mediterranean precipitation and its relationship with sea ... · mediterranean precipitation and...

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ANNALS OF GEOPHYSICS, VOL. 47, N. 5, October 2004

Key words Mediterranean – precipitation – princi-pal component – canonical correlation – trajectory

1. Introduction

The Mediterranean is characterised by hot,dry summers and warm, wet winters. The win-ter precipitation variability patterns of theMediterranean region, which extends 3600 kmfrom the Iberian Peninsula in the west to Israelin the east, show marked differences betweenthe north-western and the south-eastern areas(e.g., Conte et al., 1989; Douguedroit, 1998).

This variability can be described as a telecon-nection; an anti-correlation between the rainfallin Iberia and that in the Near East.

What influences the operation of the Mediter-ranean precipitation teleconnection? Conver-gence of moisture and regional precipitation iscontrolled by the dominant atmospheric pressurestructures. The major sources of water close tothe Mediterranean region are the Atlantic Ocean,the Mediterranean Sea itself, and further east theArabian Sea. Consequently pressure systemsover the North Atlantic, Europe and North Africamust all be considered in answering the abovequestion. Hurrell (1995) has shown that changesin the North Atlantic pressure system, indicatedby extremes of the North Atlantic Oscillation(NAO), strongly alter the moisture transportacross the Atlantic Ocean and onto the Europeanlandmass. The NAO is a measure of a pressureteleconnection and is defined as a large-scale al-

Mediterranean precipitation and its relationship

with sea level pressure patterns

Roy Thompson and David N. GreenSchool of GeoSciences, The University of Edinburgh, U.K.

AbstractThe relationship between Mediterranean precipitation and North Atlantic and European sea level pressure fieldshas been studied using statistical techniques to investigate the variability within the data. A principal componentanalysis shows the major winter precipitation variability is described by a see-saw fluctuation between the West-ern and Eastern Mediterranean. The pressure-precipitation relationships indicate that a highly variable, pressureregion situated to the south of Britain dominates this major precipitation pattern. The large-scale pressure fieldswhich facilitate the precipitation patterns have been isolated using a canonical correlation analysis. Although thewell-known major pressure centres of action in the North Atlantic are important, pressure changes in the east arefound to also control the transport of moisture across the Mediterranean to a large degree, as the presence of alarge high over Kazakhstan causes meridonial flow and impedes the passage of moisture across the Mediter-ranean. The pressure-precipitation relationships are found to be very consistent over multi-decadal, seasonal,monthly and daily time-scales with trajectory analysis confirming many of the features of the average seasonalpressure charts. This steadiness and regularity indicates that the Mediterranean precipitation teleconnection is arobust phenomenon that is affected by large-scale pressure changes to both the east and west.

Mailing address: Prof. Roy Thompson, School of Geo-Sciences, The University of Edinburgh, Grant Institute, TheKing’s Buildings, West Mains Road, Edinburgh EH9 3JW,U.K.; e-mail: [email protected]

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Roy Thompson and David N. Green

ternation of atmospheric mass between regionsof subtropical high pressure (the Azores high)and subpolar low pressure (the Icelandic low) inthe North Atlantic (e.g., Lamb and Peppler,1987). The NAO can be expected to contribute toexplaining the dominant variations in the West-ern Mediterranean (e.g., Zorita et al., 1992;Storch et al., 1993; Rodo et al., 1997; Trigo andPalutikof, 2001), however there may well be oth-er pressure systems influencing the precipitationin the Mediterranean Basin.

The spatio-temporal precipitation patternsof local regions within the Mediterranean havebeen described in some detail. Good sum-maries of precipitation and its relationshipswith circulation are available for the Central(Maheras et al., 1992; Bordi et al., 2001; Bordiand Sutera, 2002), Northwestern (Zhang et al.,1997; Ulbrich et al., 1999) and Eastern (Kuteiland Paz, 1998; Kuteil et al., 2002) Mediter-ranean. These studies point to the importance ofboth regional and local topographic influences.Here we include analyses of the less well-stud-ied areas of Northern Africa, so increasing thegeographical coverage, and concentrate onMediterranean wide effects. We are primarilyconcerned with long-term (multi-decadal)changes and variability and so only analyse siteswith published precipitation data that extend asfar back as at least 1905. The main objective ofthis paper is to study the extent and temporal be-haviour of Mediterranean precipitation patternsand their relation to sea level pressure situationsover Europe and the North Atlantic. The maintechniques employed are canonical correlationand principal component analysis.

