AVERAGE MONTHLY WIND STRESS ALONG 



COASTAL REGIONS OF THE UNITED STATES 



AND WESTERN CANADA 



By 



CHARLES W. MORGAN 

 WILLIAM H. ANTHONY 



INTRODUCTION 



In the course of work on modeling 

 the average seasonal surface current in 

 coastal regions of the United States a 

 need arose for average wind stress data. 

 Such data for the world's oceans are 

 given by Scripps Institute of Oceanog- 

 raphy (1948, 1950), Hidaka (1958) 

 and Hellerman (1967, 1968); however 

 the 5° data interval used by them is 

 felt to be too gross for use in coastal 

 work where local effects on a smaller 

 scale are important. Using a sim- 

 plification of the procedure described 

 in Hellerman (1965) the wind stress 

 was computed using a resistance law 

 (Defant, 1961; p. 422): ^ 



with stress, *t being equal to air 

 density, £*_ , times a drag coefficient, 

 C ^ multiplied by the wind velocity 

 times the wind magnitude, Wl Wj, 



The value of air density used in this 

 study was (0.0022 x Latitude + 1.136) 



x 10 



g cm" J (Hellerman, 1965). 



VALUE OF C 4 



The value of the drag coefficient, 

 C ^ , has been the subject of many in- 

 vestigations. Most have found that Ci 

 increased in some manner with in- 

 creases in the easily measured para- 

 meter of wind speed (see, for example, 



Deacon and Webb, 1962). However in- 

 vestigators have also recognized that 

 the problem is quite complex with 

 C ^ also depending on the relative 



roughness of the water surface (Rossby 

 and Montgomery, 1935, referenced in 

 Defant, 1961; p. 421) and on the sta- 

 bility of the air above the sea sur- 

 face (see, for example, DeLeonibus, 

 1971). Recent work by SethuRaman 

 and Raynor (1975) for winds of 3-10 m 

 sec" found that G± increased with 

 increases in a roughness Reynolds 

 number \J*r ty/^ (where U fr 



is surface friction velocity, "V is 

 roughness length, and y is kinematic 

 viscosity) for three distinct classes of 

 aerodynamic roughness: smooth, mod- 

 erately rough, and fully rough. They 

 also concluded, contrary to many 

 authors, that C4 did not vary with 

 mean wind speed, at least in the range 

 3-10 m sec -1 (6-20 kn); they did 



find that C^ was weakly dependent 

 on the atmospheric stability, in general 

 decreasing with increasing stability. 

 The question of which of the many 

 formulations of C^ to use in this 

 study was finally resolved in favor of 

 one used in the earlier study by 

 Hellerman (1967, 1968, Table 1). 

 Hellerman's values of C , were adopt- 

 ed from Deacon, Sheppard, and Webb 

 (1956, referenced in Hellerman 1967). 

 The values used are shown in Table 1. 

 The values for winds speeds 7-16 ton 



