Table of contents
2 The Mediterranean climate
3 Seasonal variability
3.1 Summer conditions
3.2 Winter conditions
4 The impact of the Mediterranean Sea
5 Mediterranean winds
7.2 Internet sources
The Mediterranean Sea is an almost enclosed basin with a coverage of 4000 km in west-east and 1200 km in north-south direction.1 Many European, North African and Middle Eastern states abut on this ocean and are massively influenced by it. The Mediterranean can be denoted as a cutting point between subtropical and humid climate and is influenced by both of them. This and the Mediterranean Sea lead to a very specific and individual climate around this basin.
Figure 1: The Mediterranean Sea
illustration not visible in this excerpt
In this paper the leading factors of the climate conditions are revealed, which include references to global atmospheric circulations, regional climate patterns and micro-scale descriptions of specific phenomena in the Mediterranean. Therefore I refer to different scientific articles and mainly to two specific Mediterranean monographs (Branigan; Bolle).
Because of the introducing character of this paper most phenomena are explained in a more general way and do not have reference to direct climate data. More detailed knowledge and data basis can be taken by the references and footnotes.
2 The Mediterranean climate
The Mediterranean climate type is one of the most clearly defined climate zones in the world.2 It is constituted as the dry-summer subtropical climate, which is discovered around the Mediterranean sea (between latitudes 30° and 45°) and important impacts of the Mediterranean Sea.3 In the Köppen-classification this is the Csa and Csb (dry-summer subtropical) climate category. It is influenced by subtropical and mid-latitude weather impacts.4 The variability of Mediterranean climate, which will be illustrated in this paper, is the reason, why geographers often do not use the term “Warm Temperate Western Climate” but the particular term Mediterranean Climate. Hence, there is a massive inner-annual or seasonal variation of climate patterns and a transitional character of this climate zone. Branigan (1975: 31) states 4 essential characteristics of Mediterranean climate: winter rain, summer drought, mild winters and hot summers.
In winter the region is affected by cyclones of the polar front and the Westerlies. Flocas (1984: 255) proves the occurrence of one cold front each weak (compared with one per month in spring, summer and autumn).5 Moist and warm air masses cominh from the Atlantic reach the Mediterranean and continue eastwards until the Near East (Vb-weather-conditions). So there is high precipitation and mild temperatures. Snow and freezing are rare, because they are effected by local phenomena.
Nevertheless, summer climate is determined by subtropical influence, namely the trade winds, which mean very dry and hot weather periods and aridity. The Azores high causes eastward moving dry air masses, that initiate constant wind streams over the eastern basin (s. Chapter 5).6 Moreover the Mediterranean is influenced by the Hadley circulation (see chapter 3.1). Both processes` intensity depends not only on the season, but on annual macro-scale atmospheric circulation conditions.7 Those are the main reasons for hot and dry summers in the Mediterranean.
As a consequence the Mediterranean climate can be matched to both the dry tropics and the humid midlatitudes. Spring and autumn are very brief compared with Central Europe and are influenced by the upcoming season. There can be noticed an increasing gradient of aridity in southern and eastern direction.8 This is connected with an incline of continental climate phenomena. One recognizes, for example, a great difference between the mild winters in maritime sites like western Iberia and the strong temperature variability in high continental regions like East Anatolia. Moreover the precipitation gradient runs from north-west to south-east. This can be proved by the comparison of the climate station of San Sebastian, where one can find annual humid months, and North Egypt, where <1 month is humid.9
For the temperature, there must be underlined a specific spatial distribution in the Mediterranean region. In the Northern part the land surface temperature is on average about 5° C lower than in the Southern areas. In cold mountain areas of the Alps one can find annual mean temperatures of -2° degree whereas in the hot lowlands of North Africa annual average temperatures of 22° degree occur. In the more coastal region the warmest months are delayed from July to August because of the balancing maritime effect. Moreover, the northern part of the Mediterranean must be specified. The central and eastern regions occur warmer than the western ones or the Aegean gulf. Reasons for this are the lower northern radiation, rising cool water streams caused by internal effects and the income of cooler water through rivers and the Black Sea.10
