In general, because the wind stress is a vec- 

 tor whose magnitude depends on the square of 

 the wind speed and a variable drag coefficient, 

 frequent sampling of the wind velocity--possibly 

 more than once per day--is required to obtain 

 reliable measures of the monthly wind stress. 

 The steadiness of the trades (Malkus, 1962) re- 

 duces this requirement. Nevertheless, rela- 

 tively large variations in wind speed and direc- 

 tion may occur in timespans of a few days. 

 Thus, south of lat. 15° N. the interpolated winds 

 and wind stresses of table B are heavily biased 

 by the small number of observations which, al- 

 though they may be based on correct measure- 

 ments, do not necessarily reflect the true 

 monthly mean value. 



The choice in the preparation of table B was 

 either to leave the questionable areas blank, 

 particularly those in the southeastern part of 

 the study area, or to present values based on 

 the limited observations. The latter course 

 was chosen because it draws attention to the 

 high winds that may occur, especially in the 

 southeastern part of the study area. (See the 

 climatic atlas by McDonald, 1938.) 



Inadequacies in the quality of marine meteo- 

 rological observations are well known. Visual 

 inspection of the data used in this report again 

 drew attention to the primary factors that tend 

 to reduce the quality of data. Errors in marine 

 surface weather data are not so much due to 

 incorrect reading of instruments and recording 

 of observations as to improper placement and 

 calibration of the instruments, improper tech- 

 nique of measurement, and errors in data re- 

 duction at sea. 



Errors in data reduction area primary cause 

 for wrong wind speeds and directions. For ex- 

 ample, visual inspection revealed thatanobser- 

 vation showing a westerly wind may appear in 

 areas with a large number of observations 

 showing an easterly wind. A reversal in wind 

 direction can easily occur because of an error 

 in the vector addition of the measured apparent 

 wind and the speed and direction of the ship. 

 Incorrect vector addition also results in wrong 

 wind speeds and directions, which are not so 

 readily detected as a 180° error in wind direc- 

 tion. 



Comparisons with Other Results 

 in the North Pacific 



Evaluation of the results presented in table B 

 includes comparisons with other results ob- 



tained in the North Pacific. Most suitable for 

 comparison are the tabulations of mean wind 

 stresses over the North Pacific prepared under 

 the direction of J. F. T. Saur, Jr. and John D. 

 Cochrane (Scripps Institution of Oceanography, 

 1948). These results, given in 5° square units 

 of area for each month, were based on summary 

 wind data contained in the U.S. Hydrographic 

 Office Pilot Charts. The mean wind speed in 

 integral Beaufort numbers and the frequency of 

 wind for each of the 16 points of the compass 

 are presented in the form of a wind rose. Reid 

 (1948) described the method used to obtain the 

 resultant wind stresses. Malkus (1962) has 

 demonstrated that the results obtained by the 

 "Scripps Pacific Method" are comparable with 

 those obtained by direct summation in which the 

 drag coefficient is variable. The latter is the 

 method of calculation in this paper. 



The wind stresses of table B at lat. 2° N., 

 long. 157° W. (equatorial zone), at lat. 17° N., 

 long. 152° W. (trade wind zone), and at lat. 32° 

 N., long. 167° W. (in the zone of the North Pa- 

 cific pressure ridge), together with the SIO 

 (Scripps Institution of Oceanography) results 

 for the same locations are shown in figures 2, 

 3, and 4, respectively. 



For each of the examples, except Tx in the 

 trade wind zone, the mean annual values of the 

 2 study years and of the SIO "climatic year'' 

 are in good agreement. Values of individual 

 months show relatively large departures from 

 the values based on climatic data, particularly 

 at lat. 17° and 32° N. Large fluctuations of the 

 wind stresses from, month to month are to be 

 expected at lat. 32° N., where they reflect sen- 

 sitivity to changes in the location of the North 

 Pacific high-pressure region. 



In the trade wind zone, the large month-to- 

 month and year-to-year variations of Tx were 

 unexpected, particularly in view of the small 

 variations of the monthly stress values of the 

 climatic year. Closer inspection of figure 3 

 reveals that magnitudes of Tx during the study 

 period were 1 year out of phase: absolute mag- 

 nitudes were low during summer and autumn 

 1963 and high during summer and autumn 1964; 

 conversely , absolute magnitudes were high dur- 

 ing winter and spring 1964, but low 1 year later. 

 If only the 2 years 1963 to 1965 are used to ob- 

 tain an average value for each month, the ex- 

 treme range in Tx is reduced from about 2 to 

 0.85 dynes cm."^. Thus, a flat climatic curve, 

 resulting from large year-to-year fluctuations 



