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NOAA Technical Report NMFS 134

Water Temperatures and

Climatological Conditions

South of New England, 1974-83

Reed S. Armstrong (editor)

U.S. Department of Commerce

March 1998
u.s. DEPARTMENT

OF COMMERCE

WILLIAM M. DALEY

SECRETARY

National Oceanic and

Atmospheric AdIninistration

D. James Baker

Under Secretary for

Oceans and Atmosphere

National Marine

Fisheries Service

Rolland A. Schmitten

Assistant Administrator

for Fisheries

The NOAA Technical Report NMFS

(ISSN 0892-8908) series is published by

the Scientific Publications Office, National Marine Fisheries Service, NOAA,

7600 Sand Point Way N.E., Seattle, WA

98115-0070.

The Secretary of Commerce has determined that tlle publication of this series is necessary in tlte transaction of tlte

public business required by law of tlIis

Department. Use of funds for printing of

tlus series has been approved by tlte Director of tile Office of Management and

Budget.

NOAA

Technical

Reports NMFS

Technical Reports of the Fishery Bulletin

Scientific Editor

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Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

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Editorial Conunittee

Dr. Andrew E. Dizon National Marine Fisheries Service

Dr. Linda L. Jones National Marine Fisheries Service

Dr. Richard D. Methot National Marine Fisheries Service

Dr. Theodore W. Pietsch University of Washington

Dr.Joseph E. Powers National Marine Fisheries Service

Dr. Titn D. Smith National Marine Fisheries Service

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The NOAA Technical Report NMFS series of the Fishny Bulletin carries peer-reviewed, lengthy original research reports, taxonomic keys, species synopses, flora

and fauna studies, and data intensive reports on investigations in fishery science,

engineering, and economics. The series was established in 1983 to replace two

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in limited numbers to government agencies, both federal and state. They are also

available in exchange for other scientific and technical publications in the marine

SCIences.
NOAA Technical Report NMFS 134

A Technical Report of the Fishery Bulletin

Water Temperatures and

Climatological Conditions

South of New England, 1974-83

Reed S. Armstrong (editor)

March 1998

U.S. Department of Commerce

Seattle, Washington
Suggested reference

Armstrong, Reed S. (ed.) 1998. Water temperatures and climatological

conditions south of New England, 1974-83. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS 134, 43 p.

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CONTENTS

Abstract

Papers

J. LOCKWOOD CHAMBERLIN

REED S. ARMSTRONG

GRAYSON B. WOOD

STEVEN K. COOK

J. LOCKWOOD CHAMBERLIN

MERTON C. INGHAM

REED S. ARMSTRONG

Ocean temperature monitoring: project background and

some applications to fishery ecology

Bottom temperatures on the continental shelf and upper slope:

means, standard deviations, and anomalies

Comparison ofsurface and bottom water temperatures with

air temperatures

Vertical thermal structure of midshelf waters

Maximum and minimum bottom temperatures on the

continental shelf and upper slope

Mean monthly wind stress and Ekman transport conditions

Variations in the shelf-slope front and the influence of

warm-core Gulf Stream rings

iii

v

1

7

15

21

27

31

39

Abstract

From 1974 through 1983, we conducted monitoring to

provide the first long-term, year-round record of sea

water temperatures south of New England from surface

to bottom, and from nearshore to the continental slope.

Expendable bathythermograph transects were made approximately monthly during the ten years by scientists

and technicians from numerous institutions, working

on research vessels that traversed the continental shelf

offsouthern New England.

Ten-year (1974-83) means and variability are presented for coastal and bottom water temperatures, for

mid-shelf water column temperatures, and for some

v

atmospheric and oceanographic conditions that may

influence shelf and upper-slope water temperatures.

Possible applications of ocean temperature monitoring

to fishery ecology are noted. Some large departures

from mean conditions are discussed; particularly notable during the decade were the response of water

temperatures to the passage of Gulf Stream warm-eore

rings, and the magnitude and persistence of shelf-water

cooling associated with air temperatures in three successive very cold winters (1976-77, 1977-78, and 1978-79).

Manuscript accepted 30January 1995.

Ocean Temperature Monitoring:

Project Background and Some Applications

to Fishery Ecology

J. LOCKWOOD CHAMBERLIN*

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

28 Tarzwell Drive

Narragansett, Rhode Island 02882-1199

Introduction

This volume is the culmination of a decade of environmental monitoring and analysis initiated in 1974 as part

of the Marine Resources Monitoring, Assessment, and

Prediction (MARMAP) Program of the National Marine Fisheries Service (NMFS). Planning for MARMAP

emphasized that changes in abundance and distribution ofliving marine resources can be satisfactorily understood only through analysis of the influence of both human activity and natural variation in environmental conditions. The plans also recognized that far more information is routinely gathered on human activities, such as

fishing, coastal dredging and filling, and pollution, than

on the natural variability of the physical environment.

Temperature has long been recognized as a major

factor in fishery resource ecology (Taylor et aI., 1957).

From 1966 through about 1976, surface temperature

was surveyed monthly with infrared radiometers carried

on Coast Guard aircraft flying standard transects over the

width of the continental shelffrom Cape Cod to Florida

(Deaver, 1975). Lacking, however, were year-round, crossshelf measurements of water temperature below the surface and, particularly, at and near the bottom, where most

finfish and shellfish are concentrated offthe northeastern

United States (Grosslein and Azarovitz, 1982).

We selected the continental shelf and slope south of

New England along 71°W longitude for a multi-year

monitoring project. This area was chosen because it

appeared that most of the desired data could be obtained economically there by using expendable bathythermographs (XBT's) and by enlisting the cooperation of scientists who traverse these waters on the numerous research vessels home ported at Woods Hole,

Massachusetts, and Narragansett, Rhode Island. Furthermore, because average water movement in the area

is alongshelf from off southern New England to the

Middle Atlantic shelf (Bumpus, 1973; Mayer et aI., 1979),

it was assumed that conditions in the former area were

fairly representative of those in the latter.

The research reported here owes a great deal to the

scientific talent and diligence of Robert Wylie Crist

(1950-83), who served as project manager for 8 yr until

his untimely death. As well as being an exemplary researcher and manager, Lee was a warm and cheerful

colleague, always ready to give assistance. To his memory

we dedicate this technical report.

The project benefitted at its inception in 1974 from

the numerous bathythermograph sections across the

southern New England shelf and slope obtained by

Redwood Wright, then at Woods Hole Oceanographic

Institution, in his study of variability in the offshore

limits of shelf water south of New England and Cape

Cod (Wright, 1976). Numerous scientists cooperated in

making XBT observations throughout the lO-yr project,

as acknowledged in the annual summary reports cited

in Armstrong (l998a). The major contributions were

59 sections taken from vessels of the National Oceanic

and Atomospheric Administration (mostly by NMFS

scientists), 47 taken from Woods Hole Oceanographic

Institution vessels, 38 from vessels of the Graduate

School of Oceanography, University of Rhode Island,

16 from U.S. Coast Guard ships, 10 from ships of the

Soviet Union, and 9 from Polish vessels. The project

obtained a total of approximately 196 temperature sections from 176 cruises by 35 different ships.

Before the southern New England ocean temperature data could contribute to understanding of fishery

ecology, various analyses were necessary. Results of the

monitoring were published in annual reports for each

of the ten years (see citations in Armstrong, 1998a).

These include contoured vertical sections of temperature for individual vessel transects as well as contoured

time-depth diagrams of bottom temperatures.

* Now retired. Present address: 278 Maine St., Brunswick, ME 04011.
2 NOAA Technical Report NMFS 134 _

In this volume, Armstrong (1998a), Wood (1998),

Cook (1998), and Chamberlin (1998) extend the analysis to interannual comparisons of water temperatures.

Also included are analyses of air temperature (Wood,

1998), winds (Ingham, 1998), and variability in the

offshore boundary ofshelfwater in relation to seasonality and the passage of warm-core rings (Armstrong,

1998b), factors expected to influence water temperatures and for which there were fairly consistent records

throughout the project. Each author presents the average conditions for the decade, representing the normal

annual cycle, along with an analysis of the variability,

and discusses some large departures from mean conditions in association with other data.

Project Background _

Previous to this research, the general pattern and seasonal cycle of temperatures in waters south of New

England were well known (e.g., Bigelow, 1933; Ketchum

and Corwin, 1964). On the basis of daily observations at

shore stations, it was known that short-term variations

in coastal water temperature are associated with changes

in atmospheric weather conditions, particularly insolation

and surface air temperature (Taylor et aI., 1957) and

winds (Lauzier, 1965). However, long-term trends in shorestation temperatures on the northeast coast depend largely

on the relative position and degree of mixing of coastal

and oceanic water masses (Colton, 1968, 1969). Analyses of bottom temperature on the shelf and upper slope

had shown, by examining monthly averages of longterm archive data, that annual temperature range generally decreases and annual temperature extremes occur later with increasing depth (Walford and Wicklund,

1968; Colton and Stoddard, 1973).

The pool ofminimum-temperature bottom water that

persists throughout the warm season at mid-depth on

the southern New England and Middle Atlantic shelf

was first charted by Bigelow (1933) and later corroborated by Ketchum and Corwin (1964) as persistent "winter water." From year to year this mass of bottom water

varies in intensity; within years it varies in cross-sectional area, onshore-offshore position, and extent of

contact with the bottom (Whitcomb, 1970).

In contrast to the marked annual warming and cooling of bottom water at shallow to middle depths on the

shelf, temperature on the outer shelf and upper slope,

where the slope-water mass is dominant (Wright, 1976),

was known to have an annual range of only a few degrees Celsius. Schroeder (1963) noted that greater variations were the result of irregularly occurring advective

processes.

The famous mass mortality of tilefish, Lopholatilus

chamaeleonticeps, from March through May 1882 (Collins,

1884) stimulated interest in advective processes on the

outer shelf and upper slope. Millions of tilefish were

found dead at the surface in a band extending about

310 km from off Montauk Point, Long Island, to southern Delaware. Although no pertinent water temperatures were measured at the time, biological evidence

indicated that the mortality was caused by abnormally

cold water temporarily flooding the bottom (Bigelow

and Schroeder, 1953). Hachey (1955) pointed out that

the probable source of such water was off Nova Scotia,

where cold «4°C) coastal water of Labrador origin

normally occupies the bottom along the upper continental slope east of Emerald Bank. Only a far greaterthan-normal westward advection ofthis cold water could

have caused the tilefish disaster; however, similar advections of lesser magnitude have often been observed

on the slope off Nova Scotia westward ofEmerald Bank.

Furthermore, in March 1966 an unusually extended

advection of the Labrador coastal water was observed

on the upper continental slope south of Georges Bank

and New England, where bottom temperatures were below 6°C to a depth of 200 m (Colton et aI., 1968).

During the ten years of this temperature monitoring

project, there was no recognizable incursion of the cold

Labrador coastal water into waters south ofNew England.

At bottom depths of 160-200 m the lowest recorded

temperature was about 9°C, lasting for perhaps a few

weeks during early 1977 (Armstrong, 1998a, Fig. 5).

This 10-yr minimum may be attributed to the persistently cold air temperatures during fall 1976 through

January 1977 (Wood, 1998, Fig. 4).

At the beginning of the project, research by the U.S.

Naval Oceanographic Office on Gulf Stream warm-core

rings (Gotthardt, 1973) was providing a new perspective on bottom temperature variation on the outer shelf

and upper slope. These rings, large deep-water features, had been discovered decades earlier and were

recognized as forming from Gulf Stream meanders that

detached from the stream (Iselin, 1936). Nevertheless,

the potential impact of a ring on bottom temperatures

on the fishing grounds was discovered only when XBT

sections were obtained across the outer shelf and upper

slope in the New York Bight at less-than-monthly intervals during most of each year in 1972-74 (Applications

Research Division, 1974). Of particular pertinence to

fishery ecology were sections showing marked effects of

a Gulf Stream warm-core ring on bottom temperatures

over the upper slope and outer shelf. Where the ring

contacted bottom, temperatures were elevated well

above normal values. Similar elevations of bottom salinities and current speeds were inferred because rings

are composed of high-salinity oceanic water and have

strong anticyclonic circulation (Gotthardt, 1973).

The Naval Oceanographic Office also pioneered the

use of infrared satellite data, airplane-deployed XBT's,
__________________________ Chamberlin: Ocean Temperature Monitoring 3

and current measurements taken from research ships

to track rings and to investigate their subsurface structure and dynamics (Gotthardt, 1973). Significant findings included:

1. The rings usually formed southeast ofGeorges Bank

and seemed to be irregular rather than seasonal in

time of appearance.

2. Following formation, the rings tended to travel erratically in speed and direction, but with net movement generally opposite to that of the Gulf Stream.

Thus the proximity of each ring to the outer continental shelf and upper slope varied during its progression past the fishing grounds of Georges Bank,

southern New England, and the Middle Atlantic

Bight, usually travelling only a few kilometers per day.

3. Some rings persisted for several months and, from

an initial surface diameter of around 150 km, decreased in size and strength through their lifetime.

In view ofthese findings, a specific goal of this project

was to examine the frequency, duration, magnitude,

and distribution of ring influence on the shelf-slope

fishing grounds. Annual summary reports (cited in

Armstrong, 1998a) document the irregular times and

rates of passage of the 34 rings that moved through the

slope water south of New England in 1974-83. The

number of rings passing each year ranged from 3 to 7.

Armstrong (1998a, b) and Cook (1998) in this volume

also report on ring passage during this project.

