Synoptic Effects on the Local Winds in Todos Santos Bay: A Case Study

Abstract

Synoptic winds, obtained from surface pressure maps, and local surface winds, registered around Todos Santos Bay, are analyzed and filtered to isolate the high-frequency band (diurnal scale) and the low-frequency band (greater than a few days) to study, independently, their physical phenomena. Two methods of analysis are used: rotary spectra and empirical orthogonal functions (EOF). The surface data show the highest energy at diurnal frequency, indicating a strong sea-land breeze regime. On the other hand, the synoptic data show the highest energy at longer periods. In general, the low-frequency synoptic wind is much stronger than the low-frequency local surface wind. The differences are explained by means of the frictional effect and the higher energy dissipation over land than over sea. The EOF fire mode of the low-frequency wind data, representing 77% of the total variance, shows the characteristics of the most energetic phenomena, i.e., the synoptic regime. The correlation between synoptic and surface eigenvectors is best for the island data; poor correlations are found between the synoptic eigenvector and those for the city and shoreline sites. Similarly, the synoptic-reduced surface wind, obtained by the power law equation, has the best correlation with the island data. The EOF first mode of the high-frequency wind data, representing 62% of the total variances, shows the characteristics of the sea land breeze eigenvectors.