Análisis de la influencia de las condiciones sinópticas sobre la precipitación en Canarias. Aproximación basada en análisis de componentes principales.

Abstract

The rainfalls in the Canary Islands are very important for the social and economic life of their population. However, the precipitation has different influences depending on the islands or the zones: there are desert areas and very wet ones. The Alisios winds have an important influence in the humidity of the islands, especially the occidental ones, that generally have more relief than the rest. That is caused by the Azores anticyclone, which has more impact in summer months. Nevertheless, this is not the principal cause of the heavy rains in the archipelago: that is principally due to atmospheric disturbances that destroy the stability that generate these winds. In this project, the main objective is to characterize the impact of some weather types (WTs) in the precipitation of the islands making use of two different databases, WRF and SPREAD, in the period 1st January 1995 – 31st December 2004, which involves ten years of data. To achieve this goal the Principal Components Analysis will be used to determine 4 regions or components, also known as principal components (PC), using the precipitation values registered in those databases. The Jones’s equations and rules will be also applied, which define a classification method to identify different WTs depending on the pressure disturbances in a specific day. To determine this last is necessary the sea level pressure values available in NCEP/NCAR Reanalysis-1 database. To apply the PCA, a region must be defined to make possible the calculation of the PC. The chosen area is 27.025o N - 29.975o N, 13.025o W - 18.975o W (see figure 2). Each component has an associated explained variance, which is related to the corresponding amount of information. It is important to know that the calculated components will be rotated. For the determination of the WTs, another region must be defined. Now, because of the spatial resolution of NCEP/NCAR database, the zone is defined between the coordinates 20o N – 40o N, 10o W – 25oW (see figure 3). With the information that will be extracted from the results of applying the Jones’s method tables of values will be computed. The calculation of percentiles will be also applied to determine what WTs are more important for intense precipitation in the Canary Islands. After finishing all this procedure, the results will be discussed. First of all, the WTs classification has detected 1266 days of anticyclonic type (WT2), 1234 of directional types (WT1), 993 of hybrid types (WT0), 74 days of cyclonic type (WT3) and 86 days undefined (U) (see figures 18 and 19). With that, the regions obtained applying the PCA will be analysed. To do this, the percentile 95 will be studied, because it discriminates the light rains against the heavy ones: • WRF database: (see figure 6 and table 13) − W-PC1: this component is formed by Fuerteventura and Lanzarote. There, the WTs that usually cause heavy rains are the west winds (23.67 % of the accumulated precipitation in the 10 analysed years), the cyclonic type (16.56 % of the accumulated precipitation) and the northwest winds (16.41 % of the accumulated precipitation). Other WTs that are less important are the east (14.47 % of the accumulated precipitation) and the northeast winds (10.14 % of the accumulated precipitation). 4 − W-PC2: this one is formed by the north-western islands of the archipelago, except the northeast of Tenerife and La Palma. The most important WTs in this zone are the cyclonic type (30.32 % of the accumulated heavy precipitation) and the west winds (25.03 % of the accumulated precipitation). Also the east (12.86 % of the accumulated precipitation) and northeast (10.38 % of the accumulated precipitation) winds have influence. − W-PC3: this component considers the island of Gran Canaria and a part of the coast of Santa Cruz de Tenerife. In this region the cyclonic type is again very important (27.55 % of the accumulated precipitation), and also the northeast (18.72 % of the accumulated precipitation), east (18.28 % of the accumulated precipitation) and west winds (17.45 % of the accumulated precipitation). − W-PC4: the last component is formed by the northeast of La Palma and Tenerife, a region characterized by laurisilva that stands out because of its moisture. The northeast winds are now the most important WT in the region (22.00 % of the accumulated precipitation). The cyclonic type (21.18 % of the accumulated precipitation) and the west (16.40 % of the accumulated precipitation) and east winds (14.29 % of the accumulated precipitation) are also important for the rains in the zone. • SPREAD database: (see figure 13 and table 14) − S-PC1: this region includes almost all Fuerteventura, the south of Gran Canaria and some parts in the east of Tenerife. The WTs that cause the heaviest rains in this zone are the west winds (34.15 % of the accumulated precipitation) and the cyclonic type (33.01 % of the accumulated precipitation). Both WTs amount nearly two thirds of the intense rains in the region. With less prominence are the east winds (11.55 % of the accumulated precipitation). − S-PC2: this component is formed by the north-western islands, except the north and northeast of Tenerife, and the west of Gran Canaria. There, the most important WTs are the cyclonic type (35.86 % of the accumulated precipitation) and the west winds (28.27 % of the accumulated precipitation). The southwest winds have also importance (9.82 % of the accumulated precipitation), but much less than the aforementioned WTs. − S-PC3: this component considers Lanzarote and the north of Fuerteventura. Now, the west winds are so important (43.45 % of the accumulated precipitation), practically the double of the contribution of the cyclonic type (22.63 % of the accumulated precipitation). The east winds have less importance (9.98 % of the accumulate precipitation). − S-PC4: this region includes the north of the capital islands. The most important WTs are the cyclonic type (23.62 % of the accumulated precipitation) and the northwest (18.41 % of the accumulated precipitation) and west winds (15.81 % of the accumulated precipitation). The northeast (14.60 % of the accumulated precipitation) and north winds (10.36 % of the accumulated precipitation) are important too, but to a lesser extent. 5 The regions W-PC2 and S-PC1 are quite similar, and also the components W-PC4 and S-PC4 are slightly similar. Also, for both databases the cyclonic type is very important for the heavy rains in the archipelago, and the west and east winds too. In the other hand, the anticyclonic type hardly ever is the cause of intense precipitations in the Canary Islands, but the south and southeast winds have less importance than it. Finally, the northeast winds are important in the entire components calculated using WRF database, but if the SPREAD database is used, these winds have more importance in the north of the capital islands.