An empirical classification of weather types in the ... - Springer Link

Theor. Appl. Climatol. 66, 161±171 (2000)

Institute of Geography, Martin-Luther-University Halle-Wittenberg, Halle, Germany

An empirical classi®cation of weather types in the Mediterranean Basin and their interrelation with rainfall T. Littmann With 3 Figures Received January 29, 1999 Revised March 28, 2000 Summary This paper presents a classi®cation of weather types in the Mediterranean Basin based on cluster analysis of the daily occurrences of several surface pressure centers and the subjective identi®cation of 500 hPa trough axis positions (1992±1996). The procedure results in 20 types that explain 69% of overall pressure center variance and which are consistent with the seasonal succession of regional circulation. The development of weather types in winter is primarily controlled by the eastward propagation of barotropic waves while departures from the zonal ¯ow pattern in summer tend to be linked to blocked stationary pools. H1types with anticyclonic circulation in the Western Mediterranean and cyclonic ¯ow in the eastern part are well interrelated with zonal and anticyclonic general weather types in Central Europe. H2-types featuring a weak Azores Anticyclone interrelate with a variety of meridional circulation types after the Hess and Brezowski (1969) classi®cation. The 20 types explain rainfall variance in the core Mediterranean regions (as de®ned by principal components) to a high degree while rainfall variance in marginal regions is in¯uenced by circulation patterns not being typical for the Mediterranean Basin.

1. Introduction Daily classi®cation of synoptic situations may complete regional mean value climatology in three ways: It may enhance our understanding of dynamic meteorological processes such as rainfall occurrence; it may help to explain regional differences of climate, and it may enable shortterm forecasting if meteorological elements show

signi®cant correlations with any given synoptic type (Wanner, 1980). The choice of parameters for any classi®cation should, however, be objective, clear, and as simple as possible. In synoptic climatology, subjective methods operating on a regional scale normally involve the most important centers of action, air masses and fronts for parameterisation like dominant anticyclones and cyclogenetic centers. Objective methods mostly use gridded data on the regional upper tropospheric pressure and wind ®elds or numeric and stochastic classi®cation techniques (Wanner, 1980). Any subjective approach will make its application to individual situations dif®cult as the climatologist faces the problem of assigning the actual surface pressure ®eld to its abstraction in a given weather type (Jacobeit, 1985; Wanner, 1980) because atmospheric modes are continuous and any delimitation of boundaries between classes must be arbitrary (Barry and Perry, 1973). Numerical approaches, on the other hand, tend to result in so many types, classes, and subgroups if the overall explanation of parameter variance is still to be satisfactory that they are hardly applicable at all (Jacobeit, 1985). In this context, of the two stages of synoptic climatological research (classi®cation of daily synoptic constellations and their evaluation with the reality of local weather elements) the classi®cation stage is the most time consuming one (Yarnal, 1984).

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For example, a subjective de®nition of general weather types (Grosswetterlagen) for Central Europe by Hess and Brezowski (1969) resulted in 29 types while an objective approach presented recently by Dittmann et al. (1995) based on cyclonic and anticyclonic ¯ow on the 1000 hPa and 550 hPa levels, large-scale air¯ow on the 700 hPa level, and on the vertical integration of precipitable water from gridpoint data arrived at 40 different types for Central Europe. There have been limited approaches to regional classi®cation of weather types for the entire Mediterranean Basin. Butzer (1960, after Jacobeit, 1985) closely linked his dynamic investigation of Mediterranean climate to the Central European ,,Grosswetterlagen`` without producing signi®cantly individual regional patterns. The Meteorological Of®ce's (1962) classi®cation presented 5 main weather types and 4 subtypes based on the geographical position of controlling centers of action (anticyclones, cyclones). Urbani and D'Angiolino`s (1974) classi®cation resulted in only 7 circulation types over Europe and the Mediterranean on the 700 hPa-level while Goossens (1986) and Maheras et al. (1992) linked long-term ¯uctuations of temperature and rainfall to a few synoptic patterns. In this context, CorteReal et al. (1995) presented a detailed analysis of the interrelation of such ¯uctuations and both surface and mid-tropspheric circulation over the Mediterranean based on canonical correlation. They showed non-seasonal temperature anomalies to be much more coherent with atmospheric ¯ow patterns as compared to rainfall ¯uctuations, especially since regional temperature anomalies are anti-phase. Atlantic blocking and cyclogenesis in the western Mediterranean results in positive anomalies in the eastern part, and vice versa (``Mediterranean Oscillation''). More regional and local classi®cations are largely devoted to regional climatological phenomena such as rainfall. Gazzola (1969, after Jacobeit, 1985) separated 22 surface and 500 hPa con®gurations, respectively, explaining nearly 70% of spatial rainfall distribution in Italy. Using characteristic positions of controlling anticyclonic centers and tracks of cyclones, Maheras (1985) presented 5 anticyclonic, 6 cylonic, and 4 mixed types of circulation over northern Greece that correspond to 11 individual weather types obtained from principal component analysis. For