2. Methodology

An overview of the steps employed in ouranalyses is given in the flow chart of fig. 1.Monthly values of precipitation for stationsaround the Mediterranean region spanning thetime period 1905-1990 were obtained from theGlobal Historical Climatology Network Data-base (Vose et al., 1992). The winter months,December through to March, were chosen forstudy as these are the classic months forMediterranean precipitation and because more

pronounced precipitation and pressure variabil-ity occurs in the winter season (Hurrell, 1995).Sites were selected on the basis of complete-ness of the records and on the need to achieve awide geographical spread. Forty-four satisfac-tory sites were found. We consider this data setto be the best obtainable, providing a good bal-ance between geographical extent, complete-ness and series length.

The collection of long-term precipitationdata is fraught with difficulties and manyrecords are discontinuous, due to either a breakin recording or a change of recording site.Missing data can cause severe problems in cor-relation and time-series analysis. This is espe-cially true for the statistical techniques used inthis project, principal component analysis andcanonical correlation analysis, as both requirecomplete data sets.

15100 mean monthly precipitation valueswere available to us, however on account of thelong time period under analysis there were in-evitably many missing values. To form com-

Fig. 1. The major stages of the analysis. GHCN == Global Historical Climatology Network; CRU = Cli-mate Research Unit, University of East Anglia.

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Mediterranean precipitation and its relationship with sea level pressure patterns

plete records an imputation was executed. Themost conservative approach is to use the meanof the available data. However we have fair cor-relation between the mean monthly precipita-tion at nearby sites (r 2 typically 0.55) and so aless conservative approach was preferred. Weconsidered different geographical regions sepa-rately and then used a median polish technique(Velleman and Hoaglin, 1981) to estimate themissing data month by month. In practice forregions with poor site correlation (e.g., NorthAfrica; r 2∼0.3) our interpolation method willgive results similar to using the time-series

mean, while for regions with good spatial co-herence (e.g., Iberia; r 2∼0.8) results will besimilar to those obtainable by multiple regres-sion. The solid circles in fig. 2c,d are typical ex-amples of our imputation. Missing monthlymeans for two recent years were imputed forLisbon, while missing data for eight war yearsand one recent year were imputed for Limassol.

The winter monthly mean-sea-level pres-sure data were obtained on the 5° latitude by10° longitude grid of Jones (1987). We selecteddata extending from 50°W to 60°E and from20°N to 65°N covering the North Atlantic, Eu-

Fig. 2a-d. a) The 1st canonical function (precipitation) index. b) The 1st precipitation principal component in-dex. c) The precipitation record at Lisbon, Portugal. d) The precipitation record (scale inverted) at Limassol,Cyprus.

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b

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rope, North Africa and the western extent ofAsia. The data were initially checked for outly-ing values, which were removed. The pressuredata grid was almost complete (<1% data miss-ing) and any outliers were replaced using theimputation scheme outlined above.

To allow comparison between individualstation records for both sets of data a normali-sation is required to remove the bias due tosome regions having larger mean values for thevariables than others. As we are only concernedin the variability of precipitation and pressure,dividing each record through by the mean valueof that time-series provides a simple normalisa-tion with the advantage that all values are stillpositive reducing problems in the subsequentanalysis. It should be noted that the normalisedpressure and precipitation variables still havedifferent ranges. In particular the coefficient ofvariation (standard deviation of time-series/arithmetic mean of time-series) is approximate-ly two orders of magnitude larger for the pre-cipitation data compared to that of the pressuredata. This causes no mathematical problems inthe subsequent analyses except that certain val-ues, as noted in the text below, are not directlycomparable.

The two data sets are moderately large andtherefore for an efficient analysis they requireda reduction in size, whilst maintaining the max-imum information content possible. PrincipalComponent Analysis (PCA) provides the idealcandidate for such data compression (e.g., Jol-liffe, 1990) and was applied individually toboth the precipitation and pressure data sets.PCA worked well on our two data sets allowingus to study only the first four principal compo-nents of the pressure data and the first threeprincipal components of the precipitation data.The dimensions of the analysis were consider-ably reduced and yet 83.5% of the pressurevariance and 51.2% of the precipitation vari-ance was retained.

To gain insight into the relationship betweenthe two sets of data, pressure and precipitation,a Canonical Correlation Analysis (CCA) wasperformed. CCA quantifies the strength of therelationship between two data sets by maximis-ing the correlation between linear combinationsof the variables (e.g.,, Hair et al., 1995). The

problem is to find the linear combination of pre-cipitation variables that has maximum correla-tion with a linear combination of pressure vari-ables. To simplify proceedings it is beneficial touse the two data sets in their principal compo-nent co-ordinates. From the CCA a time-seriescan be constructed which emphasises the yearsat which the pressure and precipitation patternsare highly or inversely correlated. We also foundit helpful to consider only the six most extremeyears, for both the high and low canonical func-tion values. For these extreme years mean pres-sure maps were constructed. These maps can beeasily interpreted, alongside meteorologicalrecords, to show which large scale features ofthe pressure field give rise to the Mediterraneanprecipitation teleconnection. Storch and Zwiers(1999, p. 382) provide us with a formal defini-tion of a teleconnection pattern. The mean pres-sure maps can also be compared with maps ofindividual events (see Section 3.3).