The rate of warm-core ring production in the Gulf

Stream was particularly high during and after the record

cold winters ofl976-77 and 1977-78, when GulfStream

transport strengthened through deepening of the permanent thermocline in the northwestern North Atlantic (Worthington, 1977; Mizenko and Chamberlin,

1979; Celone and Chamberlin, 1980). Thus, in the

same years that cold weather chilled the shelf waters

below normal (1977 and 1978; see Fig. 5 in Armstrong,

1998a), numerous rings moved large volumes of warm

Gulf Stream water into the slope water, and in some

cases into contact with the bottom on the shelf and

slope. In contrast, in 1984, the year after temperature

monitoring offsouthern New England was terminated,

only one ring passed south of New England (Price,

1985) .

Application of Ocean Temperature

Monitoring to Fishery Ecology _

For over 20 years, NMFS has accepted nationwide scientific resource assessment as an ongoing responsibility.

Accordingly, the ocean temperature monitoring project

was not undertaken simply to investigate environmental conditions on the fishing grounds south of New

England. Rather, it was to provide a basis for analyzing

the influence of temperature on annual differences in

the distribution, movement, reproduction, and other

aspects of fish populations.

Examples for discussion ofsuch temperature analysis

are taken from Grosslein and Azarovitz (1982), who

present distribution maps and summarize ecological

information for numerous commercial finfish and shellfish. Among these are the following eight demersal

species which undertake seasonal onshore and offshore

migrations across the continental shelf south of New

England and off the Middle Atlantic states: silver hake,

Merluccius bilinearis; red hake, Urophycis chuss; black sea

bass, Centropristis striata; scup, Stenotomus chrysops; butterfish, Peprilus triacanthus; summer flounder, Paralichthys

dentatus; longfin squid, Loligo pealei; and northern

shortfin squid, Illex illecebrosus. Furthermore, these authors suggest that temperature is a primary environmental factor controlling the seasonal migrations of

the majority of these species.

Nonetheless, the actual undertaking of temperature

control analysis by fishery ecologists may be hindered

by the lack of offshore temperature monitoring data

heretofore. Accordingly, the shoreward spring migrations of the species listed above are discussed here to

exemplifY the use of temperature data from south of

New England, and also to point out how variable the

influence of temperature conditions may have been

during the ten years of monitoring.

In slope water, which is typically present at bottom

depths below 150 m, bottom temperature is an unlikely

stimulus ofshoreward spring migration for overwintering demersal fish. One reason for this is the small

seasonal change of bottom temperature: at depths of

160 to 200 m, the 10-yr mean annual cycle had a range

ofless than 1°C (Chamberlin, 1998, Fig. 1,2). A second

reason is that this low-amplitude annual cycle lagged

almost 6 mo behind coastal air and sea-surface temperature cycles (Armstrong, 1998a, Fig. 3; Wood, 1998,

Fig. 1, 2; Chamberlin, 1998, Fig. 3, 4). Thus, on the

upper continental slope, where the 10-yr mean annual

maximum occurred in late January and the minimum

in early August, spring was a season not of warming but

ofslight cooling.

In contrast to conditions on the upper slope, bottom

temperatures increase during spring on the outer shelf,

where shelfwater is normally present (Armstrong, 1998a,

Fig. 3), supporting the concept that rising temperatures stimulate shoreward migrations of demersal fish

in spring (Grosslein and Azarovitz, 1982). With decreasing depth from 150 to 80 m, the range of mean

annual bottom temperatures increases from about 1° to

8°C. Yet the time of year of mean minimum bottom

temperature varies little at these depths, occurring in
4 NOAA Technical Report NMFS 134

mid-April, about 2 mo later than at the coast (Armstrong,

1998a, Fig. 3; Chamberlin 1998, Fig. 2, 3).

Contoured time-depth diagrams of 10-yr extreme

maximum and minimum bottom temperatures (Chamberlin, 1998, Fig. 5) give a general impression of how

variable the influence of bottom temperature on shoreward spring migrations may be. For example, during

the winter demersal fish sensitive to low temperatures

may be driven to deeper water in cold years than in

warm years, and thus start their shoreward spring migrations from farther offshore (as much as 70 m greater

bottom depth). These diagrams also indicate the extreme differences in bottom temperature that fish would

have encountered during 1974 to 1983 while migrating

shoreward in the spring near 71°W longitude. For example, if a migration in late April began on the bottom

at depths greater than 120 m, under minimum temperature conditions the fish would have encountered water of

about 6°C at a bottom depth of 120 m, water of less than

4°C at bottom depths of about 80-40 m, and near the

coast, water no warmer than 8°C. In contrast, under maximum conditions the fish would have found bottom water

warmer than 12°C to the 80-m isobath, water of 6°C near

40 m, and water of about 11°C near the coast.

The practical objectives of resource assessment, however, require more specific species-by-species and yearby-year analysis of temperature influence on shoreward

spring migrations. Especially applicable to such analysis

are diagrams of annual bottom temperature anomalies

(Armstrong, 1998a, Fig. 5) and bottom temperatures

(see citations in Armstrong, 1998a). Differences among

the ten years are most readily seen in the anomaly diagrams. For example, April bottom temperatures on the

outer shelf notably exceeded the 10-yr mean in 1979,

1980, and 1982, but were below the mean in 1975 and

greatly so in 1978. The published annual bottom temperature diagrams provide more detailed information.

A primary biological consideration is to determine

the adequacy oflandings data in documenting the progression of shoreward migrations of various species in

waters south of New England. Are these data sufficiently comprehensive to reveal annual differences in

the routes and timing of these migrations? Landings

data may also be used to address the question ofwhether

various species remain near bottom throughout migration, thus passing through cold-core bottom water on

the shelf, or whether some or all avoid the coldest water

by swimming above it.

Results from the NMFS bottom trawl survey in spring

1974 suggest that the timing of shoreward fish migrations south of New England may be quite variable from

year to year. Annual spring surveys have aimed to complete the sampling of migrant species in their wintering

grounds on the slope and outer shelf in April, before

most fish have started their shoreward spring migration. Nevertheless, in 1974, a year of warm winter bottom temperatures, the survey showed that by the end of

April, seven of the eight migrant species listed above (at

the beginning of this section) had reached the inner

shelf south of New England in appreciable numbers

(Grosslein and Azarovitz, 1982). Results for the exceptional species, Illex illecebrosus, were inconclusive due to

low sample size throughout the survey area. Early arrival of the seven species is consistent with the aboveaverage bottom temperatures across the shelf during

January through March, followed by near-average temperatures in April (Armstrong, 1998a, Fig. 5).

Data from temperature monitoring may be used to

investigate the influence of temperature not only on

the route and timing of shoreward spring migrations,

but also on related aspects of reproduction. For example, in such species as red hake, scup, butterfish, and

longfin squid, which are known to begin spawning soon

after their shoreward migration (Grosslein and Azarovitz,

1982), spawning location and timing will be affected by

route and timing of migration. Furthermore, to the extent that gonad ripening in migrant species is controlled

by temperature, where and when spawning occurs will

vary from year to year as a result ofdifferent temperatures

on offshore wintering grounds and on the shelf during,

and subsequent to, shoreward migration.

Understanding the effect of annual variation in location and timing of spawning on the reproductive success of various fish species is important in recruitment

studies because of the restrictive food requirements of

some groundfish larvae. Cushing (1975, 1982) pointed

out that during a critical early stage in planktonic development ofsome species, survival of larvae may depend

on encountering sufficient concentrations of an essential planktonic food. Clearly such encounters depend

on the spatial and temporal distribution of the forage

organism and on where and when fish larvae are spawned.

Analyzing the role of temperature in ecology is simpler with sedentary benthic species such as the sea

scallop, Placopecten magellanicus; ocean quahog, Arctica

islandica; Atlantic surfclam, Spisula solidissima; and waved

whelk, Buccinum undatum, than with mobile finfish and

squid. For example, diagrams of lO-yr extreme maximum and minimum bottom temperatures (Chamberlin,

1998, Fig. 5) approximate the range of temperature

tolerated during 1974 to 1983 by a lO-yr-old clam found

in 1984 near 71°W. Furthermore, in conjunction with

comprehensive information on the geographical distribution (including bottom depth) of species, data on

extreme bottom temperatures and annual bottom temperature anomalies, as well as annual bottom temperature, can be used to estimate temperatures necessary

for repopulation (Hutchins, 1947; Chamberlin and

Stearns, 1963). Such an approach should be directly

applicable to species like the waved whelk whose young
Chamberlin: Ocean Temperature Monitoring 5

are not planktonic, but emerge in the crawling state

from egg capsules attached to the substrate.

These examples ofthe potential application of ocean

temperature monitoring to fishery ecology deal with

only one of the factors influencing distribution, abundance, and recruitment offishes and shellfishes in southern New England waters. Consideration must also be

given to natural variations in other environmental factors and to degradation of the environment resulting

from human activities. Of most immediate concern,

however, is the impact of severe overfishing which has

decimated several of the most valuable finfish species

since the 1960's (Grosslein and Azarovitz, 1982; Murawski, 1991; Sissenwine and Cohen, 1991).

Acknowledgment

Special appreciation is due to numerous individuals

who collected data on cruises throughout the ten years

of this ocean monitoring effort and to those at cooperating institutions who kept us informed about cruise

schedules and put us in contact with cruise parties.

Without their support and cooperation this research

project could not have succeeded.

Literature Cited

Applications Research Division.

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Stream Mon. Summary 9(7).

Armstrong, R. S.

1998a. Bottom temperatures on the continental shelf and

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R. S. Armstrong (ed.), Water temperatures and climatological conditions south ofNew England, 1974-83, p. 7-14. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 134, 43 p.

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1998. Mean monthly wind stress and Ekman transport conditions. In R. S. Armstrong (ed.), Water temperatures and

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1936. A study of the circulation of the Western North

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Oceanographic Inst. Contrib. 4( 4), 101 p.

Ketchum, B. H., and N. Corwin.

1964. The persistence of "winter" water on the continental

shelf south of Long Island, New York. Limnol. Oceanogr.

9:467-475.

Lauzier, L. M.

1965. Long-term temperature variations in the Scotian Shelf

area. Int. Comm. Northwest Atl. Fish. Spec. Pub\. 6:807-816.
6 NOAA Technical Report NMFS 134

Mayer, D. A., D. V. Hansen, and D. A. Ortman.

1979. Long-term current and temperature observations on

the middle Atlantic shelf.]. Geophys. Res. 84:1,776-1,792.

Mizenko, D., and]. L. Chamberlin.

1979. Gulf Stream anticyclonic eddies (warm core rings) off

the northeastern United States in 1977. Ann. BioI. 34:

39-44.

Murawski, S. A.

1991. Can we manage our multispecies fisheries? Fisheries,

16(5):5-13.

Price, C. A.

1985. Anticyclonic warm core Gulf Stream rings off the northeastern United States during 1984. NAFO SCR Doc. 85/

19, Ser. No. N960, 12 p.

Schroeder, E. H.

1963. North Atlantic temperatures at a depth of 200 meters.

Serial Atlas of the Marine Environment, Folio 2, 2 p., Appendix 11 p., 9 pIs. American Geographical Society.

Sissenwine, M. P., and E. B. Cohen.

1991. Resource productivity and fisheries management of

the northeast shelf ecosystem. In K. Sherman, L. M. Alexander, and B. D. Gold (eds.), Food chains, yields, models,

and management of large marine ecosystems, p. 107-123.

Westview, Boulder, CO.

Taylor, C. C., H. B. Bigelow, and H. W. Graham.

1957. Climatic trends and the distribution ofmarine animals in

New England. U.S. Fish Wildl. Servo Fish. Bull. 57:293-345.

Walford, L. A., and R. I. Wicklund.

1968. Monthly sea temperature structure from the Florida

Keys to Cape Cod. Serial Atlas of the Marine Environment, Folio 15, 2 p., Appendix 28 p., 16 pI. American Geographical Society.

Whitcomb, V. L.

1970. Oceanography of the Mid-Atlantic Bight in support of

ICNAF; September-December 1967. U.S. Coast Guard

Oceanogr. Rep. 35, 157 p.

Wood, G. B.

1998. Comparision of surface and bottom water temperatures with air temperatures. In R. S. Armstrong (ed.), Water temperatures and climatological conditions south of

New England, 1974-83, p. 15-20. U.S. Dep. Commer.,

NOAA Tech. Rep. NMFS 134, 43 p.

Worthington, L. V.

1977. Intensification of the Gulf Stream after the winter of

1976-77. Nature (Lond.) 270:415-417.

Wright, W. R.

1976. The limits of shelf water south of Cape Cod, 1941-

1972. ]. Mar. Res. 34:1-14.
Bottom Temperatures on the

Continental Shelf and Upper Slope:

Means, Standard Deviations, and Anomalies

REED S. ARMSTRONG*

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

28 Tarzwell Drive

Narragansett, Rhode Island 02882-1199

Introduction Data and Methods

Seasonal and between-year variations in bottom water

temperatures are frequently cited as important environmental factors influencing the distribution, abundance,

and reproductive success ofdemersal and benthic marine

organisms. The approximately monthly collections ofsubsurface temperature data offsouthern New England during 1974-83 offer an opportunity to describe bottom temperatures and examine differing hydrographic regimes

for bottom- and near-bottom-dwelling marine organisms

across the width of the continental shelf and upper slope.

The work reported here is the completion of a multiyear analysis initiated by Robert W. Crist, to whose

memory this volume is dedicated. The methods used in

this section were generally developed by him, and the

results are mainly based on analyses that he began.

Results presented here represent ten years (1974-

83) of expendable bathythermograph (XBT) measurements of bottom temperatures. Monitoring was conducted along a transect on and near 71°W longitude,

across the shelf and into the adjacent slope water (Fig. I).