Israel, Aelion (1958) described 8 different synoptic types that occur with high-magnitude rainfall events over the area. Applying principal component analysis to a set of surface and aerological parameters, Ronberg and Sharon (1985) identi®ed 18 weather types. Koplowitz (1973) arrived at 47 surface pressure types and 23 500 hPa-types for the same area using composit analysis. Recently, Goodess and Palutikof (1996) identi®ed 8 major circulation types from sea level pressure data that interrelate closely to daily rainfall in southeastern Spain. In the light of previous work, we can clearly indicate a de®cit in the classi®cation of synoptic types for the entire Mediterranean that is statistically objective but also serves practical purposes such as the understanding of regional rainfall patterns or the correlation of regional circulation types with the larger Atlantic-European circulation. 2. Methods Basic elements of large-scale synoptic patterns are pressure cells, either anticyclonic or cyclonic (dynamic frontal depressions or thermal lows). Such elements were identi®ed on the surface charts of the daily European Meteorological Bulletin (German Weather Service) for the sampling period from November 1992 to June 1996 (1338 days). These were for the region under consideration (10 W to 40 E and 30 N to 45 N) and were compiled as to their location. Additionally, the axis position of 500 hPa-troughs was taken from the respective charts. Table 1 presents the centers of action used for classi®cation in this paper. Data on the daily occurrence of these elements were then stored in binary form (1 present, 0 not present). Such a procedure does not completely eliminate subjectivity in the determination of synoptic elements (cf. Barry and Perry, 1973). However, there is no ambiguity when the elements are indicated on the charts as individual pressure cells (i.e., clearly delimitated by isobars in the same way as they are interpolated from gridded pressure data). On days with transitional situations, i.e. the establishment or vanishing of a pressure cell, the element was assigned to either the preceeding or following pattern. Methods of data analysis are discussed in the respective sections. Monthly rainfall data for the regional 1 1 grid were supplied by the WMO World Center of

An empirical classi®cation of weather types in the Mediterranean Basin and their interrelation with rainfall

Precipitation Climatology's data®les, additional daily rainfall data for 32 stations along the coastal areas of Morocco, Algeria, Tunisia, Lybia, Egypt, Lebanon, Turkey, and Cyprus from the NCAR (Boulder) internet ®les.

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tance and Ward's algorithm for cluster fusion was applied to the data. For the evaluation of model results and further practical purposes, the following three criteria were found useful: 1. The internal (residual) variance within each cluster was limited to < 10%. Such clusters provide suf®cient homogeneity for further interpretation. 2. The individual variance of each synoptic element should be explained suf®ciently by any cluster solution. A minimum of > 50% of element variance must be explained if the solution is to hold for the majority of individual cases. 3. The total number of clusters should not be too high. Although a high number of clusters reduces internal variance, the overall classi®cation demands a reasonable and representative quantity of types.

3. Classi®cation of weather types It should be emphasized that in terms of classi®cation techniques the present approach presents a mixture of subjective (identi®cation of relevant pressure cells, Table 1) and numerical procedures (see Dittmann et al., 1995). For empirical classi®cation purposes, a raw data set of binary data as described above rather limits the availability of statistical procedures for stochastic classi®cation. Methods based on proximity measures such as correlation and factor analyses cannot be applied to the nominal scale as the total variance in case of the equal appearance of two paramaters will be zero. Thus, a hierarchical cluster analysis using the squared Euclidian dis-

The ®rst cluster solution matching the ®rst criterion resulted in 13 clusters explaining 60% of

Table 1. Elements used for the classi®cation of weather types Element

Type

Description

H1

anticyclone

H2

anticyclone

H3

anticyclone

H4

anticyclone

H5

anticyclone

H6

anticyclone

L1

thermal depression

L2

dynamic depression

L3

dynamic depression

TR1 TR2

no trough on 500 hPa trough or remnant pool (``Western Trough'')

TR3

trough or remnant pool (``Central/Eastern Trough'')

TR4

trough or remnant pool (``Black Sea Trough'')

