An Observational/Modeling Study of the Western Gulf of Maine

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Project Type: 
Research
Project Number: 
R/CE-108
Inception Date: 
1994
Completion Date: 
1998
Theme Area: 
Coastal Ecosystem and Public Health
Sustainable Aquaculture

Participants:

Daniel Lynch Dartmouth College Co-Principal Investigator
Wendell Brown University of Massachusetts - Dartmouth Principal Investigator

Students Involved:

Hui Feng UNH - Department of Earth Sciences
Monica Holboke Dartmouth College - Thayer School of Engineering
Objectives: 

1) Describe the variability in the structure of the coastal current

2) Determine the relative roles of tidal, wind, and freshwater-induced forcing on the variability of the density stratification, along-coast flow, and across-shelf dispersion of the coastal current

3) Describe the existence of and variability in the location of suspected branch points in the coastal current

4) Determine the natural "residence times" of water parcels in the coastal current

Methodology: 

The general approach is to develop a modeling capability that will spatially resolve a coastal current that is varying temporarily due to episodic/distributed river discharge and wind forcing. Observed river discharge and winds will be used to force a primitive equation model of the region. Observations of the coastal ocean will be used to verify the model. The verified model will be used to explore the coastal current system response to a variety of forcing scenarios.

Rationale: 

During the spring and early summer, the coastal current may be the primary mechanism for distributing nutrients as well as "red tide" and other toxics along the coast between Penobscot Bay and Georges Bank.

Accomplishments: 
Objectives 1 and 2:
Describe the variability in the structure of the coastal current and;
Determine the relative roles of tidal, wind and freshwater-induced forcing on the variability of the density stratification, along-coast flow and the across-shelf dispersion of the coastal current;
a. New Moored Observations: 1994-1996
The UNH field observation program consisted of deployments supported by the RMRP and Sea Grant (SG). The RMRP-supported measurements were a (1) bottom pressure/temperature (BP/T) instrument in Cape Porpoise harbor during 1994 and 1995; (2) BP/T instrument and a current meter at 4m depth during spring and summer 1994 and a SG temperature/conductivity chain during spring 1995 on the 107 m isobath near Cape Porpoise - in the "coastal current"; and (3) BP/T instrument and T/C chain and in 1996 a string of current meters during September -October 1996 in Wilkinson Basin - the interior western Gulf of Maine (for details see Table 1a). 
Table 1a. Observation Site Information
Name
Station ID
Latitude (deg N)
Longitude (deg W)
Site Depth (m)
Sensor Type
Sensor Depth (m)
Wilkinson Basin
WB
42.79
69.75
270
T/C
31
T/C
71
T/C
121
T/C
171
T/C
220
Bottom P/T
269
Cape Porpoise
CP3
43.23
70.20
105
VACM/T
4
T/C
31
T/C
71
Bottom P/T
105
Cape Porpoise Harbor
CP0
43.35
70.43
5
Bottom P/T
4
 

The pressure time series come from the UNH component of the RMRP-sponsored 1994 and Sea Grant-sponsored 1995 western Gulf of Maine field program. The bottom pressures (BP) at the sites in Table 1a were measured about 1 meter above die bottom with Paroscientific Inc. pressure gauges mounted on independent bottom frames. The measured bottom pressures were corrected for temperature fluctuations according to laboratory calibrations and measured bottom temperatures.
 
 

b. Operational Data: 1994-1996
 
 

We obtained National Data Buoy Center (NDBC) 3-hourly winds, atmospheric pressure, air temperature, National Ocean Service (NOS) hourly sea levels and US Geological Survey (USGS) daily river discharges.
Our measured bottom pressures in offshore waters were augmented by synthetic subsurface pressure (SSP) records at National Ocean Service (NOS) coastal sea level stations (Table 1b). These coastal SSP records were obtained by summing measured sea level (in equivalent pressure) and a nearby atmospheric pressure record. Coastal SSP and BP are the same quantity when water column density-related changes in pressure are found to be negligible, which they usually are at 2 to 10 day "weather-band" periods.
Table 1b. National Ocean Service Sea Level Observation Site Information
 
