omsap public meeting september 1999 circulation and water properties in massachusetts bay rocky...
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OMSAP Public MeetingSeptember 1999
Circulation and Water Properties in Massachusetts Bay
Rocky GeyerWoods Hole Oceanographic Institution
September 22, 1999
InflowOutflow
Light
ConcernsEcologicalEcological Nutrients Contaminants Organic Material Food Chain Community Structure Living Resources
Human Health Contaminants Bacteria Viruses Bioaccumulation
SEDIMENT
Mammals
Infauna
Piscivorous Fish
Zooplankton
Phytoplankton
Planktivorous Fish
Epibenthos
Demersal Fish
Regeneration
DetritusParticulate
Microbes
Dissolved
WATER COLUMN
Sources RiversRivers BoundaryBoundary Nonpoint EffluentsEffluents
Gas ExchangeExchangeN2, | O2, CO2
ATMOSPHERE
N, P, Si, ON, P, Si, O22, CO, CO22 Microbes
OMSAP Public MeetingSeptember 1999
Physical Environment Gulf of Maine Circulation
There is a general counterclockwise circulation in the Gulf of Maine, with inflow from the Scotian shelf, flow to the southwest along the coast of Maine towards Massachusetts Bay. Some of the water sweeping past Cape Ann enters Massachusetts Bay and contributes to a counter-clockwise circulation in Massachusetts Bay.
The blue arrows are near-surface currents and the red arrows are deep currents. The general tendency is for a counter-clockwise circulation, sweeping south past the mouth of Mass Bay.
OMSAP Public MeetingSeptember 1999
Physical Environment
The circulation of Massachusetts Bay is also counter-clockwise
Mean flows and variability based on moored data from the Massachusetts Bays Program
OMSAP Public MeetingSeptember 1999
Physical Environment Surface Drifter Trajectories
71° 0' 70°50' 70°40' 70°30' 70°20' 70°10'
41°50'
42° 0'
42°10'
42°20'
42°30'
42°40'
April 91
July 90 April 90
Oct 90 Jan 91
OMSAP Public MeetingSeptember 1999
Physical Environment Alexandrium abundance May 1993
OMSAP Public MeetingSeptember 1999
Physical Environment Seasonal Variation of Temperature
Temperature across Massachusetts Bay, from Boston Harbor to Stellwagen Bank varies seasonally (data from the Mass Bays Program)
Note the warmest bottom water occurs in October
80
60
40
20
Boston Stellwagen
4
Apr 28 1990
de
pth
, m
80
60
40
20
Jul 25 1990
de
pth
, m
80
60
40
20
10
Oct 17 1990
de
pth
, m
-70.9 -70.85 -70.8 -70.75 -70.7 -70.65 -70.6 -70.55 -70.5 -70.45
80
60
40
20
Feb 5 1991
longitude
de
pth
, m
OMSAP Public MeetingSeptember 1999 jan feb mar apr may jun jul aug sep oct nov dec
0
5
10
15
20
25
de
gre
es C
Temperature at N-21
near-surface near-bottom
Physical Environment Seasonal Temperature Cycle Baseline Period 1992 - 1998
Near-surface and near-bottom temperature seasonal cycle at the outfall site is relatively uniform both in timing and magnitude
Wintertime surface and bottom temperatures are close to freezing
Temperatures begin to warm up in the beginning of April
Surface water warms much faster than the bottom water
Temperature differential between the near-surface and near-bottom temperature greater more than 10° C by August
Surface water cools in the fall, but the bottom water continues to warm slowly until the water becomes well mixed, usually in mid- to late October
OMSAP Public MeetingSeptember 1999
Physical Environment Annual Temperature Cycle Baseline Period 1992 - 1998
1992 1993 1994 1995 1996 1997 1998 19990
5
10
15
20
25
year
de
gre
es C
Near-surface and near-bottom temperature, N-21
near-surface near-bottom
Surface water temperature shows nearly the same pattern every year Variations in the bottom water occur, particularly in the maximum temperature reached In 1994, 1995, and 1996, the bottom water was considerably warmer than in the other years
– around 12 C During 1992, 1993 and 1998 the temperatures reached about 8 C These differences appear to be explained by the wind forcing Years with more southerly winds tend to have colder bottom water temperatures due to
upwelling phenomena
OMSAP Public MeetingSeptember 1999
Physical Environment Upwelling
Satellite image of upwelling during southwesterly winds
OMSAP Public MeetingSeptember 1999
Physical Environment Upwelling Index
1992 1993 1994 1995 1996 1997 1998
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4Upwelling Index: summer N-S wind stress
Pa
*1
0-3
1992 1993 1994 1995 1996 1997 1998 19990
5
10
15
20
25
year
de
gre
es C
Near-surface and near-bottom temperature, N-21
near-surface near-bottom
OMSAP Public MeetingSeptember 1999
Physical Environment Freshwater Inputs
Distant sources• Rivers entering the Gulf of
Maine• Scotian Shelf, which carries
the input from the Gulf of St. Lawrence
• More northerly sources
Local inputs• Charles River • MWRA Deer Island outfall.
OMSAP Public MeetingSeptember 1999
Physical Environment Seasonal Salinity Cycle Baseline Period 1992 - 1998
Seasonal pattern of salinity is not as regular as that of temperature,• Tendency is for the salinity to decrease during the spring, usually reaching a
minimum in May • Both surface and bottom salinity vary seasonally, • Surface salinity is usually lower than the bottom salinity.
