Exploratory Analysis of Surface Winds in the Equatorial Western Pacific and El Niño

Abstract

Exploratory data analysis is used to examine several key characteristics of surface wind along a ship track in the equatorial western Pacific. A month-by-month examination is undertaken, based on daily ship data from a recent 30-year record (1958–87). The characteristics considered here include the expected frequency of occurrence, the intensity, the range of variability, and the extreme value of the wind from the eight-point compass directions. A hodograph and constancy of monthly mean wind vectors are also presented. Results from the 30-year climatology suggest that the equatorial western Pacific is affected by monsoonal and trade flows from each winter hemisphere and by the eastern Pacific subtropical highs during the transition season (i.e., May). Equatorial westerlies peak in November (20%) and December (18%). Composite analysis reveals the further influence of the trade flows from the northwest Pacific from January to May, and the Southern Hemisphere influence from June to September during a year when ENSO has occurred. The reliability of the ENSO composite has been tested using a Monte Carlo simulation technique. Westerlies indeed increase their frequency of occurrence from November of the antecedent year to November of the ENSO year. This increase, however, is small relative to the decrease in easterlies in the same period. Westerly wind events are examined in terms of their duration and timing of occurrence. Westerly wind events with a period of 5–7 days do occur more often than events with a longer duration, but their frequency of occurrence has reduced substantially during El Niño years.