regional & global scale patterns associated with rainfall anomalies in israel baruch ziv uri...
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REGIONAL & GLOBAL SCALE PATTERNS
ASSOCIATED WITH
RAINFALL ANOMALIES IN ISRAEL
Baruch ZivUri DayanChaggi Rot
Yohanan Kushnir Yehouda Enzel
Ziv et al (2006) Regional and Global Atmospheric Patterns Governing Rainfall in the Southern Levant, Int. J. Clim., 26
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MOTIVATION
•The sensitivity of the region to water shortage •The linkage of the rainfall to single synoptic system facilitates the analysis
Enzel et al. (2003) showed that the Dead-Sea level responds to Israel rainfall. Therefore:•If tele-connection is found between Israel rainfall and the conditions in other regions, the Dead Sea level can serve as proxy for past climate variations for these regions
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12 stations with continuous record were selected
2/3 of the annual rainfall is obtained in DJF
Therefore, each season is represented by DJF rainfall
Study period: 1950-2002
The study area and the rain stations used
Average annual rainfall
Mediterranean Sea
Dead Sea
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Most of the rainfall results from Cyprus Lows
Their signature appears in the Long-Term Mean SLP
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Cyprus Low is most pronounced in composite maps for days with ~50 mm
in Israel
Dates
500 hPa SLP
The low is accompanied by pronounced upper-trough
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Difference(wettest-driest)
Main Feature: WE-EM dipole
Seasonal Signature: composite SLP maps
10 wettest years – Cyprus Low dominates
10 driest years – Cyprus Low is hardly
discerned
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Composite 500-hPa gph
Difference(wettest-driest)
Main Feature: WE-EM dipole
10 wettest years – Main trough at East-
Med
10 driest years – Main trough at Central Med
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Correlation maps of Israel rainfall (DJF) with:
700 mb temp
500 mb gph
SLP
Main Feature: WE-EM dipole!
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The correlation patterns resemble the “Mediterranean Oscillation” (Dunkeloh and
Jacobeit, 2003)
Similar dipole in the upper-level
The SLP resembles the AO
Associated rainfall anomaly:
negative over the MG,
except for the SE Med
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Correlation with known global circulation
Level Period Location of Dipole
Name Corr
700 hPa 1950-2002
England and – Caspian Sea
E Atlantic/ W Russia(EA-WR)
0.51
500 hPa 1958-1998
North Sea - N. Caspian Sea
North Caspian Pattern (NCP)
0.40
700 hPa 1950-2002
Europe - NE China
Polar/ Eurasian 0.30
SST 1950-2002
Tropical Pacific
ENSO 0.29
700 hPa 1979-2002
Gulf of Alaska-Hudson Bay
Tropical North Hemisphere
-0.24
Sea Level
1950-2002
Azoreans-Iceland
North Atlantic Oscillation (NAO)
0.09
Kutiel&Benaroch
Price et al. Ben Gai, Eshel The E coast of Med’s rainfall is not
determined by NAO
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-0.4
-0.4
-0.6
Correlation with 925 hPa temperature
Prominent remote feature: Pacific Warm Pool
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Global 50 hPa zonal-wind correlation with Israel DJF rainfall
Band-anomalies along polar stratospheric jets
in both N & S hemispheres
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Height-latitude correlation cross-section of zonally averaged zonal wind with Israel DJF
rainfall
The stratospheric anomalies at both sub-polar regions are prominent
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Correlations farther from Israel are weaker, but remote distinct patterns are discerned:
• North and South stratospheric polar jets
• SST over the tropical & subtropical W Pacific
Suspecting that direct relationships between them and Israel exist, we searched for an indirect, through the W-Europe-E-Med dipoleWe constructed a time series for each factor and derived 500 hPa gph correlation map, searching for a fingerprint of the WE-EM dipole
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500 hPa gph Correlation maps for DJF with
In all of them the WE-EM dipole appears
S Polar JetPacific Warm Pool
N Polar Jet
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Explanations for the indirect tele-connections
N Polar JetGraf et al. (1994) found this relationship, suggested that this wave is enhanced by the increase in stratospheric reflectivity when the jet intensifiesIndo-Pacific Warm PoolIncrease in SST at certain region along the ITCZ enhances the Hadley Circulation there, subsequently the subtropical ridge downwind, such as over W Europe, with the associated enhanced EM troughS Polar JetNo explanation exists
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Nino 1+2
NAO
In both the WE-EM dipole is insignificant
500 hPa gph correlation maps for other global circulations previously suggested to
affect Israel rainfall:
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Correlation between global-scale systems and the EM
upper-trough
EMUppertrough
North Polar Jet
R=-0.50
South Polar Jet
R=-0.38
Pacific Warm Pool
R=-0.38R=0.41
R=0.66R=0.61
All factors - EM trough R = 0.69
EM trough–Israel rainfall R = 0.74
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CONCLUSIONS
The EM upper-trough explains > 0.54 of the DJF rainfall variation in the N half of IsraelIt is activated by 3 external factors: N Polar jet ; S Polar jet ; Indo-Pacific Warm Pool
The EM trough is combined with ridge over W Europe, hence:Cold & wet winters in south Levant coincide with warm & dry winters in W Europe and vice versa
No significant tele-connection exists between Israel rainfall and NAO or ENSO, direct or indirect
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SPECULATION
Since the Dead-Sea level reflects the rainfall variations in Israel, the anti-correlation between Israel & W-Europe in
the winter suggests that:
Under similar climatic regime periods of rise in the Dead-Sea level indicates
retreat of icebergs in Western Europe and S-West Scandinavia
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10 wettest years
10 driest years
250 hPa speed of composite
windThe Atlantic & African jets are more
detached in wet years
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Stratospheric Polar Jet (50 mb)
Normal
10 Dry Years
10 Wet Years
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Fig. New: Tropical-Subtropical moisture transport1. Enhanced Hadely circulation over W-africa – positive, over
E-Africa – negative2. Wave No. 2 seems to play a role
400 mb
600 mb850 mb
Correlation maps with Spec. Humidity
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Another global correlation map with Israel DJF rainfall - 250 hPa meridional wind
Note the dominance of wave 5 in NH and 4 in SH
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Fig. 4: Latitudinal-height cross-section of zonally averaged geopotential height anomaly over 80°S-80N – 1000-50 hPa domain for: a) the 10 driest seasons and b)
the 10 wettest seasons
a
b
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Height-latitude cross-section of the correlation between the
zonal average of geopotential height and rainfall in Israel
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Correlation with jets 150 mb North STJ
50 mb S- PJ