Local impact of the freshwater inflow on Massachusetts Bay
jan feb mar apr may jun jul aug sep oct nov dec27
28
29
30
31
32
33
psu
Salinity at N-21
near-surface near-bottom
OMSAP Public MeetingSeptember 1999
Physical Environment Annual Salinity Cycle Baseline Period 1992 - 1998
Surface and bottom salinity at the outfall site show a distinct drop every year• Large interannual differences in the magnitude of the decrease are evident
Lowest salinities (as low as 28 psu) were observed in the spring of 1998• 1998 unusual in that the low salinity water persisted longer into the summer than other years• 1998 was the wettest year of the monitoring program
Differences between surface and bottom salinity• maximum during the spring, and • tends to vanish during the winter, when the water column becomes well mixed
1992 1993 1994 1995 1996 1997 1998 199928
29
30
31
32
33
year
psu
Near-surface and near-bottom salinity, N-21
near-surface near-bottom
OMSAP Public MeetingSeptember 1999
Physical Environment Seasonal Stratification Baseline Period 1992 - 1998
Density stratification shows a pronounced seasonal cycle due to the combined effects of temperature and salinity
Maximum stratification usually occurs in July and August, due to the seasonal warming of the surface layer.
Magnitude of the density difference is typical of mid-latitude coastal waters that are too deep to be mixed by the tides.
Stratification usually vanishes during the • some instances when enough low-salinity water was near the surface to produce
stratification.
jan feb mar apr may jun jul aug sep oct nov dec0
0.5
1
1.5
2
2.5
3
3.5
4Stratification at N-21
sigm
a-t
OMSAP Public MeetingSeptember 1999
1992 1993 1994 1995 1996 1997 1998 19990
0.5
1
1.5
2
2.5
3
3.5
4
sig
ma
-t
Stratification at N-21
total stratifiction
temperature-stratification
Physical Environment Annual Stratification
Baseline Period 1992 - 1998
Variation of stratification (density) shows a regular seasonal pattern• Only slight interannual variation observed
Stratification is usually initiated in the spring by salinity effects By the middle of the summer it is dominated by thermal stratification Most of the interannual density variability comes from the variations in salinity
stratification 1998 is notable due to the large amount of freshwater inflow to the system
The blue line is the total stratification; green line is the contribution of temperature to the stratification
OMSAP Public MeetingSeptember 1999
1992 1993 1994 1995 1996 1997 1998 1999
15
10
5
0
tem
pe
ratu
re
1992 1993 1994 1995 1996 1997 1998 1999
6
8
10
12
dis
so
lve
d o
xyg
en
Bottom Temperature and Dissolved Oxygen, N-21
Physical Environment Annual and Inter-annual Variation of Dissolved Oxygen
Dissolved oxygen in the bottom water at the outfall site is inversely correlated with the variation of near-bottom temperature.
1994 and 1995 had warmest near-bottom waters and the lowest DO values during baseline Possible causes
• Upwelling that replenishes the deep dissolved oxygen values. • More Gulf of Maine water is advected in during years with weak upwelling(deep GOM water
appears to be lower in DO than waters of the same depth in Massachusetts Bay) • Biological processes
OMSAP Public MeetingSeptember 1999
Physical Environment Hydrodynamic Modeling USGS Model Grid
OMSAP Public MeetingSeptember 1999
Physical Environment Hydrodynamic Modeling Model-Data Comparison
OMSAP Public MeetingSeptember 1999
Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Winter Conditions
Higher concentrations are associated with the old outfall, mainly in Boston Harbor The farfield dilution is projected to be virtually the same when the outfall is online Effluent will extend to the surface in dilute concentrations from the new outfall
during the winter, due to the absence of stratification
A hydrographic section from Boston Harbor outfall to Cape Cod Bay for the present discharge location (upper panel)and the new outfall (lower panel) shows
OMSAP Public MeetingSeptember 1999
Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Summer Conditions
Trapping of the effluent in surface layer during the summer at the Harbor outfall location Effluent from the new outfall is trapped below the thermocline in the summer Difference is likely to reduce the impact of nutrient loading from the outfall on the
ecosystem
A hydrographic section from Boston Harbor outfall to Cape Cod Bay for the present discharge location (upper panel)and the new outfall (lower panel) shows
OMSAP Public MeetingSeptember 1999
Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Plan view, Surface
OMSAP Public MeetingSeptember 1999
Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Plan view, mid-depth
OMSAP Public MeetingSeptember 1999
Physical Environment Effect of outfall on near-field
OMSAP Public MeetingSeptember 1999
Physical Environment Summary
Massachusetts Bay is part of the larger circulation regime of the Gulf of Maine • Currents, water properties, and biology are strongly influenced by the
conditions in the Gulf of Maine • Interconnection applies to the outfall site as well as Massachusetts Bay as
a whole
Large seasonal variation in stratification is the dominant characteristic of the water properties of Massachusetts Bay • Well-mixed conditions in the winter • Strong stratification in the summer
OMSAP Public MeetingSeptember 1999
Physical Environment Summary
Interannual variations in bottom water temperature and dissolved oxygen at the outfall site appear to be related to wind forcing • Persistent southerly winds during the summer lead to colder bottom
temperatures and higher DO• Weaker southerly winds lead to warmer bottom waters and lower DO
USGS circulation model results • main influence of the new outfall will be a reduction of the impacts of
the effluent in Boston Harbor • Farfield will not be significantly altered