Azores High in an easterly position over Spain and the western Mediterranean Basin as far as Italy Azores High in a westerly position far offshore over the Atlantic Ocean Central European Anticyclone north of the Alps over Germany and France East European Anticyclone located either over the Baltic Sea, Poland, or the Balkans as far as Northern Greece Libyan Anticyclone, a high pressure cell located over Libya and Northern Egypt Extensions of the Siberian Anticyclone as far as Eastern Europe, the Black Sea, and Turkey Large thermal low centered over the Arabian Peninsula and the Persian Gulf Cyclones generating in the western to central Mediterranean Basin (Gulf of Genoa, Tyrrhenian Sea) as well as intrusions of Atlantic frontal systems into this area Cyclones generating in the central to eastern Mediterranean Basin (``Cyprus Low'') Completely zonal upper ¯ow, no barotropic waves Barotropic wave with its trough axis centered over the offshore East Atlantic or over Western Europe or stationary remnant of this wave Barotropic wave with its trough axis centered over Central Europe, the central or eastern Mediterranean or stationary remnant of this wave Barotropic wave with its trough axis centered over the Black Sea, either meridional or with NE-SW-orientation into the eastern Mediterranean or stationary remnant of this wave

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overall element variance. Although the Azores High parameters H1 and H2 and the East European High H4 were well represented in this model, others such as Western or Central Mediterranean cyclogenesis (L2) were not. Increasing the number of clusters led to a parallel increase of explained element variance until a total number of 20 clusters. In this second model, internal cluster variance is below the 5%-limit and nearly all parameters are represented following the second criterion. High variability is still found for the Libyan Anticyclone H5 as well as for intrusions of the Siberian Anticyclone H6, and for cyclogenesis in the Western Mediterranean (L2). The mean explained variance of 69% could not be increased further by increasing the number of clusters (a 30 cluster solution showed mean explained variance of 73% but would not be easy to handle). Following a 2-test (2 5089, 152 degrees of freedom), the best-®t 20 cluster model is signi®cant with p < 0.01. Although being signi®cant on this basis, it should be mentioned that this solution may not be stable because of the rather limited time series. What is more, subjectivity in parameterisation may also not be eliminated by the statistical approach. Interpretation of the cluster model is based on two selective prerequisites: 1. Signi®cant synoptic elements within a cluster are those that show a high positive deviation from their expected cluster frequency following the 2-test mentioned above. 2. The correlation of signi®cant elements within a cluster (i.e., their similarity of appearance) should be high enough to be primary features. In case of low interrelation, elements are classi®ed as secondary phenomena. For this purpose the Russel and Rao similarity coef®cient for binary data (a binary point product in terms of distance correlation) was applied and the similarity of appearance is expressed as a r2-coef®cient. Figure 1 shows the results of the empirical classi®cation. In all clusters except one the position of the Azores High is the dominating primary feature. Therefore, it is used as a ®rst-order characteristic for the denomination of types followed by the regional surface ¯ow type (anticyclonic A or cyclonic C). The H1-group of synoptic types appears in 54% of all observed

days. As to be expected from its dynamic boundary conditions (strong circulation associated with the anticyclone's seasonal southerly position), 55% of all H1-cases occur in winter (November to February), 21% in spring (March and April as transitional months with many winter characteristics), and only 24% in summer. In winter, most H1-situations are accompanied with either regional cyclogenesis or Atlantic frontal intrusions. However, it is only the Cyprus Low (L3) in types H1-C4 and H1-C5 that appears in 100% of those situations whereas cyclogenesis in the Western and Central Mediterranean is far more variable in winter. The H2-group (Azores High in westerly position over the Atlantic in its seasonal northerly position) occurs in 42% of all cases. Except for the H2-A2 type with weak zonal ¯ow mostly in summer, high pressure over the Western Mediterranean is not dominant. 13% of H2-cases occur in winter, 35% in spring and early summer (March to May), and 52% from June to September. Following the Meteorological Of®ce's (1962) description, the dominance of disturbed H2-types, i.e. all except H2-A2 that show western to central trough intrusions, is an effect of the northward extension of the Azores High towards the British Isles. Cyclogenesis and frontal intrusions somewhere in the Mediterranean occurred in 64% of all days over the observation period. 56% happened to be H1-type situations and 44% were H2-types. While this distribution does not show any of the two groups to be more cyclogenetic, winter cyclones were primarily found within the H1-group (39% of all cyclones). Cyclones in H2-situations are much more typical in spring (24% vs. 12% H1-types) and summer (16% vs. 5% H1-types). Linked to the seasonal structures of the main centers of action, the individual weather types may show both high and low seasonality themselves. Figure 2 shows only highly signi®cant departures from the expected relative frequency of type (percentage distribution over the months of the year) following 2-testing. In winter, types exclusively linked to the presence of the Siberian Anticyclone in the Eastern Mediterranean and simultaneous cyclogenesis in the western part associated with a Central Trough (H1-C1, H1-C5, H1-C7) occur with singular signi®cance. Winter is also the season where the Central European Anticyclone may signi®cantly control ¯ow over the Mediterranean (H2-A3)


