Regional and Global Atmospheric Patterns Governing

8/9/2019 Regional and Global Atmospheric Patterns Governing

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INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 26: 5573 (2006)

Published online 2 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1238

REGIONAL AND GLOBAL ATMOSPHERIC PATTERNS GOVERNING

RAINFALL IN THE SOUTHERN LEVANT

BARUCH ZIV,a,* URI DAYAN,b YOHANAN KUSHNIR,c CHAGGI ROTHb and YEHOUDA ENZELb,d

a Open University of Israel, Ramat-Aviv, Israelb Department of Geography, The Hebrew University of Jerusalem, Israel

c Lamont Doherty Earth Observatory of Columbia University, USAd Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel

Received 3 August 2004

Revised 22 May 2005

Accepted 13 June 2005

ABSTRACT

This study attempts to find a linkage between the interannual variations of the rainfall measured in 12 stations spreadover the northern half of Israel (the southern Levant) and atmospheric circulations ranging from regional to global scale.The analysis was done for the midwinter months, December February, in which two-thirds of the annual rainfall occurs,during the years 19502002. The study is based on composite maps for extremely dry/wet seasons and on maps ofcorrelation between atmospheric variables and the rainfall time series.

Our results show that an upper-level trough extending from Eastern Europe toward the Eastern Mediterranean (EM)is closely linked with the seasonal rainfall over the study area. This is expressed by a correlation of 0.74 betweenthe 500-hPa geopotential height at 32.5 N, 35 E and the rainfall. This trough has two effects on the southern Levantrainfall: one is the dynamics implied by the upper trough and the other is the cool advection over the EM impartedby the northwesterly flows induced by the trough. The latter presumably enhances the atmospheric instability when itsweeps over the warmer waters of the EM. The upper trough was found to be associated with three global factors: thepolar stratospheric jets, in both the Northern and Southern Hemispheres, and the SST variations over the western tropical

Pacific Ocean, represented by the Pacific Warm Pool index.The EM trough is accompanied by a ridge covering western Europe, so that cold and wet winters in the southernLevant coincide with warm and dry winters over western Europe and vice versa. Copyright 2005 Royal MeteorologicalSociety.

KEY WORDS: teleconnection; correlation maps; Mediterranean climate; Levant; global circulations; rainfall variability

1. INTRODUCTION

The eastern and southern parts of the Mediterranean Basin, located along the boundary separating Mediter-

ranean climate from semiarid and arid climates, experience relatively high year-to-year precipitation variations

(Lockwood 1988; Alpert et al., 2002). The southern part of the eastern coast of the Mediterranean, i.e. the

southern Levant, suffers from water shortage directly related to precipitation deficiencies. The rainfall in thatregion is confined to the winter season, i.e. OctoberMay, as is common for Mediterranean climates, and

results mainly from passages of extratropical cyclones called Cyprus Lows (Sharon and Kutiel, 1986; Alpert

et al., 1990). From water resources point of view, rainfall deficits of 10% and 20% of the annual average are

considered a hydrological drought and an extreme drought, respectively.

Recent studies of Middle East history suggest that many dramatic demographic and cultural events owe their

occurrence to drastic climatic variations (Neumann and Parpola, 1987; Ken-Tor et al., 2001; Cullen et al.,

* Correspondence to: Baruch Ziv, Department of Natural Sciences, The Open University of Israel, Klausner 16, Tel Aviv, Israel;

e-mail: [email protected]

Copyright 2005 Royal Meteorological Society


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56 U. DAYAN ET AL.

33N

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34E 35E

Egypt

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Research PointsRain Volumemm

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Figure 1. The study area and the 12 rain stations used. The contours are isohyets of the annual rainfall

2002; Enzel et al., 2003). What we lack is a better record of past regional climatic variability. However,a long record, of over 2000 years, of the Dead Sea level has recently been recovered (Bookman et al.,

2004), and as Enzel et al. (2003) show, such a record is a reliable measure for the integrated rainfall over

the northern half of Israel (hereafter, the study area, Figure 1). It is therefore desirable to determine the

relationship between the winter regime over northern Israel and the broader regions and thus extend the use

of the Dead Sea level record as a proxy for the past climatic conditions over larger parts of the climate system.

Deepening the understanding of the mechanisms governing the annual variability of the rainfall in northern

Israel and its linkage to remote regions can thus help improve our understanding of past climatic variability.

