Surface Winds from Tropical Pacific Islands—Climatological Statistics

Abstract

Multidecadal time series of surface wind observations from tropical Pacific islands have been examined in order to investigate the space and time scales of variability. Climatological monthly means and variances are compared with comparable means and variances derived from ship observations; usually the means agree to within ?1 m s?1 in speed and ?10 degrees in direction. Annual and semiannual cycles differ in detail. Island zonal wind variances are often significantly larger, by up to 10 m2 s?2 near the equator between September and December; because of the spatial coherence of the island results, these discrepancies are believed to result from the poor high-frequency sampling typical of ship data. A substantial near-equatorial zonal wind variance maximum is shown to be related to ENSO period variability; excluding ENSO time periods leaves a relatively spatially uniform variance of ?5 m2 s?2 over a broad region. The frequency distribution of variance, derived from daily-averaged data, exhibits considerable geographical variation. Within a few degrees of the equator the most energetic zonal wind variability is found in a broad band extending from about 3- to 60-day periods, with maximum at about 10 days; there is also significant interannual power in records located west of 170°W. There is occasionally a local variance maximum in the range of 30- to 60-day periods. Within this near-equatorial region, the meridional wind variance is roughly half the zonal wind variance and is found primarily between about 3-day and 6-day periods and at the annual period. Poleward of about 5 degrees of latitude, the interannual variability in zonal wind diminishes sharply, and the zonal and meridional wind variances become increasingly comparable. The zonal wind energy level in the 3- to 60-day band decreases as one moves farther from the equator, until the more energetic winds typical of subtropical latitudes arise. Coherence calculations typically show zonal wind coherence significant at the 95% level at all energetic periods when islands axe within 200?300 km of each other meridionally, and within 1000?1500 km zonally. The meridional wind tends to be less coherent. A minimal sampling array for tropical surface wind variability in this region should have meridional sampling about every 2° and zonal sampling about every 15° for the zonal wind, and perhaps half these distances for the meridional wind.