WOODS HOLE LABORATORY REFERENCE DOCUMENT NUMBER 84-24

TEMPERATURE STRUCTURE AND SURFACE SALINITY

IN THE GULF OF MAINE DURING 1983

by

Catherine Jewell

Northeast Fisheries Center Woods Hole Laboratory

Woods Hole, Massachusetts 02543

August 1984

MARMAP Contribution MED/NEFC 84-30

INTRODUCTION

This report presents the monthly temperature and surface

salinity data from the Gulf of Maine collected during 1983 by the

Ship of Opportunity Program (SOOP). This program originated in

1975 as part of the joint effort of the Atlantic Envi ronmental

Group (AEG) in Narragansett, Rhode Island and the Northeast'

Fisheries Center (NEFC) laboratory in Woods Hole, Massachusetts.

The main objective of this program is to monitor the monthly

changes in water temperature and sea surface salinity along

designated transect routes in the Gulf of Maine. Reports on

results from earlier years of this program are in Jewell and

Mountain (1983), Wri ght (1981), Caine (1980), Ki rschner (1980),

Pawlowski (1978, 1979).

Observations i~ 1983 were made by M/V YANKEE CLIPPER

operating between Boston, Massachusetts to Cape Sable, Nova

Scotia. Dates of these cruises are listed in Table 1. Locations

of the transects are shown in Figure 1.

METHODS

Expendable bathythermograph (XBT) probes were dropped at

hourly intervals along the transect route. A surface water

sample for salinity analysis was also taken at each XBT

station. The salinities were determined by AEG using a

Conductive salinometer. Bottom depths were read from XBT traces

':'1 I

J.

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when possible. In other cases, the station position was plotted

on a bathymetric chart and a depth estimate was obtained.

Distance between the stations were calculated for use in plotting

temperature cross-sections. The XST traces were read to a tenth

of a degree resolution and temperature cross-sections were drawn

with a 1°C contour interval. In areas where data is missing,

isotherms and estimated depths were plotted using a dashed line

(Figures 2-8). The surface temperature and salinity along the

transects were plotted for each station on each section and are

organi zed by seasons (Fi gu res 9-12).

The surface salinity data were also averaged regionally

along each transect and the values for each region were plotted

throughout the year (Figure 13). The five regions used were

coastal Massachusetts and coastal Nova Scotia (with depths less

than 100 m), central gulf west of 69°W, the central gulf between

69° and 68°W, and the central gulf east of 68°W. For the three

central gulf regions, the area weighted mean temperature below

100 m depth was determined by use of a planimeter and the values

plotted (Figure 14). The contoured temperature sections will be

discussed by season: winter (January-March), spring (Apri 1-

June), summer (July-September), and fall (October-December).

I I

I ~

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RESULTS

By late January wind mixing and cooling induced convection

had restored homogeneity to almost all depths. As winter pro­

gressed cooler temperatures were observed in the more coastal

areas giving rise to vertically isothermal conditions above 100 m

and increasing temperatures progressing from the shores

seaward. These horizonal gradients continued to intensify until

mid-April. Coastal temperature lows of 1.6°C near Cape Sable and

3.1°C near Boston were recorded during mid-April and early March,

respectively. Most offshore surface waters were 5 to 6°C during

this time. As the surface waters continued to cool, warmer

waters >7°C occupied the deeper basins throughout most of the

winter.

A vertical temperature inversion developed by early Mai as

solar warming resumed and the horizonal gradient decayed. The

surface warming was greater over the western half, which lead to

a 6-7°C difference between the warmer coastal Boston waters and

the cooler waters off the Cape Sable coast. A cold pool with

waters <5°C began to form in early May and transformed into cold

cells which occupied the intermediate layers between 40-120 m

later in the month. The continual surface warming acted to

isolate this cold intermediate water. Warm (>8°C) Slope Water

oCcupied Crowell Basin in June. The coastal bottom water

COntinued to remain cooler and uneffected by surface water

warming. The increased solar isolation caused greater warming

With increased stratification and reduced vertical mixing.

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The rapid warming trend established in early June brought

surface temperature increases of 5°C for most stations east of

68°W and 1-2°C for stations west of 68°W by late June. This

created a strong vertical temperature gradient in the west with

isotherms surfacing to the east. These conditions intensified

until late August for coastal Nova Scotia where a high

temperature of 16.5°C was recorded. For coastal Massachusetts,

the station high was 19.4°C. The bottom waters continued .to show

little effect of the seasonal warming cycle.