It is important to emphasise that a statisticalanalysis based solely on principal componentand canonical correlation analysis basically on-ly serves to show that patterns of the analysedfields covary. By itself it does not demonstratethe patterns of major statistical weight are dy-namically significant (being, for example, linearor non-linear normal modes); not that there is acause and effect connection between the co-varying variables; or that the dominant patternsof co-variation (as identified, by the canonicalcorrelation analysis) are dynamically signifi-cant. So in addition to analysing mean monthlydata, daily and sub-daily records were studiedfor specific synoptic conditions. The dailyrecords chosen for comparison with the meanmonthly charts were selected as objectively aspossible. The year and month of an event wereselected from the exceptionally high and lowvalues of the first CCA timeseries. The day ofthe event was chosen as that with the highestrainfall at either Lisbon (Portugal) or Paphos(Cyprus), depending on which polarity of theprecipitation teleconnection was being consid-ered. Lisbon and Paphos were chosen to repre-sent typical West and East Mediterranean sitesrespectively.

Surface pressure maps were constructed togive a synoptic overview of the conditions over

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Europe, at the time of the precipitation event, andback and forward trajectories were produced us-ing the British Atmospheric Data Centre trajec-tory service to give an indication of the directionand speed of airflow towards and away from thetwo sites. For this analysis an initial pressure lev-el of 700 mb was chosen over the two recordingstations as a compromise corresponding very ap-proximately to an average height of precipitationformation while being removed from surfaceboundary layer levels. The height along the tra-jectory path was calculated from the pressurelevel data using the hypsometric equation.

3. Results

3.1. The Mediterranean precipitationteleconnection

A spatial view of the Mediterranean precip-itation teleconnection is provided by the 1stPCA loadings map (fig. 3) giving an indication

of the geographical extent of the first-order pre-cipitation variability. The zero loading linetrends south-west to north-east, with the load-ing size increasing with distance from the zerocontour. The time series of the first principalcomponent (fig. 2b) indicates how the variabil-ity in the precipitation across the Mediter-ranean, shown in fig. 3, has changed over the 85year timespan since 1905. The PCA index iscentred on a value of one, therefore high indexyears such as 1936 show unusually high precip-itation in the west and low rainfall in the east,whereas low index years such as 1989 show thereverse situation. The index shows a slight ten-dency to rising values prior to the 1960s/1970s,and falling values thereafter.

3.2. CCA and the synoptic pressure situations

The canonical correlation analysis took theprecipitation PCA discussed above and a PCAof the corresponding pressure field and calcu-

Fig. 3. The geographical form of the 1st precipitation principal component. The contours represent equal com-ponent loading values and indicate that the regions of maximum variability occur towards the west and south-east extents of the Mediterranean. The smooth contours were drawn by fitting a lowess smoothing function(Cleveland, 1979) to the loadings of the first principal component. Black dots represent the forty-four precipita-tion recording stations used in the analysis.

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lated the strength of the relationship betweenthe two sets. The resulting time series (fig. 2a)is centred on a value of 0.24, and shows thevariability of the first canonical function. ThisCCA time series is extremely well correlatedwith the precipitation first principal component(r = 0.86), giving an indication that the precipi-tation teleconnection is highly influenced bythe pressure situation.

The geographical patterns of the CCA show-ing the positions of maximum response betweenthe variability in the pressure and precipitationfields over the 85 year study are shown in fig.4a,b. The first precipitation canonical function isremarkably similar to the first precipitation prin-cipal component indicating that the predomi-nant Mediterranean precipitation variability iswell correlated with the pressure changes over

Fig. 4a,b. a) The geographical form of the first precipitation canonical function. The contours indicate equalloading values. Increasing loading values represent increased correlation between the variations of precipitationat that locality and the pressure field. b) The geographical extent of the first pressure canonical function. Thecontours indicate equal loading values. Increasing loading values represent increased correlation between thevariations of pressure at that locality and the whole precipitation field. Thus regions of high loading are partic-ularly responsive. N.B.: the loading values in a) and b) are not directly comparable due to the choice of normal-isation (see text for further details).

