3.1.2 Climatic classification

To analyse the different climatic forms present in the Coastal meseta two main classification methods were adopted: Emberger and Rivas Martinez. In general Emberger classification is the most famous approach in the Mediterranean climate while the Rivas Martinez one is used in order to have a general overview.

The Rivas Martinez methodology (Rivas Martinez et al., 1999) determines a generic world-wide climate classification in five macrobioclimates (tropical, Mediterranean, temperate, boreal and polar) on the basis of bioclimatic indexes. The bioclimatic type of the Mediterranean macrobioclimate, with the relative indexes, are described in Table 15.

For the Coastal meseta the different climatic parameters and the bioclimatic indexes have been calculated on the basis of the most recent data (Table 16).

Comparing the continentality and the ombrothermic indexes, the Mediterranean bioclimates could not be classified on the basis of the table above. A simple classification, in types and subtypes, was carried out only on the basis of the continentality index:

• Hyperoceanic (Ic=0-11): extremely hyperoceanic (0-3), euhyperoceanic (3-7), and barey hyperoceanic (7-11);
• Oceanic (Ic=11-21): euoceanic (11-18) and semicontinental (18-21);
• Continental (Ic=21-65): subcontinental (21-28), eucontinental (28-45), and hypercontinental (46-65).

Analysing the continentality index, it is evident that the Atlantic coast climate is oceanic-semicontinental except for the Kenitra station which falls in the continental-subcontinental type because of the high temperature. Also the lower plateau and the higher plateau are fall into the continental type and range between continental-subcontinental and continental-eucontinental.

Table 15 - Rivas Martinez classification for the Mediterranean macrobioclimate

Because Ic is more than 18, the Itc index was calculated and since its value was less than 120, the classification of the thermotypes was carried out using the termicity index. According to this, the Atlantic coast and the lower plateau fall in the thermomediterranean type while the upper plateau in the mesomediterranean type.

The ombrothermic index (Table 17) allowed us to classify the climate of the region as dry and semiarid in the Atlantic coast and in the lower plateau, dry and subhumid in the higher plateau.

In 1930 Emberger elaborated a synthetic expression for the Mediterranean climate (Daget, 1977), classifying this kind of climate on the base of three important climatic parameters: precipitation, temperature, and evaporation. The precipitation (P) is represented by the annual precipitation (mm). For temperature, the mean of the maximum temperatures of the hottest month in the year (M) and the mean of the minimum temperatures of the coldest month (m) in the year were considered because vegetation growth is strictly related to these thermal limits. The temperature parameter is represented by the quotient (M/2+m/2). Evaporation is represented by the temperature range (M-m) because evaporation frequently increases with it; this parameter expresses the continentality of a climate (Calvet, 1964).

Table 16 - Continentality and termicity indexes (Ic and It)

Table 17 - Ombrothermic indexes (Io)

All these climatic parameters are represented in the pluviothermic quotient of Emberger, calculated with the following formula in which the temperature values are expressed in absolute degrees (K):

Carrying out the pluviothermic quotient, in general the Mediterranean climate is more arid when the quotient is smaller. An improvement in the Mediterranean climate classification was made by the author utilising both the Q₂ and the minimum temperature of the coldest month (m). In fact this last parameter expresses in a generic and useful way the degree and the duration of the critical frost period: the lower is the m value the more severe these conditions are. These two parameters are graphically represented in the climatogram, a graphic where on the x and y axis the m and the quotient values are respectively represented.

In the present work the classification of Coastal meseta has been carried out through the climatogram of Emberger obtained for Morocco by Sauvage (1961). In this climatogram a further subdivision has been done through the comparison between the climatic classification and the observations on the Mediterranean vegetation. In fact six different zones called bioclimatic stages have been recognised (according to the increasing aridity: per-humid, humid, sub-humid, semi-arid, arid, and saharian).

The pluviothermic quotient, the Emberger parameters, and the results of the bioclimatic classification are shown in Table 18. The thermic variant, another distinction in each bioclimatic stage, was considered according to the values of m for the Mediterranean area: variants with hot winters for m>7, with temperate winter for 3<m<7, and cool winter for 0<m< 3.

Table 18 - Emberger classification

Analysing the results it appears that the climate of Coastal meseta is quite homogeneous; in fact most of the Emberger quotients fall in the semi-arid climatic stage, except the ones collected by Rabat, Kenitra, Casablanca, and Oulmes stations and classified as subhumid climatic stage.

Comparing the classifications obtained with the two different approaches, the climate of the study area is semi-arid for Emberger and dry for Rivas Martinez.

According to the mentioned climatic parameters, it seems that the Emberger classification fits the situation of the study area better. These results apparently confirm what was previously stated regarding the best adaptability of the Emberger classification to the Mediterranean climate.