In short, the interest in studying this region stems from the following reasons:

The high sensitivity of the water budget of Israel to interannual variations in the rainfall over the region;

The single synoptic situation favorable for the vast majority of the rain in the study region, the Cyprus Low

(described in Section 2), suggests that the rainfall anomalies can be explained by a distinct atmospheric

process; Since the Dead Sea level represents the integrated rainfall over our study area, if linkage is found between

the rainfall variations and the conditions in remote regions, the Dead Sea level would serve as proxy for

past climatic variations over these regions.

Many previous studies have dealt with the climatic variations over the EM, rainfall variability in particular,

and their relation to large-scale atmospheric circulations (e.g. Stanhill and Rapaport, 1988; Kutiel and Paz,

1998; Price et al., 1998; Ribera et al., 2000; Krichak et al., 2000; Eshel and Farrell, 2000, 2001; Maheras

et al., 2001; Ben-Gai et al., 2001; Kutiel and Benaroch, 2002; Kutiel et al., 2002). However, a comprehensive

explanation for the rainfall regime over the southern Levant in the global context is still missing.

Owing to the proximity of the North Atlantic Oscillation (NAO) and its documented effects on the northern

and western Mediterranean Basin (e.g. Hurrell and van Loon, 1997; Cullen et al., 2002), it was just natural

Copyright 2005 Royal Meteorological Society Int. J. Climatol. 26: 5573 (2006)


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ATMOSPHERE CIRCULATION AND RAINFALL OVER SOUTHERN LEVANT 57

to search for a possible linkage between its variability and climatic variability over the EM (e.g. Eshel and

Farrell, 2000, 2001; Kutiel and Benaroch, 2002; Xoplaki et al., 2003). Cullen et al. (2002) showed correlation

between the NAO and the Middle East annual rainfall, but they set the southern boundary of their study regionat 37.5 N so that the southern Levant was excluded. Ben-Gai et al. (2001) found high correlations between the

winter mode of the NAO and temperature and sea level pressure (SLP), i.e. R = 0.8 and +0.9, respectively,

in Israel. However, none of the cited studies found any significant correlation between this oscillation and

the rainfall over the southern Levant.

Searching for alternative teleconnections, Price et al. (1998) found a significant correlation (R = 0.59)

between the rainfall in Kfar Giladi (at the northern tip of Israel) and El Nino for the years 19751995.

However, for earlier years the correlation was rather poor.

Kutiel and Benaroch (2002) defined an atmospheric index, i.e. the 500-hPa geopotential height (gph)

difference between the North Sea and the Northern Caspian region (NCP), which resembles the main features

of the East Atlantic/Western Russia (EA/WR) pattern (Branston and Livezey, 1987). While the NCP index

was found capable of differentiating between below- and above-normal temperatures over the EM (Kutiel

et al., 2002), its ability to explain the rainfall variations over this region was found less promising, except forIsrael, for which it was found most crucial. Krichak et al. (2002) found that the NAO and the EA/WR have

a combined effect, so that when they are both in their positive phase, a rainy season occurs in the EM, and

vice versa.

In this paper, we start from a brief description of the climatological background of the region (Section 2).

The rainfall data and methodology are specified in Section 3. Next, the interannual variations in the rainfall

and its relationship with the Cyprus Low phenomenon are shown through composite maps for extreme

anomalous winters and correlation maps (Section 4). This discussion is followed by correlation maps of

global coverage, together with a correlation analysis with known global indices, which are used to identify

remote teleconnection patterns. The results are integrated, attempting to isolate a small number of global

patterns that are linked to the winter rain regime over the study region (Section 5). Section 6 summarizes the

main findings and their significance. The main conclusions are outlined in Section 7.

2. CLIMATOLOGICAL BACKGROUND

The synoptic-scale system affecting the EM that is responsible for most of the annual rainfall is an extratropical

cyclone the Cyprus Low (Sharon and Kutiel, 1986; Alpert et al., 1990), exemplified in Figure 2. The rain is

formed within cold air masses of European origin that enter the region from the northwest. While moving over

warmer Mediterranean waters, the air masses gain moisture and become conditionally unstable. The strong

thermal effect is essential in cyclone dynamics over this region (Shay-El and Alpert, 1991). The dynamics

associated with the cyclone itself, together with that implied by the intersection of the westerly flow with

the shoreline and, later on, with the mountain ridges, results in intensive rainfall over the Levant (Sharon

and Kutiel, 1986). The majority of Cyprus Lows approaches the region from the West Mediterranean (WM,

Air Ministry, 1962), but some of them form near Cyprus (e.g. Alpert et al., 1995). The surface low over the

EM is generally accompanied by high pressure over the WM and southwestern Europe (Figure 2(a)). Theconcurrent upper-level system (Figure 2(b)) consists of a pronounced trough extending toward southwestern

Turkey, slightly to the west of Cyprus, and a pronounced ridge over West Europe (WE, hereafter). The upper

trough over the EM induces cold advection aloft into the Cyprus Low region.