I describe the mean annual cycle over the ten-year

period and variations from the mean for each year,

from the beach to 200 m depth. Cook (1985) employed

some of these same XBT data, from 1977-81, along

with complementary data from the New York Bight, to

describe and compare bottom temperatures and cold

pool conditions between the two areas. Other descriptions of bottom water temperatures that include southern New England shelf waters have generally presented

spatial distributions of monthly mean values (Walford

and Wicklund, 1968; Colton and Stoddard, 1973) or

seasonal mean temperatures (Schopf, 1967; Davis, 1979)

based on irregularly spaced data collected over a wide

area of the continental shelf.

Data presented here are derived from diagrams of bottom temperature contoured on a depth-time coordinate plane and published in annual summary reports

for each year, 1974-83 (Armstrong, 1986; Chamberlin

1976, 1978; Crist, 1984; Crist and Armstrong, 1985;

Crist and Chamberlin, 1979a, b, 1980, 1981, 1983). The

method used to construct annual bottom temperature

diagrams is described by Chamberlin (1976). Vertical

temperature sections were prepared from XBT data

obtained by ships transiting along 71 OW or within about

30' on either side of this meridian (Fig. I). From the

sections, intersections of isotherms with the bottom

were tabulated, plotted on a diagram of bottom depth

versus time of year, and contoured (Fig. 2). To complete the bottom temperature diagram to the shore (0

m bottom depth), sea surface temperatures were included from daily observations at the NOAA National

Ocean Service tide station at Newport, Rhode Island,

and from a recording thermograph placed on the bottom (20 m depth) by the NMFS Northeast Fisheries

Center at the navigation tower on Brenton Reef in the

mouth of Narragansett Bay (Fig. I).

During 1974-83, an average of 19 cruises per year

were conducted, ranging from a minimum of 15 in

1980 to a maximum of 23 in 1983. The time between

cruises ranged from less than a week to approximately

two months. The timing of the passage of warm-core

Gulf Stream rings across the transect, as determined

from interpretations of satellite infrared imagery, was

depicted on the diagrams in the annual summaries.

I Now retired. Present address: 309 Tom Fripp Rd., St. Helena Is.,

SC 29920.

7
8 NOAA Technical Report NMFS 134

60\

I

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,

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I

/

/

I ,

4 0°0a1--;-:..,...-=-""-..,-.,+_'-=-----::-"J4f"Hti¥,.4AH-.A----=----->.,o:+---+-

.- ,

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,

Figure I

Area along 71°00'W south of New England (shaded region) where

expendable bathythermograph (XBT) data were collected for bottom

temperature analyses, 1974-83. Also shown are locations of the recording thermograph for 20-m bottom temperatures (Brenton Tower, Rl)

and the NOAA tide station where shore temperatures were recorded

(Newport, Rhode Island). Dept.h cont.ours in met.ers.

For this report, the bottom temperature diagrams for

each of the ten years were hand-digitized by recording

temperature for every tenth day along each lO-m isobath

from 0 to 200 m. The annual diagrams extended to 400

m depth, but because coverage beyond the 200-m

isobath was more sporadic, this compilation was not

extended beyond 200 m. Also, definition of the bottom

temperature field at depths greater than 200 m was

considered questionable because of the relatively rapid

change in bottom depth with distance at those depths

in relation to XBT station spacing. The bottom temperatures at lO-day, lO-m-depth intervals were averaged

at each grid point to produce ten-year means, and

standard deviations around the means were calculated.

In addition, anomalies of bottom temperature for each

year were calculated by subtracting the lO-yr mean values from the data for individual years at each datedepth grid point.

The 10-yr mean bottom temperatures and standard

deviations were contoured on diagrams of bottom depth

versus time of year to depict the mean annual cycle and

variability about the mean. Also, anomaly values for

each year were con toured on diagrams of bottom depth

versus time of year to reveal patterns in departures

from the mean through the decade. Information on

the presence of warm-core Gulf Stream rings in the

slope water and on the transect was taken from the

annual summary reports.
______________ Armstrong: Bottom Temperatures on the Continental Shelf and Upper Slope 9

V> 150

C'

W

.-

W

:E 200

I

.-

0-

W

0 250

Figure 2

Bottom temperature (0C) on the continental shelf and slope south of New England during 1982.

Vertical sections of expendable bathythermograph (XBT) data are numbered sequentially across

the top. Heavy dots mark inshore and offshore limits of XBT sampling for each section. Horizontal lines at the bottom indicate duration ofwarm-core rings south of New England during the year

(from Crist and Armstrong, 1985).

Results

Ten-year-mean minimum annual bottom temperatures

occurred earliest and were coldest at the coa~t; they

occurred progressively later and were warmer with increasing depth (Fig. 3A). Maximum annual bottom

temperatures were earliest and warmest at the coast;

they occurred progressively later and, generally, were

cooler with increasing depth. A secondary maximum

occurred at midshelf depths (centered at 70 m bottom

depth). Amplitude of the annual cycle was greatest at

the coast (20.S°C) and decreased with increasing depth.

At the shelf break (lOO-m isobath) the amplitude was

about 5°C; on the upper slope there was only a weak

signal of an annual cycle, with the amplitude amounting to less than 1°C at bottom depths greater than 150 m.

Variability in bottom temperatures around the tenyear means (Fig. 3B) was highest in winter, with the

largest standard deviations in February near the 100-m

isobath. The largest difference in anomalies for the
10 NOAA Technical Report NMFS 134

11 A

0

50

100

13

150 12

V')

W

I- 200

W

:E

I

I- 0

0-

W

Q 50

100

150

200

Figure 3

A) Ten-year mean annual cycle of bottom temperature (0C), and B)

annual cycle of standard deviation (0C) from the 10-yr mean of bottom

temperatures on the continent.al shelf and upper slope south of New

England, 1974-83.

same date was also at this depth in late February, ranging from an anomaly of-5°C in 1979, when the bottom

temperature was 5°C, to a +5°C anomaly in 1975, when

the bottom temperature was 15°C (Fig. 4). These extremes in anomaly values coincided with the occurrence of the largest anomalies in monthly air temperatures during the ten years: January 1975 and February

1979 (Wood, 1998, Fig. 4), excluding February 1981,

when no bottom temperature data were available. In

the winters and early springs of 1977, 1978, and 1979,

high negative anomalies developed around the lOO-m

isobath, and bottom waters tended to remain cooler

than average through summer and into fall over much

of the shelf and upper slope (Fig. 4). These events were

associated with three successive unusual years of very

cold winter air temperatures (Wood, 1998).

At times, large anomalies in bottom temperature developed with the passage of warm-core rings. For example, in March 1982, warm-core ring 82-B approached

the shelf off southern New England, raising bottom

temperatures near the 80-m isobath from about 2.5°C

in March to about 11°C in April (Fig. 2) with a warm

water intrusion from offshore. In May, with offshore

flow in the wake of the ring, 80-m bottom temperatures

fell to less than 7°C (Crist and Armstrong, 1985). Associated with the ring's passage, bottom temperature

anomalies in excess of +5°C developed in April 1982 on

the outer shelf and influenced temperatures shoreward

to about the 30-m isobath (Fig. 4). By mid-May, in the

wake of the ring, the pattern reversed, with negative

anomalies prevailing over the outer shelf. The impact

on bottom temperatures from the aperiodic passage of

warm-core rings is highly variable, depending on the

dynamics and structure of the ring, its duration in the

area, and proximity of the ring to the continental shelf

and slope (Chamberlin, 1982).
1974 1976

Figure 4

Anomalies (0C) in bottom temperature for each year, 1974-83, referenced to the 1O-yr mean annual cycle (Fig, 3). Along the bottom of each graph are indicated

the times of vertical sections of expendable bathythermograph data (tick marks) and the presence and duration of warm-core Gulf Stream rings in the slope

water (horizontal bars). Shaded regions are warmer (positive anomaly) than the 10-yr mean.

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Figure 4 (cont.)

Anomaly (OC) in bottom temperature,
Armstrong: Bottom Temperatures on the Continental Shelf and Upper Slope 13

1982

0

50

100

150

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Q:::

W 200

IW

:r:

I0- 0

W

0

50

100

150

200

1983

Figure 4 (cont.)

Anomaly (0C) in bottom temperature.

Minimum variability in bottom temperatures occurred

during March-May and August-October in nearshore

waters (0-30 m), duringJune-August at the shelfbreak

(100 m), and July-September at greater depths (Fig. 4).

The lowest standard deviations were in July-September

at bottom depths of about 150 m and greater, where

anomalies generally remained within 1°C of the mean

throughout the ten years.

Literature Cited

Armstrong, R. S.

1986. Bottom temperatures on the continental shelf and slope

south of New England during 1983. Ann. BioI. 40: 14-16.

Chamberlin,]. L.

1976. Bottom temperatures on the continental shelf and slope

south of New England during 1974. In]. R. Goulet Jr.

(compiler), The environment of the United States living

marine resources-1974. U.S. Dep. Commer., NOAA/

MFS MARMAP Contrib. 104, Section 18, p. 1-7.

1978. Temperature structure on the continental shelf and

slope south of New England during 1975. In]. R. Goulet

and E. D. Haynes (eds.), Ocean variability: effects on U. S.

marine fishery resources-1975. U.S. Dep. Commer.,

NOAA Tech. Rep. NMFS CIRC-416, p. 271-281.

1982. Application ofsatellite infrared data to analysis of ocean

frontal movements and water mass interactions off northeastern United States. NAFO Sci. Coune. Stud. 4, p. 21.

Colton,j. B.,Jr., and R. R. Stoddard.

1973. Bottom-water temperatures on the continental shelf,

Nova Scotia to New Jersey. U.S. Dep. Commel'., NOAA

Tech. Rep. NMFS CIRG376, 55 p.

Cook, S. K.

1985. Temperature conditions in the cold pool 1977-81: a

comparison between southern ew England and New York

transects. U.S. Dep. Commer., NOAA Tech. Rep. NMFS

24,22 p.
14 NOAA Technical Report NMFS 134

Crist, R. W.

1984. Bottom temperatures on the continental shelf and slope

south of New England in 1981. Ann. BioI. 38:17-21.

Crist, R. W., and R. S. Armstrong.

1985. Bottom temperatures on the continental shelf and slope

south of New England during 1982. Ann. BioI. 39:16--18.

Crist, R. W., and]. L. Chamberlin.

1979a. Temperature structure on the continental shelf and

slope south of New England during 1976. In]. R. Goulet

and E. O. Haynes (eds.), Ocean variability in the U.S. fishery

conservation zone, 1976. U.S. Oep. Commer., NOAA Tech.

Rep. NMFS CIRC-427, p. 315-335.

1979b. Bottom temperatures on the continental shelf and

.slope south of New England in 1977. Ann. BioI. 34:21-27.

1980. Bottom temperatures on the continental shelf and slope

south of New England in 1978. Ann. BioI. 35:25-30.

1981. Bottom temperatures on the continental shelf and slope

south of New England in 1979. Ann. BioI. 36:25-28.

1983. Bottom temperatures on the continental shelf and slope

south of New England in 1980. Ann. BioI. 37: 18-20.

Davis, C. W.

1979. Bottom-water temperature trends in the Middle Atlantic Bight during spring and autumn, 1964-76. U.S. Oep.

Commer., NOAA Tech. Rep. NMFS SSRF-739, 13 p.

Schopf, T.].

1967. Bottom water temperature on the continental shelf off

New England. U.S. Geol. Surv. Prof. Paper 575-0, p. 192-

197.

Walford, L. A., and R. 1. Wicklund.

1968. Monthly sea surface temperature structure from the

Florida Keys to Cape Cod. Serial Atlas of the Marine Environment, Folio 15. American Geographical Society.

Wood, G. B.

1998. Comparison ofsurface and bottom water temperatures

with air temperatures. In R. S. Armstrong (ed.), Water

temperatures and climatological conditions south of New

England, 1974-83, p. 15-20. U.S. Oep. Commer., NOAA

Tech. Rep. NMFS 134, 43 p.
Comparison of Surface and Bottom

Water Temperatures

with Air Temperatures

GRAYSON B. WOOD

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

28 Tarzwell Drive

Narragansett, Rhode Island 02882-1199

Introduction

The difference between the annual seasonal cycles of

bottom and surface temperature is regarded as an important controlling factor in vertical mixing of the water column on the continental shelf (Ingham et aI.,

1982). The air-sea interactions involved in the warming and cooling of the ocean by the atmosphere, and

vice versa, have been studied by various authors

(Bjerknes, 1960; Laevastu, 1960; Laevastu et aI., 1976;

Niiler and Kraus, 1977). In the mid-Atlantic Bight area

south of New England, temporary but recurring features such as the "cold pool" are largely the result of

surface-layer temperature changes (Bigelow, 1933;

Ketchum and Corwin, 1964). Colton (1968) found that

coastal sea surface temperature could be used as an

indicator of anomalous water-column conditions in shelf

waters offshore of New England.

Monthly records of air temperature, coastal water

temperature, and surface and bottom water temperatures at the midshelf depth of 70 m were compiled to

represent atmospheric and water temperature conditions offsouthern New England in 1974-83. These data

were used to display and compare mean annual cycles

and between-year variability during the ten-year period.

Data and Methods

Four time series were selected to represent air and

water temperature patterns off southern New England

in 1974-83:

Air Temperature

Monthly averages of 3-hourly observations by the National Weather Service, National Oceanic and Atmospheric Administration (NOAA) at Providence, Rhode

Island (about 30 km inland) were obtained from Local

Climatological Data (LCD) reports prepared and issued by the National Climatic Data Center, NOAA. For

comparison with ten-year means, we utilized the longterm, length-of-record (1905-83) monthly means for

air temperatures at the Providence weatherstation, found

in LCD reports.