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the area chosen. The first pressure canonicalfunction map indicates in which regions thepressure variability contributes most to the pre-cipitation changes. The form of the canonicalfunction is at first sight unexpected. Rather thantwo centres of action over the East and WestMediterranean or a single maximum lying in be-tween the two foci of precipitation variability,major regions of pressure variation well to thenorth are found. One elongated focus, centred atan approximate position of 80°N and 10°W, ex-tends from South Greenland across the NorthPole to Northern Russia. It is similar in shape tothe area of high correlation between Portugueserainfall and North Atlantic sea level pressure asgiven by Ulbrich et al. (1999), but with apromontory extending towards Siberia. Thenegative polarity of loadings indicates that pres-sure variations in a high index year (wet in thewest/dry in the east) will tend to be unusuallyhigh pressure over Greenland and over NorthernRussia. The second focus is situated over the ap-proaches to the English Channel (fig. 4a,b) and

is of opposite (i.e. positive) polarity indicatingthat the pressure variations in this area are anti-correlated with those to the north. In high indexyears higher pressure than usual over the Eng-lish Channel guides maritime air and precipitat-ing weather systems north westwards awayfrom Iberia.

The form the canonical function takes canbe clarified by examining the six most extremeyears for both polarities of the index. Figure5a,b plots the mean winter sea-level pressuremaps for these two sets of extreme conditions.Figure 5a presents the situation causing highprecipitation in the Western Mediterranean andlow precipitation in the east. The Azores highand Icelandic low pressure systems are dis-placed south of their mean positions causing aseries of isobars to run west-east directly acrossthe Iberian Peninsula, providing a route formoisture from the Atlantic Ocean to impingeupon Spain and Portugal. Towards the east ofthe Mediterranean, a large high-pressure ridge(a western bulge of the Siberian high-pressure

Fig. 5a. The mean winter pressure map associated with the six highest canonical function years (precipitationin the west, dry in the east). All contours labelled in millibars.

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system), referred to here as the Kazakhstanhigh, provides a block causing meridional air-flow. The col on the ridge between the Azoresand Kazakhstan highs lies to the south of theMediterranean over Libya.

Figure 5b indicates the situation leading toincreased precipitation in the east and dry con-ditions in the west. The Azores high pressuresystem is displaced to the north and extendsover the Iberian Peninsula preventing airflowacross the Atlantic and onto the westernMediterranean where below normal precipita-tion is recorded. Consequently tight isobarstrend north eastwards from the Atlantic acrossNorthern Scotland and then onwards acrossScandinavia and Finland. Situated in the east-ern Mediterranean is a distinct pressure mini-ma, the Cyprus low. This region has been linkedto Mediterranean cyclones (Alpert et al., 1990),the associated cyclonic flow causes high pre-cipitation in Israel and the surrounding area(e.g., Zangvil and Druian, 1990). Further eastthe Kazakhstan high is more subdued and dis-

placed somewhat to the south. The ridge be-tween the Azores and Kazakhstan highs nowlies to the north of the Mediterranean with abroad col over Hungary and Romania.

3.3. Individual events

In addition to examining the 6-year meanwinter circulation of the extremes of ourMediterranean precipitation teleconnection, wehave also analysed individual precipitationevents to see how the sub-daily dynamic situa-tion compares to the long-term average.

A high rainfall event recorded in Lisbon on27th January 1979 was chosen as an example ofbehaviour in a year of high CCA index. January1979 was wet across Iberia and dry, or moder-ately dry, in the Eastern Mediterranean (com-pletely dry at Cairo). The synoptic pressuremap, trajectory path and trajectory height forthis one event can be seen in fig. 6a. The pres-sure map for 26th January 1979 (the 24-h peri-

Fig. 5b. The mean winter pressure map associated with the six lowest canonical function years (precipitationin the east, dry in the west). All contours labelled in millibars.

(i)

(ii)

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Fig. 6a. i) The pressure map for 26th January 1979. ii) The forward and back trajectories for the 700 mb levelat Lisbon (crosses) and Paphos (open circles). The markers are placed at 6 hourly intervals. iii) The trajectoryheights for Lisbon (crosses) and Paphos (open circles). Solid symbols 00:00 on 27/01/1979.

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Fig. 6b. i) The pressure map for the 13th February 1990. ii) The forward and back trajectories for the 700 mblevel at Lisbon (crosses) and Paphos (open circles). The markers are placed at 6 hourly intervals. iii) The trajec-tory heights for Lisbon (crosses) and Paphos (open circles). Solid symbols 00:00 on 14/02/1990.

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od leading up to the precipitation event) showsa well formed high in the east extending fromKazakhstan restricting the Mediterranean air-flow and generating anticyclonic circulationacross the Eastern Mediterranean Sea. The Ice-landic low pressure system and the Azores highare displaced much further to the south thantheir mean positions so allowing depressions topass across the Atlantic along more southerlytracks than normal onto the Iberian peninsula.The 26th sees lows passing over the Canariesand the Faeroes and a cold front approachingLisbon. The 3-day back trajectory for Lisbonshows cyclonic circulation and rising air arriv-ing from the SW (ii and iii in fig. 6a). The tra-jectory is consistent with rainfall in Lisbon andwith the air passing low over the Atlantic gath-ering moisture before rising to condense andprecipitate over the land. The forward trajecto-ry from Lisbon shows the air continuing to as-cend and being carried north-west away fromthe Mediterranean illustrating how the moisturefrom the Atlantic does not reach Israel orCyprus in such high index years.