3. DATA COLLECTION AND METHODOLOGY

The study period extends over 52 years starting from 1950. Twelve rain gauges distributed over the northern

half of Israel were used. They were divided into two groups; four represent the north of Israel and eight

represent its center (Figure 1). The spatial correlation between the seasonal rainfall of the member stations

belonging to each group was calculated and found to exceed 0.8, indicating that they represented well the



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58 U. DAYAN ET AL.

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Figure 2. Composite maps for ten days with heavy rainfall in Israel: (a) SLP (hPa); (b) 500-hPa gph (m). These images are provided

by the NOAACIRES climate diagnostic center, Boulder, Colorado, USA, from their web site http://www.cdc.noaa.gov/



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regions of interest. These stations have a relatively good temporal coverage (only 4% of the individual

monthly observations are missing). The study addresses the variability of rainfall in the midwinter months,

December, January and February (DJF), in which over 65% of the annual rainfall occurs (Atlas of Israel,1970). In order to overcome gaps in the data and to compensate for the variation in the average monthly

rainfall among the various stations, the monthly rainfall anomalies at each station were normalized separately.

Then the normalized anomalies were averaged separately in two regions, referred to as north and central

Israel (Figure 1). Finally, the regional anomalies were averaged to create a single rainfall time series. The

latter step was taken in order to equally represent each of the two regions. To correct for the log-normal

distribution of rainfall, the values were finally log-transformed.

The atmospheric fields used in our analysis were extracted from the NCEP/NCAR reanalysis archive (Kalnay

et al., 1996; Kistler et al., 2001). The monthly numerical indices representing the global teleconnection

patterns used in this study were taken from NOAA site http://www.cdc.noaa.gov/ClimateIndices/corr.html,

and are based on the analysis of Branston and Livezey (1987). To study the relationship between global

fields and the rainfall time series, we used correlation maps based on DJF averages. Considering the length

of the study period (52 years) and assuming that the seasonal values of the atmospheric fields are not seriallycorrelated, a correlation of |R| > 0.3 is significant at the 97% level.

The composite maps used in this study are extracted separately for the ten driest winters (1960, 1984, 1999,

1951, 1955, 1973, 1966, 1979, 1996 and 1991) and the ten wettest winters (1992, 1988, 1952, 1983, 1980,

1974, 1964, 1967, 1993 and 1969), with the year being that of the corresponding January (e.g. 1960 means

December 1959 February 1960). The average deviation from the long-term mean was +53% for the wettest

years and 38% for the driest. Composite maps allow studying the full circulation (not only anomalies) and

difference between the wet and the dry years. They are also commonly used to resolve relationships between

large-scale atmospheric circulations and meteorological or hydrological phenomena. Kahana et al. (2002), for

example, successfully identified the typical synoptic patterns inducing major flash floods over southern Israel.

Xoplaki et al. (2003), analyzing extreme warm and cool summers in Greece, showed that the variability of

the large-scale atmospheric circulation strongly influenced the variability of the temperature there.

4. REGIONAL TELECONNECTIONS

Figures 3(a) and (b) show the composite SLP fields for the ten driest and ten wettest years (hereafter the dry

and wet composites, respectively). Both composites exhibit one closed cyclone in the central Mediterranean,

over Italy, and another over Cyprus. In the wet composite (Figure 3(b)) the cyclonic center over Cyprus

is much more pronounced than that over Italy, in contrast to the dry composite (Figure 3(a)), in which the

eastern center is barely detectable. This demonstrates the intensified cyclonic activity over the EM in wet

years, in agreement with Enzel et al. (2003), who found an increase in frequency of Cyprus Lows in wet

years. The gap between the two cyclonic centers suggests that there is a tendency for cyclones to be formed

over Cyprus in wet years rather than to enter this region from the west.

A closer examination reveals another difference between the two composites, this one over southwestern

Europe. The ridge that extends from the Azores toward Europe, separating the westerlies that prevail overnorthwest Europe from the central Mediterranean cyclone, is far more pronounced in the wet years than in the

dry years. This reflects a higher degree of separation between the European and the Mediterranean cyclone

tracks in the years that are wet in the south Levant.

