42 



can be followed from one survey to the next. It is of interest to note 

 that because of the comparative simplicity of pattern of this year's 

 maps it is possible to get an idea of the seasonal change in the strength 

 of the Atlantic Current. An examination of the five current maps 

 shown in figures 23 to 27 yields the qualitative information that the 

 northward encroachment and strength of the Atlantic Current 

 apparaently increased progressively up to the time of the third map 

 (fig. 25, May 17-23, 1940) and then decreased. According to the 

 ideas set forth by Iselin ^ the Gulf Stream system expands during 

 periods of decreasing energy and contracts during periods of increasing 

 energy. As the Atlantic Current shown in figures 23 to 27 represents 

 the outer edge of the Gulf Stream system in this region, the inter- 

 pretation of the above noted fluctuation, in terms of the Gulf Stream 

 system mechanism proposed by Iselin, would be that during the 

 period from March 30 to June 18, 1940, the energy of the system 

 decreased to a minimum about May 17-23, after which it increased 

 to the end of the period. Iselin has demonstrated the negative 

 correlation between the energy of the Gulf Stream system off the 

 American coast and the sea level as indicated by American coastal 

 tide gages. From both tide gage records and determinations of the 

 volume of flow of the Gulf Stream past the section from Montauk 

 Point to Bermuda, the seasonal variations show a secondary minimum 

 of energy of that sector of the Gulf Stream system occurring during 

 the month of May. Iselin 's report is a preliminary one and recognizes 

 the need for considering whether or not evidence from other sectors of 

 the Gulf Stream system support or contradict the tentative con- 

 clusions. The fluctuations of the Atlantic Current pointed out here 

 are of interest chiefly because they are from another sector and 

 although they are of a qualitative nature they at least do not contra- 

 dict Iselin's hypothesis. If they can be accepted as evidence they 

 seem to indicate that there is little if any phase difference between the 

 American sector and the Grand Banks sector. From the standpoint 

 of forecasting conditions in the Grand Banks sector for ice patrol 

 purposes this is unfortunate if true. 



In previous bulletins of this series an investigation of the seasonal 

 variations of the Labrador Current in the Grand Banks region has 

 been reported on from year to year. Certain sections, called T, U, 

 and W have been repeatedly occupied. These are defined as follows: 

 Section T, running southeasterly from about 46°20' N., 49°00' W.; 

 section U, extending east and west about the 45th parallel; and section 

 W, running south off the Grand Banks in about longitude 50° W. 



2 Iselin, C. O'D. Preliminary report on long-period variations in the transport of the Gulf Stream system. 

 Papers in Phys. Oceanog. and Met. M. I. T. and W. II. O. I., vol. VIII, No. 1 (July 1940) Cambridge and 

 Woods Hole. 