Coastal Sea Surface Temperature

Daily measurements of sea water temperature taken at

a tide station off Newport, Rhode Island, were acquired

from the National Ocean Service, NOAA. The tidal observation station is located in 9 ft of water about 5.5 kIn

inland from the mouth of Narragansett Bay. We calculated the mean sea-surface temperature for each month

of the ten years for which there was an adequate temporal distribution of observations.

Midshelf Surface and Bottom

Water Temperatures

Monthly mean values of water temperature at the surface and on the bottom, at a location approximately

midway across the continental shelf, were determined

from expendable bathythermograph (XBT) transects

made at or near 71°W longitude during 1974-83.

Monthly mean values were derived from XBT data plots

for water temperatures at the 70-m isobath (midshelf)

in Cook (1998).

For each of the four sets of monthly data, 10-yr

monthly means and standard deviations about the means

were calculated and plotted to display average annual

cycles and seasonal patterns in air and water temperature variability. Anomalies for each month of the ten

15
16 NOAA T~chnical Report NMFS 134

veal'S were calculated by subtracting the monthly means

from the values for individual months. Anomaly values

were plotted to portray conditions for the decade and

to display associations between anomalies for different

data sets. For each year, the temperature and time of

the coldest and warmest months were tabulated.

Results

Minimum temperatures typically occurred in February

2.t the coast and about a month later throughout the

water column at midshelf (Table 1,2; Fig. 1,2). Annual

minimum water temperatures were lowest at the coast

and highest at 70-m bottom depth. At midshelf, the

minimum usually occurred a month later on the bottom than at the surface. Minimum air temperatures

usually occurred in the same month as coastal water

temperatures, but were lower.

Maximum annual air temperatures typically occurred

in July and about a month later for the sea surface at

both coast and midshelf (Table 2; Fig. 2). Maximum

temperatures on the bottom did not occur until fall

and were about the same as those for surface waters,

which were then cooling.

In general, the lowest minimum water temperatures

occurred in the years of coldest atmospheric conditions, and the warmest water minimums were associated with t.he warmest. winters on land. However, t.here

was little correlation among t.he four data sets wit.h

respect to higher and lower annual maximum temperatures. The amplitude of the annual cycle (difference

bet.ween maximum monthly temperat.ure in Table 2

and minimum monthly t.emperat.ure in Table 1) for

water temperat.ures on the bottom at. t.he 70-m isobath

was about. one-third t.hat for air t.emperature and onehalf t.he:- amplitude of the sea-surface cycle at. either

midshelf or coastal locat.ions. The length of warming

and cooling seasons in at.mospheric records and in the

sea-surface temperature records were about. t.he same

(about 6 mo), but t.he season ofwarming on t.he bottom

at midshelf was twice as long (8 mo) as the cooling

season (4 mo), with about half the warming on the

bottom in t.he months of October and ovember

(Fig. 1).

As an indication of how representative at.mospheric

condit.ions were in 1974-83, t.he annual cycles ofmonthly

mean air temperature at Providence, based on the 79-yr

(1905-83) record and on dat.a for 1974-83, are plotted

in Figure 2. The t.wo curves are within O.soC for all

months except January and October, when the lO-yr

means are about. 1°C cooler than the long-t.erm values.

Variabilit.y around 10-yr mont.hly means t.ended to be

greater in land air temperat.ure and in bottom-water

temperat.ure at the 70-m isobath than in sea-surface

temperature (Fig. 3). Land air, coastal sea-surface, and

70-m bottom temperat.ures showed greatest. variability

in December t.hrough February (and int.o March for

bottom t.emperat.ures), with the least variability during

spring and summer months. Also, the largest. monthly

temperature anomalies generally occurred during the

winter months (Fig. 4). Sea-surface temperature at

midshelf did not. exhibit clear seasonal patterns ofvariability (Fig. 3c). The high standard deviation in June

Table I

Annual minimum mean monthly temperature (OC) and month of occurrence for southern New England, 1974-83.

\,Vater

Air Coast 70-m isobath

Providence, RJ Surface Surface Bottom

Temp. Month Temp. Month Temp. Month Temp. Month

1974 -1.7 Feb 3.3 Jan 5.2 Mar 6.2 Apr

1975 -0.9 Feb 3.6 Feb 5.3 Feb 5.5 Apr

1976 -4.7 Jan 2.4 Jan 4.fi Mar 5.6 Apr

1977 -6.2 Jan 0.0 Jan 2.1'1 Feb 2.7 Feb

1978 -5.5 Feb 0.6 Feb 2.6 Mar 2.6 Mar

1979 -6.8 Feb l.l Feb 2.5 Feb 2.4 Mar

1980 -2.9 Feb 2.6 Feb 3.7 Mar 4.3 Mar

1981 -6.5 Jan 0.9 Feb 3.8 Mar

1982 -5.8 Jan 1.7 Jan 2.7 Feb 3.4 Mar

1983 -0.3 Jan 3.3 Feb 5.4 Mar 6.4 Apr

Average -4.1 Jan-Feb 2.0 Feb 3.9 Mar 4.3 Mar-Apr

10-yr mean -2.9 Jan 2.2 Feb 4.0 Mar 5.0 Mar
__________ Wood: Comparison of Surface and Bottom Water Temperatures with Air Temperatures 17

Table 2

Annual maximum mean monthly temperature (OC) and month of occurrence for southern New England, 1974-83.

Water

Air Coast 70-m isobath

Providence. RI Surface Surface Bottom

Temp. Month Temp. Month Temp. Month Temp. Month

1974 22.6 Jul 21.1 Aug 21.3 Aug 13.9 Nov

1975 23.5 Jul 21.1 Jul 20.5 Aug 13.5 Nov

1976 21.1 Jul 20.6 Aug 21.0 Aug 14.1 Oct

1977 ~3.5 Jul 21.7 Aug ~2.2 Aug 15.9 Nov

1978 22.1 Jul 21.4 Aug 22.7 Aug 13.7 Nov

1979 23.1 Jul 21.4 Aug 21.6 Jul 14.8 Nov

1980 23.8 Jul 21.7 Aug 22.1 Aug 14.7 Oct

1981 24.2 Jul 21.1 Jul 21.6 Aug 11.6 Nov

1982 23.1 Jul 20.6 Aug 19.7 Aug 15.3 ov

1983 24.8 Jul 21.1 Jul 20.4 Aug

Average 23.2 Jul 21.2 Aug 21.3 Aug 14.2 ov

10-yr mean 23.2 Jul 21.1 Aug 21.2 Aug 14.0 ov

A 5 o N o IJ

MONTH

I

M

I

A

I

M

I

F

and temperatures on the bottom at midshelf remained

much lower than average most of the time. Most distinctive of these three cold winters was that of 1976-77,

when air temperature was more than 3°C cooler than

I

J

I

o

2 -

4 -

18

22 -

20 -

U 16

o

L.LJ 14

0::

::::l 12 .....

<l:

0:: 10

L.LJ

a..

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Figure I

Ten-year (1974-83) mean annual cycle of water temperature off southern

ew England for coastal surface waters off ewport, RI (Shore Station

Surface), surface waters at the midshelf location of the 70-m isobath (70-m

surface) and bottom waters at 70-m depth at mid-shelf (70-m bottom).

(>2°C) for sea-surface temperature

at the 70-m isobath was associated

with the presence of unusually cold

water in the surface layer in June

1982 (Cook, 1998), which produced

a temperature anomaly of about -

4°C (Fig. 4). This was associated with

the coldest June since 1926 in the

Providence air-temperature records

and unusually low sea-surface temperatures for the month off most of

the northeastern United States

(Wood and Tang, 1988).lftheJune

1982 value were excluded from the

calculation, the June standard deviation would be reduced to 1.3°C,

making it more similar to values for

midshelfsea-surface temperature for

other months (Fig. 3c).

Monthly anomalies based on 10-

yr means of air temperatures (Fig.

4) were mostly due to large monthto-month changes, but water-temperature anomalies exhibited more

prolonged and persistent patterns.

One prominent feature in the anomaly diagrams is the three consecutive cold winters of 1976-77, 1977-

78, and 1978-79, when cold air temperatures prevailed

throughout the northeastern United States (Ingham,

1982). During these three years, winter water temperatures at the surface were generally lower than average,
18 NOAA Technical Report NMFS 134

average during November-January. Water temperatures

during these months, and continuing into early spring,

were apparently much lower than average at all three

locations. A prolonged period oflower-than-average air

temperature developed in August 1981 and lasted

through August 1982, when air-temperature anomalies

were positive for only two of the thirteen months. During this period, cooler-than-average conditions persisted

- - - - 10 YR MEAN (1974-83)

-- LONG TERM MEAN (190.5-83) 25

U 20

0

I.U 15 c.::

::::>

I-« 10

c.::

UJ

0- 5

UJ

I- 0

-5

0 M A M A SON 0

MONTH

Figure 2

Mean annual cycle of air temperature at Providence, RI from IO-yr (1974-83)

and long-term (1905-83) monthly means.

4

3

°C 2

4

0c 3

2

PROV. AIR

J F M A M J J A SON 0

NOS TIDE

JFMAMJJASOND

4

3

°c

2

4

0C 3

2

70 M SST

J F M A M J J A SON 0

70 M BOTTOM

J F M A M J J A SON 0

Figure 3

Yariability (standard deviation) around IO-yr (1974-83) monthly means

of water and air temperature (Fig. I, 2). PROY.AIR = air temperature

at Providence, RI; NOS TIDE = sea-surface temperature at Newport,

RI; 70 M SST =sea-surface temperature at midshelf (70-m isobath); 70

M BOTTOM = bottom temperature at midshelf (70-m isobath).
__________ Wood: Comparison of Surface and Bottom Water Temperatures with Air Temperatures 19

+6

+4

+2

0

-2

-4

PRay. AIR

0

0

+2

>- .-J

<! 0

0 -2

Z

<!

w

0::: +2 ::J

c:::r 0

0:::

W

a.. -2

W -4 70 M SST

+4

+2

0 ~~ -2

-4 I

1974 75 76 77 78 79 80 81 82 83

Figure 4

Monthly anomalies ofair and water temperatures, referenced to the 10-yr monthly

means for 1974-83 (Fig. 1,2). Positive values are warmer than the 10-yr mean

and negative (shaded) are colder. A blank space represents absence of data for

the month. PROV.AIR = air temperature at Providence, RI; NOS TIDE = seasurface temperature at Newport, Rl; 70 M SST = sea-surface temperature at midshelf (70-m isobath); 70 M BOTTOM = bottom temperature at midshelf (70-m

isobath).
20 NOAA Technical Report NMFS 134

for the sea surface at the coast and midshelf. On the

bottom at midshelf there were also negative anomalies

during this time, but the pattern was interrupted in

spring 1982 by the intrusion of warm water from oft~

shore associated with the passage of a warm-core ring

(Armstrong, 1998).

Literature Cited

Armstrong, R. S.

1998. Bottom temperatures on the continental shelf and upper slope: means, standard deviations, and anumalies. In

R. S. Armstrong (ed.), Water temperatures and climatological conditions south of New England, 1974-83, p. 7-14. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 134, 43 p.

Bigelow, H. B.

1933. Studie, uf the water, on the cuntinental shelf. Cape

Cod to Chesapeake Bay. t, The cvcle of temperature. Pap.

Phys. Oceanugr. Meteorol,Woods Hole Oceanographic Inst.

Contrib. 2(4),135 p.

Bjerknes,J.

1960. Ocean temperatures and atmospheric circulation.

WMO Bull. 9(3):151-157.

Colton,j. B..Jr.

1968. A comparison of current and long-term temperatures

of continental shelf waters, Nova .">cotia to Long Island.

ICNAF Res. Bull. 5:110-129.

Couk, S. K.

1998. Vertical thermal structure of midshelfwaters. In R. S.

Armstrong (ed.), Water temperatures and climatological

conditions south of. ew England, 1974-83, p. 21-25. U.S.

Dep. Commer., NOAA Tech Rep. NMFS 134,43 p.

Ingham, M. C.

1982. Weather conditions and trends in the Maine-Virginia

coastal and offshure areas during 1970-79. NAFO Sci.

Coune. Stud. 5:33-37.

Ingham, M. c., R. S. Armstrong,j. L. Chamberlin, S. K. Cook,

D. G. Mountain, R.j. Schlitz,j. P. Thomas,j.j. Bisagni,

j. E. Paul, and C. E. Warsh.

1982. Summary of the physical oceanographic processes and

features pertinent to pollution distribution in the coastal

and offshore waters of the ortheastern United States, Virginia to Maine. U.S. Dep. Commer., NOAA Tech. Memo.

I\MFS-F/NEC-17, 166 p.

Ketchum, B. H., and N. Corwin.

1964. The persistence of "winter" water on the continental

shelf suuth uf Long Island, New York. Limnol. Oceanogr.

9:467-475.

Laevastu, T.

1960. Factors affecting the temperature of the surface layer

of the sea. Soe. Sci. Fenn. Comm. Physico-Math 25, 136 p.

Laevastu, T.,j. Harding, K. Rabe, and S. Larson.

1976. Ocean-atmosphere interactions off the northeast coast

of orth America. Am. Soc. Limnol. Oceanogr. Spee. Syrnp.

2:35-43.

Niiler, P. P., and E. B. Kraus.

1Y77. One dimensional models of the upper ocean. In E. B.