The contrast between the two groups of winters is further elucidated by the map showing the SLP difference

between them (wet minus dry, Figure 3(c)). This map exhibits a continuous increase in pressure difference

from a low of0.5 hPa over Cyprus to a high of+5.5 hPa over the Atlantic.

The similarly calculated 500-hPa composites (Figures 4(a,b)) indicate that in each group of extreme years

a distinct trough is found over the Mediterranean. In the driest years its axis is found near 20 E (in the central

Mediterranean) and in the wettest along 35 E (the Levant). These locations are consistent with the locations

of the main surface cyclones for the pertinent groups. This implies that northwesterly flows characterize the

EM in the wettest years and westsouthwestern flow in the driest ones and demonstrates the role of cold



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Figure 3. Average SLP (hPa): (a) for the ten driest years; (b) for the ten wettest years and (c) for the difference between them

(wettest driest)

advection associated with Cyprus Lows in enhancing rain in the south Levant. As is the case for SLP, the

500-hPa gph difference map between the two groups (Figure 4(c)) emphasizes this contrast. Here, unlike for



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Figure 3. (Continued)

SLP, a higher symmetry is found between the anomaly amplitude observed over the Cyprus region (45 m)

and that over Western Europe (hereafter WE, +60 m).

A pronounced difference was found also in the structure of the near-tropopause Atlantic subtropical jet,

represented here by the average monthly magnitude of the 250-hPa wind vector. In the wet composite

(Figure 5(b)), the jet orientation is turned counterclockwise by more than 10 compared to the orientation in

the dry composite (Figure 5(a)), so that its northeastern exit points to the north of British Isles rather than

toward the Bay of Biscay, as is found in the driest years. As a result, the degree of separation between

the Atlantic and the North African segments of the subtropical jet is considerably more pronounced in the

wettest years. Moreover, in the wettest years WE and the WM are anticyclogenetic regions, being under both

the right sector of the Atlantic Jet exit region and the left sector of the African Jets entrance region. The

situation is different in the driest years, when WE is located to the left of the Atlantic Jet exit and so becomes

cyclogenetic. These findings agree with the extension of the Azores Ridge over the western parts of Europe

and the Mediterranean in the wettest year and the dominance of the Italian cyclone in the driest.

The above findings are further examined through correlation maps that emphasize the anomalous circulation

feature related to southern Levant rainfall. The SLP correlation map (Figure 6(a)) shows a widespread positivecorrelation pattern over WE and WM with a maximum correlation (R > 0.5) over France and an insignificant

negative correlation center over Cyprus. This is consistent with the large difference found between the two

groups of extreme years over WE compared to the small pressure difference found over Cyprus (Figures 3(c)).

The weak signal in the SLP over the EM is also consistent with Enzel et al. (2003), who found that in the

wettest years the average minimum SLP of the Cyprus Lows was higher than in the driest years, which is

cancelled by their higher frequency in the wettest years. However, the high positive correlation with WE

encircling the relatively lower values over the EM is a manifestation of the enhanced activity of the Cyprus

Lows in the wet years compared to the dry ones.

In the 500-hPa correlation map (Figure 6(c)), the positive correlation center over WE looks more or less

similar to the surface one, but the negative center over the EM becomes most prominent, with an extreme

value of 0.74 at 32.5 N, 35 E. Similar values were obtained also at higher levels up to 250 hPa. The



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60N

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Figure 4. As in Figure 3, but for 500-hPa gph (m)

intensification of the surface cyclonic system with height confirms that it is an equivalent barotropic, cold,

low-pressure system, and so stresses the contribution of upper-level cold air to the rain formation over the

region discussed in Section 2. Indeed, the correlation of the 700-hPa temperature (and 500 hPa) exceeds 0.7



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60N

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Figure 4. (Continued)

over the same region (Figure 6(b)). The SLP and 500-hPa correlation maps (Figure 6(a) and (c), respectively)

resemble the main features of the first canonical correlation patterns for winter type coupled variability(precipitation and geopotential heights) explaining the largest part of the Mediterranean rainfall variability

(Dunkeloh and Jacobeit, 2003). This regional pattern is referred to as the Mediterranean Oscillation (MO)

(Conte et al., 1989), and has opposite pressure/height and rainfall anomalies between the western and eastern

Mediterranean area.

