NOAA TR NMFS SSRF-643 A UNITED STATES DEPARTMENT OF COMMERCE PUBLICATION ^i-'O'Co^ V -T NOAA Technical Report NMFS SSRF-643 U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Marine Fisheries Service Surface Winds of the Southeastern Tropical Atlantic Ocean ^ JOHN M. STEIGNER and MERTON C. INGHAM SEATTLE, WA. October 1971 irfne Bio(o|icai Uboratory LIBRARY SEPiai972 Woods Hole, Mass. NOAA TECHNICAL REPORTS National Marine Fisheries Service, Special Scientific Report-Fisheries Series The major responsibilities of the National Marine Fisheries Service (NMFS) are to monitor and assess the abundance and geographic distribution of fishery resources, to understand and predict fluctuations in the quantity and distribution of these resources, and to establish levels for optimum use of the resources. NMFS is also charged with the development and implementation of policies for managing national fishing grounds, develop- ment and enforcement of domestic fisheries regulations, surveillance of foreign fishing off' United States coastal waters, and the development and enforcement of international fishery agreements and policies. NMFS also as- sists the fishing industry through marketing service and economic analysis programs, and mortgage insurance and vessel construction subsidies. It collects, analyzes, and publishes statistics on various phases of the industry. The Special Scientific Report — Fisheries series was established in 1949. The series carries reports on scien- tific investigations that document long-term continuing programs of NMFS, or intensive scientific reports on studies of restricted scope. The reports may deal with applied fishery problems. The series is also used as a medium for the publication of bibliographies of a specialized scientific nature. NOAA Technical Reports NMFS SSRF are available free in limited numbers to governmental agencies, both Federal and State. They are also available in e.xchange for other scientific and technical publications in the marine sciences. Individual copies may be obtained (unless otherwise noted) from NOAA Publications Section, Rockville, Md. 20852. Recent SSRF's are: 586. The Trade Wind Zone Oceanography Pilot Study. Part VII: Observations of sea birds March 1964 to June 1965. By Warren B. King. June 1970, vi -I- 136 pp., 36 figs., 11 tables. 591. A bibliography of the lobsters, genus Homarus. By R. D. Lewis. January 1970, i -f 47 pp. 592. Passage of adult salmon and trout through pipes. By Emil Slatick. January 1970, iii + 18 pp., 8 figs., 12 tables. 594. Seasonal and areal distribution of zooplankton in coastal waters of the Gulf of Maine, 1967 and 1968. By Kenneth Sherman. July 1970, iii -|- 8 pp., 6 figs., 3 tables. 595. Size, seasonal abundance, and length-weight re- lation of some scombrid fishes from southeast Florida. By Grant L. Beardsley, Jr., and William J. Richards. May 1970, iii + 6 pp., 5 figs., 2 tables. 596. Fecundity, multiple spawning, and description of the gonads in Sebastodes. By John S. MacGregor. March 1970, iii + 12 pp., 6 figs., 7 tables. 597. Fur seal investigations, 1967. By Bureau of Commercial Fisheries Marine Mammal Biological Laboratory. March 1970, vii + 104 pp., 31 figs., 79 tables. 599 Diagnostic characters of juveniles of the shrimps Penaeus aztcciis aztcctis, P. duoraruni diiorarmn, and P. brasiliensis (Crustacea, Decapoda, Penaei- dae) . By Isabel Perez Farf ante. February 1970, iii -I- 26 pp., 25 figs. 600. Birectilinear recruitment curves to assess in- fluence of lake size on survival of sockeye salmon (Oncorhyiichus yierka) to Bristol Bay and fore- cast runs. By Ralph P. Silliman. March 1970, iii + 9 pp., 13 figs., 2 tables. 601. Effect of flow on performance and behavior of chinook salmon in fishways. By Clark S. Thomp- son. March 1970, iii + 11 pp., 8 figs., 3 tables. 602. Biological characteristics of intertidal and fresh- water spawning pink salmon at Olsen Creek, Prince William Sound, Alaska, 1962-63. By John H. Helle. May 1970, iii -|- 19 pp., 11 figs., 5 tables. 603. Distribution and abundance of fish in the Yakima River, Wash., April 1957 to May 1958. By Ben- jamin G. Patten, Richard B. Thompson, and Wil- liam D. Gronlund. June 1970, iii + 31 pp., 26 figs., 37 tables. 604. The flora and fauna of a basin in central Florida Bay. By J. Harold Hudson, Donald M. Allen, and T. J. Costello. May 1970, iii -|- 14 pp., 2 figs., 1 table. 605. Contributions to the life histories of several penaeid shrimps (Penaeidae) along the south Atlantic Coast of the United States. By William W. Anderson. May 1970, iii -|- 24 pp., 15 figs., 12 tables. 