season duration (T > 5°C) over the conterminous US (dots), southwestern US, (rhombs), and northwestern U.S. (squares)
Warm Season Duration
120
140
160
180
200
220
240
260
280
300
1960 1970 1980 1990 2000 2010Years
Day
s
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Changes in derived-temperature characteristics over Northern Eurasia during the past 54 years (within the former USSR boundaries; period 1951-2004)
All trend estimates are statistically significant at 0.01 or higher levels
Characteristic Trend estimates, %/54 yrs Former USSR Siberia & Russian Far
East south of 66.7°N
Heating-degree days -7 -7 Degree-days below 0ºC -15 -14Degree-days above 15ºC 11 20Duration of the growing season
T> 10ºC 9 14T> 5 ºC 8 12
Duration of the frost-free period 8 11
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Major permafrost-free zone of the Russian Federation and its
precipitation changes
Groisman and Rankova 2001
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Eastern Siberia.Regionally averaged January and July precipitation
prior and after bias correctionEaster n S iber ia , no rth o f 5 5 N
0
2 0
4 0
6 0
8 0
10 0
12 0
192 0 194 0 196 0 1980 200 0 2020Years
Mon
thly
pre
cipi
tatio
n, m
m
Jan .Raw Ja n_co rr July.co rr July.Raw
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Seasonal frequency of occurrence of daytime Cumulonimbus and Cumulus
Sun et al (2001, J. Climate)
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Regions in the Central U.S. where statistically significant annual increases in very heavy precipitation were reported and counts of
the upper 0.3% of daily rain events (with return period of 4 years)
00.10.20.30.40.50.60.70.80.9
1
1880 1900 1920 1940 1960 1980 2000 2020years
Day
s w
ith p
reci
pita
tion
abov
e 99
.7%
-ile
Linear trends (1893-2006 and 1967-2006) are 22%/114yrs and 27%/40yrs respectively
Update of Groisman et al. 2005
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Trends in number of extreme precipitation days derived from 95th percentile
More Extreme events
(Zhai et al. 2007)
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Thus, in high&mid latitudes we have:• Up to two-digit (%) increase in temperature
derivatives evapotranspiration may ↑ ;• Earlier snowmelt & more frequent thaws
more cold season precipitation become unavailable in the warm season;
• Only moderate increase in precipitation but increase in thunderstorm activity more warm season precipitation goes into runoff;
• All the above possibility of drier summer conditions increase in forest fire danger.
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DYNAMICS OF FIRES NUMBERS AND BURNED AREA (PROTECTED TERRITORY OF RUSSIA)
Korovin and Zukkert 2003, updated
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Potential Forest Fire Danger Increase. Russian Far East south of 55°N
Groisman et al. 2007
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May-July drought index for the major cereal-producing region Southwestern Siberia & Northern Kazakhstan
(Mescherskaya & Blazhevich, 1997, updated to 2006)
-60-40-20
020406080
1860 1900 1940 1980 2020
Perc
ent
Linear trend: 16%/100yrs
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Rationale to look on the last 4 decades
• a disproportional increase in precipitation coming from intense rain events
• an extension of the vegetation period with intensive transpiration
• an insignificant change in total precipitationAll the above could lead to prolonged
periods without precipitation (even when seasonal rainfall totals increase)
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Summer frequency of wet days and days with heavy rains. Asian part of Russia
Sun and Groisman 2000
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Frequency of rainy days over the northeastern quadrant of the
contiguous United States (both show a decrease during the past 30 years).
• Observations(Northeast and Midwest regions) for 1908-2002.
• ECHAM4 (35N-45N; 75W-85W;adapted from Semenov & Bengtsson 2002)10 year running mean values for1900-2090. 2000
This picture is enlarged in the next slide.
Groisman et al. 2005
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Rainy days over the northeastern quadrant of the U.S.
2000
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Frequency of rainy days over the northeastern quadrant of the
contiguous United States (both show a decrease during the past 30 years).
• Observations(Northeast and Midwest regions) for 1908-2002.
• ECHAM4 (35N-45N; 75W-85W;adapted from Semenov & Bengtsson 2002)10 year running mean values for1900-2090. 2000
Groisman et al. 2005
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Lengthy intervals of “dry” days without sizeable rain during the warm
season (when daily T > 5°C)
We assess the following quantities:• Fraction of the warm season (%) with dry
day intervals above 20, 30, 60, 90 days, etc.• Number of days within these dry intervals
during the warm season
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Station map and regional partition used
Southwestern U.S.