The most extreme correlation found (0.74) for the 500-hPa gph over any of the fields examined

demonstrates the EM upper-level trough (or the EM trough) as the major synoptic-scale factor for rainfall in

the study region.

5. TELECONNECTIONS ON THE GLOBAL SCALE

To assess the global circulation anomalies with which south Levant rainfall is associated, we first calculatedthe correlations between our rainfall data set and the standard global teleconnection indices (described in

Section 1 and details in Appendix A). The maximum correlation for the entire period was found with

the EA/WR index, R = 0.51. A marginal improvement in the correlation was gained by combining the

EA/WR and Tropical/Northern Hemisphere (TNH) indices, yielding R = 0.54. The NCP (not part of the

standard set), especially customized for Israel (Kutiel and Benaroch, 2002), yielded only the second best

correlation, 0.4. The ENSO indices yielded a correlation smaller than 0.3 and the NAO, a value of only

0.09.

Correlation maps of global coverage, including both hemispheres, were extracted for various fields such

as temperature, wind components and gph for various levels up to the stratosphere. As one may expect,

the correlation centers located farther from the vicinity of the study area are weaker and less distinct than

those described above in Section 4. However, several remote distinct patterns were found. We assumed



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64 U. DAYAN ET AL.

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Figure 5. 250-hPa wind speed (m s1) for (a) the ten driest years and (b) the ten wettest years. The jet axes are denoted by thick arrows.

Note the pronounced detachment of the Atlantic jet from the African jet in the wet years (b)

that these remote patterns are associated with the EM trough, and so indirectly with southern Levant

rainfall.

The first is a negative center of over 0.4 in the west part of the tropical Pacific, between 120 and

180 E, found in the 925-hPa temperature correlation map (Figure 7). This pattern resembles the temperature

correlation pattern with the Pacific Warm Pool pattern (PWP hereafter), defined by SST anomaly at the

large pool of the global ocean warmest temperatures, over 29 C (Thunell et al., 1994). The time series for

the PWP was taken from the NCEP/NCAR database (http://www.cdc.noaa.gov/ClimateIndices/corr.html#NP).

The vicinity between this pattern and the El Nino suggests that they might be correlated, but the correlation

between them is only 0.25.

The other two distinct patterns were found in the stratospheric zonal wind field, represented here by

the 50-hPa level (Figure 8): One is a band of positive correlation with a maximum of >0.3, meandering



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Figure 6. Correlation of Israel rainfall in DJF with (a) SLP, (b) 700-hPa temperature and (c) 500-hPa gph regional coverage (extracted

from http://www.cdc.noaa.gov/correlation/)



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66 U. DAYAN ET AL.

0.3

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Figure 7. Correlation of 925-hPa temperature with Israel rainfall in DJF global coverage. In the lower left corner is the correlation with

the Pacific Warm Pool (PWP)

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Figure 8. As in Figure 7, but for 50-hPa zonal wind component

around 65 N, along the North Polar Night Jet (NPNJ), which was found highly correlated with the index

of the Arctic Oscillation (taken from NCEP/NCAR data base). The other is a zonal band of negative

correlation (>0.4) along 60 S, the poleward margins of the austral summer South Polar Jet (SPJ). A

detailed analysis shows that in the wettest years the SPJ shifts poleward and intensifies. The antisymmetric

distribution with respect to the equator contradicts the symmetry found in the correlation maps of the El



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100090S 30S60S EQ 30N 60N 90N

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Figure 9. Longitudeheight cross section of the correlation for DJF between zonally averaged U-wind and Israel rainfall

Nino (not shown), suggesting that the linkage between the latter and the rainfall in the study area is rather

weak.

The zonal bands that appear in the 50-hPa correlation map are further emphasized in the latitudepressure

cross section (Figure 9). The correlation pattern shows two distinct extrema, a major negative one around

50 S and other around 40N, and resembles the EOF-2 of the zonally averaged zonal wind (not shown), with

a reversed sign. The correlation between the EOF-2 time series and the EM trough index was found to be

0.58. That EOF-2 time series when correlated with 500-hPa gph gives back a pattern similar to the WEEM

dipole (not shown).To further explore the implication of these stratospheric variations on the WEEM dipole, a time series

of the Northern Hemisphere Polar Night Jet (NPNJ) was extracted from the average zonal wind at the three

maxima along the jet axis (where the greatest correlation was found), i.e. the north shore of Alaska, Iceland

and western Siberia. A time series for the SPJ was constructed from the average zonal wind along 60 S,

where the axis of the correlation band was found.