Kraus (ed.), Modelling and Prediction of the Upper Layers

uf the Ocean, p. 143-172. Pergamon.

Wood, C. B., and V. Tang.

1988. Sea-surface temperature anomalies off the northeastern U.S.A. during 1981-86. NAFO Sci. Coune. Rep. 88/

84, Ser. No. N 1536, 7 p.
Vertical Thermal Structure of MidshelfWaters

STEVEN K. COOK*

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

28 Tanwell Drive

Narragansett, Rhode Island 02882-1199

Introduction

A location at 70 m depth, about midway across the

continental shelf, at 71 oW longitude was selected to represent the vertical temperature structure of shelf waters

offsouthern New England through the ten years 1974-

83. The 70-m isobath is also approximately in the center of the cold pool (Houghton et aI., 1982). I examined temperature through time and from the surface to

the bottom to portray the annual cycle and betweenyear variation in midshelfwaters.

Data and Methods

To portray the annual water-column temperature structure at midshelf off southern New England, we employed a method similar to that of Hughes and Cook

(1984) for NewYork Bightshelfwaters. Vertical profiles

of temperature at the 70-m isobath at and near 71°W

longitude were constructed from vertical temperature

sections of expendable bathythermograph (XBT) data

previously constructed for each cruise that made XBT

drops in that area. These vertical temperature sections

were published in a series of annual reports of bottom

temperatures off southern New England (see references in Armstrong, 1998).

Temperature values at the 70-m isobath were plotted

and contoured on diagrams oftime ofyear versus depth

for each of the ten years. The con toured diagrams were

digitized for each 10m of depth from the surface to the

bottom (70 m), and for each 20th day through the year.

The temperature values at each 20-day, 10-m interval

were averaged over the ten years and plotted, with

standard deviations, on a diagram of depth versus time

of year to show the 10-yr mean annual cycle. The 10-yr

mean temperatures for each date/depth grid point

were subtracted from the annual values; the differences

between the annual and 10-yr means were contoured

on diagrams of depth versus time of year to display

annual temperature anomalies. In addition, minimum

and maximum surface and bottom temperatures and

their dates of occurrence in each year and in the lO-yr

mean annual cycle were tabulated.

The analysis employed XBT data from 1974 through

1983 from a total of 179 cruises, on average about 18

cruises per year, ranging from 13 in 1976 to 23 in 1979.

The interval between cruises ranged from less than a

week to slightly more than two months.

Results

The 10-yr (1974-83) mean annual cycle of water column temperature on the southern New England shelf

(Fig. 1) shows the same seasonal pattern as that described by Ketchum and Corwin (1964) for shelfwaters

off eastern Long Island in 1956-59. From late fall

through winter, the waters were vertically almost isothermal, and cooling proceeded steadily until the time

of the annual minimum in March. Vertical thermal

stratification was established in April. With rapid warming at the surface, but much more gradual warming in

deeper waters, the thermocline intensified through

spring and summer. During summer the thermocline

deepened, and with vertical mixing driven by surface

cooling in fall, continued to deepen until reaching the

bottom in October. Bottom temperatures rose until the

annual maximum was reached in November, when fall

overturn was completed, and the water column returned

to being nearly isothermal from surface to bottom.

The annual minimum in 10-yr mean temperature at

midshelf occurred in March, with waters O. 7°C cooler at

the surface than along the bottom (Fig. 1, Table 1).

During the period of maximum warming at the surface

(May-August), warming rates were about 4°C/mo there

but only about I°C/mo on the bottom. During summer

1 Present address: Ocean and Atmospheric Research, NOAA, 8604

La Jolla Shores Dr., La Jolla, CA 92037.

21
22 NOAA Technical Report NMFS 134

days later and 7.5°C cooler than at the surface. Amplitude of the annual cycle of temperature at the 70-m

isobath was about half as large along the bottom as at

the surface. Mainly because of the later annual maximum on the bottom, the warming season on the bottom (about 8 mo) was almost twice

that at the surface (about 5 mo).

Variability between years in water

column temperature at the 70-m isobath is shown in the contoured diagrams of standard deviation (Fig. 1)

and in the anomaly plots for the 10

years (Fig. 2). Highest variability was

during summer, in a layer centered

at about 10 m depth inJune, which

deepened to about 25 m in September (Fig. 1). This layer nearly coincided with the center of the thermocline (Fig. 1). Because ofthe large

vertical temperature gradient in the

thermocline, relatively small changes

in thermocline depth could produce

large temperature anomalies. For example, during summer and early fall

1977, alternating cells of large positive and negative temperature anomalies (Fig. 2) corresponded to shortperiod changes in thermocline depth.

Departures of thermocline depth in

1977 from 10-yr mean positions (using 15°C as an indicator of thermocline depth) were about 15 m

50

50

Figure 1

Ten-year (1974-83) mean annual cycle of water-column thermal structure

(0C) at midshelf (70 m boltom depth) off southern New England (above)

and annual cycle of standard deviation (0C) from the 10-yr mean (below).

I

I0...

W

a

l/)

Q::

W

IW

:E

the thermocline was most intense, with a vertical temperature gradient of about 0.5°C/m. Annual maximum

temperature occurred in August at the surface and was

progressively later and lower with increasing depth,

such that the annual maximum at the bottom was 100

Table I

Annual minimum and maximum surface and bottom temperatures and their dates of occurrence at midshelf (70 m

boltom depth) off southern New England, 1974-83.

Annual minimum Annual maximum

Surface Bottom Surface Bottom

Temp. Julian Temp. Julian Temp. Julian Temp. Julian

(OC) day (OC) day (OC) day (0C) day

1974 4.8 80 4.9 90 21.5 230 15.2 320

1975 4.4 60 5.0 85 21.7 220 14.4 330

1976 4.2 70 5.2 105 21.5 220 14.8 290

1977 2.2 50 2.2 50 23.3 250 17.0 310

1978 2.2 80 2.2 80 24.1 '230 15.5 330

1979 1.8 60 1.8 60 24.1 210 16.4 330

1980 3.4 70 3.6 90 22.8 225 16.3 290

1981 3.6 70 3.9 100 22.5 210 13.8 310

1982 2.0 50 2.4 65 20.2 235 16.0 330

1983 5.3 80 5.6 100 22.1 250

Average 3.4 67 3.7 82 22.4 228 15.5 316

10-yr mean 3.9 65 4.6 75 21.3 220 13.8 320
________________________ Cook: Vertical Thermal Structure of Midshelf Waters 23

...l..

T

I

I

I

I

I

M

I

o

lI')

o

lI')

N

I

o

z

o

V')

u..

-.. - - - ~i~O ON

o

a

.l..

I

1

(S~313 W) Hld3G

o

lI')

o

lI')

o

o

z

a

o

o

deeper than average in June

and 5 m deeper in August

(periods of positive temperature anomalies) and almost

10 m shallower in July and

mid-September through early

October (negative anomalies). Crist and Chamberlin

(1979) thought that these

variations probably resulted

from changes in shelf water

circulation associated wi th

the passage of two warm-core

rings and possible advection

of cooler waters from southern Georges Bank.

A secondary maximum in

standard deviation values

(Fig. 1) occurred in winter,

particularly in the deeper

portion of the water column.

High variability in winter bottom temperatures derived

from large between-year differences in annual minimum

water temperatures and dates

of occurrence of the minimum (Table 1), and was associated with large variability

in fall-winter air temperatures

(Wood, 1998). Annual minimum bottom temperatures

(Table 1) in the relatively

warm winters of 1976 and

1983 occurred one to almost

two months later than in years

when air and annual minimum bottom temperatures

were coldest (1977, 1978,

1979, and 1982).

Typically, Gulf Stream

warm-core rings in 1974-83

had little influence on water

temperatures as far onto the

shelf as the 70-m isobath.

However, there was notable

ring influence, as discussed

earlier, from the passage of

two rings during the summer

of 1977. In November 1977

an intrusion of 17°C Gulf

Stream water was carried onto

the shelf by a ring (Crist and

Chamberlin, 1979) and produced the highest annual
24 NOAA Technical Report NMFS 134

maximum bottom temperature of

the decade (Table I). During the

cold water conditions prevailing

during the winter and spring ofl982

(Fig. 2), an intrusion ofslope water

witha warm-core ring (Crist and

Armstrong, 1985) brought anomalously warm water onto the bottom,

raising bottom temperatures from

about 3°C in March to over lOoC in

April.

Other significant events In the

annual anomaly diagrams occurred

in the summers of 1980, 1982, and

1983. During summer the meridional component of mean monthly

wind stress was typically northward;

however, in August 1980 and JuneAugust 1983, the meridional component was southward, resulting in

westward Ekman transport (Ingham,

1998). During both periods, air temperatures were warmer than average (Wood, 1998), but water temperatures were cooler than 10-yr

means, particularly in upper layers

(Fig. 2, 1980 and 1983). The water

temperature anomalies may have derived from wind-driven transport of

cooler water from the east. The largest water temperature anomaly (-6°C)

of the 10 years occurred in surface

waters in June 1982 (Fig. 2), associated with one ofthe coldestJunes in

air temperature ofthe century (Wood,

1998). In August 1982, near-bottom

waters were more than 4°C warmer

than average as the result of an intrusion of slope water from offshore

(Crist and Armstrong, 1985).

In general, during 1974 through

1983, temperature anomalies tended

to be more consistent from surface

to bottom during periods when the

waters were unstratified (late fall

through winter). Anomalies for waters above and below the thermocline were often of opposite sign

during spring and summer, when

the water column was stratified. Also,

positive or negative anomalies that

developed during winter were more

likely to persist into spring and summer in sub-thermocline waters than

in surface waters.

IIIII1

o

II') o

II')

_ c-~---7

I,~

NM

II

zo

\I)

oo

...... o

-}----,

-l---NI

II

NI

(S~313W) Hld30

o

II') o

II')

oo

:E:~~;~~;{~;f.;~\'.::':...

I .0. 0 0 + •••••

o

00

0-
_________________________ Cook: Vertical Thermal Structure of MidshelfWaters 25

1982

50

V)

a::::

UJ

UJ

1983

I

J

a.... 0

UJ

0

50

Figure 2 (cont.)

Anomalies (OC) in water column temperature structure.

Literature Cited

Armstrong, R. S.

1998. Bottom temperatures on the continental shelf and upper slope: means, standard deviations, and anomalies. In

R. S. Armstrong (ed.), Water temperatures and climatological conditions south ofNew England, 1974-83, p. 7-14. U.S.

Dep. Commer., NOAA Tech. Rep. MFS 134,43 p.

Crist, R. W., and R. S. Armstrong.

1985. Bottom temperatures on the continental shelf and slope

south of New England during 1982. Ann. BioI. 39:16-18.

Crist, R. W., andJ. L. Chamberlin.

1979. Bottom temperatures on the continental shelf and slope

south of New England in 1977. Ann. BioI. 34:21-27.

Houghton, R. W., R. Schlitz, R. C. Beardsley, B. Butman, and

J. L. Chamberlin.

1982. The Middle Atlantic Bight cold pool: evolution of the

temperature structure during summer 1979. J. Phys.

Oceanogr. 12:1,019-1,029.

Hughes, M. M., and S. K. Cook.

1984. Water column thermal structure across the shelf and

slope southeast of Sandy Hook, New Jersey, in 1981. Ann.

BioI. 38:14-17.

Ingham, M. C.

1998. Mean monthly wind stress and Ekman transport conditions. In R. S. Armstrong (ed.), Water temperatures and

climatological conditions south of New England, 1974-83, p.

31-38. U.S. Dep. Commer., NOAA Tech. Rep. NMFS 134, 43 p.

Ketchum, B. H., and N. Corwin.

1964. The persistence of "winter" water on the continental

shelf south of Long Island, New York. Limnol. Oceanogr.

9:467-475.

Wood, G. B.

1998. Comparison ofservice and bottom water temperatures

with air temperatures. In R. S. Armstrong (ed.), Water

temperatures and climatological conditions south of New

England, 1974-83, p. 15-20. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS 134, 43 p.

Maximum and Minimum Bottom Temperatures on the

Continental Shelf and Upper Slope

J. LOCKWOOD CHAMBERLIN*

Northeast Fisheries Science Center

National Ma'rine Fisheries Service, NOAA

28 Ta'YZwell Drive

Narragansett, Rhode Island 02882-I I 99

Introduction

Herein I present an analysis of maximum and mInImum bottom temperatures across the continental shelf

and slope south of New England, from the shore to 200 m

depth, for 1974-83. I utilize the same data used by

Armstrong (1998). Complementing the analyses in Armstrong (1998), Cook (1998), and Wood (1998), this

examination provides an additional perspective on the

temperature regime of the fishing grounds south of

New England. The significance to fisheries research of

temperature extremes, and some examples of the utility of this information in fisheries ecology, are discussed in Chamberlin (1998).

Data and Methods

Annual Maximum and Minimum

Temperatures and Dates of Occurrence

For each year, 1974-83, maximum and minimum water

temperatures on the bottom, and their dates of occurrence, were determined for each 10-m interval of bottom depth from values derived by Armstrong (1998)

from bottom temperature diagrams for each year. The

diagrams were based on vertical temperature sections

from expendable bathythermograph drops along and

near 7loW longitude. Armstrong (1998) interpolated

bottom temperatures for each 10-day period and each

10-m increment of bottom depth from the coast (O-m

bottom depth) to the 200-m isobath.

Ten-Year Maximum and Minimum Temperatures

and Dates of Occurrence

From the mean temperatures at 10-day, lO-m bottom

depth in tervals derived by Armstrong (1998), the 10-yr

maximum and minimum water temperatures and dates

of occurrence were determined for each 10-m depth

increment.