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Fig. 1. Weather types in the Mediterranean following a 20 cluster-solution. Bold lines denote surface pressure systems and circulation, thin lines 500 hPa-structures, and broken lines secondary features. The Russel and Rao-coef®cients indicate the similarity of occurrence of the respective synoptic elements

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T. Littmann

Fig. 2. Signi®cant monthly relative frequency of weather types. Types in box denote singular signi®cant occurrence of a given type within one season only

whereas type H2-A2 also occurs in spring as a transitional feature of East European high pressure. Other cyclonic types occurring not only in winter are interrelated with troughs over the Mediterranean but not with the presence of the Siberian Anticyclone (Fig. 2). These types may also occur signi®cantly in summer in the case of Atlantic blocking while most anticyclonic types are concentrated on the summer months. The transitional seasons indeed show a mixture of winterly cyclonic and summerly anticyclonic types. According to regional climatic descriptions (cf. LineÂs EscardoÂ, 1970; CantuÂ, 1977; Furlan, 1977; Taha et al., 1981), this solution seems consistent in terms of the seasonality of regional pressure cells. In addition, clustering the frequency distribution of synoptic elements used in this analysis (Table 1) by the months of the year resulted in a very clear de®nition of climatic seasons for the Mediterranean region. In early winter (November, December) the Azores High tends to be over the Western Mediterranean while the East European High is already established. Intrusions of the Siberian Anticyclone are not signi®cant until December. The depression over the Persian Gulf is less frequent but cyclones in the Eastern Mediterranean dominate. Upper ¯ow may still be zonal or show Central and Black Sea Troughs. Core winter (January) is characterised by the virtual absence of the Azores High anticyclonic in¯uence but intrusions of the Siberian Anticyclone are dominant in the eastern half. Central and Eastern Troughs lead to local cyclo-

genesis over the entire Mediterranean. In late winter and spring (February to April) the Azores High is still far out over the Atlantic. As a secondary pressure center, the Libyan Anticyclone may be above its mean frequency but the thermal depression over Arabia is not yet developed. Cyclogenesis in the entire Mediterranean is still active while zonal upper ¯ow shows no sigi®cant deviation from its annual mean. Transitional cases occur in May and September only. The Azores High is in a western position over the Atlantic but the thermal depression over Arabia and the Persian Gulf is present. Frontal passages may reach the Western Mediterranean or Genoa cyclones develop under a Western Trough. In early summer (June, July) the Azores High has moved towards the Western Mediterranean and may coalesce with the Central European Anticyclone at times. The Arabian Low is fully developed and Mediterranean cyclogenesis is below all averages. Core summer (August) shows another shift of the Azores High towards the Atlantic and the Arabian depression deepens to its maximum. Zonal upper ¯ow is dominant and cyclones are con®ned to the northernmost parts of the region. Autumn (October) is an exceptional period as the Azores High frequently cannot be exactly located. High pressure builds up over Eastern Europe and Central Asia while the region is ¯anked by a Western Trough and the Black Sea Trough. Cyclonic centers are not signi®cant. It may be concluded that such seasonal structures imply large-scale opposing patterns of air ¯ow


over the Western and Eastern Mediterranean Basin (cf. Corte-Real et al., 1995). Weak ¯ow over the entire region (August) seems mainly an effect of the westernmost position of the Azores High over the Atlantic but not so much of its northerly position as described by the Meteorological Of®ce (1962). 4. Interrelation of weather types with Mediterranean rainfall A prerequisite of the investigation of Mediterranean weather types and rainfall is the identi®cation of rainfall regions. For this purpose, 1 1 gridded monthly rainfall totals for 7 W to 37 E and 30 N to 45 N over the observation period were aggregated by means of PCA. A varimaxrotated solution with 92% explained variance resulted in 10 PCs that represent regions showing highly similar rainfall regimes from 1992 to 1996 (Fig. 3, Table 2). PC 2 (Gulf of Biscaye, northeastern Spain, southern France; Gulf of Genoa, Northern and Central Italy, Corsica, Sardinia, and the Adriatic Coast) shows high rainfall with a long rainy season starting as early as September (maximum rainfall) until April while summer rainfall regularly occurs. A similar seasonal pattern is found in PC 5 (North African Coast and Tell Atlas regions), PC 9 (Southeast Spain), and even PC 10 (Moroccan High Atlas and adjacent parts of the northwestern Sahara). However, the rainy season is shorter in these regions as compared to

