National Oceanic and
Atmospheric Administration
United States Department of Commerce


 

FY 1997

Westerly wind events in the tropical Pacific, 1986–1995: An atlas from the ECMWF operational surface wind fields

Vecchi, G.A., and D.E. Harrison

NOAA Tech. Memo. ERL PMEL-109, NTIS: PB97-188213, 222 pp (1997)


We present an atlas of results from an analysis of Westerly Wind Events (WWEs) in 10 years of the European Centre for Medium Range Weather Forecasting (ECMWF) 10-m wind field analysis. We compare the analysis wind field to the Comprehensive Ocean-Atmosphere Data Set (COADS) climatology and to wind time series from the Tropical Atmosphere-Ocean (TAO) array. We present a classification scheme for the westerly wind variability we observe in the data, using eight types based on the location of the maximum westerly winds. We construct a composite for each type of event, and show the results for a 21-day composite period for each one, both for winds and anomalies. A simple propagating Gaussian model satisfactorily describes the evolution of zonal wind anomaly for each type of event; we estimate the scales of each composite event by fitting the model to each composite. Several instances of wide-spread westerly wind anomaly are identified and described, but these "mega"-WWEs have few features in common. We discuss the WWEs that occurred during the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) Intensive Observing Period (IOP) and show the extent to which our composite events are able to reproduce the major westerly wind features of the IOP. We describe the frequency of occurrence of each type of WWE for each year of this record and by monthly climatology; we find that several types of events are negatively correlated with the annual mean Troup Southern Oscillation Index (SOI), and that the stronger WWEs often have a statistically significant seasonality. We believe that our composites offer a useful framework for representing the sort of westerly wind variability that occurs in the western and central tropical Pacific and can provide a basis for further study of the importance of such winds in the climatological and interannual variability of this part of the world ocean. However, we note that this decade was dominated by years in which the SOI was negative; reliable frequency statistics and SOI correlations may require a longer analysis period.




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