The 500-hPa gph maps of correlation with the three indices, NPNJ, PWP and SPJ (Figure 10(a) (c),

respectively) show, indeed, distinct fingerprints of the WE EM dipole. The reversal found in the signs of the

correlation centers for the PWP and the SPJ with respect to those found for the NPNJ reflects only the fact

that each of them supports the EM trough when it is in its negative phase.

Figure 11 shows schematically the statistical relationship between each of the teleconnection patterns and

the EM trough, represented by the 500-hPa gph at 32.5 N, 35 E. The highest correlation is found with the

NPNJ (R = 0.50). The SPJ and PWP yield R = 0.38 each. It is worth noting that both the PWP and SPJ

are not correlated with the NPNJ (|R| < 0.11) but are correlated with each other (R = 0.38). The multiplecorrelation between the three patterns and the EM trough is 0.69 and their correlation with the rainfall itself

is R = 0.53. The indirect dependence of the rainfall in the study region on the three remote factors, via the

EM trough, is manifested by a multiple-regression experiment. When the three global factors were added to

the EM trough, their multiple correlations with the rainfall increased negligibly from 0.74 (for the EM trough

alone) to 0.75.

Following the teleconnection suggested to exist between the EM rainfall and the El Nino Southern

Oscillation (ENSO) and NAO circulations (Section 1), a 500-hPa gph correlation map was extracted for

each of them (Figure 12(a) and (b), respectively). The El Nino (represented by SOI) yields a weak dipole

signature, but the WE center is shifted to the south of Europe and the EM one is shifted to the east and is

very weak (


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68 U. DAYAN ET AL.

80N

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10N120W 100W 80W 60W 40W 20W 0 20E 40E 60E 80E 100E 120E

0.3

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00

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Figure 10. Correlation of 500-hPa gph for DJF with (a) Northern polar jet, (b) Pacific Warm Pool and (c) Southern polar jet

6. SUMMARY AND DISCUSSION

This study aims to link the interannual variations in the winter rainfall over the northern half of Israel,

representing the south Levant, to atmospheric circulations of synoptic and global scale. The present study

extends the previous ones in two aspects:

It examines the relationships with the entire known set of global oscillations beyond those that have beenstudied so far.

It extends the study domain beyond the troposphere and the Northern Hemisphere, covering also the

stratosphere and Southern Hemisphere.

6.1. Main findings

Our study shows that an upper trough, extending from Eastern Europe toward the eastern coast of the

Mediterranean, plays the central role in modulating the seasonal rainfall over the southern Levant. This is

expressed by a correlation of0.74 between the 500-hPa gph at 32.5 N, 35 E and the seasonally averaged

log-rainfall for DJF. This trough has a dual effect in enhancing the rainfall over the study region. One

is the dynamics implied by its associated Cyprus Low and the other is the cold advection imparted by



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South Polar JetPacific Warm Pool

North Polar Jet

EM troughIsraelrainfall 0.74

All factors withEM trough 0.69

R=0.50

R=0.66

R=0.38

R=0.41R=

0.38

R=0.61

EMtrough

Figure 11. Correlation between global-scale systems and the EM upper trough

the northwesterly flow over the EM basin. When this advection occurs over the warmer waters of the

Mediterranean, it produces conditional instability over the southern Levant.

The EM trough was found to be linked to a pronounced ridge covering the WE and the British Isles.

The WE ridge was found to be accompanied by an anticlockwise abnormal deflection of the Atlantic Jet,

accompanied by its pronounced detachment from the North African subtropical Jet, implying that WE is thenlocated under the anticyclonic sectors of both jets. We propose that the process that leads to the Levant rainfall

derives from the generation of low-frequency interseasonal perturbations in the main Atlantic storm-track exit

region over the British Isles. This is a region where baroclinic disturbances become equivalent barotropic

ones and slow down to become also quasi-stationary (Kushnir and Wallace, 1989). It is a phenomenon that

often results in blocking. When this happens, it generates a downstream dispersion of energy in the form of

Rossby waves that then supports the EM trough. If a positive anomaly (anticyclone) develops over the British

Isles, the downstream effect over the Levant is an opposite phase disturbance (a cyclone) and then it rains in

Israel. The reverse is also true. In addition, a positive correlation, which increases with height, of the rainfall

in the south Levant with temperature was found over WE. The above implies that rainy winters in the south

Levant are associated with mild, warm and dry winters over WE.