The corresponding trajectory path for Pa-phos, Cyprus for the same six-day period showsdry air circulating around the northern edge ofan anticyclone before being transported off theSahara and passing towards the north-east. Theflow corresponds well with the winter trajecto-ries studied by Dayan (1986).

Turning to a low canonical function year.Paphos experienced a high rainfall event on the14th February 1990 (fig. 6b). The pressure mapshows a large area of high pressure centred overthe Canaries and extending over the IberianPeninsula with depressions crossing to thenorth of Scotland and a low pressure system ex-tending into the eastern Mediterranean. There isa high pressure system to the east extendingfrom the Persian Gulf to Kazakhstan, howeverthis is much smaller than in the example above.A series of lows pass through the East Mediter-ranean around this time. One formed overNorth Italy on the 11th and during the 13th to15th another passed to the north of Cyprus, anassociated cold front producing the torrentialrain in Paphos on the 14th. The trajectory pathsshow air movement consistent with this pres-sure situation. Air crossing the Atlantic is

forced around the high pressure system, passingover the Bay of Biscay into the WesternMediterranean. This air has been at a low heightacross the ocean gathering up water, as it pass-es into the Mediterranean it rises slowly until itreaches approximately 3000 m over Cyprus, be-coming the source for the precipitation event.

The air passing over the Iberian Peninsula atthis time also travels across the Atlantic, how-ever it moves slowly and descends as it reachesland due to it being trapped within the Azoreshigh pressure system. This descending air hasno potential for precipitation.

4. Discussion

4.1. Pressure influences on the teleconnection

The principal component analysis showsthat the Mediterranean precipitation telecon-nection is the dominant form of precipitationvariability across the whole region in the wintermonths. The canonical correlation analysis pro-vides two synoptic pressure situations that givea first order explanation of this variability.

High precipitation in the Western Mediter-ranean occurs due to the moisture transport offthe North Atlantic impinging upon the westernseaboard of North Africa and Iberia. This is adirect consequence of the southward shift of theIcelandic low and Azores high pressure systems(fig. 5a) comparable to the motions described inthe work of Sahsamanoglou (1990). At thesame time, the Kazakhstan high provides ablock for any westerly airflow across theMediterranean, forcing meridional flow north-wards over Eastern Europe and Russia and al-lowing the Eastern Mediterranean to remaindry. The meridional flow causes air tempera-tures to be some 1.5°C warmer than average inthe region extending from Rumania to EasternTurkey. At the same time, in North Europe, azone extending from Iceland across the North-ern Baltic to Northern Russia experiences airtemperatures 1.5°C below average. These tem-perature anomalies further demonstrate the re-gional extent and consistency of the pressurepattern producing the Mediterranean precipita-tion teleconnection.

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Conversely with an extended high pressuresystem over the Iberian Peninsula, as in fig. 5b,Spain and Portugal remain dry as moisture car-ried off the Atlantic is advected north west-wards onto Northern Europe. Air temperaturessome 2.0°C above normal are produced overmuch of Sweden and North Finland by the flowfrom the south-west. In contrast air tempera-tures over Eastern Turkey are some 2.0°C cool-er than average. The isobars surrounding theextended Azores high exhibit some curvatureacross Northern Europe leading to the possibil-ity of moisture entering the Eastern Mediter-ranean either by transportation from the At-lantic or from secondary depressions formedwithin Mediterranean areas in the lee of theAlps (e.g., Buzzi and Tibaldi, 1978). The tra-jectory of fig. 6bii indicates that the former oc-curred on the 13th of February 1979. TheCyprus low in fig. 5b indicates that depressionsand cyclonic flow are present in the East whichremove considerable heat and moisture fromthe water (Shay-El and Alpert, 1991). This is il-lustrated by the rising airmass in the trajectoryof fig. 6b as it approaches the Cyprus region.

