Observing long-period fluctuations of surface winds in the tropical Pacific: initial results from island data

The utility of studying low-frequency surface weather phenomena with long time series of meteorological data from Tropical Pacific islands is demonstrated. The wind stress changes associated with El Nino events in the period 1950-1978 are examined at seven locations. Zonal wind stress anomalies at the Equator near the date line often exhibit strengthening and subsequent weakening of the trade winds prior to each El Nino, as originally suggested by Wyrtki. An exception is the weak 1963 El Nino, which is preceded by meridional wind stress anomalies at the Equator. The strongest zonal and meridional wind stress anomalies, however, occur well after the first occurrence of anomalously warm water off the coast of Peru for each El Nino, in agreement with prior analyses of merchant marine data. Away from the Equator, variability of the wind stress anomalies from one El Nino to the next is strong, leading to numerous discrepancies with published profiles of the mean El Nino wind changes. Power spectra of wind stress from three island stations are compared with concurrent wind stress spectra computed from merchant marine data. Many disparities are found which can be attributed to (sometimes severe) aliasing in the ship data. Possible aliasing errors in the ship data time series are estimated by randomly subsampling the island data in order to mimic the ship data sampling. Sampling criteria, which depend upon the scientific application, are suggested in order to limit the alias noise in the ship data to acceptable amounts.