Three distinct correlation patterns with the rainfall in the study region were detected also in remote regions:

one over the tropical Western Pacific (temperature) and the other two along the SPJs (50 hPa, zonal wind) in

both hemispheres. The multiple correlation between the intensity of the EM trough and these three patterns

was found to be 0.69. The dominant among them is the north polar jet, having a correlation of 0.50 with the

EM trough. The intensity of the NPNJ is better correlated with the EM trough than the Arctic Oscillation

does (R = 0.38). It should be stressed here that the former has also the advantage of being a physical entity.

6.2. Discussion

The statistical relationships shown here indicate that the system that is definitely responsible for rainfall

over the southern Levant on the seasonal time scale is the EM trough. Therefore, we assume that the other

systems that apparently affect the rainfall over the study region do so by affecting the EM trough. The

physical mechanisms that connect the remote factors to the EM trough are not trivial. Concerning the North

Polar Jet, Graf et al., (1994) showed through numerical simulations and data analysis a clear relationship



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70 U. DAYAN ET AL.

100W 80W 60W 40W 20W 0 20E 40E 60E 80E 100E

70N

60N

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40N

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0(a)

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0.30.3

0

Figure 12. As in Figure 10, but for (a) SOI and (b) NAO

between the EM trough and the intensity of the NPNJ, and described the former as a mode that is enhanced

by the latter. Concerning the PWP, Thunell et al., (1994) stated that this pool of warm water plays an

important role in modulating low-latitude climate throughout the IndoPacific region. Seager et al. (2003)

show that anomalously higher SST at the tropics enhances the Hadley Circulation, including the Subtropical

Jet (STJ). The latter can affect, among others, the Mediterranean and the EM trough as well (this has not

been shown yet). The physical mechanism connecting the southern summer polar jet with the Levant is

unclear.

The poor correlation found between the rainfall and the NAO by Ben-Gai et al. (2001) and in this study

is inconsistent with Eshel and Farrell (2000, 2001), who found that the rainfall variability over the EM is

explained by modulations of a North AtlanticMediterranean teleconnectivity. Eshel (2002) found correlations

with the 700-hPa gph, reaching 0.8 over the Balkan and +0.4 over Iceland. Such high correlations andspatial distributions were not found for the Levant, suggesting that the EM, as defined by Eshel and Farrel

(2000, 2001), does not coincide with the Levant. Indeed, their study domain was bounded between 3242N

and 2236 E, in which only 3 of the 16 rain stations they used were located in the Levant, and the

majority was distributed over Turkey and Greece. The linkage found by Krichak et al. (2002) with the

NAO and EA/WR has a limited relevance for our study region for two reasons. One is that the domain

they used, 3336N, 3437 E, only partly overlaps with our study area. The other is related to the

rainfall data they used, i.e. the NCEP/NCAR reanalysis. The capability of the latter to represent our study

region was examined by correlating the monthly rainfall in the 32.5 N, 35 E grid-point with our data for

the study period. The correlation found was 0.59 only, implying that gridded smoothed data is not the

optimal source for rainfall analysis for the east coast of the Mediterranean, characterized by sharp spatial

variations.



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The Levant, though being a part of the EM, differs in its rainfall regime from the major part of the latter.

This is related to the difference between them in the wind direction found as favorable for rain formation,

southerly for the EM (Eshel and Farrel, 2000, 2001) and westerlynorthwesterly for the Levant (as impliedby our Figures 2a and 3b). This is further emphasized by the average 700-hPa wind anomaly for the ten driest

years in our study region, which we found to be southerly (1 1.5 m s1).

Eshel and Farrel (2000) explained the rainfall-enhancing effect of southerly winds as due to air ascendance

while moving along the upward-tilting isentropes that suppresses the subsidence that prevails over the region.

Eshel and Farrel (2001) also showed that this ascendance changes the atmospheric thermodynamic profile,

reducing the mean column static stability that subsequently intensifies the generation of rain. The role of the

prevailing wind on the rainfall was also addressed by Kutiel et al. (2002). They stated that the impact of the

meridional wind component on the rainfall over the EM is complex, that regions exposed to the southern

maritime influence (e.g. Peloponnesus, Greece, western Turkey) experience more rainfall when the wind is

abnormally southerly and that the reverse holds for regions subjected to northern maritime influence (such

as Crete and the Black Sea coast of Turkey). They found that the wind-direction impact on the rainfall is

most crucial in Israel, where during the negative NCP (implying southerly wind anomaly) the rainfall isconsiderably reduced (4456%) with respect to the positive phase.