The areas of pressure variability which af-fect the precipitation teleconnection most arehighlighted by the large loading regions in thefirst canonical correlation map (fig. 4b). Thecentral area of high variability correlation, cen-tred over the English Channel approaches, islinked to the translation of the Azores highpressure system. The English Channel centre ofaction is the position that is affected most bythis movement. When the Azores high is promi-nent and moves northward from its mean posi-tion (approximately 30°N and 35°W: Sah-samanoglou, 1990) this region underlies highpressure (e.g., fig. 5b) but when the Azores highand the Icelandic low move southwards the re-gion is overlain by a low-pressure system. Asexplained above this alters the moisture trans-port across the Iberian Peninsula. As this vari-ability affects the movement of moisture intoand across the Western Mediterranean, so thepressure variability in Northern Russia affectsthe eastern end of the teleconnection. As thepressure increases in Northern Russia the Kaza-khstan high blocks any East Mediterraneanstorm tracks and airflow across the Mediter-

ranean (fig. 5a). This coincides with the south-ward migration of the Azores high causing lowpressure to be present in the English Channel.This difference in pressure between the westernand eastern centres of action is illustrated by thedifference in polarity between the canonicalfunction loadings of the two centres of action infig. 4b. A year in which there is unusually highpressure in Northern Russia corresponds to ayear of unusually low pressure in the EnglishChannel approaches. The opposite situation oc-curs when the Kazakhstan high wanes and theAzores high strengthens and pushes northwardover the English Channel region.

Calculation of the canonical function pair offig. 4a,b involves a long series of matrix manip-ulations including scaling, extraction of princi-pal components and estimation of canonicalloadings. The whole sequence is then followedin reverse to generate the individual canonicalcomponents. A very instructive exercise, whichcan be used to confirm that the above matrix al-gebra has been performed correctly, is to sub-tract the pressure field of fig. 5a (the six highestindex years) from that of fig. 5b (the pressurefield of the six lowest index years). Here the re-sult, as expected, is an almost identical patternto that of fig. 4b.

Recently Palutikof et al. (1996) and Qua-drelli et al. (2001) have investigated the rela-tionship between Mediterranean precipitationand 500 hPa geopotential height. AlthoughQuadrelli et al.’s (2001) results only spanned17 years, and so they were in doubt about thelong-term robustness of their results, our 85-year sea level pressure/precipitation relation-ships are in excellent agreement with their link-ages to large-scale upper-air circulation anom-alies. By contrast, Palutikof et al. (1996) em-phasise the 500 hPa surface over the Mediter-ranean Sea itself as an indicator of atmosphericpressure which relates to Mediterranean precip-itation variability.

4.2. The second canonical pair

The second canonical pair highlights themajor correlation between the pressure andrainfall data sets under the constraint that this

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correlation is orthogonal to the first canonicalpair. The second precipitation canonical func-tion has high loadings over the North Africansites and loadings close to zero over Portugal tothe west and Israel to the east. The area of highloadings is centred over the region in whichthere is most difference between the first prin-cipal component (fig. 3) and the first precipita-tion canonical function (fig. 4a). This can beexplained due to the fact that the second canon-ical function, which perturbs the consistencybetween the canonical function and the princi-pal component is describing a second order ef-fect. The first canonical function has a r 2 valueof 0.79 while the second has a r 2 value of only0.22.

The second pressure canonical functionshows that the area of large pressure changeswhich is predominantly correlated with the sec-ond-order precipitation variation in North Africais situated well to the north of the Mediter-ranean, with a centre over Northern Scandi-navia. The zero contour of the map of this sec-ond pressure canonical function passes alongthe whole length of the Mediterranean Sea. Soagain, we find it is the broad regional or farfield, pressure variations that principally orches-trate Mediterranean precipitation variability.

4.3. The link between monthly and daily analyses

The pressure charts in fig. 5a,b are the aver-age of six mean winter (DJFM) pressure chartschosen due to their high or low canonical indexvalue. Therefore, they are an extremelysmoothed version of the actual pressure situationat any particular time within the winter period.Nevertheless the main pressure patterns are ro-bust, representative features. For example thelow-pressure centre that lies between Crete andCyprus, in fig. 5b, occurs as a closed contourfeature in three out of the four individual wintermonths (December through March). A distinc-tive, closed contour, Eastern Mediterranean lowis also present in five of the six individual win-ters that were averaged to make up fig. 5b. Fur-thermore the mean pressure charts of fig. 5a,bcan be compared with the pressure maps and tra-

jectory analyses of fig. 6a,b that shows plots rep-resenting actual events. These type events showbroadly similar features to those of the monthlyanalysis.

The most important dynamical features con-sistent with high precipitation in the west andlow precipitation in the east are the high overKazakhstan linked with low pressure over theAtlantic, both of which are present in the dailychart (fig. 6a). This situation causes a strongeast-west pressure gradient and air to flow offthe ocean onto the Iberian Peninsula, before itpasses north as the block creates meridonalflow. The equivalent air mass passing overCyprus at the same time has its origins in thedry regions of the Northern Sahara. The backtrajectory stays at an approximately constantheight allowing no advection of moisture.Dayan (1986) classified this type of flow as of‘inland North African type’.

