= 
PR MAY oct NOV DEC 
1520 25 510 15 20 25 510 15 20 30 510 15 20 25 S10 15 20 25 
Teo Leal T Toplist Talulentanks “1963! Toate) 
1964 
GULF OF MAINE ~-—— --- 
1965 
GEORGES BANK Sain 
' 1966 
| - 
~~ F967 
1968 v 1968 
1969 | 1969 
j 
1970 1970 
omc SS 
1971 1971 
— [Rae 
1972 1972 
9,7 eee Woe nse ll 
| \ 
| 1974 | 374 | 
1975 1975 
| 
Figure 4.—Dates of spring and autumn cruises in the Gulf of Maine 
and Georges Bank, 1963-75. 
temperatures are largely influenced by highly variable 
surface conditions. The dates of cruises on Georges Bank 
were fairly consistent in autumn, but spring cruises had 
two rather distinct groupings—March dates for 1969-72 
and April dates for the remaining 4 yr. In the Gulf 
autumn cruise dates nearly all overlapped temporally 
and spring cruises occurred mostly when the bottom- 
water temperatures remain virtually stable and isolated 
from surface effects (Bigelow 1927; Colton and Stoddard 
1973). It was assumed in an earlier report (Davis*) that 
timing of the cruises was not critical to the estimation of 
major temperature trends because most of the data were 
collected during or close to the seasonal maxima or 
minima in temperatures (Colton and Stoddard 1972). In 
this report, however, the cruises have been adjusted to 
common reference dates (April 20 and November 9 for 
the Gulf; April 4 and November 7 for the Bank). Average 
bottom temperatures by 10-day intervals at Portland 
and Nantucket Lightships (Fig. 5) were used to cal- 
culate adjusted values for the Gulf of Maine and Georges 
Bank, respectively. The mean temperatures observed on 
each cruise for the whole area and its subareas were ad- 
justed by adding or subtracting the differences in the 
mean temperatures at the appropriate lightship on the 
reference dates and on the middates of the cruise and 
subarea samplings. Magnitude of the adjustments was 
mostly less than 0.4°C in the Gulf of Maine, but was 1°C 
or more on Georges Bank, especially in the autumn (Figs. 
6, 10, 14, 18; Tables 2-5). Finally an annual index for 
both the Gulf of Maine and Georges Bank was obtained 
by averaging the spring and autumn indices (Fig. 22). 
The reader is alerted to keep in mind which in- 
dex—observed or adjusted—is used in the text. Ad- 
justed values are intended primarily for examining 
trends and observed values for comparing habitat con- 
ditions, i.e., TCI distributions. 
‘Davis, D.W. 1976. Spring and autumn bottom-water temperatures 
in the Gulf of Maine and Georges Bank, 1968-1975. Int. Comm. North- 
west Atl. Fish. Res. Doc. 76/85, 14 p. 
12 
11 
2 
= al© 
WwW 
= OF 
ra 
F 
g 8 Nantucket 
' 7 Lightship 
= [ Portland 
fe) = brawn 
E a (1956 — 1968 ) iightsu 
fea) 5b (1956 —1969) 
MONTH 
Figure 5.—Mean annual cycles of bottom-water temperatures at 
Nantucket (1956-68) and Portland (1956-69) Lightships (data from J. 
Chase, Woods Hole Oceanographic Institution). 
RESULTS 
Spring Temperatures, 1968-75 
Gulf of Maine—Spring.—Adjusted mean bottom- 
water temperatures for the Gulf of Maine showed a 
warming trend after the coldest year, 1968, with peak 
temperatures of 6.5°C in 1970 and 6.4°C in 1974 (Fig. 6). 
Annual decreases of 0.3°C were observed in 1971 and 
1975, while the largest annual increase of 0.8°C occurred 
in 1970 and accounted for over 70% of the total 8-yr range 
of 1.1°C (5.4-6.5°C). The 1968-75 mean index of 6.1°C 
was about 1°C colder than in 1955 and 1956, but 1°C 
warmer than in 1965 and 1966 as reported by Schopf 
(1967), and also 1°C warmer than the 1962-72 long-term 
Adjusted 
Observed 
BOTTOM -WATER TEMP °C 
Spring — 
GULF OF MAINE 
1968 "69 ZO) 7A) eee ae 
Figure 6.—Observed and adjusted mean bottom-water temperatures 
in the Gulf of Maine during the spring, 1968-75. 