606. Annotated references on the Pacific saury, Colol- abis .'idira. By Steven E. Hughes. June 1970, iii + 12 pp. 607. Studies on continuous transmission frequency modulated sonar. Edited by Frank J. Hester. June 1970, iii -j- 26 pp. 1st paper. Sonar target classification e.xperiments with a continuous- transmission Doppler sonar, by Frank J. Hester, pp. 1-20, 14 figs., 4 tables; 2d paper, Acoustic target strength of several species of fish, by H. W. Volberg, pp. 21-26, 10 figs. 608. Preliminary designs of traveling screens to col- lect juvenile fish. July 1970, v + 15 pp. 1st paper. Traveling screens for collection of juvenile Continued on inside back cover. ,^0ATM0SP^, NOflfl ""^'■^T^STcP"' U.S. DEPARTMENT OF COMMERCE Maurice H. Stans, Secretary NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION Robert M. White, Administrator NATIONAL MARINE FISHERIES SERVICE Philip M. Roedel, Director NOAA Technical Report NMFS SSRF-643 Surface Winds of the Southeastern Tropical Atlantic Ocean JOHN M. STEIGNER and MERTON C. INGHAM Marine Pi-!oglc,| L.bar.i-iy LIBRARY SEP 1 3 1972 Woods Hole. Mass. SEATTLE, WA. October 1971 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 35 cents The National Marine Fisheries Service (NMFS) does not approve, rec- ommend or endorse any proprietary product or proprietary material mentioned in this publication. No reference shall be made to NMFS, or to this publication furnished by NMFS, in any advertising or sales pro- motion which would indicate or imply that NMFS approves, recommends or endorses any proprietary product or proprietary material mentioned herein, or which has as its purpose an intent to cause directly or indirectly the advertised product to be used or purchased because of this NMFS publication. CONTENTS Page Introduction 1 Data 2 General characteristics of the surface wind field 2 Interpreting the charts 3 Acknowledgments 3 Literature cited 3 111 Surface Winds of the Southeastern Tropical Atlantic Ocean By JOHN M. STEIGNER, Instructor Department of Natural Sciences St. Petersburg Junior College St. Petersburg, Florida 33733 and MERTON C. INGHAM, Director Oceanographic Unit U.S. Coast Guard Washington, D.C. 20390 ABSTRACT This publication presents mean monthly surface wind velocities by 5° rectangles (Marsden square quadrants) for the eastern tropical Atlantic Ocean from the African coastline at approximately lat 5° N to lat 20° S, and from long 15° E to long 20° W. The publication is intended to serve principally as an information resource for studies of air-sea interaction and related seasonal variation of the near-surface oceanic envi- ronment. It can also be used to outline potential areas and seasons of fishing operations. INTRODUCTION The southeastern tropical Atlantic Ocean has been the site of several recent oceanographic and fishery studies which have added to the knowledge of the oceanic environment and bi- ota of this area. There have not been com- parable meteorological investigations in the area. Adequate atlases of average meteorolog- ical conditions at the ocean surface exist for the North Atlantic Ocean (U.S. Navy, 1955; U.S. Naval Oceanographic Office, 1963), but those for the South Atlantic (Supplement "C" to H.O. No. 261, Naval Air Pilot West Central Africa — including Cape Verde, Ascension, and St. Helena Islands) are inadequate in geo- graphical and temporal coverage. This pub- lication was constructed to correct the inade- quacy, at least for surface wind data, by ' Contribution No. 204, National Marine Fisheries Service, Tropical Atlantic Biological Laboratory, Mi- ami, Fla. 33149. presenting mean monthly wind velocities by 5° rectangles (Marsden square quadrants) for the eastern tropical Atlantic Ocean south of lat 5° N. This publication is intended to serve prin- cipally as an information resource for studies of air-sea interaction and related seasonal var- iation of the near-surface oceanic environment. It is reasonable to hypothesize, for example, that the seasonal variation in the surface wind field is the cause of the variation of the position of an oceanic front which annually ranges be- tween lat 1° and 17° S. This hyiwthesis can be tested by correlating the mean monthly surface wind data with mean monthly sea- surface temperature data for the tropical At- lantic Ocean as portrayed by Mazeika (1968). The relationship between the surface wind field and sea-surface temperature also is