1078 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1957 



slightly retarded. In the open water of the continental shelf there is often 

 a strong stratification of water masses and the vernal cycle is either 

 strongly retarded (by several months) or completely obscured. 



On the open shelf the bottom-temperature cycle is controlled by 

 shifting currents and wedges of water which flow in along the bottom 

 from the open ocean. In some areas these changes go through essentially 

 the same cycle each year. The Irish Sea and the continental shelf west 

 of Scotland are areas where the cycle is so regular that one can safely 

 predict the bottom temperature for a given month within an accuracy 

 of ±1.5-2°C. On the other hand, the bottom temperature on the 

 Grand Banks changes radically from day to day and week to week. It 

 is possible to show that on the Grand Banks summer temperatures are 

 on an average 2° colder than winter bottom temperatures. The day-to- 

 day temperature changes can amount to 5°C or more. 



In one particularly well-studied area off Halifax, Nova Scotia, an in- 

 teresting complication has been discovered. In this 10,000-sciuare mile 

 area the bottom temperatures had been studied for about twenty-five 

 years, observations having been taken at different times of the year. 

 The bottom temperature was considered known to 1°C. It has more 

 recently been found that on occasion, the 8°C water is displaced upward 

 by an underflow (incursion) of 2°C water which suddenly lowers the 

 bottom temperature by 6°C. However, after a few weeks the bottom 

 temperature again approaches the usual value of 8°C. Such incursions 

 of contrasting water (both cold and warm) from the open ocean are as 

 yet only partially understood. 



The maximum amplitude of temperature changes in bays and near 

 shore areas of both seasonal and erratic nature often approaches 20°C. 

 On the outer shelves 8°C would be the maximum change expected. 



It is now well established that the average temperature over wide 

 areas in the North Atlantic has undergone a gradual increase for the 

 past one hundred and fifty years. The average bottom temperature on 

 the Nova Scotian shelf increased 2°C between the early and mid-nine- 

 teen thirties and the late nineteen forties. Similar changes have beeni 

 reported for the area near Iceland. At present, no sure way of predicting! 

 the future longterm changes of temperature is available. Changes of 1° 

 per decade may be experienced. 



4.3.3 Dee/p Water {Depth more than 200 Meters) 



A search was made for all deep sea temperature measurements taken 

 with accurate thermometers in depths greater than 2,000 meters, from 



I 



