On the climatology of surface wind direction frequencies for the central Chilean coast

The main synoptic meteorological features influencing Chile are: (a) Pacific anticyclone (PA), (b) coastal low (CL), (c) what has been called enhancement of coastal low (ECL) a nucleation of the CL frequently observed during summer in Central Chile, and (d) subtropical lows (STL) and subpolar lows (SPL). The SPL will not be considered in the present paper. All four features can be identified with a typical mean sea-level pressure pattern. A detailed discussion of this meteorological scenario can be found in Saavedra and Foppiano (1992a). In particular, the ‘high wedge’ observed on the continent, the location and maximum pressure of which can vary considerably from day to day, has been considered representative of the interaction between the four features. Weatherwise, ‘good weather’ is usually to be found north of the wedge, while ‘bad weather’ occurs south of it. A detailed description of a climatic scenario (see Fig. 1) keeping the corresponding features of the meteorological scenario has also been given by Saavedra and Foppiano (1992a). The main characteristics of the monthly mean ‘high wedge’ are expressed, to a first approximation, in terms of the location of maximum monthly mean pressure along the Chilean coast (LMP). Saavedra (1980) first precisely defined the LMP (Fig. 2) on the basis of coastal monthly mean values of pressure published by Whittaker (1943), who used observations for the 1911-1940 period (this period includes fourteen El Niño events: four strong, six moderate and four weak (Quinn et al. 1978)). The latitude of the LMP can be used as a pointer, which divides the country into two regions. The climatic properties of these regions may be associated with the meteorological properties already mentioned. Moreover, the LMP can then be considered as an index of the monthly mean spatial interaction of the meteorological features. The annual evolutions of the LMP’s latitude and pressure have been discussed by Saavedra and Foppiano (1992b) in terms of only two empirical Fourier components: annual and semi-annual. The semi-annual components give the observed LMP’s evolutions their characteristic shapes, i.e. a faster (slower) change of the LMP’s latitude (of pressure at the LMP) from summer to winter than from winter to summer. Details of the LMP’s capacity as a climatic descriptor for Concepción (36.8°S; 73.1°W) are given in Saavedra (1985; 1986). Prohaska (1952), Pittock (1971; 1980), Minetti et al. (1982) and Minetti and Vargas (1983; 1992) have used similar indices for various applications. On the climatology of surface wind direction frequencies for the central Chilean coast