Annual and Nonseasonal Variability of Monthly Low-Level Wind Fields over the Southeastern Tropical Pacific

Abstract

The time and space variability of low-level winds over the Southeast Tropical Pacific (SETP) region is described for the 6-year period 1974?80. The data set consists of monthly averaged low-level cloud-motion vector winds supplemented by coastal surface winds and pressure-related indices at fixed stations, and by long-term averages of ship-reported surface winds. The data are statistically analyzed in terms of correlations and empirical orthogonal functions (EOF's). The annual cycle of the SETP low-level wind speeds is most prominent north of 15°S, with minimum and maximum intensifies during the January?March and July?September periods, respectively, in phase with the larger scale southeast trades. South of 15°S the annual variability is small and characterized by minimum wind speeds from May through July, when the anticyclonic circulation center is at its northernmost position (?26°S). There is an SETP core region of maximum wind speeds that annually migrates (along 85°W) from 20°5 in January?March to 15°S in June?August. Peru coastal winds are seasonally in phase with the southeast trade circulation at Talara (4°5) and San Juan (15°5) but considerably out of phase at Chimbote (9°5), Lima (12°S) and Tacna (17°S). The nonseasonal component of the SETP circulation is characterized by areal coherence over the 15?30°S subregion. When the SETP circulation is unseasonably weak (strong), so is the intensity of the high-speed core, and both the core and the anticyclonic circulation center tend to lie north (south) of their climatological positions. A weak SETP circulation with northward lying positional characteristics prevailed during the 1976?77 El Nin?o. Nonseasonal variability is poorly correlated or uncorrelated between the coastal winds and various measures of either the basinwide or SETP circulations. Monthly anomalies of the low-level cloud-motion winds at 20°S, 85°W are an effective index of the nonseasonal atmospheric, circulation of the SETP region.