Processes Influencing the Transport and Fate of Contaminated Sediments in the Coastal Ocean-Boston Harbor and Massachusetts

Home , Cape Cod, Cape Cod Bay, Massachusetts Bay

26 Section 4: Oceanographic Setting

By Bradford Butman, Richard P. Signell, John C. Warner, and P. Soupy Alexander

The ocean currents in Massachusetts Bay mix and time because of the complex bathymetry and coastal transport water and material in the bay, and exchange geometry, and because of the multiple processes (for water with the adjacent Gulf of Maine. The currents example wind, river runoff, and currents in the Gulf of can conceptually be separated into tidal currents (which Maine) that drive the flow and change seasonally. fluctuate 1–2 times each day), low-frequency currents The oceanography of Massachusetts Bay may caused by winds and river runoff (which typically be conceptually separated into four seasonal intervals fluctuate with a period of a few days), and a residual (following Geyer and others, 1992) based on the wind current (steady over a few weeks). Field observations and surface waves (fig. 4.1); the temperature and thermal (Butman, 1976; Geyer and others, 1992; Butman and stratification of the water column (fig. 4.2); the salinity, others, 2004a; Butman and others, 2006) and simulations salinity stratification, and horizontal salinity gradients of the currents by numerical hydrodynamic models (for caused by river discharge (fig. 4.3); and the density example Signell and others, 1996; Signell and others, 2000) provide descriptions of the flow pattern, strength, stratification, which results from the temperature and and variability of the currents. Field observations salinity distribution (fig. 4.4). From November through provide measurements of the currents at selected March (winter), the water column is vertically well- locations during specific periods of time, whereas model mixed, and the wind and surface waves are the largest of simulations provide a high-resolution view of the often the year. In April and May (spring), the freshwater runoff complex spatial pattern of the flow. Direct observations from rivers is largest, and the water column is vertically and numerical simulations provide complementary salt-stratified. In spring, there may also be horizontal descriptions of coastal currents that vary in space and salinity gradients in the surface layers caused by the

WINTER SPRING SUMMER FALL WINTER 15 A 10

IN METERS 5 PER SECOND WIND SPEED,

0.3 B 0.2

0.1 WIND STRESS, SQUARE METER IN NEWTONS PER 0 3 C 2

IN METERS 1 SIGNIFICANT WAVE HEIGHT, WAVE

0 JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

Figure 4.1. Monthly averaged (A) wind speed, (B) wind stress, and (C) significant wave height at NOAA Buoy 44013 (see fig. 3.1 for location). Vertical bars show one standard deviation (computed using hourly data) on each side of the mean value (dot). Data from the period November 1989 through December 2002 (Butman and others, 2004a). Section 4: Oceanographic Setting 27

WINTER SPRING SUMMER FALL WINTER

18 DEPTH BELOW SURFACE

16 5 METERS 23 METERS 14

TEMPERATURE, IN DEGREES CELSIUS TEMPERATURE, 6

0 JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

Figure 4.2. Monthly averaged water temperature at 5 and 23 m below the surface at the long- term monitoring site LT-A in western Massachusetts Bay (see fig. 3.1 for location). Vertical bars show one standard deviation (computed using hourly data) on each side of the mean value (dot). Data from the period November 1989 through December 2002 (Butman and others, 2004a).

rivers that discharge freshwater along the coast. From uniform throughout the bay (fig. 4.5A). Tidal currents June through August (summer), the seasonal thermocline are largely bi-directional and flow into and out of the

develops, the vertical density stratification reaches a bay (fig. 4.5B). The strength of M2 tidal current is less maximum, and the winds and surface waves are the than 0.20 meters per second (m/s) except in the Race weakest of the year. From September through October Point Channel, across the southern portion of Stellwagen (fall), the water cools, and the vertical stratification of Bank, and in the entrance to Boston Harbor where it the water column decreases from the strong summer exceeds 0.5 m/s (fig. 4.5B). stratification to well-mixed winter conditions. The low-frequency fluctuations in current are Tidal current and elevation in Massachusetts Bay are driven primarily by the wind and by horizontal density dominated by semi-diurnal tides. The typical tidal range gradients. In Massachusetts Bay, these fluctuations are is about 2.6 m and varies from about 1.1 m to 4.1 m. typically 0.1 m/s (figs. 4.6A, 4.6B), generally larger than At LT-A (fig. 3.1), the amplitudes of the semi-diurnal the 0.01–0.05 m/s residual flow. Although the winds are

tides M2 (period 12.42 hours), N2 (period 12.66 hours) strongest in winter, the low-frequency current fluctua- and S2 (period 12.00 hours) are 1.29 m, 0.29 m, and tions are largest in spring and summer (fig. 4.7). The 0.20 m, respectively, and the amplitudes of the strong currents in spring are driven by density gradients