167

the northern component 2. This western Mediterranean pattern is fairly consistent with regional rainfall dynamics reported for Spain (LineÂs EscardoÂ, 1970) and Italy (CantuÂ, 1977). Depending on the geographical position of the southern branch of the circumpolar vortex, the Iberian Peninsula encounters the intrusion of Atlantic frontal systems that account for 54% of total rainfall from September to November and may move southeast towards North Africa. Northern Italy experiences winter rainfall mainly from Genoa-type cyclones that form under a westerly trough while summer rainfall is an effect of regional cyclogenesis in the Po Valley. PC 1 is a large and astonishingly homogeneous rainfall region including the entire Eastern Mediterranean (Tunisia, Sicily, Southern Italy, Greece, Asia Minor and Black Sea region, the Levantine and Egyptian coasts as far as Cyrenaica). Rainfall is much lower, a compared to PC 2, starts later (November is the month of maximum rainfall) and terminates in March. This is because it is mainly linked to the de¯ection of cyclonic tracks and Tyrrhenian cut-off lows towards Greece and Cyprus after the establishment of the Siberian Anticyclone and of the large thermal contrast over the Eastern Mediterranean (Furlan, 1977; Taha et al., 1981). A similar seasonal pattern is found in region 8 (Moulouya Basin and the westernmost Tell Atlas with a rainfall maximum in February and March) which is quite different from adjacent PC 5 and PC 9. Contrary to this, PC 3 (Central and Southern Spain and the

Fig. 3. Mediterranean rainfall regions, November 1992±June 1996. Varimax-rotated PCA-solution for gridded monthly means

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T. Littmann

Table 2. Rainfall variance of Mediterranean rainfall regions (PCs as in Fig. 3) explained by the frequency of synoptic types (%) following individual one-way analyses of variance (Anova) Synoptic Type

PC 1

PC 2

PC 4

PC 3

PC 10

PC 5

PC 9

PC 6

PC 8

PC 7

H1-A1 H1-A2 H1-A3 H1-A4 H1-C1 H1-C2 H1-C3 H1-C4 H1-C5 H1-C6 H1-C7 H2-A1 H2-A2 H2-A3 H2-C1 H2-C2 H2-C3 H2-C4 H2-C5 H3-H4 total 1 total 2

17 ÿ4 2 72 ÿ10 23 14 5 0 ÿ1 ÿ8 ÿ11 0 0 ÿ6 ÿ1 7 ÿ2 74 8 73 76

ÿ14 20 0 ÿ24 ÿ25 ÿ7 ÿ2 ÿ4 5 ÿ2 ÿ2 ÿ10 ÿ3 0 51 1 ÿ10 3 ÿ1 ÿ15 68 82

ÿ11 13 13 ÿ8 ÿ43 ÿ6 ÿ5 ÿ4 49 4 8 ÿ5 ÿ1 0 28 4 10 ÿ1 ÿ2 ÿ4 65 69

37 ÿ5 15 ÿ4 ÿ17 ÿ12 0 ÿ9 ÿ9 5 2 ÿ17 2 ÿ5 ÿ8 7 ÿ11 14 ÿ5 ÿ15 40 59

ÿ6 ÿ3 12 ÿ18 ÿ17 ÿ13 0 ÿ4 4 1 9 ÿ26 ÿ5 ÿ6 ÿ13 10 ÿ6 30 ÿ4 ÿ15 38 57

ÿ9 ÿ2 4 13 ÿ15 ÿ5 2 ÿ5 ÿ3 ÿ9 ÿ22 ÿ5 8 ÿ2 ÿ7 ÿ1 ÿ22 32 30 ÿ16 67 76

ÿ8 6 ÿ6 ÿ8 ÿ20 6 ÿ6 18 ÿ2 ÿ6 14 ÿ33 2 ÿ1 ÿ6 ÿ1 ÿ34 ÿ3 37 ÿ20 54 68

ÿ9 ÿ5 ÿ5 ÿ21 38 ÿ4 0 ÿ5 ÿ3 ÿ1 ÿ21 ÿ15 ÿ2 5 ÿ8 ÿ5 24 ÿ5 8 18 54 69

ÿ6 ÿ5 ÿ2 ÿ8 30 ÿ13 ÿ2 14 ÿ2 0 ÿ13 ÿ14 ÿ2 ÿ5 ÿ16 ÿ2 22 ÿ3 ÿ3 ÿ21 49 86

21 ÿ5 ÿ4 12 23 ÿ5 0 ÿ11 4 ÿ3 16 ÿ13 ÿ3 29 ÿ3 ÿ1 ÿ23 ÿ4 1 13 47 63

Notes: Positive values mean positive departures from the rainfall series mean, negative values negative departures. Vertical lines group PC-clusters that show high similarity (italics) as to the spectrum of synoptic types that effect positive monthly departures from the respective series mean. Total 1 is the percentage of explained PC-series variance when applying a design of 2 factors and 1 covariate using the 3 highest positive variance values. Total 2 results from a multiple Anova using the the 12 highest positive and negative values from the PC-spectrum as 2 factors and 10 covariates.