The crucial importance of moisture advection for Israel was stressed also by Kahana et al., (2002), who

explained episodes of heavy rains in southern Israel under northwesterly flow by intensive moisture transport

from the Mediterranean Sea, in spite of the unfavorable dynamics implied by the negative vorticity advection

aloft. Ulbrich et al. (1999) also stressed the importance of moisture supply as an essential factor explaining

the rainfall variability in Portugal. We therefore suggest that moisture advection is a crucial rain factor in

the Mediterranean coastal region. Following the above, the findings of Eshel and Farrel (2000, 2001) may

be interpreted somewhat differently; i.e. the southerly winds, in addition to their dynamic effect, impart also

moist advection toward Greece and western Turkey.

The discrepancy between the high correlation with ENSO found by Price et al. (1998) compared to the

relatively low correlation found here stems mainly from the difference between the study period for which

they found the higher correlation, i.e. 19751995 and ours (Table AI). An increase in correlation with time

was also found for the PWP index. A similar trend was also found by Chiang and Kushnir (2000) in thecorrelation between the ENSO and Brazil rainfall index for April May (CPI AM). They explained the

increased correlation in the late twentieth century by an increase in amplitude of the SST anomalies over the

tropical Pacific during that period (and similarly during the 1920s).

The limited degree of variability explained by atmospheric circulations (0.55) can be attributed to two types

of factors. One is the noisy nature of the rainy season, as reflected by the intraseasonal low correlation, i.e.

|R| < 0.16 between the three pairs of the winter months. The other is related to local effects, such as the

year-to-year differences in the amount of heat released by the winter cooling of the upper layers of the EM

waters (Tzvetkov and Assaf, 1982).

7. CONCLUSIONS

Our results lead to the following conclusions:

The major factor that modulates the winter rainfall over the northern half of Israel is the EM upper trough,

which explains over 0.54 of the rainfall variance.

The EM trough coexists with a ridge covering Western Europe.

The EM trough is linked with three global factors: both SPJs, the northern and southern, and the SST over

the tropical Western Pacific (The PWP).

Cold and wet winters in the south Levant are associated with warm and dry winters over Western Europe

and vice versa.

The linkage between the level of the Dead Sea and the rainfall over the Levant, together with the relationship

found between the winter conditions over the Levant and WE, leads us to hypothesize that under the present



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72 U. DAYAN ET AL.

climatic regime, periods in which the Dead Sea level rose indicate reduced ice accumulation over the icebergs

in WE and southwest Scandinavia and vice versa.

ACKNOWLEDGEMENTS

This research was supported by The Ring Family Foundation Fund for Atmospheric Research, Grant No.

3014282, provided through the Hebrew University Multidisciplinary Center for Environmental Research. We

thank the Department of Climate, the Israel Meteorological Service, for the rainfall data. Special thanks are

due to Michal Kidron from the Cartographic Laboratory of the Department of Geography at the Hebrew

University of Jerusalem for her assistance in preparation of the figures.

APPENDIX A: CORRELATION WITH KNOWN GLOBAL INDICES

Correlations between our rainfall data set and 11 indices, representing global circulations that have been

previously analyzed with respect to the study region, were calculated. The indices were derived from thetwice-daily Northern Hemisphere 700-hPa level (Branston and Livezey, 1987), except for the NCP, derived

from the 500-hPa level (Kutiel and Benaroch, 2002), and ENSO, derived from the SST data. The five

oscillations that yielded the highest correlations are listed in Table AI.

Table AI. Correlation between DJF log-normalized rainfall in Israel and the four known oscillations (Branston and

Livezey, 1987), together with the NCP index (Kutiel and Benaroch, 2002), which yields the highest score

Correlation

p-values

Level Period Location of

major dipole

Full name Abb. name

0.51p < 0.005 700 hPa 19502002 EnglandCaspian Sea East Atlantic/Western Russia EA/WR

0.40p = 0.005 500 hPa 19581998 North Sea N. Caspian Sea North Caspian pattern NCP

0.30p = 0.05 700 hPa 1950 2002 Europe NE China Polar/Eurasian P/E

0.29a p = 0.05 SST 1950 2002 Tropical Pacific Southern Oscillation ENSO

0.24p = 0.05 700 hPa 19502002 Gulf of Alaska Hudson Bay Tropical/Northern Hemisphere TNH

a Following Price et al. (1998), see Section 1 above, the correlation for 19751995 is 0.54.

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