diurnal tides K1 (period 23.92 hours) and O1 (period associated with lenses of freshwater from river runoff 25.82 hours) are 0.11 m and 0.14 m, respectively. The through Boston Harbor and from rivers that discharge amplitude and phase of the tidal elevation is nearly into the Gulf of Maine to the north of Massachusetts 28 SECTION 4 December 2002(Butmanandothers,2004a). flow andhourlydataforsalinity)oneachsideofthemeanvalue (dot).DatafromtheperiodDecember1989through Bay (seefig.3.1forlocation).Vertical barsshowonestandarddeviation(computedfromdailyobservationsofriver 23 mbelowthesurface(10abovebottom)atlong-term monitoringsiteLT-A inwesternMassachusetts Figure 4.3. SALINITY, IN PRACTICAL MERRIMACK RIVER DISCHARGE, SALINITY UNITS IN CUBIC METERS PER SECOND 1,000 250 500 750 30 31 32 33 0

(A) MonthlyaverageddischargefortheMerrimackandCharlesRivers, and B A JAN. DEPTH BELOW SURFACE WINTER 23 METERS 5 METERS FEB. MAR. APR. SPRING MAY JUNE SUMMER JULY AUG. SEPT. FALL OCT. Merrimack River Charles River (B) NOV. salinityat5mand WINTER DEC. 0 25 50 75 100

CHARLES RIVER DISCHARGE, IN CUBIC METERS PER SECOND 29

WINTER SPRING SUMMER FALL WINTER Figure 4.4. Monthly 1,027 averaged water density A at 5 and 23 m below the 1,026 surface (A), and the

density difference between 1,025 densities at 23 m and 5 m DEPTH BELOW (B) at long-term monitoring 1,024 SURFACE site LT-A in western

5 METERS Massachusetts Bay SECTION 4 1,023 PER CUBIC METER (see fig. 3.1 for location). 23 METERS DENSITY, IN KILOGRAMS DENSITY, Vertical bars show 1,022 one standard deviation

2.0 (computed using hourly data) on each side of the B mean value (dot). Data 1.5 from the period December 1989 through November 1.0 2002 (Butman and others, 2004a). CUBIC METER 0.5 IN KILOGRAMS PER DENSITY DIFFERENCE,

0 JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

Bay (Butman, 1976; Geyer and others, 1992). Along the Massachusetts Bay; a water particle takes about 15 days western shore of Massachusetts Bay and in the Gulf of to reach the southern part of Cape Cod Bay from the Maine Coastal Current, the low-frequency currents are area of the Massachusetts Bay outfall offshore of Boston strongest in the along-coast direction, but in the north- (fig. 1.5). The flow in central Massachusetts Bay is very western corner of Massachusetts Bay, over Stellwagen weak (less than a few centimeters per second); a particle Bank, and in the center of Cape Cod Bay, the low-fre- takes more than 100 days to travel from Cape Ann to quency currents do not have a strong preferred orienta- Cape Cod Bay. tion (the ellipses in figs. 4.6A, 4.6B are more circular). In summer, at a depth of 5 m, which is above the Information on the residual flow pattern in Massa- seasonal thermocline, there is a southerly flow into Mas- chusetts Bay comes from field observations (figs. 4.6A, sachusetts Bay south of Cape Ann (fig. 4.9A). In con- 4.6B) and from model simulations. Simulations of the trast to the winter pattern however, all this flow occurs in currents in Massachusetts Bay with a three-dimensional the center of Massachusetts Bay, is joined by upwelled hydrodynamic model (Signell and others, 1996) for water from the northwest corner of Massachusetts Bay, winter (defined as November through February) and exits to the east across Stellwagen Bank; no surface (fig. 4.8) and summer (defined as June through August) flow enters Cape Cod Bay. Along the western shore, (figs. 4.9A, 4.9B) and averaged for the years 1990–1992, there is a weak flow to the northwest. The flow pattern in provide a picture of the flow pattern in Massachusetts Cape Cod Bay includes a clockwise circulation centered Bay that is quite consistent with the limited field southeast of Plymouth Harbor and a smaller counter- observations (figs. 4.6A, 4.6B). In both seasons, the clockwise circulation south of Provincetown. At 25-m Gulf of Maine Coastal Current flows to the southeast at depth, which is below the seasonal thermocline, the 0.05–0.1 m/s to the east of Stellwagen Bank. In winter, principal features of the residual flow are the southeast- some of the Maine Coastal Current enters Massachusetts ward Maine Coastal Current, and a southerly flow along Bay south of Cape Ann, initially flowing to the south at a the western side of Massachusetts Bay at about the 40-m few centimeters per second (fig. 4.8). Some of the inflow isobath (fig. 4.9B). In contrast to the near-surface flow, turns eastward and exits Massachusetts Bay across this southerly flow feeds a counterclockwise flow in Stellwagen Bank, some continues southward across Cape Cod Bay that exits north of Race Point. In winter Massachusetts Bay into Cape Cod Bay, and some flows and summer, with the exception of the Gulf of Maine southwestward toward Boston, then turns southeastward Coastal Current, the residual flows are less than 0.05 m/s toward Cape Cod Bay. The residual flow is strongest, and particle trajectories are most influenced by the tidal about 0.05 m/s, in the current along the western shore of currents and the low-frequency flow. 30 SECTION 4 compare welltothepredictedamplitudeandphase. Observations, obtainedatlocationsmarkedbyredtriangles, varies byabout4 tide. ThephaseoftheM (ROMS). ThebackgroundcoloristheamplitudeofM calculated withtheRegionalOceanModelingSystem Figure 4.5 N O T S O B A 0 0 40-METER DEPTHCONTOUR TIDAL OBSERVATIONS 5 10 (A) . 10 MILES M Plymouth 20 KILOMETERS