Atlantic parts of Morocco) shows a very short rainy season from October to January (Atlantic frontal systems along the southernmost position of the circumpolar vortex, cf. LineÂs EscardoÂ, 1970) while rainfall can be above average again in May. PC 4 is much drier but shows the same rainfall months while maximum rainfall occurs in October, December and February. In between this short rainy season pattern and the long season regime reaching deep into northwest Africa is the continental Saharan PC 7 with no similarity to any of the Mediterranean types (rainfall in March and September above average). Also quite different is Bulgaria and the western Black Sea with rainfall in all seasons but high amounts in winter (November, December) and summer (May, June). In this context, it is interesting that Corte-Real et al. (1995) found a major boundary nearly identical with that of the main types PC 1 and PC 2 for non-seasonal rainfall anomalies representing the mid-tropospheric Mediterranean Oscillation enforced by the opposition of cyclonic ¯ow over the eastern Atlantic and western

Mediterranean and anticyclonic ridges over the eastern part and vice versa. How much variance of rainfall in each region can be explained by the respective monthly frequency of our 20 synoptic types? Table 2 shows results of individual oneway analyses of variance computed for factor values of the 10 rainfall PCs as well as their aggregation with respect to relevant rainfall types. Generally, regions with a longer rainy season show relatively good interrelations with the synoptic types. 82% of PC 2 rainfall variance over the observational period is explained by an analysis of variance design including up to 12 synoptic types as factors and covariates. The best interrelation is shown for H2-C1 which is characterized by a trough over the region and Genoa-type cyclones in early winter. Although anticyclonic, also H1A2 explains higher rainfall in PC 2 in spring. This may be the effect of a simultaneous intrusion of Atlantic fronts into the northern part of the Genoa Basin or the formation of local Po Valley cyclones (cf. CantuÂ, 1977). In PC 4 (the Tunisian and Cyrenaica coasts; 69% explained rainfall


variance) H2-C5 is a signi®cant type and H1-C5 with highly variable cyclogenesis under a remnant pool in the Central Mediterranean may lead to rainfall in winter while H1-A2 should not feature rainfall events except for the occurrence of Saharan depressions in early spring. All other rainfall regions with an early start of the rainy season in September show high interrelations with fairly similar synoptic types featuring regional cyclogenesis. PC 5 (North African coast) rainfall variance is mainly explained by H2-C4 in September when Tyrrhenian cyclones follow a southerly track or with H2-C5 cyclones forming under a westerly trough. The same type H2-C5 seems to be responsible for most of region 9 positive rainfall departures in southeast Spain, which is grouped along with PC 5. PC 1 with a late onset of the rainy season in November is interrelated with a few types that explain 73% of rainfall variability. H2-C5 cyclones may lead to rainfall in the western parts as far as Greece (cf. Furlan, 1977) as in PCs 5 and 9 while H1-C2 is a typical winterly rainfall pattern in the eastern Mediterranean. For PC 8 around the Moulouya Basin H2-C3 with westerly cyclones provides a good explanation for late winter rainfall while winterly H1-C1 may lead to rainfall in this region only in case of frontal passages in a more westerly position than indicated by this type. More easterly cyclonic tracks of the same type effect the same pattern in PC 6 (the southern Black Sea region). With the rainy season becoming shorter, the interrelations also become weaker. In PC 3 (the Atlantic parts of Spain and Morocco) and PC 10 (High Atlas) 40% and 28% of rainfall variance, respectively, are explained by H1-A1 in winter and by H2-A3 in May. Both types do not imply any regional cyclogenesis but may allow for the intrusion of Atlantic fronts. In this way, PC 3 is not part of Mediterranean rainfall regimes (cf. LineÂs EscardoÂ, 1977). Winter and spring types H2-C4 and H2-C2 denote positive departures in both regions when cyclones follow a southerly track or frontal passages may be associated with a westerly trough. Peripheral PC 7 (northern Sahara) shows 47% of explained variance by H2-A3, H1-A1, and H1-A4 which, however, do not provide any plausible explanation in terms of regional cyclones or frontal passages. Rainfall events in this region seem to occur randomly within such situations.

