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

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

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

Most of the rainfall results from Cyprus Lows

Their signature appears in the Long-Term Mean SLP

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

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

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

Correlation maps of Israel rainfall (DJF) with:

700 mb temp

500 mb gph

SLP

Main Feature: WE-EM dipole!

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

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

-0.4

-0.4

-0.6

Correlation with 925 hPa temperature

Prominent remote feature: Pacific Warm Pool

Global 50 hPa zonal-wind correlation with Israel DJF rainfall

Band-anomalies along polar stratospheric jets

in both N & S hemispheres

Height-latitude correlation cross-section of zonally averaged zonal wind with Israel DJF

rainfall

The stratospheric anomalies at both sub-polar regions are prominent

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

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

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

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:

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

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

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

10 wettest years

10 driest years

250 hPa speed of composite

windThe Atlantic & African jets are more

detached in wet years

Stratospheric Polar Jet (50 mb)

Normal

10 Dry Years

10 Wet Years

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

Another global correlation map with Israel DJF rainfall - 250 hPa meridional wind

Note the dominance of wave 5 in NH and 4 in SH

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

Height-latitude cross-section of the correlation between the

zonal average of geopotential height and rainfall in Israel

Correlation with jets 150 mb North STJ

50 mb S- PJ