Consistency between the seasonal mean anddaily charts is also apparent for the opposite sit-uation in which the Eastern Mediterranean issubject to higher rainfall. The high pressureover Portugal in fig. 6b causes the Atlanticmoisture to travel north and then turn aroundthe pressure maximum and back into theMediterranean. The low pressure system thenextends into the eastern region generating cy-clonic flow and transportation of moisture overIsrael, Cyprus and coastal regions of Egypt.Thus as discussed by Zangvil and Druian(1990) rainfall over Israel is largely affected bythe trajectory of air prior to the rain event. Mar-itime air trajectories are linked with rain events.However, our canonical pressure patternswould suggest that flow from the Atlantic is al-so of importance in addition to Zangvil andDruian’s (1990) postulated source as moistureevaporation from the Mediterranean. At thesetimes there is also airflow across the Atlanticonto Lisbon, however it is travelling extremelyslowly within the high pressure system and issinking as it passes over the land (fig. 6biii),thereby allowing no precipitation. Once againthe fact that the same features can be identifiedin both the seasonal average and daily synopticcharts suggests that a stable recurrent phenom-enon orchestrates the Mediterranean precipita-tion teleconnection.

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4.4. Mediterranean precipitation and pressure teleconnections

It has been well documented by previousworkers that the NAO causes precipitationanomalies in the Western Mediterranean area(e.g., Lamb and Peppler, 1987; Hurrell, 1995).However, the present study indicates that pres-sure variability in the east, far removed from theNorth Atlantic, has a large influence on the pas-sage of moisture through the entire Mediter-ranean region. The relative importance of theKazakhstan high and Cyprus low have possiblybeen previously underestimated in terms oftheir effect on Mediterranean precipitation. Themoisture which forms the precipitation comesfrom the west as seen in all the trajectories (fig.6a,b), however it is the presence of a block orweakened anticyclone in the east which deter-mines the extent of any meridonial flow, andtherefore the extent of any Eastern Mediter-ranean precipitation. The influence of this east-ern precipitation can be seen clearly on the timeseries (fig. 2a-d). Comparing the CCA index(fig. 2a) to the two individual precipitationrecords (fig. 2c and 2d) it can be seen that dif-ferent temporal sections have distinct featuresfrom one or either of these rainfall time-series.For example the variation in the PCA index(fig. 2b) around 1945 closely follows that of theLisbon precipitation record whereas the varia-tion around 1970 has more in common with thePaphos time series, these variations can then befollowed into the CCA time-series.

Rodgers (1997) suggests that the NAO haslittle influence on the North Atlantic stormtracks and that the major changes in these tracksare consistent with sea level pressure changes inthe Bay of Biscay region. This part of the At-lantic is identifiable in fig. 4b as having a largeinfluence on the passage of moisture from theAtlantic into the Mediterranean, as seen by our‘English Channel’ centre of action centredslightly northwards of Rodgers’ in the Bay ofBiscay. When there is low pressure over the Bayof Biscay (fig. 5a), the Azores high is pushed tothe south allowing moisture to pass over the At-lantic and onto the Iberian Peninsula causinghigh precipitation. High pressure over the Bayof Biscay extends across Spain and Portugal re-

ducing the precipitation in this area. The stormtracks from the Atlantic in this situation pass tothe north of the Bay of Biscay and then turnsouth into the Mediterranean, as seen by the tra-jectories in fig. 6b. The influence of pressurechanges over the Bay of Biscay is described interms of a series of North Atlantic teleconnec-tions summarised by Rodgers (1990). RecentlyKutiel and Benaroch (2002) have identified anddescribed the upper level atmospheric telecon-nection pattern (NCP) at the 500 hPa level be-tween the North Sea and the Caspian areas asthe most significant in the European area. Theassociated circulation patterns are found to havea major and direct impact on the mean surfaceair temperature over the Eastern Mediterranean.However the impact of the NCP on rainfall ismore complex (Kutiel et al., 2002).

The strength of our canonical correlationbetween Mediterranean precipitation variabilityand the large scale pressure field over an 85year period, coupled with a very similar rela-tionship on annual, monthly and even dailytime-scales, suggest it is a robust phenomenonthat would be worth investigating further. Inparticular more work needs to be done on theinfluence of pressure variations in the east, in-cluding the Siberian high, before a completepicture of the link between Mediterranean pre-cipitation and sea-level pressure changes is ob-tainable. Winter precipitation persists furthereast through Iraq, Iran, Turkestan and onthrough Tashkent. However, the precipitationteleconnection patterns are rather weak throughthis area. To the south winter precipitation pat-terns change rapidly and there is less scope foradditional studies. We were unable to userecords on the desert margins (even sites suchas Cairo on the Nile Valley proved unworkable)i.e. our analyses in NE Africa were largely re-stricted to the classic Mediterranean zonewhich only occupies a very narrow coastal stripwhere the Libyan and Egyptian deserts en-croach on the Mediterranean Sea (Quezel,1981). The strong canonical pressure loadingsover Greenland and Russia, both areas well tothe north of the Mediterranean, point to theneed of also extending the geographical cover-age of the sea-level pressure variations and pre-cipitation patterns northwards. Indeed we have