Range of Maximum and Minimum Temperatures

and Lengths of Warming and Cooling Seasons

From the interpolated values compiled for each year,

the 10-yr ranges in annual maximum and minimum

and the range in days between the earliest and latest

occurrences of annual maximum and minimum were

calculated for each lO-m increment of depth along the

bottom. For each depth interval, I calculated differences between the maximum and minimum in the 10-

yr mean annual cycle, and between the warmest and

coldest values throughout the ten years. In addition,

the length of the warming and cooling seasons at each

bottom depth interval was determined as the number

of days between the maximum and the minimum.

Maximum and Minimum Bottom Temperatures

Ten-year extreme maximum and minimum temperatures at each 10-day, 10-m grid point were determined.

Results

The patterns of maximum and minimum bottom temperatures in the lO-yr mean annual cycle, and the variability in magnitude of extremes and dates of occurrence

during the ten years are depicted by graphs (Fig. 1-5) in

which bottom conditions are plotted against depth from

the coast (O-m bottom depth) to the 200-m isobath.

A principal feature of maximum and minimum mean

bottom temperatures (Fig. 1) is the general decrease in

* Now retired. Present address: 278 Maine St., Brunswick, ME 04011.

27
28 NOAA Technical Report NMFS 134

25

20

u

°

w

a::

2 15

<!

a::

w

CL

W

I-- 10

o

I--

I--

5

MAXIMUM

25

U

20

w

C)

Z

<!

"" 15 w

""

f-

<!

""w

a... 10

::2:

w ;- f-

::2: TEN-YEAR

0 MEAN f- 5 f0

en

50 100 150 200

BOTTOM DEPTH (m)

O+,.--,....,.-,--,-.---,....,.-,--,--,-,-,-,---,--,-,.-,r-r-o

o

MINIMUM

o

-2

o 50 100 150

BOTTOM DEPTH (m)

200

Figure 2

Ranges of bottom temperature extremes plotted in

Figure 1. The lower cu",e represents the 1O-yr mean

annual cycle; the upper cu",e represents 1O-yr extremes.

Figure I

Ten-year (1974-83) maximum and minimum bottom

temperatures (0C) on the shelf and upper slope south

of New England. Cu",es represent the 1O-yr mean annual cycle; vertical bars indicate the range of extreme

values over the 10 years.

the maximum and increase in the minimum with depth,

with a corresponding decrease in annual range from

19.5°C at the shore to less than 1°C below 150 m (Fig.

2). Also prominent in Figure 1 is the marked contrast

between maximum and minimum cu",es in rate of

change of bottom temperature with depth. A steep

linear decrease (7.5°C) ofthe maximum from the shore

to 40 m corresponds with the depth range over which

the seasonal thermocline intersects bottom. The more

gradual, near-linear rise (1.8°C) in the minimum fron:

shore to 60 m reflects the mixed-water column in the

cold season. Maximums were nearly constant from 40

to about 100 m, followed by a gradual and uniform

decrease of2.2°C from 100 to 200 m. Minimum bottom

temperatures display a gradual and nearly linear in··

crease (1.8°C) from the shore to 60 m, followed by a

steep ascent (8.0°C) from 60 to 150 m. At depths greater

than 150 m, minimum bottom temperatures decrease

with increasing depth, and generally parallel the maximum curve.

Beyond midshelf, the pattern of ranges in extreme

bottom temperatures is increasingly dissimilar from the

pattern shown by the mean extremes (Fig. 1). The

particularly high maximum (l7°C) from 50 to 70 m

resulted from above-normal bottom warming during

the autumn overturn in October 1980 (Armstrong, 1998,

Fig. 5; Cook, 1998, Fig. 3). This is also reflected in the

increased range of extreme bottom temperatures at

50-70 m (Fig. 2). From 100 to 140 m, the large ranges

in the minimum are largely the result of the record

cold winters of 1977, 1978, and 1979, as discussed in

Armstrong (1998). At depths greater than 150 m, maximum and minimum bottom temperatures are often

outside the range of extremes (Fig. 1). In shallower

waters, extremes generally occur around the times of

maximum and minimum temperature in the lO-yr mean

annual cycle. However, at depths greater than 150 m,

extremes are typically associated with short-period events

(such as the passage of warm-core rings) which could

occur at almost any time of the year. Fluctuations in

bottom temperature stemming from these irregular

events dominate the seasonal signal in the lO-yr means.

The times of year of both maximum and minimum

bottom temperature on the shelf tend to occur later

with increasing depth (Colton and Stoddard, 1973; Bowman and Wunderlich, 1976). The 10-yr maximum oc-
____ Chamberlin: Maximum and Minimum Bottom Temperatures on the Continental Shelf and Upper Slope 29

J

M

A

M

F

J

D

N

OI

fS Z

A 0

J

J

M

A

M

F

J

D

N

50 100 150 200

BOTTOM DEPTH (m)

MAXIMUM

MINIMUM

o

o

o

100

200

100

200

300

300

W

f-

«

o

Z

«

Figure 3

Ten-year (1974-83) maximum and minimum bottom temperatures

on the shelf and upper slope south of New England. The curves

connect dates of maximum (upper) and minimum (lower) temperatures in the lo-year mean annual cycle. Vertical lines are the ranges of

dates throughout the ten years. Vertical bars appear when the same

mean maximum or minimum temperature occurred on multiple dates.

100

250

Z Z

0 0

"'-;;;- « >- '"« >- UJ 0 150 UJ 0

"'0 '" 0

0 <5 200 0 6 ZZ Z Z ~-

"" 0 « 0 3 200 u

150

0 50 100 150 200

BOTTOM DEPTH (m)

Figure 4

Length ofwarming and cooling seasons on the bottom on the shelf

and upper slope south of New England. Values are differences in

date of occurrence of minimum and maximum bottom temperatures in the lO-yr mean annual cycle, 1974-83 (Fig. 3).

curs progressively later with increasing depth

along the bottom over the entire depth range

from 0 to 200 m, with the steepest gradient

in date of occurrence on the inner shelf

(Fig. 3).

The time lag with depth for lO-yr minimum is less regular than for the maximum

(Fig. 3). There is only a I-mo lag from 0 to

70 m, followed by an increase in rate of lag

between 70 and 90 m. Dates for the minimum are about the same from 100 to 150 m,

but there is a 3-mo lag between 150 and 160 m.

From 160 to 200 m, there is a IS-day lag. At

160 m depth, there are two dates for the 10-

yr minimum: one in April continuing the

progression of dates in shelf water, and the

second in July, associated with the time of

the minimum in slope water.

At 200-m bottom depth, the seasons are

essentially reversed from coastal conditions.

The lO-yr maximum occurs in late January,

about 180 days (5.9 mol later than at the

coast, while the minimum is in early August,

about 175 days (5.8 mol later than at the

coast.

The seasonal bottom temperature cycle at

170-200 m is of small range (<1 0c) and is

overshadowed by short-term, nonseasonal

temperature variations (Fig. 1). This is reflected in the large range in date of annual

maximum and minimum (Fig. 3).

Three distinct patterns in the length of

warming and cooling seasons along the bottom (Fig. 4) reflect the differences between

times of maximum and minimum bottom

temperatures. At the coast, warming and

cooling seasons were each about 6 mo; the

period of warming increased with depth to

more than 8 mo at midshelf (60-70 m). The

warming season lasted about 7.5 mo at 80-

90 m, increasing to almost 9 mo at 160 m,

with corresponding decreases in period of

cooling. In slope water at 160-200 m, the

warming and cooling seasons were each

about 6 mo long, as at the coast. However,

seasons in the slope water were essentially

reversed from those at the coast.

The contour diagrams in Figure 5 present

extreme maximum and minimum bottom

temperatures, regardless of year of occurrence, for each 10-day/l0-m increment of

bottom depth. The largest differences in

maximum and minimum temperatures were

over the mid- to outer continental shelf, at

bottom depths of 60 to 110 m, with the ex-
30 NOAA Technical Report NMFS 134

MAXIMUM

50

E

I

fc...

UJ

o

100

150

200,-L--------L-..L-L----__--l-__-----l

MINIMUM

100

150

200---'--...l..-----L--------L-L--....l.-_-L..__::::...-_.<.:.J

Figure 5

Extreme maximum and minimum bottom water temperatures (Oe)

on the shelf and upper slope south of New England, 1974-83.

treme difference of more than 9°e at 110m in February. Major differences between the extreme maximum

and minimum occurred on the outer shelf and were

associated with the contrast between cold bottom conditions in winter 1977 and spring-summer following

the cold winter of 1978; and warm conditions on the

bottom in the winters or early springs of 1975, 1976,

and 1982 (Armstrong, 1998).

Literature Cited

Armstrong, R. S.

1998. Bottom temperatures on the continental shelf and upper slope: means, standard deviations, and anomalies. In

R. S. Armstrong (ed.), Water temperatures and climatologccal conditionssouth ofNew England, 1974-83, p. 7-14. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 134,43 p.

Bowman, M.]., and L. D. Wunderlich.

1976. Distribution of hydrographic properties in the New

York Bight apex. In M. G. Gross (ed.), Middle Atlantic

Continental Shelf and the New York Bight. Arner. Soc.

Limnol. Oceanogr. Spec. Symp. 2:58-68.

Chamberlin,]. L.

1998. Ocean temperature monitoring: project background

and some applications to fishery ecology. In R. S. Armstrong

(ed.), Water temperatures and climatological conditions

south of New England, 1974-83, p. 1-6. U.S. Dep.

Commer., NOAA Tech. Rep. NMFS 134, 43 p.

Colton,]. B.,Jr., and R. R. Stoddard.

1973. Bottom-water temperatures on the continental shelf,

Nova Scotia to New Jersey. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS CIRC-376, 55 p.

Cook, S. K.

1998. Vertical thermal structure of midshelf waters. In R. S.

Armstrong (ed.), Water temperatures and climatological

conditions south of New England, 1974-83, p. 21-25. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 134, 43 p.

Wood, G. B.

1998. Comparison ofsurface and bottom water temperatures

with air temperatures. In R. S. Armstrong (ed.), Water

temperatures and climatological conditions south of New

England, 1974-83, p. 15-20. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS 134, 43 p.
Mean Monthly Wind Stress and

Ekman Transport Conditions

MERTON C. INGHAM*

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

28 Tarzwell Drive

Narragansett, Rhode Island 02882-1199

Introduction

The ocean surface layer is strongly influenced by

highly variable wind conditions immediately above

the surface. Mixing, evaporation, cooling, and

transport are all heavily dependent on the winds.

I analyzed wind velocity, wind stress, and Ekman

transport conditions for a point representative of

the midshelf (70-m isobath) south of New England, near 700

W.

Data _

The Pacific Fisheries Environmental Group

(PFEG) of the National Marine Fisheries Service

utilizes the computers and databases of the Fleet

Numerical Oceanography Center in Monterey,

California to compute wind stress, Ekman transport, and several other parameters from surface

atmospheric pressure records (see Bakun, 1973,

for methods). PFEG issues a monthly list which

includes mean wind-stress components and Ekman transport components on a 3° grid. I utilized

data for the grid point at 39°N 72°W, located in

slope water about 230 km south ofMontauk Point,

Long Island. Because the computations of wind

stress and Ekman transport at a point depend on

computed pressure-field gradients in the surrounding area, the stress and transport values for a point

are representative ofthis area. The area represented

by 39°N 72°W extends about 150 km (half the

distance to the next grid point) in all directions,

and northward nearly to the 70-m isobath on the

shelf offsouthern New England (Fig. 1).

Seale

I------<

100

10- 3 dynes/em2

Figure 1

Ten-year (1974-83) mean monthly wind stress (in 10-3 dyne/

cm2) offsouthern New England. Vectors shown are for stress

values at 39°N 72°W (triangle) and are representative of the

area encompassed by the dashed line.

* Now retired. Present address: 11 Hawthorne St., South Dennis, MA

02660.

31
32 NOAA Technical Report NMFS 134

Scale

t------<

100

metric tons/sec/km

Figure 2

Ten-year (1974-83) mean monthly Ekman transport (in metric ton/sec' km) offsouthem New England. Vectors shown are

for transport values at 39°N 72°W (triangle) and are representative of the area encompassed by dashed line.

Table I

Ten-year (1974-83) monthly means and standard deviations (SO) of wind stress components at 39°N now (10-3 dynes/cm2).

Meridional Zonal Meridional Zonal

10-yr Meridional 10-yr Zonal lO-yr Meridional 10-yr Zonal

Month mean SD mean SD Month mean SD mean SD

Jan -357.1 285.6 +261.0 230.2 Jul + 47.5 50.5 +158.2 96.6

Feb -404.6 374.7 +206.8 239.8 Aug + 24.4 48.0 + 70.8 46.2

Mar -141.4 205.4 +176.4 194.4 Sep - 23.5 35.7 + 5.9 34.6

Apr -19.9 159.1 +293.5 209.7 Oct - 93.1 95.4 + 17.5 71.0

May + 49.8 89.5 + 87.3 93.4 Nov -219.9 227.6 +188.4 211.7

Jun + 42.5 56.9 +110.3 107.5 Dec -235.2 165.2 +222.0 187.6

Mean monthly wind stress, Ekman transport, and

wind velocity vectors for 1974-83 were computed and

plotted (Fig. 1,2,3) to establish general seasonal patterns.

Wind stress and Ekman transport data were taken directly

from the PFEG lists. Wind velocity was back-ealculated

from wind stress using relations in Bakun (1973, p. 3).