relevant to studies of the distribution of tuna and other pelagic fishes. Fishery scientists have tried to relate the distribution of tunas to the surface 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 Marsden square quadrants 0012 and 0011 and the northern halves of Marsden square quad- rants 2012, 3011, 3002, and 3001 or about Wr of the wind field's area during 40 ';f of the year. In contrast to the small seasonal movement of the oceanic high pressure cells, the strong thermal reaction of the African continent causes its low pressure cell to move from a Northern Summer latitude of lat 25° N to its Southern Summer latitude of 15° S. Through- out the year, this low pressure cell affects sur- face winds of the area just ofl"shore from Cape Palmas to Luanda. Over these coastal waters this strong low pressure cell changes the flow of the South Atlantic High from southeasterly to predominantly southerly and southwesterly. The west-east oriented area formed by Mars- den square quadrants 3011, 3002, and 3001 has only a small seasonal change in wind speed. A comparison of the wind fields for February and August shows that this area has only a 0 to 5''r change in frequency of wind speed greater than 10 knots; the ai'ea of zero change is about one-half Marsden square quadrant in size and is centered over Marsden square quad- rant 3002. In contrast, a sharp gradient of seasonal change in wind frequency occurs to the north and south, and (more steeply) to the northeastward towards Port Harcourt. Also centered over Marsden square quadrant 3002 is an area of convergence (Trewartha, 1968, p. 100, Fig. 337), which is about the size of one Marsden square quadrant. This area of convergence persists months after the larger ITC, earlier described, has moved northward out of the wind field. Further study could pos- sibly correlate other interface parameters with this coincident area of small change of con- vergence and frequency of wind speed. INTERPRETING THE CHARTS The wind data are portrayed in the first section by wind rose charts. Wind rose charts are interpreted as follows: To determine the percentage of winds in a Marsden square quadrant from any one di- rection, measure the total length of the wind rose arm pointing in that direction and com- pare that length with the scale at the upper left of each chart. The percentage of winds of any force group from any one direction may be determined in the same manner. Thus in January in Marsden square quadrant 3011 the wind blew from the southeast 53';^ of the time; and 17 /f of the time this southeast wind was of Beaufort Force 4 (11-16 knots). To determine the total frequency of winds falling within a certain force group, measure the length of that force group on the scale lo- cated in the Marsden squai-e quadrant. Thus in January in Marsden square quadrant 3011 the wind was of Beaufort Force 2-3 (4-10 knots) 61% of the time. The percentage frequency of wind speed is contoured in the second and third sections. The second section shows the percentage fre- quency of wind speeds of Beaufort Force 3 (10 knots) or less. The third section shows the percentage frequency of wind speeds of Beaufort Force 4 (16 knots) or less. ACKNOWLEDGMENTS Robert W. Wilder and Dennis C. Yazell, students of St. Petersburg Junior College, as- sisted in plotting wind roses from computer data. LITERATURE CITED FLOHN, H. 1969. Climate and weather. (Translated from the German by B. V. de G. Wal- den.) McGraw-Hiil Book Co., New York, 253 p. MAZEIKA, P. A. 1968. Mean monthly sea surface temper- atures and zonal anomalies of the trop- ical Atlantic. Ser. Atlas Mar. Environ. Am. Geogr. Soc. Folio 16. TREWARTHA, G. T. 1968. A.n introduction to climate. 4th ed. McGraw-Hill Book Co., New York, 408p. U.S. NAVAL OCEANOGRAPHIC OFFICE. 1963. Oceanographic atlas of the North Atlantic Ocean. Section IV. Sea and swell. U.S. Nav. Oceanogr. Off., Publ. 700, 227 p. U.S. NAVY. 1955. Marine climatic atlas of the world, Volume 1. North Atlantic Ocean. NA- VAER (Nav. 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I | , , i i , I, ,1 i 1 1 i , I i^^^ w"i''LiLinj"i"ua:jj ini^<; 16 20" 20 20 Figure 14. — Percentage frequency of occurrence of winds of speeds of Beaufort Force 3 or less, January-June. 17 2Cf Figure 15. — Percentage frequency of occurrence of winds of speeds of Beaufort Force 3 or less, July-December. 18 ^-^ Beaufort • April Force 4 or Less • • -r U " • • • • • \ ^ • • ^^v • . \ • \ • e.^ \ * • • • ■0 • • iC'' \ 20' 10° W 6 " E lb 20 2