The Maine Coastal Current: Spring climatological circulation

Daniel R. Lynch, Monica J.Holbke and Christopher E.Naimie

1 Introduction

1.1 Gulf of Maine (GoM) 1.2 Maine coastal current (MCC)

1.1 Gulf of Maine--Position

Semi-enclosed

Fig1.Gulf of Maine Position(Google earth)

1.1 Gulf of Maine—general coastal sea phenomena

Westward current along its northern margin runoff, frontal structure

Cyclonic circulation in the interior Gulf

Throughflow from cape Sable, topography, seasonal dense slope water from Northeast Channel, baroclinic circulation

Cape Sable Island

Fig2.Gulf of Maine topography

1.2 Maine Coastal Current

three branch points Great South Channel

Penobscot, Cape Ann, Great South Channel

seven segments Eastern, Jordan, Western, Massachusetts, Stellwagen, Nantucket, and Georges Bank.

Fig3. Schematic of the Maine coastal current （ 24. ） .

Penoboscot

Cape Ann

Great South Channel

George Bank

1.3 Gulf of Maine& Maine Coastal Current—what’s unknown

The detail dynamics and interactions (local and regional winds, real topography) in the GoM is unexploited

Motivation--What’s the mean pattern and variation of Maine Coastal Current in spring season( high runoff along the coast( Frank et al,1992))?

Dynamical factors controlling the MCC?

2. Objective

Investigate the climatological structure and dynamics of MCC during spring

Compare the difference between two spring periods—March-April (freshwater runoff the primary, strong wind) vs. May-June (surface heating dominant, low wind)

Unravel the various dynamical influences contributing to the spring climatology—local( freshwater runoff, local wind, tidal rectification)

and remote (Gulf-wide wind response, throughflow from the Scotian Shelf, and basin-scale baroclinicity)

3. Tools/Procedure

3-D prognostic Finite Element Model

Domain: In order to compute Gulf-wide circulation in the two spring period, a mesh with broad geographic coverage is used, including the whole George Bank and the Scotian Shelf

a refined coastal mesh is introduced to examine the coastal region with increased dynamical and topographic detail using the focus period of March-April and May-June, providing resolution of order 1km in the shallow part of MA and Cape Cod Bays, and grows to roughly 3km in the deeper regions of MA Bay and to 5km in the Gulf of Maine, which facilities enhanced topographic realism.

Biologically initiative fields from various routines Hydrostatic free surface partially mixed vertically fully nonlinear

transporting momentum, heat, salt and two turbulent variables

The baroclinic and barotropic parts are resolved in tidal time. Mixing Scheme--Vertical : a level 2.5 turbulence closure scheme;

horizontal mixing : a mesh and shear-dependent eddy viscosity.

Resolution--horizontal : variable using unstructured meshes of triangles; vertical: discretization is nonuniform which allows tidal-time tracking of the free surface and proper resolution of surface and boundary layers.

Resolution

Fig4. Model mesh

1km,2km,5km

Shallow=> high resolution

Boundary given intuitively=>problematic

4. Results

Annual Climatological cycle March-April& May-June spring circulations Dynamical influences on the Maine coastal current

Significant bimonthly variations

Fig. Residual surface elevation (m) along the boundary for all six bimonthly periods

There is a significant barotropic pressure variation of order 5cm along the margin of the costal currentaccompanying detailed and variable interaction between in and the interior Gulf.

4.1 Results—Climatological Circle ///Black and White

supports the widely accepted view that Gulf of Maine cyclonic circulation is persistent, and Maine coastal current is the shoreward extent of GoM.

significant intra-annual variations of GoM The intensity and spatial structure of the inflow from the Scotian Shelf, the strength and spatial pattern of George Basin-Centered cyclonic gyre, smaller scale cyclonic and anti-cyclonic features, the two exit points at Great South and Northeast Channels and the anti-cyclonic recirculation over Georges

The Gulf-wide circulation is the simplest during November-December (ND)

from ND to July-August (JA) throughflow is reduced, and interior recirculation becomes localized over Georges Basin.

September-October (SO) appears to reflect a transition towards the conditions of ND

4.2 Results– Spring Season

The result well presents the coastal current in spring, confirming the existence of branch points of Penobscot Bay, east of Cape Ann, and at Great South Channel ( Black and White).

difference between March-April and May-June circulation. Both of the offshore transport at the Penobscot Bay at the Penobscot branch point and the along-shore transport at Cape Ann are strong, dominant features in March-April, but are weak in May-June.

<=combined recirculation Inner gyre in Jordan Basin Cyclonic gyre Proves the MCC segments: clear offshore@P

enoboscot Branches

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Anti-cyclonic from Saint John

Geo

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The circulation is different between MA-MJ spring period

The circulation is different between MA-MJ spring period—Drifters

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Details of the circulation patterns are givenDynamical factor are analyzed

Difficulty

4. Results--Physical influence testbed (MA)

Simulations of MA period as testbed with selected forcing ‘turned off’ are used to investigate the role of separate physical influences

the tide is computed alone with all other forces removed to identify the isolated influence of barotropic tidal rectification.

4.Results

Dynamical influences on MCC

Tides=> eastern branch

Wind =>small impact near-shore( upwelling downwelling, Ekman )

River => decrease high pressure anomaly near-shore

Baroclinic=>offshore, exchanges between pathways, unorganized exchange, exaggerated current, quiescent in western

Cape Ann

Penoboscot Bay

Barotropic boundary pressure=>mismatch ,near-shore intact within 100isobath, Penoboscot(moving shorward),rearrange pathways Baroclinicity and barotropic pressure must be well balanced

5.Conclusion

(1) large scale features of GoM circulation (2) the principal observed MCC features (3) a testing ground to help construct the modeling of MCC and to u

nderstand the dynamical influences impacted on the MCC

Thanks!