The 10°C isotherm which was around 25 m depth in August

gradually progressed downward in the water column as solar

radiation decreased, surface cooling began and greater vertical

mixing occurred.

By October the isotherm h~d penetrated 50 m. By November

sufficient cooling had occurred that the 10° isotherm

disappeared. The intermediate water warmed very slowly with cold

6°C water remaining in isolated cells until late November. By

November wind mixing and cooling acted to overturn the surface

waters, helping to restore homogenous conditions to depths well

below 50 m for most areas. This continued throughout the

season. December saw near isothermal conditions at all· stations

With temperatures averaging 7.7°C and only minor intrusion of

warm water (>8°C) at the mid-depth regions.

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surface Salinities -Surface salinities were highest during the winter months for

all areas, with averages between 33.0-33.5% with the exception of

the coastal Nova Scotian area which was 1-2% lower than the other

areas. Local coastal runoff and net precipitation and

evaporation were responsible for the lower salinity observed in

spring. These low salinity values contfibuted to the development

of the intense temperature gradients in late spring and early

summer by enhancing the near surface density stratification. The

central waters, being removed from local freshening, showed more

constant salinity values and demonstrated the usual trend of

decreasing in spring and recovery in late summer and early

fall. By December the central waters were between 0.4-0.7% lower

than January salinity values. Coastal Nova Scotian waters showed

variable salinities over most of the year with high and low

values oscillating between consecutive sections. These erratic

values can be for the most part dependent on the dominance of the

fresher Scotian Shelf water or the Gulf of Maine surfac2 waters

during the coastal Nova Scotian sampling.

!ottom Layer Temperature

The average temperatures below IOU m depth are shown in

Figure 14. Minimum for the area occurs in early May givng a

total average of 5.6°C. A warming trend began in late June and

Continued till December producing a slow rise of less than 1°C

Per month. Both waters were progressively warmer from west to

east. Several factors contribute to the observed bottom water

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temperatures. The main influence appears to be the inflow of

warm and high salinity Slope Water on to the shelf. The degree

of influence is dependent upon the location and bathymetry. The

eastern half, being closer to the Northeast Channel, is more

affected by offshore waters than the western half.

Another factor which aids in the effective warming of the .

eastern bottom water is the degree and intensity of vernal

warming •. The gradual increase in temperature experienced in the

eastern half, enabled a more thorough overturn of heat, where as

the intense stratification towards the western half acted to trap

the heat above the thermocline layers, thus preventing overturn

by an effective density gradient. This can be further supported

by the slow warming and breakup of the cold intermediate water,

which exists into late November in the western waters.

DISCUSSION AND COMPARISON

The 1983 sections for the Gulf of Maine exhibited the same

general features in the seasonal cycles that were observed in

previous years. To summarize, winter sections were characterized

by isothermal conditions above 50 m with horizonal gradients

extending from shores seaward. Spring sections showed freshening

and warming of surface waters, thermocline development, and

formation of the cold Gulf of Maine intermediate water. Summer

sections exhibited intense thermocline development above 50 m and

Vertical warming. Fall sections displayed thermocline breakdown,

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cooling and wind induced mixing, warming of the intermediate

layer and dispersion of the cold water cells, along with bottom

water warming •. Minor differences from previous years are

discussed below.

surface Waters

The winter of 1983 appeated to begin with a longer duration

with milder temperatures then previous years. Sea surface data

from the central waters appeared 1-2°C warmer then in the past,

while coastal water temperatures were more comparable. The onset

of solar warming occurred somewhat later then it did in the 1981

and 1982 sections. Once seasonal warming did resume, it did so

with a greater intensity as sea surface temperatures climbed 2-

4°C approximately every two weeks at stations west of 68°W, while

those near Nova Scotia showed a more variable rate of increase.

An early peak in temperature occurred in July in the western

half, followed by a decrease of 1-4°C and a second peak in early

September. A temperature minimum was observed near Cape Sable in

early July. Similar features were also observed in June of 1981

and August of 1982 and are probably due to the influx of cold

Scotian Shelf water into the area.

Warmer conditions continued for a longer duration in 1983

than in 1982 with temperatures remaining high over most of the

transect well into September. Cooling was rapid in October and

November with the establishment of isothermal conditions to

depths above 50 m.