169

5. Conclusions It was shown that the classi®cation of 20 different weather types based on a subjective identi®cation of controlling pressure cells and statistical analysis presented in this paper provides a reasonable solution in terms of the seasonal structures of regional mid-tropospheric and surface circulation patterns. The weather types also explain regional rainfall patterns in the core areas of the Mediterranean Basin to a large extent. However, it should be clearly stated that these interrelations predominantly provide climatological information that may not be useful for short-term forecasting. Comparing the cluster solution to the Meteorological Of®ce's (1962) classi®cation gives some good correspondence of types although this classi®cation incorporates synoptics over a larger region such as the Northern Atlantic and Scandinavia. The Meteorological Of®ce's type A may be easily compared to cluster types H2-C2, H2-C3, and H2-C5; subtype A1 (including the East European High) to H2-C1. Being a typical cold air outbreak situation with regional frontogenesis in the Western Mediterranean, type A corresponds exclusively to the H2-group in winter and spring. Type B is matched by type H1C3; subtype B1 by H1-C4 or H1-A1, H1-C4, and H2-C4. However, subtypes B2 and B3 correspond to only one cluster (H1-C2). Types C and E mainly match the H1-group such as H1-C1, H1C5, H1-C7 (type C), and H1-A2, H1-A3, H1-C6 (in case of a blocking remnant upper pool), H2A1, and H2-A2 (type E). Type D may correspond to a mixed group of H1-A4, H2-A3, and H3-H4. It may be concluded that the numerical classi®cation presented here (20 types) does not contradict the Meteorological Of®ce's (1962) qualitative classi®cation (9 types) but de®nes synoptic situations in the Mediterranean context with a higher spatial resolution. As was shown before, the 20 types are well controlled by the pattern of cirumpolar vortex circulation and the corresponding spatial arrangement of pressure centers that also encompass the larger European context. In this way, it may be expected that there are some basic interrelations with Hess and Brezowski's (1969) catalogue of general weather types. A 2test applied to the observational period proved the overall interrelation to be signi®cant while a contingency test showed only 48% of Mediterra-

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T. Littmann

nean weather types variance explained by the ,,Grosswetterlagen``. Most H1-types, with zonal ¯ow over the Western Mediterranean and meridional ¯ow over the central and eastern parts mainly associated with troughs and regional cyclogenesis, are fairly well interrelated (> 50% explained variance) with either westerly or anticyclonic circulation over Central Europe. On the other hand, the interrelation with the Western Mediterranean seems more complex: both westerly and anticyclonic types over Central Europe may coincide with anticyclonic ¯ow in the Western or the entire Mediterranean Basin at all seasons. Contrary to these H1-situations, H2types naturally are more or less meridional and thus correlate with southerly or a variety of mixed circulation types over Central Europe. From our rather limited time series, some characteristics of persistence, recurrence and succession of weather types can be concluded. Grouping of the weather types mean persistence and recurrence intervals reveals that most cyclonic types occurring signi®cantly in both winter and summer, and showing L2-cyclones, developed under a Western Trough or stagnant pool over the Tyrrhenian Sea have an intermediate persistence of 1.7 days but rather short recurrence intervals (22 days; types H1-C2, H1-C6, H2-C1, H2-C3, H2-C5). On the other hand, winterly types H1-C1 and H1-C4 associated with high pressure cells over Eastern Europe or Asia and Central or Black Sea Troughs show low recurrence (26 to 51 days) but are persistent (2 days). L2-cyclogenesis under a Central Trough is also the main characteristic of the remaining cyclonic types in winter and the transitional seasons which show both low persistence (1.4 days) and recurrence (33 to 47 days). Zonal ¯ow and anticyclonic types may occur in all respective groups. As barotropic waves normally propagate from the Eastern Atlantic into Asia within a few days and related pressure cells may build up, move and vanish in the same time span, low persistence of types is to be expected except of situations where the much more persistent Siberian Anticyclone is present in the Eastern Mediterranean. In this context, a few typical successions of weather types correspond to the propagation of barotropic waves, e.g. H2C1 is followed by H1-C3 when the Western Trough narrows, H1-C1 is often replaced by H1C2 when the Central Trough vanishes and leaves