Monthly means and standard deviations were computed for meridional and zonal components of wind

stress for the 10-yr period (Table 1, Fig. 4, 5). Anomalies from monthly means were computed for each month

of the 10-yr period (Fig. 6, 7). Monthly anomalies (in

standard deviations for each month) were computed;

those months showing an anomaly more than one standard deviation from the mean in either or both components are listed in Table 2. Monthly mean values

of wind stress components, wind stress anomalies,
_________________ Ingham: Mean Monthly Wind Stress and Ekman Transport Conditions 33

Scale

t---i

100

em/sec

Figure 3

Ten-year (1974-83) mean monthly computed surface wind

velocity (in em/sec) offsouthern ew England. Vectors shown

are for values at 39° 72°W (triangle) and are representative

of the area encompassed by dashed line.

Table 2

Anomalies greater than one standard deviation from IO-yr monthly mean wind stress components (meridional or zonal) at

39°N 72°W. (Anomaly values are in standard deviations).

Anomaly Anomaly Anomaly Anomaly

Date Date Date Date

(YYMM) Meridional Zonal (YYMM) Meridional Zonal (YYMM) Meridional Zonal (YYMM) Meridional Zonal

7401 1.02 -0.85 7702 0.97 1.36 7904 0.06 -1.12 8104 2.53 1.38

7406 -2.24 -1.18 7705 -1.01 0.57 7905 1.03 -0.20 8109 -1.69 2.37

7410 -1.10 0.44 7707 -0.56 -1.12 7910 1.32 2.09 8111 -2.31 0.04

7502 1.94 -0.29 7708 0.72 2.06 8001 -1.22 -0.84 8112 -2.44 1.55

7504 -2.59 1.08 7712 1.25 -0.86 8004 0.95 -1.07 8204 -0.13 1.08

7508 -1.05 0.07 7801 1.10 1.36 8006 0.83 1.80 8209 -0.55 -1.12

7512 0.36 -1.02 7802 -1.01 -0.31 8007 2.09 1.84 8210 -1.95 -1.49

7602 1.79 2.03 7805 -0.20 -1.01 8010 -1.10 -1.08 8301 -0.14 -1.25

7605 1.94 1.84 7806 0.99 1.38 8011 -0.43 1.05 8302 -0.69 -1.50

7606 2.28 0.17 7809 -1.44 -1.04 8101 -1.27 0.53 8304 0.48 -1.03

7609 1.16 0.96 7811 0.07 -1.38 8102 1.26 -0.83 8305 1.41 1.48

7701 -1.74 2.52 7812 0.19 1.82 8103 -2.89 2.53 8307 -1.71 1.52
34 NOAA Technical Report NMFS 134

STANDARD DEVIATION

300~ 200

100

o -,.....,--.----.-,.....,--.----.-,.....,--.----.---,

300

V)

V)

W

011:

.....

V)

FMAMJ JASONDJ

MONTH

Figure 5

Ten-year (1974-83) mean and standard deviation of

monthly zonal wind stress (+ eastward) in 10-3 dyne/

cm2, off southern New England in the vicinity of 39°N

now.

..... 150

-:{

Z 100

2 50

o

-50 -'--.---.r--,......,....-.---,-,.....,--.---,-,...."'T'"""""

o

Z 250

3: 200

F M A M J ASONDJ

400

300 STANDARD DEVIATION

200

V) 100 V)

w

011: 0 .....

V) 100

0 50

Z

0

-50 .....

-:{ -100

Z

0 -150

0 -200 011:

w -250

-300

-350

-400

-450

MONTH

Figure 4

Ten-year (1974-83) mean and standard deviation of

monthly meridional wind stress (+ northward) in 10-3

dyne/cm2, off southern New England in the vicinity of

39°N 72°W.

and Ekman transport components are presented in

Table 3.

Results

Plots of wind stress and wind velocity (Fig. 1, 3) show

the commonly recognized (e.g., Meserve, 1974; Williams and Godshall, 1977; Beardsley and Boicourt, 1981}

seasonal patterns off southern New England: northwesterly winds during all but the summer months, when

southwesterly winds prevail. The Ekman transport vectors (Fig. 2) show a similar seasonal progression but 90°

to the right of the wind stress vectors.

Examples of the linkage between windfield variation

and water column changes can be found most readily

by examining the most anomalous winter windfield

conditions. One rather long (8-mo) period of negative

anomalies in zonal wind stress was September 1982-

April 1983 (Fig. 7, Table 3). The largest anomalies in

this period occurred in January-February 1983, when

normal eastward wind stresses were replaced by strong

westward components (Table 3). Bottom water temperature anomalies along 71°W during SeptemberMarch (Armstrong, 1998) were positive inshore oflOO m

depth, probably because of the reduction in seasonal

wind-caused cooling.

Another multi-month period of anomalies in zonal

wind stress occurred in November 1976-March 1977,

when there were relatively strong positive anomalies

(Fig. 7). InJanuary and February 1977, these anomalies

were 2.52 and 1.36 standard deviations, respectively,

from the monthly means (Table 2). This period of

unusually strong westerly winds also was noted in an

analysis of wind stress data from eleven grid points in

the northwestern Atlantic during the 1970's (Ingham,

1982). During this time period, there were also record

cold air temperatures in the middle Atlantic states, and

bottom water temperature anomalies along 71°W

(Armstrong, 1998) were negative, especially duringJanuary and February 1977, when they were strongly negative from the coast to beyond the 200-m isobath. Similarly, water column temperatures at mid-shelfwere generally cooler than average from surface to bottom (Cook,

1998). The low water temperatures resulted from the

cooling effect of unusually strong, eastward wind stresses

(cold, westerly and northwesterly winds).

Mean summer wind stresses (Fig. 1) show northward

components (southerly winds), but in August 1980 and

June-August 1983 the meridional components were

southward instead (Table 3). Coincident with these

unusual wind stresses, water temperatures at midshelf,

especially in the upper water column, were unusually cool

(Cook, 1998). The southward wind stress components

may have caused a westward transport (Ekman transport)

of cooler water from Nantucket Shoals to 71oW.
________________ Ingham: Mean Monthly Wind Stress and Ekman Transport Conditions 35

700

600

V")

V") 500 w

CI:: 400 lV")

u.. 300

0 0

Z 200

>- .....J 100 «

~...J 0

0« -100 ZZ

«0 -200

0

CI:: -300 w -400

-500

-600

YEAR

Figure 6

Monthly anomalies from IO-yr (1974-83) mean meridional wind

stress (+ northward) in 10-3 dyne/cm2, off southern New England

in the vicinity of 39°N now.

V")

V")

w

CI::

u..1- o V")

>-0 .....JZ

«-

~~

O.....J Z« «Z

o

N

600

500

400

300

200

100

o+---.hlWl",lI,,_~~1J.,

-100

-200

-300

-400

-500

-600

YEAR

'80 '81 '82 '83

Figure 7

Monthly anomalies from the 10-yr (1974-83) mean zonal wind

stress (+ eastward) in 10-3 dyne/cm2, offsouthern New England in

the vicinity of 39°N now.

Ingham: Mean Monthly Wind Stress and Ekman Transport Conditions 37

Table 3 (continued)

Stress Stress Anomaly Ekman Transport

Date

(WMM) Meridional Zonal Meridional Zonal Meridional Zonal

7806 99 259 56.5 147.8 -28 11

7807 36 68 -11.5 -90.2 -7 4

7808 66 52 41.6 -18.8 -6 7

7809 -75 -30 -51.5 -35.9 3 -8

7810 -41 9 52.1 -8.5 -1 -5

7811 -203 -104 16.9 -292.4 11 -22

7812 -204 564 31.2 342.0 -62 -22

7901 -164 367 193.1 106.0 -40 -18

7902 -855 248 -370.4 41.2 -27 -93

7903 -6 62 135.4 -114.4 -7 -1

7904 -11 58 8.9 -235.5 -6 -1

7905 142 69 92.2 -18.3 -7 16

7906 0 7 -42.5 -103.3 -1 0

7907 59 90 11.5 -68.2 -10 6

7908 46 67 21.6 -3.8 -7 5

7909 7 -20 30.5 -25.9 2 1

7910 33 166 126.1 148.5 -18 4

7911 -5 40 214.9 -148.4 -4 -1

7912 -273 288 -37.8 66.0 -31 -30

8001 -707 68 -349.9 -193.0 -7 -77

8002 -823 287 -338.4 80.2 -31 -90

8003 -64 93 77.4 -83.4 -10 -7

8004 131 69 150.9 -224.5 -7 14

8005 9 139 -40.8 51.7 -15 1

8006 90 304 47.5 193.7 -33 10

8007 153 336 105.5 177.8 -37 17

8008 -18 128 -42.4 57.2 -14 -2

8009 9 25 32.5 19.1 -3 1

8010 12 94 105.1 76.5 -10 -1

8011 -317 410 -97.1 221.6 -45 -35

8012 -347 142 -111.8 -80.0 -15 -38

8101 -720 382 -362.9 121.0 -42 -79

8102 -11 8 473.6 -198.8 -1 -1

8103 -735 669 -593.6 492.6 -73 -80

8104 167 583 402.2 289.5 -64 18

8105 -31 0 -80.8 -87.3 0 -3

8106 41 193 -1.5 82.7 -21 4

8107 24 78 -23.5 -80.2 -8 3

8108 -5 2 -29.4 -68.8 0 -1

8109 -84 88 -60.5 82.1 -10 -9

8110 -179 5 -85.9 -12.5 0 -20

8111 -746 196 -526.1 7.6 -21 -81

8112 -639 513 -403.8 291.0 -56 -70

8201 -335 274 22.1 13.0 -30 -37

8202 -302 1 182.6 -205.8 -0 -33

8203 -50 67 91.4 -109.4 -7 -6

8204 -41 519 -21.1 225.5 -57 -4

8205 -16 21 -65.8 -66.3 -2 -2

8206 6 34 -36.5 -76.3 -4 1

8207 55 179 7.5 20.8 -20 6

8208 10 101 -14.4 30.2 -11 1

8209 -43 -33 -19.5 -38.9 4 -5

8210 -279 -88 -185.9 -105.5 10 -30

8211 -52 256 167.9 -163.4 -3 -6

8212 -121 57 114.2 -165.0 -6 -13

8301 -396 -28 -38.9 -289.0 3 -43

continued
Literature Cited

Armstrong, R. S.

1998. Bottom temperatures on the continental shelf and upper slope: means, standard deviations, and anomalies. In

R. S. Armstrong (ed.), Water temperatures and climatological conditions south ofNew England, 1974-83, p. 7-14. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 134,43 p.

Bakun, A.

1973. Coastal upwelling indices, West Coast ofNorth America,

1946-71. U.S. Dep. Commer., NOAA Tech. Rep. NMFS

SSRF-671, 103 p.

Beardsley, R. C., and W. C. Boicourt.

1981. On the estuarine and continental shelf circulation in

the Middle Atlantic Bight. In Evolution of Physical Oceanography-Scientific Surveys in Honor of Henry Stommel,

Chap. 7. MIT Press, Cambridge, MA.

Cook, S. K.

1998. Vertical thermal structure of midshelf waters. InR. S.

Armstrong (ed.), Water temperatures and climatological

conditions south of New England, 1974-83, p. 21-25. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 134, 43 p.

Ingham, M. C.

1982. Weather conditions and trends in the Maine-Virginia

coastal and offshore areas during 1970-79. NAFO Sci.

Counc. Stud. 5:33-37.

Meserve,j. M.

1974. U.S. Navy marine climatic atlas of the world. Vol. 1-

North Atlantic Ocean. Naval Weather Service Detachment,

U.S. Naval Weather Service Command, Washington, D.C.,

371 p.

Williams, R. G., and F. A. Godshall.

1977. Summarization and interpretation of historical physical oceanographic and meteorological information for the

Mid-Atlantic region. U.S. Dep. Commer., NOAA, Washington, D.C., 295 p.
Variations in the Shelf-Slope Front and the

Influence ofWarm-core Gulf Stream Rings

REED S. ARMSTRONG*

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

28 Tarzwell Drive

Narragansett, Rhode Island 02882-I I 99

Introduction

Variations in the position of the shelf-slope front and

interactions ofanticyclonic, warm-eore GulfStream rings

with shelf waters can introduce major changes in currents and water masses over the outer continental shelf

and upper slope (Morgan and Bishop, 1977; Ingham et

aI., 1982) and, at times, the m~dshelf (Crist and

Chamberlin, 1979). Because of the different origins

and water mass characteristics ofshelf and slope water,

the front represents an ecological boundary for living

marine resources. Shifts in the front's location may

expose marine organisms, particularly bottom-dwelling

organisms, to unusual or stressful environmental conditions. Also, since ocean fronts are typically regions of

convergent flow, the shelf-slope front may be an area

where drifting plankton accumulate, and therefore a

region where foraging pelagic fish (Ingham, 1976) and

marine mammals (Waring et aI., 1993) concentrate.

The shelf-slope front is the permanent boundary

that separates less saline and usually cooler shelf water

from slope water. On the east coast ofthe United States,

the front is located over or near the outer edge of the

continental shelf, is continuous from Cape Hatteras,

North Carolina, to Georges Bank, and is present

throughout the year. It extends from the surface to the

bottom, sloping so that the surface expression is located about 25 to 55 km seaward of the bottom contact

(Beardsley and Flagg, 1976). The horizontal salinity

gradient across the front is usually about 1 to 2 parts

per thousand; the horizontal temperature gradient at

the bottom is about 2°_4°C and at the surface is 4°_6°C

(Bowman and Wunderlich, 1977). Wright (1976) examined seasonal variation in the position of the front

off southern New England and found that the bottom

contact was usually near the 100-m isobath, shifting

shoreward in fall. At the surface, the front's position

was more variable, ranging from 52 km seaward of the

100-m isobath in winter to 72 km seaward ofthe isobath

in late summer (Wright, 1976). Both short-term and

seasonal excursions of the front of more than 100 km at

the surface off southern New England have been reported (e.g., Armstrong, 1985) and have been attributed to a variety of oceanographic and meteorologic

influences. As the front shifts shoreward, shelf water is

replaced by usually warmer and more saline slope water. With seaward shifts in the front, shelf water may be

carried off the continental shelf.