Eastern U.S.
Gulf
North Mexico
SE Canada
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Station map and regional partition used
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Climatology.Percentage of the warm season included in the prolonged dry days episodes of (a) 30 and more days; (b) the warm season
duration (days with mean daily T > 5°C).
ba
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What does it mean, when 2% of the warm season is occupied by dry episodes that
are 30 or more days long?
• Example: warm season = 200 days therefore during the past 55 years, we observed: 200 x 55 x 0.02 = 220 such days or maximum 7 such dry episodesduring the past 55 years with return period of ~ 8 years ([220/30] = 7).
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Distribution function of the prolonged dry interval durations above 30 (Eastern U.S.)
and 60 days (Southwestern U.S.).
Eastern U.S.
0
5
10
15
20
25
30
30 40 50 60Duration of the dry episode, days
f(x),
perc
ent
Southwestern U.S.
0
5
10
15
20
25
30
60 70 80 90 100 110 120Duration of the dry episode, days
f(x),
perc
ent
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Fraction of the dry day episodes with 1-month or more length during the warm season during the past 40 years
area-averaged over the Eastern United States
Red dashed line: linear trend (1.1 % per 40 years)
0
2
4
6
8
10
1960 1970 1980 1990 2000 2010Years
Perc
ent
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Fraction of the dry day episodes with 2-month or more length during the warm season during the
past 40 years area-averaged over the southwestern United States (dots), CA and NV (triangles), and TX, OK, NM, and AZ (squares)
Red dashed line: linear trend (4.3 % per 40 years)
0
10
20
30
40
50
60
1960 1970 1980 1990 2000 2010Years
Perc
ent
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Northern Mexico, fraction of strings of dry days longer than 60 days
Red dashed line: linear trend (9.4% per 40 years)
0
20
40
60
80
1960 1970 1980 1990 2000 2010Years
Perc
ent
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Red dashed line: linear trend (7.3% per 40 years)
020406080
1960 1970 1980 1990 2000 2010Years
Perc
ent
Gulf coast of Mexico, fraction of strings of dry days longer than 30 days
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Southeastern Canada, fraction of strings of dry days longer than 20 days
Red dashed line: linear trend (1.3% per 40 years)
02468
10
1960 1970 1980 1990 2000 2010Years
Perc
ent
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30 and above days in dry episodes (20 for SE Canada)
60 and above days in dry episodes
Regions where dry episode frequency is increasing during the past 40 years
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Lowland Central Asia, fraction of strings of dry days longer than 50 days
0
20
40
60
80
1940 1950 1960 1970 1980 1990 2000 2010Years
Perc
ent Poor data
coverage
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Mountainous Central Asia, fraction of strings of dry days longer than 50 days
0
20
40
60
80
1940 1950 1960 1970 1980 1990 2000 2010Years
Perc
ent
Poor data coverage
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Russia east of 85°E, south of 55°N
Dry episodes above 30 days during the warm seasonPlus several droughts (severe fire weather conditions in the 2000s)
-1
1
3
5
7
9
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005Years
Perc
ent
Linear trend 0.9% per 40 yrs; R2 = 0.13
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Regions with more humid conditions (blue), regions where potential forest fire danger has increased in the 20th century (red), the region where agricultural droughts have increased (circled), and the region where prolonged dry episodes have
increased (rectangled).
Major wheat producing
area in Northern Asia
Changes in the surface water cycle listed below have been statistically significant in the 20th century
Mescherskaya & Blazhevich (1997), Dai et al. (2004), Zhai et al. (2005), Groisman et al. (2005, 2007),
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SummaryDuring the past four decades over the contiguous US:
– the duration of prolonged dry episodes has significantly increased over the Eastern and Southwestern United States.
• For the same period, the increase in the frequency of prolonged dry episodes has expanded to neighboring regions of Mexico and Canada
• Over the southern part of Asian Russia prolonged 1-month-or-longer dry episodes has significantly increased.
• The changes are a new phenomenon, are observed on the background of the relatively “wet” period, and is consistent with the notable change in rainfall rate distribution (increase in intense rainfall frequencies while mean precipitation grows slower or does not increase at all (e.g., Southern Siberia).
• Studies of this type are possible only in the regions with a dense daily precipitation network.