a stationary pool. H1-C2 in turn is replaced by H1-C4 under the same condition that high pressure over Eastern Europe is followed by an Eastern Trough system (including the Cyprus Low). The cyclonic summer type H2-C5 most frequently turns into the blocked type H1-C6, secondarily into H2-C1. H1-C7 shows a similar tendency to leave a remnant pool and consequently is followed by H1-C6. These patterns clearly show that the development of weather types in winter is primarily controlled by the formation and eastward propagation of barotropic waves and the subsequent arrangement of pressure cells while in summer the prevailing zonal ¯ow is interrupted by rare cases of trough intrusion which, however, do not propagate readily but tend to leave blocked stationary pools under which strong depressions develop that may lead to heavy rainfall. Rainfall patterns in the Mediterranean core areas (PCs 1 and 2) are well explained by the interrelated synoptic types (more than 75% of rainfall variance can be explained) while the interrelations become less clear as the rainfall regions become more peripheral to the main cyclogenetic centers and cyclonic tracks in the Mediterranean Basin. Our results show that in most cases neighbouring regions show high similarity with respect to those synoptic types being responsible for positive departures from rainfall grand totals. However, it should be emphasized that there are no linear or non-linear interrelations of rainfall amounts and the frequency of the respective synoptic types. A better statistical interrelation should be expected when comparing the frequency of types and of rainfall events, irrespective of rainfall amounts. Acknowledgements I would like to thank the German Weather Service (DWD) for supplying rainfall data, Mr. J. Kalek for assistance in weather chart data compilation, Mr. U. Traeger for data preparation, and Mrs. E. SchroÈter for drawing the maps. I am also grateful to the reviewers of TAC for their critical comments and advice. References Aelion E (1958) A report of weather types causing marked dust storms in Israel during the cold season. Hakirya: Met Service Israel, Series C, Misc Papers 10 Barry RG, Perry AH (1973) Synoptic climatology. Methods and applications. London: Methuen, 555 pp

An empirical classi®cation of weather types in the Mediterranean Basin and their interrelation with rainfall CantuÂ V (1977) The climate of Italy. In: Wallen C (ed) Climates of Central and Southern Europe. (World Surv Climatol 6). Amsterdam: Elsevier, pp 127±184 Corte-Real J, Zhang X, Wang X (1995) Large-scale circulation regimes and surface climatic anomalies over the Mediterranean. Int J Climatol 15: 1135±1150 Furlan D (1977) The climate of Southeast Europe. In: Wallen C (ed) Climates of Central and Southern Europe. (World Surv Climatol 6). Amsterdam: Elsevier, pp 185± 236 Goodess CM, Palutikof JP (1996) Daily rainfall scenarios for Southeast Spain. Abstract 28th Internat Geogr Congress The Hague August 1996 unpubl Goossens C (1986) Regionalization of the Mediterranean climate. Theor Appl Climatol 37: 74±83 Hess P, Brezowski H (1969) Katalog der Groûwetterlagen Europas. Ber Dt Wetterdienst 113: Offenbach, Wetterdienst, 136 pp Jacobeit J (1985) Die Analyse groûraÈumiger StroÈmungsverhaÈltnisse als Grundlage von Niederschlagsdifferenzierungen im Mittelmeerraum. (WuÈrzburger Geogr Arb 63) WuÈrzburg, Inst Geography 145 pp Koplowitz R (1973) An objective classi®cation of synoptic pressure-®eld patterns of the Eastern Mediterranean Basin for use in synoptic-climatological studies. Thesis, The Hebrew Univ of Jerusalem, unpubl 152 pp LineÂs Escardo A (1970) The climate of the Iberian Peninsula. In: Wallen C (ed) Climates of Northern and Western Europe. (World Surv Climatol 5) Amsterdam: Elsevier, pp 195±239

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Maheras P (1985) Correspondences between types of circulation and weather types. Application to the area of Thessaloniki. Z Meteorol 35: 26±35 Maheras P, Balafoutis C, Va®adis M (1992) Precipitation in the central Mediterranean during the last century. Theor Appl Climatol 45: 209±216 Meteorological Of®ce (1962) Weather in the Mediterranean, vol 1, 2nd ed. London: Her Maj Stat Of®ce, 225 pp Ronberg B, Sharon D (1985) An objective weather typing system for Israel: a synoptic climatological study. 9th Conf. on Probability and Statistics in Atmosph Sci, Virginia Beach, unpubl, 5 pp Taha MF, Harb SA, Nagib MK, Tantawy AH (1981) The climate of the Near East. In: Takahashi K, Arakawa K (eds) Climates of Southern and Western Asia. (World Surv Climatol 9). Amsterdam: Elsevier, pp 183±246 Urbani M, D'Angiolino G (1974) Tipi di circolazione nella media troposfera sull' Europa e sul Mediterraneo. (Serv Met Roma, Nota Tech 21) Rome: Italian Met Serv, 64 pp Wanner H (1980) GrundzuÈge der Zirkulation der mittleren Breiten und ihre Bedeutung fuÈr die Wetterlagenanalyse im Alpenraum. In: Oeschger H, Messerli B, Svelar M (eds) Das Klima. Berlin: Teubner, pp 117±124 Yarnal B (1984) A procedure for the classi®cation of synoptic weather maps from gridded atmospheric pressure surface data. Computers & Geosciences 10: 397±410 Author's address: PD Dr. Thomas Littmann, Institute of Geography, Martin-Luther-UniversitaÈt Halle-Wittenberg, Domstrasse 5, D-06108 Halle (Saale), Germany.