Changes of the prevailing trade winds over the islands of Hawaii and the North Pacific

Abstract

Changes in the frequency and intensity of the prevailing northeast and east trade winds from 1973‐2009 are analyzed from four land stations in the Hawaiian Islands. A nonparametric robust trend analysis indicates a downward trend in northeast trade wind frequency since 1973. At the Honolulu International Airport, northeast trade wind days usually occurred 291 days per year 37 years ago are observed to occur only 210 days per year in 2009. In contrast, the frequency of the east trade winds has increased over the past 37 years. Comparison of observations from four ocean buoys with land stations for the last 26 years (1984–2009) is presented. The northeast trade frequency is found to decrease for all eight stations while the east trade winds are found to increase in frequency. These results are similar to the longer (1973–2009) data set. Most buoys revealed an increase in trade wind speeds since 1984. The NCEP/NCAR reanalysis II data are used to analyze surface winds and sea level pressure (SLP) over the north Pacific. A northeast to east shifting of winds and an increase in SLP is found to occur from the 1980s to the 2000s epoch. Linear trends in reanalysis II from 1980 to 2009 indicated a strengthening of northeast trade winds over the Hawaiian Islands and in the subtropical eastern North Pacific with an extension of increased northerlies off the California coast. Meanwhile, southeast trades in the eastern North Pacific reduced their strength. Changes in trades in the western Pacific are relatively small.