The Mediterranean is constituted by a large-scale decreasing intensity of precipitation northwesterly to southeasterly. The dryness of southeast Mediterranean regions is connected with a much more intensive total evaporation compared with northwest patterns. Hence, the precipitation gradient is modified by meso- and micro-scale impacts, which is in the Mediterranean mainly the relief (almost two thirds are orographic rain causing mountains). So there is a great local diversity of precipitation levels.11 In some areas (Anatolia or Algeria) a continental climate, which is caused by orographic rain shadows, can be found.12 Xoplaki et al. (2004: 63) disclaim a connection of Mediterranean sea surface temperature and precipitation variability patterns. They carve out the connection between the North Atlantic Oscillation (NAO) and local precipitation amounts.13
Here a short explanation about the NAO is given. The NAO refers to the north southern distribution of air masses (pressure gradient) between two centers of action: the Icelandic Low and the Azores High. This phenomenon has its greatest occurrence in winter, when the atmosphere is dynamic. In this season the NAO is responsible for more of one third of the North Atlantic pressure variances. Hurrell14 and Wanner15 et al. carried out, that the NAO has massive impacts on the temperature patterns and the precipitation and storm amounts of the Northern Hemisphere. Since Flohn et al. (1998: 23) a strong NAO index leads to a warming and a raising moisture in Southern Europe and the Mediterranean.16
Another phenomenon, which is connected to Mediterranean precipitation patterns, is El Nino Southern Oscillation (ENSO), which describes the properties of the Pacific atmosphere ocean circulation, but has large macro-scale impacts. So precipitation anomalies in Southern Europe can be assigned to different stages of ENSO. Most effects of the ENSO can be recognized in spring and autumn.17
Moreover it has to be mentioned, that the rainy periods differ southwards. In South Europe a longer precipitation stage can be measured. Contradictory to this is South Mediterranean precipitation, which is cumulative concentrated on short and intensive periods. Here lies an erosional risk, because water is rarely infiltrating and flowing on the marginal planted soil. Another precipitation phenomenon in North Africa is the occurrence of a couple of dry years in a row.18
3 Seasonal variability
The most obvious phenomenon of Mediterranean climate is a great seasonal variability. This is caused by the affects of two global circulation systems, namely the westerlies in the winter and the Inner Tropical Convergence Zone (ITCZ) in the summer. The seasonal climate occurrences in meso- and macro-scales are presented in the following chapter.
3.1 Summer conditions
In the summer, the Mediterranean is influenced by the Hadley circulation, which is determined by heated, equatorial, moist air, which moves polewards. Therefore, the Mediterranean is affected by maritime tropical (mT) and continental tropical (cT) air masses.19 In the space of the ITCZ, which is moving northwards in the summer, the moist air rains and sinks down at the Tropic of Cancer. The intensity of this process is determined by other, global atmospheric processes, which could be the Southern Oscillation or the Indian Monsoon, but these connections should not be discussed here. In wintertime he westerlies are further north and do not affect the Mediterranean climate intensive, but just in some rare “blocking” situations.20
1 Zeccheto, S.; et al.: Sea surface winds over the Mediterranean basin from satellite data (2000-2004). Meso- and local- scale features on annual and seasonal timescales; in: Journal of Applied Meteorology and Climatology 46: 6 (2007); p. 814-827, here 814.
2 Branigan, J. J.: The Mediterranean Lands; 10th edition; London 1975, p. 31.
3 Lutgens, F.; Tarbuck, E.: The Atmosphere. An Introduction to Meteorology;10th edition, Upper Saddle River 2008, p. 453.
4 Reddaways, J.; Bigg, G.:Climatic Change over the Mediterranean and links to the more general atmospheric circulation; in: International Journey of Climatology 16 (1996); p. 651-661, here 651. Lutgens, Tarbuck 2008: 454.
5 Flocas, A. A.: The annual and seasonal distribution of fronts over Central Southern Europe and the Mediterranean; in: Journal of Climatology 4: 3 (1984); p. 255-267, here 255.
6 Wagner, H. G.: Mittelmeeraum; Darmstadt; 2., vollständig überarbeitete Auflage 2011, p. 119.
7 Bolle, H. J.: Mediterranean Climate. Variability and trends; Berlin 2003, p. 25.
8 Borman, M.: Das Mittelmeerklima. Erscheinungen und Ursachen; in: Geographie heute 52 (1987), p. 11-23, here 11.
9 Wagner 2011: 119-120.
10 Bolle 2003: 34.
11 Wagner 2011: 120.
12 Branigan 1975:32.
13 Xoplaki, E.; et al.: Wet season Mediterranean precipitation variability. Influence of large scale dynamics and trends; in: Climate Dynamics 23: 1 (2004); p. 63-78, here 63.
14 Hurrell, J.W.: North Atlantic Oscillation (NAO); in: Encyclopedia of Ocean Science 4 (2001); p. 1904-1911.
15 Wanner, H.: North Atlantic Oscillation. Concept and Studies; in: Surveys in Geophysics 22 (2001); p. 321-382.
16 Flohn, H.: Behavior of the Centers of action above the Atlantic since 1881. Part II. Associations with regional climate anomalies; in: International Journal of Climatology 18: 1 (1998), p. 23-36, here 23.
17 Zeng, N.; Lau, K. M.: Euro-Mediterranean rainfall and ENSO. A seasonally varying relationship; in: Geophysical Research Letters 21: 12 (2002), p. 59.1-59.4, here 59.1, 59.4.
18 Wagner 2011: 120.
19 Branigan 1975: 34.
20 Bolle 2003: 27.