Warm-core rings are large, anticyclonic eddies that

occur in slope water and are formed from Gulf Stream

meanders that develop on the shoreward side of the

Gulf Stream. When warm-core rings approach the continental shelf, large volumes of shelf water may be removed by entrainment in the circulation of the ring

(Bisagni, 1983), and slope water and modified Gulf

Stream water may be injected onto the shelf (Chamberlin, 1982), with both processes producing displacements in the position of the shelf-slope front. Warmcore rings are about 200 km in diameter at the surface,

extend to depths of at least 2,000 m, and include currents as high as 140 cm/sec (Cheney, 1978). Typically,

the rings drift westward and southwestward through

the slope water offsouthern New England.

I compiled data on the monthly positions of the

shelf-slope front at the surface and along the bottom

and on the presence of warm-core rings in waters off

southern New England for ten years, 1974-83, to describe average annual cycles and variability.

Data and Methods

The strong thermal gradients at the surface associated

with the shelf-slope front and with warm-eore rings,

and with interactions ofrings with adjacent water masses,

* Now retired. Present address: 309 Tom Fripp Rd., St. Helena Is.,

SC 29220.

39
40 NOAA Technical Report NMFS 134

D

1

100

A

120

/' _ 50 100

E

z

0 0 80 tJ ;::

Vi B z 2100 40 8 w

u :E « 50 20 8 ::::l ./ .... V) ./ 0

'" 0 0

10

C V)

eo:

«

w

>- 5

6

z

0

Figure I

Ten-year (1974-83) average annual cycle and variability ofthe position ofthe shelf-slope front at the surface

and on the bottom and the presence of Gulf Stream

warm-core rings off southern New England. A) Mean

position of the front by month. Surface positions are

horizontal distances, measured seaward of the 200-m

isobath, and bottom contacts are depths ofintersection

of the front with the bottom. B) Standard deviation of

the frontal position, by month. C) Number ofyears when

rings were present in slope water at 71 oW longitude: 'sum

of periods of duration ofrings byJulian Day (Fig. 2).

Results

The shelf-slope front was generally farther offshore at

the surface and the bottom contact was deeper during

winter and spring (Fig. lA). In late summer and fall,

the front was farther shoreward at the surface and in

shallower bottom waters. At the surface, the front remained seaward of the 200-m isobath throughout the

year and tended to be about 50 km offshore of the 20D-m

isobath from December through May. FromJuly through

November, the front was located farther shoreward,

and was about 25 km seaward of the 200-m isobath.

Along the bottom, the front was located near the 95-m

isobath during the summer and was in deepest water,

about 110 m, from December through April. For comparison with the seasonal variation in the position of

the front at the surface, the horizontal distance beAnomalies from the 10-yr monthly means were calculated for each month of the 10-yr period (Fig. 2).

are often apparent in satellite infrared data and imagery. The presence ofwarm-eore rings offsouthern New

England during 1974--83 was determined from annual

reports of bottom temperature, as presented in Armstrong (1998), based on interpretations of satellite infrared data. Warm-core ring passages were recorded as

time of year and duration of each ring's presence along

the 71°W meridian. To examine the possibility of a

seasonal tendency in the occurrence of warm-core Gulf

Stream rings, the number of years during the 10-yr

period in which rings were present along 71°W on a

given Julian Day were summed and plotted against time

of year (Fig. 1).

Monthly mean positions of the shelf-slope front at

the surface (Fig. 1) were computed from weekly determinations of the front compiled in annual summary

reports, 1974--83 (Ingham, 1976; Gunn, 1978, 1979a, b;

Hilland and Armstrong, 1980; Hilland, 1981, 1983;

Armstrong, 1984, 1985, 1986). For the annual reports,

frontal locations along twelve bearings from Cape

Romain, South Carolina, to eastern Georges Bank were

determined from weekly satellite-derived charts. This

analysis employed weekly positions for the bearing along

the 120° azimuth from Montauk Point, because this

line was nearest the 71°W meridian employed in the

studies of warm-core ring tracks and water temperatures presented in Armstrong (1998), Chamberlin

(1998), and Cook (1998). The Montauk Point 120°

bearing line crosses the 200-m isobath about 20 km west

of 71 oW. Because the front is typically located near the

shelf break, frontal positions were recorded as departures from the 200-m isobath.

Bottom locations of the shelf-slope front were determined from the vertical cross-sections of temperature

from expendable bathythermograph (XBT) transects

across the continental shelf and slope on or near 71°W

longitude, as described in Cook (1998). On the vertical

sections, the intersection of the center of the (usually

strong) temperature gradient with the bottom was assumed to represent the offshore boundary of shelf water and was taken to represent the frontal location. Subsurface salinity observations were rarely made on the

XBT transects, therefore how accurately this thermal

gradient feature represents the shelf-slope front could

not be determined. Wright (1976) used the intersection of the lOoC isotherm with the bottom to indicate

the bottom contact of the front, but, as he noted, this is

not always accurate. For each temperature section, the

bottom depth at the frontal location was recorded and

plotted against time of year. Mid-month positions, recorded as bottom depth, were derived from these plots.

Derived monthly positions of the shelf-slope front at

the surface and along the bottom were averaged, by

month, for the 10 yr to determine mean annual cycles

and estimates of variability (standard deviation) (Fig. 1).
_____ Armstrong: Variations in the Shelf-Slope Front and the Influence of Warm-core Gulf Stream Rings 41

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983

A

+100 +100km

-50

o

j

w

II)

o

i

w

'"o

:I:

II)

-100--J---+---1---+---+---+---+---+---+---+---+--100km

~"40

:E

J:

+30m 0

I- +20 '" Q..

UJ

Cl wII)

:E 0 0 0 0

I- '" I0 -20 w co '" -30m 0

:I:

II)

-40

V)

a::

UJ +50 IUJ

:E9 0

Figure 2

Anomalies in the position of the shelf-slope front off southern New England, 1974-83,

relative to the 10-yr monthly means in Figure 1: A) at the surface, in horizontal distance,

and C) along the bottom, by bottom depth. Blanks represent absence of data. B) Presence

and duration of Gulf Stream warm-core rings at 71°W longitude.

tween the 95- and 1l0-m isobaths is about 6 km. Averaged for all months of the ten years, the front was

located 41 km seaward of the 200-m isobath at the

surface and its bottom contact was at 102 m depth

(Table 1), i.e., the front sloped so that its location at the

surface was about 65 km offshore from its bottom position. These seasonal patterns and positions for the surface and bottom contacts of the front are very similar to

those determined for 1941-72 (Wright, 1976).

Variability in the position of the shelf-slope front,

expressed as the standard deviation of average monthly

frontal position (Fig. IB), tended to be least in late

summer and fall, which was when the front was most

shoreward at the surface and in shallowest water on the

bottom (Fig. lA). However, seasonal patterns in frontal

positions are somewhat uncertain since the typical standard deviations (±35 km at the surface and ±12 m on

the bottom) were comparable in magnitude to seasonal

shifts in location of the front. The variation in annual

average frontal position during the 100yr period (Table 1)

was similar in magnitude to the 10-yr mean seasonal

difference at the surface, although bottom positions were

within 5 m of the 99-m isobath in all years except 1978.

A total of 34 warm-core Gulf Stream rings transited

the waters off southern New England during 1974-83,

with 3 ofthe rings first appearing near the end of a year

and persisting into the beginning of the following year

(Table 2). During each year, 3-4 rings were present,

except in 1977, when there were 7 rings. The average

duration for each ring at the 71°W meridian was about

35 days, ranging from about one week for a ring in 1977

to about three months for a ring in the first half of

1974. Warm-core rings were present off southern New

England from as little as 92 days in 1983 to as long as

163 days in 1977, averaging 123 days per year, or about

one-third of the time. No distinct seasonal pattern in

the presence of warm-core rings was evident, although

rings tended to occur less often during late summer

and early fall (Fig. Ie), which corresponds to the period of least variability in frontal position (Fig. IB).

They were most often present in late June, when there

were rings in six of the ten years.

Anomalies in location of the shelf-slope front at the

surface and along the bottom during 1974-83 are shown

in Figure 2 as departures from the 10-yr mean monthly

position. Also depicted in Figure 2 are the periods
42 NOAA Technical Report NMFS 134

Table 1

Average location ofthe shelf-slope front at the surface and on the bottom offsouthern New England during months when

warm-core rings were present or absent for each year, 1974-83, and for the decade.

Surface Bottom

Rings present Rings absent Rings present Rings absent

Average No. Locationl No. Location' Average No. Location2 No. Location2

location' (km) months (km) months (km) location2 (m) months (m) months (m)

1974 25 5 10 6 45 96 5 94 7 104

1975 14 5 I 5 28 102 6 92 6 III

1976 47 4 24 7 60 94 4 81 5 105

1977 15 8 12 I 25 103 9 99 3 ll4

1978 68 4 28 8 89 111 4 100 8 ll6

1979 54 4 38 6 64 103 5 96 7 108

1980 37 4 29 8 41 101 4 93 8 104

1981 62 6 26 6 98 104 5 98 6 109

1982 52 5 35 7 61 101 5 92 7 107

1983 37 5 II 7 56 103 5 95 7 109

Average 41 5 21 6 56 102 5 94 6 109

Note: number of months of values may not sum to 12 in a year because of absence of data for some months.

I Surface frontal positions are the horizontal departure of the front from the location of the 200-m isobath, with positive values to

seaward.

2 Bottom frontal positions are the bottom depth of intersection of the front with the bOltom.

when warm-core Gulf Stream rings were present in the

slope water along 71oW. In general, throughout the 10 yr,

excursions of the front at the surface and along the

bottom parallel one another. Typically, when the front

shifted to offshore of the 10-yr mean monthly position

at the surface, the bottom contact was in deeper water;

when the front was shoreward of the mean monthly

position at the surface, the bottom contact was shallower. However, there were notable exceptions to this

pattern. For example, from January through July 1977

(Fig. 2), the front was located shoreward of the 10-yr

mean monthly position at the surface, but was generally

farther offshore and in deeper water than average, or

near the average position, along the bottom. At the

surface, the farthest offshore position of the front for the

10 yr was in April 1978 (180 kIn seaward of the 200-m

isobath) and the farthest shoreward position occurred in

January 1975 and October 1982 (20 kIn shoreward of the

200-m isobath). Extremes of bottom contact of the front

were in February 1977 (deepest contact at 137 m) and in

November and December 1976 (shallowest bottom contact at about the 70-m isobath).

Excursions of the shelf-slope front from the 10-yr

mean monthly positions at both the surface and along

the bottom often tended to be associated with the presence or absence of warm-core Gulf Stream rings (Fig.

2). When rings were present, the front was usually

located father shoreward at the surface and in shallower water along the bottom. In the absence of rings,

the front tended to be farther offshore and in deeper

Table 2

Frequency and duration ofwarm-core rings present off

southern New England by year, 1974-83.

Number Duration

of rings (days)

1974 3 128

1975 4' 149

1976 3 99

1977 71 163

1978 31 105

1979 4 146

1980 3 107

1981 3 139

1982 3 102

1983 4 92

Average 3.7 123

I Includes one ring present at the end of the preceeding year

that persisted into the beginning of the noted year.

water. Horizontal shifts in frontal position in response

to the passage of rings were generally larger at the

surface than along the bottom, probably due to the fact

that the slant of the front makes it more likely to interact with rings at the surface than along the bottom.

Not all of the rings had comparable influence on

front position, which was likely due to variations in the

size and dynamics of the rings and their proximity to

the continental shelf (Chamberlin, 1982). An example
______ Armstrong: Variations in the Shelf-Slope Front and the Influence of Warm-core Gulf Stream Rings 43

of the relation between warm-core rings and shifts in

frontal location can be seen in the sequence of events

in 1980 through early 1981 (Fig. 2). During the first

half of 1980, three rings were present and the front was

well shoreward and shallower than average. From July

through December, no rings were reported, and the

front shifted to offshore at the surface and into deeper

water at the bottom. From January through April, the

front shifted shoreward and into shallower water as two

more rings drifted across 71°W longitude.

There was considerable between-year variation in annual mean position of the front at the surface and

along the bottom (Table 1), which was not clearly related to either the number of rings present during the

year or the total duration ofrings in the area (Table 2),

nor were the surface and bottom position shifts between years typically in tandem. However, when compiled for months when rings were present versus absent, the annual mean frontal position was always farther shoreward at the surface and in shallower water on

the bottom when rings were present (Table 1).

Shoreward movement of the front with the passage

ofrings results in reduction ofthe area covered by shelf

water. To estimate this reduction, the 10-yr average

frontal positions for periods with and without rings

(Table 1) were used to calculate the cross-sectional area

of shelf water from the coast to the front along 71 oW

longitude. Assuming the front slopes linearly with depth

from the surface to the bottom, the cross-sectional area

was 9.1 km2 for periods with rings and 11.6 km2 for

periods without rings, amounting to a 21 % reduction

in shelf water cross-sectional area due to rings.

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