º CAPE ANN (8minutes)acrossMassachusettsBay. 2 MODELED M 1.2 tidalelevationinMassachusettsBay, 108 AMPLITUDE, INMETERS 2 EXPLANATION tideisshownascontoursand PHASE, INDEGREES 1.3 104 2 TIDALELEVATION 1.4

106 CAPE 1.5 Point Race 108

COD 110 2

Harbors. Bank, andinthe entrancestoBostonandPlymouth are inRacePointChannel,over southernStellwagen in phasethroughoutthebay. Thestrongest tidalcurrents tide flowsintoandoutofMassachusetts Bay, essentially shows theexcursionofawater particleatmapscale.The shown inblackrotateclockwise. Thesizeoftheellipse in whiterotatecounterclockwise;currentstheellipses ellipse everytidalcycle.Currentsintheellipsesshown originates atthecenterofellipseandsweepsout major axisofthetidalellipse.Thecurrentvector (ROMS). Thebackgroundcoloristhemagnitudeof calculated withtheRegionalOceanModelingSystem Figure 4.5 N O T S O B B 0 0 40-METER DEPTHCONTOUR 5 10 (B) .

10 MILES Plymouth 20 KILOMETERS Surface M CAPE ANN 0 SPEED, INMETERSPERSECOND MODELED M 0.5 METERSPERSECOND 0.20 EXPLANATION TIDAL ELLIPSE 2 tidalcurrentellipses, 2 0.40 TIDALCURRENT

0.60 CAPE Point Race 0.80

COD 31

40 GULF OF MAINE

CAPE ANN

Maine Coastal Current

40 SECTION 4 LT-A 40 GULF OF MAINE

B S O E L L S W CAPE ANN

T MASSACHUSETTS A O G BAY E Maine Coastal Current N N

N K

LT-B EXPERIMENTS Mass Bay Circulation LT-A 40 Cape Cod Bay S Western Mass Bay Race T B E L Point L Stellwagen Bank O W

S A GoMOOS T G E O MASSACHUSETTS N USGS-MWRA N B BAY A

VELOCITY SCALE K CAPE COD 0.1 METERS PER SECOND BAY LT-B 40-METER DEPTH CONTOUR Race EXPERIMENTS Point 0 10 20 KILOMETERS Mass Bay Circulation 0 5 10 MILES CAPE COD Stellwagen Bank GoMOOS USGS-MWRA