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Subsurface and Bottom Waters -The cold intermediate layer which had formed in the western

waters by May was more persistent then in previous years. It

warmed only 1°C and maintained its identity well into November.

The duration of this cold pool indicates that heat from the

warmer surface area did not penetrate to the bottom water in the

western area. Bottom water temperature followed the same pattern

as previous years; cooling in spring and warming in summer and

fall. No significant differences in the temperature were

observed from the 1981 and the 1982 bottom water averages. In

all years, bottom waters were progressively warmer from west to

east.

Surface Salinities

Surface salinities averaged about .5-1.0% fresher during

'1983 than in 1981 and 1982 for all sections. Spring freshening

had a greater impact during 1983 than 1981 and 1982, though

return to higher values occurred around 'July and August for all

three years.

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REFERENCES

Caine, T.E. 1980. Temperature structure and surface salinity in

the Gulf of Maine during 1979. Ann1s. Bio1., Copenhagen.

36: 36-43.

Gatien, M.G.

Halifax.

1976. A study in the Slope Water region south of

J. Fish. Res. Board can. 33: 2213-2217.

Hopkins, T. and N. Garfield. 1979. Gulf of Maine intermediate

water. J. Mar. Res. 37: 103-139.

Jewell, C.S.E. and D. Mountain. 1983. Temperature structure and

surface salinity in the Gulf of Maine during 1981 and

1982. Woods Hole Lab. Ref. Doc. No. 83-31. MAR MAP Contra

MED/NEFC 83-35.

Kirschner, R.A. 1980. Temperature structure and surface

salinity in the Gulf of Maine in 1978. Ann1s. Bio1.,

Copenhagen. 35: 30-41.

MCLain, D. and M. Ingham. 1984. Sea surface temperatures in the

northwestern Atlantic in 1983. Ann1s. Biol., Copenhagen.

PawloWSki, R.J. 1979. Vertical temperature structure and sea

surface salinity in the Gulf of Maine in 1977. Annls.

Biol., Copenhagen. 34: 27-31.

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Wright, W.R. 1981. Temperature structure and surface salinity

in the Gulf of Maine 1980. Annls. Biol., Copenhagen. 37:

21-34.

Ii'

Table 1. Dates of XBT observations from MjV YANKEE CLIPPER between

Boston, Massachusetts and Cape Sable, Nova Scotia during

1983.

JULIAN DAYS

I

29 Jan. 83-03 25 May 83-12 09 Sept. 83-19 ~ ~ I

19 Feb. 83-04 11 Jun. 83-13 08 Dc t. 83-20

04 Ma r. 83-05 20 Jun. 83-14 15 Dc t • 83-21

15 Mar. 83-06 08 July 83-15 26 Dc t. 83-22

05 Ap r. 83-U7 23 July 83-16 24 No v. 83-24

12 Apr. 83-08 03 Aug. 83-17 09 De c. 83-25

09 May 83-10 20 Au g. 83-18 17 De c. 83-26

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List of Figures

Figure 1 Location of transect line across the Gulf of

Maine.

Figures 2-8 Temperature cross-sections across the Gulf of

Maine.

Figures 9-10 Surface salinity across the Gulf of Maine.

Figures 11-12 Surface temperature across the Gulf of Maine.

Figure 13 Average surface salinity across the Gulf of

Maine by geographic area.

Figure 14 Average bottom water (>100 m) temperature across

the Gulf of Maine by geographic area.

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SURFACE SALINITY (CJLo)

JAN 29 FEB 19 MAR 4 MAR 15

1983

CAPE SABLE

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APR 5 APR 12 MAY9 MAYZ5 .u. II

1983

Figure 9

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YANKEE CLIPPER SURF ACE SALINITY (960)

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1983

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SURF ACE SALINITY (960)

OCT 8 OCT 15 OCT 26 NOV 24 DEC 9 DEC 17

1983

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1983 OCT 8

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NOV 24············ DEC 9 .-.... DEC 11 ""

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COAST AL MASS.

CENTRAL GULF

1983

CENTRAL GULF between

CENTRAL GULF COAS T AL NOV A SCOTIA

42°25 1,70°49' West of 69C OO'

69°00 1 -68°00' East of 68 COO' 43°13 1,65°26'

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Fi gure 13

JAN FEB MAR APR MAY JUN TEMP (oC)

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YANKEE CLIPPER I 983

Average temperature of Bottom Water

West of 69°00' between 69°00'-68°00' East of 68000' Total

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