Station Name
Station ID
Latitude (deg N)
Longitude (deg W)
Halifax
hf
44.67
63.57
Yarmouth
ym
43.8
66.13
Eastport
epo
44.95
66.99
Portland
por
43.66
70.25
Boston
bos
42.36
71.05
Nantucket
nan
41.285
70.10
c. Shipboard Measurements: 1994-1996
We conducted a series of one-day CTD profile surveys of northern Wilkinson and Jeffreys Basins using the R/V Gulf Challenger on March 16, March 21, May 23, August 3, and October 31, 1994 (RMRP) and March 16 and March 20 1995 (SO). A total of 97 CTD casts have provided relatively high-resolution structural information on water properties along the Cape Porpoise across-sheIf transect of the moored measurements during spring, summer and autumn.
d. The Wind-Forced Gulf-Scale Response: A Historical Data Analysis
We performed a statistical analysis of the winter and summer wind-forced pressure response of the Gulf of Maine system using 1974-75, 1986-87 and 1990-91 data sets. Archived NOAA buoy-measured winds were used to objectively construct surface wind stress distribution time series for the 1986-1987 time period. The results of this study were published in: Brown, W.S. 1998. Wind-forced pressure response of the Gulf of Maine, J. Geophys. Res., 103, 30,661-30,678.
The wind-forced modeling component of that study demonstrated the importance of both remote forcing from the Scotian shelf and local gulf forcing, however under-predicted the pressure (i.e. sea level) response in the central Gulf
e. Synthesis of the Observations: Spring & Summer 1994
A synthesis of the time series measurements and hydrography (as reported in Feng, H. and W.S. Brown, 1999) has helped us to understand the variability in the structure and the forcing of the along-coast currents in the western Gulf. In particular, these results show a laterally-sheared, buoyancy-driven flow near the coast - the "coastal current," which is modulated by both the local gulf wind and remote forcing. The wind causes relatively simple 2-D upwelling/downwelling across-isobath transport during spring and more complex 3-D across-isobath upwelling/downwelling transport during summer. During spring, the along-isobath flows in the general direction of the east-west wind, first in the southwest and then later in the northeast along the coast. During summer, when the winds are weaker and vertical density stratification is very strong, the along-isobath flow is highly sheared in the vertical, with instances of deep flows counter to the wind direction.
f. Circulation Modeling
The western Gulf of Maine coastal current is strongly influenced by buoyancy and wind-forced gulf-scale current variability. Thus we used two "meshes" or grids. The gulf-scale mesh with a nominal element size of 10km extended from the deep ocean south of Georges Bank into the gulf and shelf between the Scotian Shelf off of Halifax and the New England shelf south of Watch Hill Rl. The coastal-scale mesh, characterized by its high (~lkm) resolution, focused on the coastal domain in the western gulf.
A pair of 3-D Dartmouth finite element models - one was linear and diagnostic (FUNDY) and the other was nonlinear and prognostic (QUODDY) - were run on these meshes. The models were used to simulate the gulf’s responses to both climatological and realistic forcing. In the latter case spatially/temporally variable winds, derived from NOAA/NDBC buoy-measured winds were used to force the models.
Prognostic flow patterns due to (a) seasonally-evolving density fields, and (b) the transient forcing of winds, river discharge, and Scotian Shelf inflow were generated and analyzed by Monica Holboke (1998) as part of Ph.D. Dissertation under the guidance of D. Lynch- a co-Pi in this project. The results of that work showed the countercurrent response of the deep coastal current in response to wind forcing as reported in:
Holboke, M.J. and D. R. Lynch, 1995." Simulations of the Maine Coastal Current", .Proc. ASCE 4th Int. Conf on Estuarine and Coastal Modeling. San Diego Oct. 1995.
Objectives 3 and 4
Describe the existence of and the variability in the location of suspected branch points in the coastal current  and;
Determine residence times of water parcels in the coastal current.
The locations and the variability of flow in the region of the "branch points" of the western Gulf of Maine Coastal Current were explored with model current maps and water particle trajectories in:
Lynch, D. R., M, J. Holboke, C E. Naimie. The Maine Coastal Current Spring Climatological Circulation. Continental Shelf Research. (Accepted 27 August, 1996)
The type of boundary conditions, which can be imposed on limited-area models of the coastal current (and the associated errors) as well as, the transit times between different pairs of coastal and deepwater sites were computed for different seasonal flow scenarios and during an early April 1987 storm in:
Holboke, M. J. 1998. "Modeling Realistic Events of the Maine Coastal Current", Ph.D. Dissertation, Thayer School of Engineering, Dartmouth College, Hanover NH.

Publications

Available from the National Sea Grant Library (use NHU number to search) or NH Sea Grant

Journal Article

  • Lynch, D., J. Ip, C. Naimie and F. Werner (1996). Comprehensive coastal circulation model with application to the Gulf of Maine. Continental Shelf Research 16(7):875-906.
  • Brown, W. (1998). Wind-forced pressure response of the Gulf of Maine. Journal of Geophysical Research 103(C13):30,661-30,678.
  • Lynch, D., M. Holboke and C. Naimie (1997). The Maine coastal current: spring climatological circulation. Continental Shelf Research 17(6):605-634.

Report

  • Bub, F., W. Brown and P. Mupparapu (1999). Circulation variability in the western Gulf of Maine. University of New Hampshire Ocean Process Analysis Laboratory, OPAL Technical Report UNH-OPAL-1999-001.
  • Feng, H. and W. Brown (1996). Hindcasting the Gulf of Maine wind field: a case study. University of New Hampshire Ocean Process Analysis Laboratory, OPAL Technical Report UNH-OPAL-1996-003.

Thesis/Dissertation

  • Feng, H. (1996). Wind-induced responses of the western coastal Gulf of Maine during spring and summer 1994. Master's Thesis, University of New Hampshire.
  • Holboke, M. (1998). Variability of the Maine coastal current under spring conditions. Doctoral Dissertation, Dartmouth College.

Book Chapter

  • Brown, W. (1998). Boundary flux measurements in the coastal ocean. The Sea, Volume 10, Chapter 15, Kenneth H. Brink and Allan R. Robinson, eds., pp. 399-418.

Proceeding

  • Holboke, M. and D. Lynch (1995). Simulations of the Maine coastal current. ASCE 4th International Conference on Estuarine and Coastal Modeling, San Diego, Calif., October 1995.