Surface Circulation in the Northwestern Gulf of Mexico 

 as Deduced From Drift Bottles 1 



ROBERT F. TEMPLE and JOHN A. MARTIN 2 



ABSTRACT 



Drift bottles were released monthly at predetermined stations in the northwestern Gulf of Mexico 

 from February 1962 to December 1963. Of the total 7,863 bottles released, 12% (953) were recovered 

 within 30 days after release. Analysis of the monthly recoveries revealed seasonal shifts in the flow of 

 surface waters. Between September and February the dominant flow was west along the Louisiana 

 and east Texas coasts, shifting southwest along the southern Texas coast. Between March and May, 

 currents underwent a transitional period, shifting to the north and onshore, particularly along the 

 south and central Texas coast. Converging currents, also apparent along the south Texas coast, ap- 

 peared to progress up the coast with time. In June and July the surface flow was to the northeast and 

 east. August was another transitional period with currents appearing to weaken and turning on- 

 shore. Movements of surface waters appeared directly related to prevailing winds. 



INTRODUCTION 



As part of an expanded research effort in 1962 (Kut- 

 kuhn 1963), the Bureau of Commercial Fisheries, now 

 the National Marine Fisheries Service, NOAA, initiated 

 a drift bottle study to determine the direction and rate of 

 flow of surface waters in the northwestern Gulf of Mex- 

 ico. The objectives of this study were: 1) to document on 

 a monthly basis surface current direction and velocity; 

 and 2) to attempt to relate monthly variations in current 

 direction and speed with the success or failures of the 

 yearly shrimp crops. Shrimp are planktonic, and may be 

 dependent upon currents for transportation to the estu- 

 arine nursery grounds, which are essential for successful 

 completion of their life cycle. The drift bottle study 

 reported herein began in 1962 and continued through 

 1963. 



Seasonal difference in direction and speed of surface 

 currents in the Gulf of Mexico have been generally de- 

 scribed by Smith et al. (1951), Leipper (1954), Curray 

 (1960), Chew et al. (1962), and Ichiye (1962). More recent 

 works on current include those by Drennan (1963), Dren- 

 nan et al. (1963), Armstrong et al. (1967), Watson and 

 Behrens (1970), Ichiye and Sudo (1971), 3 and Moore 

 (1973). Many of the above studies, however, although 

 contributing to an understanding of ocean currents in the 

 Gulf, were restricted to limited geographic areas and 

 completed over relatively short time periods. The data 

 presented in this report are unique in that they were 

 generated from the entire northwestern Gulf of Mexico 



'The compilation, tabulation, and analysis of part of the data reported 

 herein were supported under the Department of Interior's Bureau of Land 

 Management Interagency Agreement No. 08550-1A5-19. 



-Gulf Fisheries Center, National Marine Fisheries Service, NOAA, 

 Galveston, TX 77550. 



'Ichiye, T., and H. Sudo. 1971. Mixing processes between shelf and 

 deep sea waters of the Texas coast. Dep. Oceanogr., Texas A&M Uni- 

 versity, Ref. 71-19-T, p. 30. 



and on a monthly basis for a 2-yr period, thus providing a 

 time series heretofore unavailable. 



METHODS 



Cruises were conducted monthly with chartered 

 shrimp vessels from February 1962 to November 1963 to 

 stations located over the continental shelf (Fig. 1). The 

 monthly schedule was followed as closely as possible, the 

 only exceptions being due to adverse weather conditions 

 or mechanical breakdowns. Operations were similar be- 

 tween years except that fewer stations were occupied in 

 1963, and one vessel was used to cover the entire study 

 area rather than two as in 1962. The general overall ef- 

 fects of these modifications were that in 1963 the areal 

 coverage was slightly reduced, and the time required to 

 provide total coverage of the study area was increased. 

 The latter effect explains seeming discrepancies in the 

 date labeling of figures used in the analysis to follow. For 

 example, the April cruise in 1963 extended into May. 



Drift bottles used throughout the study were made of 

 clear glass, about 22 cm in height, 6 cm in diameter, and 

 had a capacity of about 240 cm 3 . Each bottle contained a 

 bright reddish-orange card on which was a brief message 

 in Spanish and English. A reward of 50 cents was paid for 

 the return of the card with information of location and 

 date of recovery. In most cases, half the bottles released 

 at a station were ballasted (odd numbers) and half were 

 unballasted (even numbers). Those ballasted floated at 

 or just under the surface. The reason for the use of the 

 two types of bottles was an attempt to determine the 

 direct effects of winds. 



The number of bottles released during each cruise 

 varied during the 2-yr study. Generally, 12 bottles were 

 released at each station in 1962; 4-10 bottles were 

 released per station in 1963. This modification did not af- 

 fect the rate of recovery for bottles within 30 days after 

 release for the percent recovered in 1963 was greater than 



