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Abstract
Report on the Large Scale Wind Patterns in the Tropical Pacific via EOF and CCA Analysis of NSCAT and TAO Array Data John T. Gunn and Gary S.E. Lagerloef Wind measurements derived from the NASA Scatterometer (NSCAT) and measured by the Tropical Ocean Atmosphere (TAO) buoy array are used in a comparative analysis to examine the large-scale wind patterns in the tropical Pacific on 10-day and longer time scales. First, we addressed a possible aliasing of semidiurnal wind variability by the twice daily NSCAT sampling with a statistical analysis of averaged TAO hourly winds sampled at NSCAT crossing times versus 24-hour sampling. The aliasing was small (-0.12 +/- 0.23 ms-1 and -0.03 +/- 0.24 ms-1 for zonal and meridional winds, respectively) for 10-day and longer periods. A subset (34) of the TAO buoys with sufficiently complete time series was used for large-scale comparisons. These were located predominately in the central part of the TAO array. An EOF analysis over the period from October 1996 to June 1997 found the first two modes were very similar in both the TAO data and NSCAT data sampled at TAO locations. One mode represented an equatorial velocity jet associated with the onset of the 1997-1998 El Niño, the other an acceleration of the northeast Trade and Monsoon winds. The order of these modes was reversed in the EOF of the full spatial resolution NSCAT data. This is a result of the increased spatial coverage of the full NSCAT data in the region east of 95W where the trade wind mode is strongest. The strong agreement of the first two modes, together accounting for >65% of the total variance, shows that both measurement systems provide consistent observations of the large-scale wind field. The two wind field data sets were very highly correlated (r>0.99) as evidenced by results of a canonical correlation analysis (CCA). TAO buoy data were correlated with the NSCAT data sub-sampled at TAO buoy locations, as well as with the full spatial resolution NSCAT data with similar results. A directional offset was found with the NSCAT wind vector rotated 14° to the right, relative to the TAO data, averaged over all locations. This rotation was of the same sign in both hemispheres and on the equator, ruling out an Ekman planetary boundary layer effect. A small speed bias of 0.09 ms-1 was found, but it was not significantly different from zero at the 90% confidence level. |
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