Five million ship observations have been used to obtain a description of the Pacific trade wind fields in terms of empirical orthogonal functions. The description covers the period 1950–72 in two-month intervals.
The major conclusions are as follows: 1) The core regions of the trade winds are quasi-permanent in position. 2) The winds in these regions experience large (20–40%) interannual fluctuations in strength, thus suggesting similarly large changes in the momentum and kinetic energy of the trade wind system. 3) The area of maximum trades expands and contracts at interannual time scales. 4) The ITCZ and the thermal equator in the ocean are congruent on annual and interannual time scales. 5) The northeast and southeast trades are seasonally out of phase. However, the winds immediately poleward of a given trade system fluctuate in phase with the trade system of the opposite hemisphere. The northeast and southeast trades are weakly coupled at interannual time scales. 6) The maximum interannual variations in the trades occur away from the region of maximum intensity, particularly in the western and southwest Pacific. 7) The trade wind field is closely and interactively coupled to the underlying equatorial ocean. 8) Interannual variations in the sea-level pressure field over the North Pacific lead by about a season associated changes in the zonal component of the trades.