and near-surface temperature field with vary- 

 ing success. If the relationship between tuna 

 distribution and sea-surface temperature and 

 between sea-surface temperature and wind field 

 can be determined, then synoptic wind data can 

 be used as an aid to predict the distribution 

 of tuna schools. 



Average wind data, as portrayed in this at- 

 las, also can be directly useful to commercial 

 fishermen in the rapidly developing tuna fishery 

 in the southeastern tropical Atlantic. Spotting 

 tuna schools and carrying out purse-seine fish- 

 ing operations are difficult at wind speeds above 

 15 knots, particularly in a fully arisen sea. 

 Therefore, the publication can be used to gen- 

 erally outline potential areas and seasons of 

 fishing operations. 



DATA 



The area covered by this atlas is shown in 

 Figure 1; the wind data are from 458,129 

 surface weather observations made from 1854 

 to 1966. These observations are from the same 

 sources and periods of years listed by Mazeika 

 (1968), plus observations made through 1966. 

 Observations were made by personnel of mer- 

 chant and naval vessels of several nations. 



Personnel of the National Weather Records 

 Center, Asheville, N.C., transferred the ob- 

 servations to punch cards and programmed a 

 computer print-out of data which included 

 monthly means of: 



1. Surface air temperature, 2° rectangles 



2. Air-sea temperature diflference, 2° rec- 



tangles 



3. Clouds, 2° rectangles 



4. Visibility, 2° rectangles 



5. Ceiling and visibility, 2° rectangles 



6. Precipitation types, 2° rectangles 



7. Dew-point temperature, 2° rectangles 



8. Wind speed versus air temperature, 2° 



rectangles 



9. Surface wind rose, 5° rectangles 



The above data are deposited at the National 

 Marine Fisheries Service, Tropical Atlantic 

 Biological Laboratory, Miami, Fla. The data 

 for the surface winds are the basis for this 

 publication. 



The computer print-out of wind data showed 

 for each month and for each 5° rectangle 



(Marsden square quadrant) the cumulative 

 percentage distribution of wind direction and 

 wind force. The direction is to eight points; 

 force is by five Beaufort Force groups: 0-1 

 (0-3 knots) ;2-3 (4-10 knots) ; 4 (11-16 knots) ; 

 5-6 (17-27 knots); 7-12 (28-71 knots). 



GENERAL CHARACTERISTICS OF 

 THE SURFACE WIND FIELD 



The surface wind field of the southeastern 

 tropical Atlantic Ocean is influenced by three 

 surface pressure systems: the two subtropical 

 high pressure cells over the North and South 

 Atlantic Oceans and the low pressure cell over 

 the continent of Africa. 



The northeast portion of the subtropical high 

 of the South Atlantic Ocean occupies most of 

 the geographical area covered in this study. 

 Between February and August this dominant 

 pressure system of the South Atlantic Ocean 

 strengthens and drifts about 5° northward. 

 During this period surface pressure in the vi- 

 cinity of Port Harcourt, Nigeria, changes little. 

 As a result, the surface pressure gradient be- 

 tween Port Harcourt and the southwest corner 

 of the wind field increases about 40%. As 

 might be expected, the frequency of winds 

 greater than 10 knots increases from 20 to 30% 

 over the entire oflfshore field south of lat 7° S, 

 and winds greater than 28 knots also increase 

 significantly. At the same time, the frequency 

 of onshore winds greater than 10 knots in- 

 creases at least 20% over the 5° coastal squares 

 from Cape Palmas to Accra, Grana. However, 

 south of Cape Lopez, during this interval, the 

 increase of winds greater than 10 knots is 

 negligible (from 5 to 10%). 



The subtropical high of the North Atlantic 

 Ocean influences only the northernmost area of 

 the wind field portrayed in these charts. The 

 intertropical zone of convergence (ITC) is the 

 southern boundary of the influence of this high 

 pressure cell on the wind field. The position 

 of the ITC ranges annually from lat 11° N in 

 August to lat 1° N in February-March (Flohn, 

 1969, p. Ill) . It extends eastward to the zero 

 meridian in January (Trewartha, 1968, p. 100, 

 Fig. 336), and its center line, the thermal 

 equator, is well to the north of this wind field 

 7 months of the year, April through October. 

 From November through March the ITC affects 



