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Huijie Xue, Fei Chai, and Neal R. Pettigrew

Abstract

The Princeton Ocean Model is used to study the circulation in the Gulf of Maine and its seasonal transition in response to wind, surface heat flux, river discharge, and the M 2 tide. The model has an orthogonal-curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical. It is initialized and forced at the open boundary with model results from the East Coast Forecast System. The first experiment is forced by monthly climatological wind and heat flux from the Comprehensive Ocean Atmosphere Data Set; discharges from the Saint John, Penobscot, Kennebec, and Merrimack Rivers are added in the second experiment; the semidiurnal lunar tide (M 2) is included as part of the open boundary forcing in the third experiment.

It is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the Gulf of Maine. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior gulf and the exterior shelf/slope region. From June to December, the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction. While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of the tide results in not only the anticyclonic circulation on Georges Bank but also modifications to the seasonal circulation.

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Wendell S. Brown, Neal R. Pettigrew, and James D. Irish

Abstract

The Nantucket Shoals Flux Experiment (NSFE) was a collaborative effort to measure the alongshelf transport of mass, heat, salt and nutrients from March 1979 through April 1980 with a dense army consisting of moored current, temperature and bottom pressure instruments in an across-shelf and upper-slope transect south of Nantucket Island. The pressure component of that experiment is described here.

Bottom pressure recorders were deployed at stations N1 (46 m), N2 (66 m), N4 (105 m), and N5 (196 m) during two half-year periods. spring–summer 1979 (SUMMER) and fall–winter 1979/80 (WINTER). A synthetic subsurface pressure (SSP) record was formed from atmospheric pressure and sea level observations at Nantucket Island. The low-pass filtered (periods > 36 h) or subtidal pressures were used for the subsequent analysis. It was found that Nantucket SSP and BP are very nearly equivalent for fluctuation periods less than about 50 days if steric changes in sea level, due to density changes above the seasonal pycnocline, were removed from the SSP record. However, if coastal SSP and bottom pressures are to be contrasted on seasonal time scales, then an internal pressure (or equivalent dynamic height) correction to bottom pressure is required. An empirical orthogonal function (EOF) analysis of the pressure field shows that most of the pressure variance for periods between 2 and 40 days (WINTER, 91.9%; SUMMER, 83.5%) is contained in a mode that (a) is characterized by a decreasing amplitude and small phase-lag increase at successive seaward locations and (b) is coherent with local alongshelf (75°T) wind stress. Pressure differences between stations were used to compute across-shelf pressure gradients, whose fluctuation distribution exhibits a conspicuous intensification over the outer shelf during the WINTER in contrast to the relatively uniform SUMMER distribution.

The excellent comparison found between “geostrophic currents,” which were inferred from bottom pressure differences, and suitably averaged “observed currents” represents the first direct confirmation of the quasi-geostrophy of alongshelf flow. Subtidal fluctuations of geostrophic currents are highly coherent with observed currents and lag them by periods that are consistent with typical geostrophic adjustment times for shelf flow fields. Frequency-domain EOF results indicate that most of the geostrophic current (pressure gradient) variance is contained in the primary mode, which is characterized by an outer-shelf amplitude maximum and an approximate two-day across-shelf phase lag. A variety of statistical results shows that most of the geostrophic current (pressure gradient) variance in the 2–14-day band is highly coherent with and lass alongshelf wind stress by 0–2 days in WINTER. SUMMER geostrophic currents (pressure gradients) show less coherence and phase lag with the considerably less energetic winds but larger response per unit stress than in WINTER. Such a response is consistent with a locally wind-forced shelf flow in which friction is important.

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