VELOCITY SCALE CAPE COD Figure 4.6 (A). Winter (November through February) near- 0.1 METERS PER SECOND BAY surface mean flow and low-frequency current ellipses observed 40-METER DEPTH CONTOUR in Massachusetts Bay during experiments conducted between 0 10 20 KILOMETERS 1986 and 2005. The observed winter mean flow (blue arrows) 0 5 10 MILES CAPE COD and the variability (shown as an ellipse centered around the tip of the mean-flow arrow) are shown for near-surface currents (measured 2–8 m below sea surface). Typically, the daily averaged current originates at the station symbol (colored squares) and flows toward any location within the ellipse. In Figure 4.6 (B). Summer (June through August) near-surface general, the low-frequency fluctuations are larger than the mean flow and low-frequency current ellipses observed in mean. These data were obtained from the Massachusetts Bay Massachusetts Bay during experiments conducted between Circulation Experiment (winter 1990–1991; Geyer and others, 1988 and 2004. The observed summer mean flow (blue arrows) 1992), U.S. Geological Survey-Massachusetts Water Resources and the variability (shown as an ellipse centered around the tip Authority (USGS-MWRA) long-term observations (winter 1990– of the mean-flow arrow) are shown for near-surface currents 2002 at LT-A and winter 1997–2002 at LT-B; Butman and others, (measured 2–8 m below sea surface). Typically, the daily averaged 2004a), Gulf of Maine Ocean Observing System (GoMOOS; winter current originates at the station symbol (colored squares) and 2001–2005), Stellwagen Bank (winter 1994; USGS data archives), flows toward any location within the ellipse. In general, the western Massachusetts Bay (winter 1987; USGS data archives), daily fluctuations are larger than the mean. These data were and Cape Cod Bay (winter 1986; USGS data archives). obtained from the Massachusetts Bay Circulation Experiment (summer 1990; Geyer and others, 1992), U.S. Geological Survey- Massachusetts Water Resources Authority (USGS-MWRA) long-term observations (summer 1990–2002 at LT-A and summer 1998–2002 at LT-B; Butman and others, 2004a), Gulf of Maine Ocean Observing System (GoMOOS; summer 2001–2004), and Stellwagen Bank (summer 1994, USGS data archives). 32 SECTION 4 "/34/. 0 0 0 LOW-FREQUENCY CURRENT SPEED, IN CENTIMETERS PER SECOND

40 PARTICLE PATH 5 10 12 14 10 10 DAYS 20 DAYS 0 2 4 6 8 METERS PERSECOND CURRENT SPEED 30 DAYS 40-METER DEPTHCONTOUR 10 MILES

20 KILOMETERS 0.05 JAN.

CAPE ANN DEPTH BELOW SURFACE WINTER 23 METERS FEB. 5 METERS 40 0.1 MAR.

40 APR.

SPRING Maine Coastal Current Coastal Maine MAY JUNE SUMMER JULY AUG. SEPT. FALL strongest (about0.05m/s)along thewesternshore. Massachusetts Bay, thedepth-averaged residualflowis some ofwhichentersthebaysouth ofCapeAnn.Within east ofMassachusettsBay(the MaineCoastalCurrent), the southeastof0.05–0.1m/sin theGulfofMaineto scalar currentspeed.Thesimulationsshowaflowto at everysecondgridcell;thebackgroundcoloris every 10days.Theblackarrowsshowthevectorcurrent dots alongthetracksindicatepositionsofparticles along thedashedlinetoeastofCapeAnn.Thewhite visualized bytracksofparticles(whitelines)introduced water columnisverticallywell-mixed.Theflowpattern the monthsNovember–February1990–1992,when Bay inwintercalculatedfromnumericalsimulationsfor Figure 4.8. OCT. NOV.

WINTER Depth-averaged meanflowinMassachusetts DEC. frequency current.Vertical the typical strength of the low- current speedisameasureof (fig. 3.1).Thelow-frequency western MassachusettsBay monitoring siteLT-A in the bottom)atlong-term at 5and23m(10above north currentcomponents) low-pass filteredeastand current speed(computedfrom averaged low-frequency Figure 4.7. the meanvalue(dot). 6-hour data)oneachsideof deviation (computedusing bars showonestandard

Monthly-

CAPE ANN

Maine Coastal Current

40 "/34/. 40 SECTION 4

PARTICLE PATH 10 DAYS Figure 4.9 (A). Mean flow in Massachusetts Bay in summer at 5-m water depth calculated from numerical simulations for 20 DAYS START the summer months June–August 1990–1992, when the water CURRENT SPEED column is stratified. The flow pattern is visualized by tracks 0 0.05 0.1 of particles (shown as white lines) introduced at every fourth model-grid cell. Each particle track is 20 days long; white METERS PER SECOND dots indicate the beginning of the track and the particles’ 40 40-METER DEPTH CONTOUR positions at 10 and 20 days (end). The black arrows show 0 10 20 KILOMETERS the vector current at every second grid cell; the background 0 5 10 MILES color is the scalar current speed.

CAPE ANN

Maine Coastal Current

"/34/. 40 40

PARTICLE PATH Figure 4.9 (B). Mean flow in Massachusetts Bay in summer 10 DAYS at 25-m water depth calculated from numerical simulations 20 DAYS START for the summer months June–August 1990–1992, when the water column is stratified. The flow pattern is visualized by CURRENT SPEED 0 0.05 0.1 tracks of particles (white lines) introduced at every fourth model-grid cell. Each particle track is 20 days long; white METERS PER SECOND dots indicate the beginning of the track and the particles’ 40 40-METER DEPTH CONTOUR positions at 10 and 20 days (end). The black arrows show 0 10 20 KILOMETERS the vector current at every second grid cell; the background 0 5 10 MILES color is the scalar current speed.