The kinematic and hydrographic structure of Maine Coastal Current

1. The kinematic and hydrographic structure of Maine Coastal Current Written by: Neal R. Pettigrew , James H. Churchill et al

2. Acronyms • GMCC: Gulf of Maine Coastal Current • EMCC: Eastern Maine Coastal Current • WMCC: Western Maine Coastal Current • GOM: Gulf of Maine • GOMOOS: Gulf of Maine Ocean Observation System (2001~present） • ECOHAB-GOM: Ecology and Oceanography of Harmful Algal Blooms-Gulf of Maine study • GOMRNRP: Maine Regional Marine Research program

3. 1. Introduction • Marginal Sea • Relatively Isolated; • Complex system (r=10km) • GMCC: EMCC&WMCC

4. 1 Introduction --Objective • the nature of the connection between EMCC&WMCC and the route variation of EMCC remains unsolved. • Most of former observations are sparse, large-scale surveys, and the detail observation is seldom made. • The GOM circulation’s importance to the biological productivity distribution. =>the interannual structure of EMCC frontal zone is linked to the strength of toxic events along Western GOM shore => the trajectories and pathway of HAB-laden waters of EMCC are thought to be the key element in the initiation of nearshore red tide outbreaks along both the eastern and western shores.

5. 2 Data and Methods • primarily from ECOHAB-GOM field programs (March- August 1998, March-August 2000, and June-August 2001) • Meteorological information was also garnered from several NOAA stations. • Ancillary data includes drifters and hydrographic surveys from 1994-1996 GOMRNRP and moored current measurements from GOMOOS from 2001-present. • monthly images of SST are retrieved from NOAA.

6. 3. Results and Discussions the GOM overall cyclonic circulation a pair of linked recirculated cyclonic gyres related to Jordan Basin and Geroges Basin time-scale of around 60 days high A. fundeyence routine • 3.1 The large-scale summer interior circulation April of 1994, 1995, 1996 with 5 drifters drogued at 40 meters along a line in the eastern GOM.

7. 3.1 The large-scale summer interior circulation show consistent pattern with the double gyre pattern suggested by the drifters the 34psu isohaline two dense slope water domes within the Jordan and Georges Basin separated by a low slope water topography

8. 3.1 The large-scale summer interior circulation • the two linked gyres • the shelf regions are not adequately sampled • may not present a circulation system subject to interannual and seasonal variability. • a paucity of observations in the western part tends to support the western weaker views in priority.

9. 3.2 1998 moored current measurements Five mooring with one east of Casco Bay and four in Penobscot Bay were deployed in the spring and summer of 1998.

10. 3.2 1998 moored current measurements in spring and summer, the southwestward current in the opposite direction to mean-wind stress are everywhere currents increase within the offshore direction a) sharp contrast in the strengths of the southwestward mean flows b) strong convergence alongshore requires an offshore recirculation of the currents to satisfy continuity. The low-pass filtered monthly mean currents from moorings

11. 3.3 1998 Hydrographic survey(June, July and August) GOM was significantly affected by A. fundeyence. near-surface waters of EMCC turn offshore east of Penobscot Bay undergoes abrupt transition to weak and less organized in WMCC west of Penobscot, causing dramatic discontinuity July and August are similar to June, but the offshore position is 10km west of Penobscot Bay.

12. 3.3 1998 Hydrographic survey the water properties of EMCC near-surface core are only restricted to Eastern Gulf of Maine. The hydrographic inventory of EMCC core water

13. 3.3 1998 Hydrographic survey freshened by local runoff, including streams, and non-point sources, and slowed by frictional effects. The inner boundary layer of EMCC near 50m isobath found in Western GOM-in the pycnocline of Wilkinson basin possible intrusion of eastern inner shelf water into subsurface to the western interior GOM.

14. 3.3 1998 Hydrographic survey it is also seen in WMCC actually driven by buoyant outflow of P.B River, the potential source for WMCC The July an August are similar. the inventory of river-freshened surface water of outer Penobscot Bay

15. 3.3 1998 Hydrographic survey • All of the result shows that during the summer of 1998, there is no direct A. fundeyence-laden EMCC beyond Penobscot bay. • waters that spread beyond P.B seem to subduct and might result in subsurface populations of toxic dinofalgellate as was observed.

16. 3.4 2000 moored current measurements for the spring and summer of 2000 6 current meters were deployed, 2 far from the frontal region and 4 on either side of the front.

17. 3.4 2000 moored current measurements • compared to 1998, the discrepancy between EMCC&WMCC dramatically reduced • alongshore convergence not evidence • obviates the need for offshore, suggesting much of EMCC continued on to feed WMCC.

18. 3.5 2000 hydrographic surveys In 2000 only two cruises are taken with sparse stations (one from later April to Early May, one through June). current looks weaker in EMCC than 1998 show few east-west differences GMCC is generally a continuous current without the characteristic offshore veering and along-shore discontinuity .

19. 3.5 2000 hydrographic surveys Slope waters in 2000 were significantly warmer, saltier and denser than 1998, and the depth of slope water in EMCC/WMCC is less contrast. The Penoboscot Bay outflow are found far as Cosco Bay, indicating tendency for the distribution subsurface from P.B westerwards.

20. 3.6 2001 Hydrographic survery shows intermediate case to 1998 and 2000 with a large portion offshore with leakage onto western shelf.

21. 3.7 Satellite SST anomalies (relative to 19yr mean) The WMCC/EMCC are warmer in 1998, colder in 2000, mixed in 2001. colder EMCC waters result in cold anomalies in the Western GOM(1998,2000) in 2001 both the intrusion of EMCC and offshore veering are visible

22. 3.8 Long-term current measurements east and west of Penobscot Bay Position of E,I The transport discrepancy between EMCC and WMCC is a highly feature every summer since 2001, current speed in EMCC exceeds WMCC 50%-75%（1998& 2000 summer present opposite extremes） offshore veering of EMCC arises in spring, persists throughout the summer, diminishes during the fall the importance of reverse flow in winter (eg. Feb. 2003) could have significant ecological implications.

23. 3.8 Long-term current measurements east and west of Penobscot Bay deeper alongshore convergence is out of phase with that of surface layers—The deep water veer offshore in the winter, but upper water column flows through toward the southwest (reason: seasonal along shore baroclinic pressure gradient)

24. 4. Conclusions A) Overall feature of GMCC: two linked cyclonic gyres B) GMCC is subject to strong interannual, seasonal, shorter term variability C) EMCC and WMCC connectivity variation(1998,2000,2001 summer) D) GOMOOS: typical summer is ‘ajar’ E) No direct throughflow of near-surface water from the core of EMCC to WMCC F) In winter, strong deep offshore veering of EMCC is out of phase with near-surface veering

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