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subsequently found that winter precipitationalong the eastern coast of the Black Sea corre-lates inversely with precipitation in NorthernSpain and Southern France in a very similarfashion to that of the East and West Mediter-ranean. More speculatively, links with otherteleconnections could be explored. Interesting-ly we find a significant correlation between thefirst canonical function and the Indian OceanDipole. This link, with a pattern of sea surfacetemperatures within the Indian Ocean (Saji etal., 1999), further suggests that an interactionfrom the east plays an important role in creatingthe correct conditions for the Mediterraneanprecipitation teleconnection. The well-docu-mented Southern Oscillation shows no linkwith the first canonical function but does have asignificant relationship with the second. Thisconnection, harnessed with the knowledge thatprecipitation over North-west Africa is correlat-ed with the Southern Oscillation (Ropelwskiand Halpert, 1987) and that spring precipitationin Spain shows the most significant relationshipwith the El Niño-Southern Oscillation in Eu-rope (Rodo et al., 1997), suggests that less pro-nounced Mediterranean precipitation variationsmay have links with various global pressurephenomena.

4.5. Statistical identification of pressure influences

The findings of this paper are based on theidentification of correlations between two largedata sets. The multivariate statistical methodsused provided an extremely powerful tool, al-lowing clear pictures to be obtained of the vari-ability. The consistency of results throughoutare remarkable, with multi decadal (86-year),winter, monthly and daily pressure charts allshowing the same broad features and highlight-ing the phenomena giving rise to the precipita-tion teleconnection. The consistency is suchthat even smaller local features such as theCyprus low are identifiable even on maps of 6-year (24-month) winter averages.

Consistency between independent statisticaltechniques is also found. The first precipitationprincipal component indicates the major pattern

of internal variability within the rainfall data(fig. 3). This produces an extremely similarvariability map to that of the first canonicalfunction (fig. 4a), which describes the correla-tion of the variability between the precipitationand pressure data. These two types of analysisdo not necessarily produce the same geograph-ical patterns as the former derives from climat-ic variability within a climatic parameter, whilethe later derives from the relationship betweenparameters.

5. Conclusions

The major pattern of precipitation variabili-ty in the Mediterranean is shown to be orientat-ed such that it is dipolar with maximum fluctu-ations in the north-west and south-east. The ze-ro variability line in the first principal compo-nent strikes in a south-west/north-east directionseparating the Iberian Peninsula and NorthernEurope from the Eastern Mediterranean andNorthern Africa.

Canonical correlation analysis shows that theEuropean surface pressure field and Mediter-ranean precipitation field variations are highlycorrelated with a north-west/south-east precipita-tion see-saw and an anomalous, highly variable,pressure region situated to the south of Britain.

Pressure maps for the high index years ofthe canonical function show that a high pres-sure system over West Kazakhstan causesmeridional flow, blocking precipitation fromthe Eastern Mediterranean. A simultaneoustranslation of the Azores high to the south al-lows westerly flow from the Atlantic to passover the Iberian Peninsula increasing precipita-tion.

Pressure maps for the low canonical func-tion show the Azores high translated north-wards over the Iberian Peninsula, thereby re-ducing precipitation in the Western Mediter-ranean. These average pressure maps also indi-cate the presence of a depression, the Cypruslow, situated in the Eastern Mediterranean. Itslocation is in good agreement with the findings

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of Zangvil and Druian (1990) who linked thephenomenon with high precipitation in Israel.

The winter pressure and precipitation inter-action in the Mediterranean is correlated withthe pressure variations across the North Atlantici.e. the North Atlantic Oscillation. A furtherlink with the Indian Ocean as described by theIndian Dipole Mode is also found to exist.

Our results show excellent consistency be-tween all four individual winter months and thetotal season (DJFM) over the 86-year periodstudied, as well as for single precipitationevents. The invariability over such a wide rangeof temporal scales lends confidence to the ro-bustness of the Mediterranean precipitationteleconnection and to its physical link with re-gional pressure patterns.

Acknowledgements

The data were obtained from databaseskindly provided by P. Jones of the Climate Re-search Unit at the University of East Anglia,and R. Vose and colleagues of the Global His-torical Climatology Network. The British At-mospheric Data Centre is thanked for its re-analysis database and trajectory service. Theauthors are grateful to Keith Weston who pro-vided much meteorological expertise and ad-vice concerning the synoptic analyses, especial-ly the trajectories.

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(received December 15, 2003;accepted March 4, 2004)

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