♦ * 



.0* ,^ 



(a) 



07- 



GEOSTROPHICt WIND 



(b) 



Figure 3. — (a) Configuration of data points used to 

 form pressure derivatives, lb) Diagram showing 

 the transformation of the geostrophic wind vector 

 to form an estimate of the wind near the sea surface. 



pressure fields were used as input data, thereby 

 making best use of available data within each 

 month while retaining a consistent sampling in- 

 terval for computation of the stress. The effect of 

 using monthly mean data on the absolute magni- 

 tude of the index at each location is discussed in 

 the next section. 



The monthly mean fields have been assembled 

 by FNWC from, as far as was possible, 6-hourly 

 (four per day) synoptic fields. Where this fre- 

 quency was not available, twice per day where 

 available or once per day fields were utilized. 

 This decreased frequency occurs mainly in those 

 years previous to 1963 when FNWC began to 

 produce its own analysis. To extend the series 

 back to 1946, FNWC has collected synoptic fields 

 from the sources listed in Table 1. 



Because of the different data sources and the 

 expected trend toward increased detail in later 

 pressure fields due to an expanded system of 

 meteorological observations, care should be used 

 in attempting to discern long-term trends in 

 these upwelling indices. For example, it is diffi- 

 cult to determine how much of the decreased 

 amplitude of the yearly cycle apparent at several 

 of the locations during the earliest several years 

 is real and how much is due to relative smoothing 

 of the data in the earliest pressure fields. The in- 

 tention has been to produce a series which will 

 give a useful indication of the relative intensitv 



of upwelling by month within any group of fairly 

 contemporaneous years. 



Effects of Monthly Mean Data 

 on Calculated Results 



Due to the nonlinear linkage of wind to stress 

 (Equation 4) the value of the Ekman transport 

 computed from monthly mean data will be small- 

 Table 1. — Sources of synoptic surface atmospheric pres- 

 sure fields used by Fleet Numerical Weather Central in 

 constructing monthly mean fields. 



Time 

 period 



Source 

 agency 



Remarl^s 



Jan. iy46- National Climatic Center Hand analysis of available 

 Mar. 1955 Ashville. N.C. reports: once per day. 



Apr. 1955- National Center for 

 Dec. 1959 Atmospheric Research 

 Boulder, Colo. 



Jan. 1960- National Climatic Center 

 June 1962 Asheville, N.C. 



National Meteorological 

 Center analysis digitized 

 at alternate gridpoints; 

 twice per day. 



Once per day. 



July 1962- Fleet Numerical Weather FNWC objective machine 

 Dec. 1971 Central. Monterey, Calif, analysis. Certain gaps 



filled with NCC data; 



four times per day. 



