Oct 2006 Oct 2006 Jan 2007 Jan 2007 Apr 2007 Apr 2007 Apr 2007 Apr 2007 Apr 2007 Jun 2007 Jun 2007 Jul 2007 Jul 2007 Jul 2007 Jul 2007 Oct 2007

89

10

111213

141516

Oct 2007

31.532.032.533.033.534.034.535.035.5

0 200100

0

200

100

Distance (km)

Dep

th (m

)

Tem

pera

ture

Salin

ity

Above: The standard glider sensor package included a Sea-Bird CTD, Wet Labs bb2f chlorophyll fluorom-eter and red/blue optical backscatter sensor, and a custom WHOI PAR (photosynthetically active radia-tion) sensor. Several vehicles also flew with prototype sensors including a Nortek 1 MHz ADCP, a biolumines-cence bathyphotometer, a novel fast-response CTD and a low-frequency passive sonar.

Right: The transect connects the shallow (20 m) Martha’s Vineyard Coastal Observatory with Line W, a deep moored array spanning the Gulf Stream and Deep Western Boundary Current.

Below: Seventeen cross-shelf temperature and salinity sections obtained between October 2006 and October 2007. Note that each 200 km section corresponds to approximately 7 days of transit time. Most sections show evidence of the shelfbreak front, a ubiquitous hydrographic feature of the continental shelfbreak at which cool, fresh shelf water meets the warm and saline water of the continental slope.

| | | | | | || | | | | |

0 20 40 60 80 100 120 140 160 180 20060

80

100

120

140

160

180

20021 22 23 24 25 26 27

0 20 40 60 80 100 120 140 160 180 20031.5

32.0

32.5

33.0

33.5

34.0

34.5

35.0

35.5| | | | | | || | | | | |

21 22 23 24 25 26 27

0 20 40 60 80 100 120 140 160 180 20022.5

23.0

23.5

24.0

24.5

25.0

25.5

26.0

26.5

27.0

27.5| | | | | | || | | | | |

21 22 23 24 25 26 27| | | | | | || | | | | |0

50

100

150

200

Dep

th (m

)

0 20 40 60 80 100 120 140 160 180 2008

9

10

11

12

13

14

15

1621 22 23 24 25 26 27

0 20 40 60 80 100 120 140 160 180 20060

80

100

120

140

160

180

200| | | | | | | || | | | | | |

13 14 15 16 17 18 19 20

0 20 40 60 80 100 120 140 160 180 20031.5

32.0

32.5

33.0

33.5

34.0

34.5

35.0

35.5| | | | | | | || | | | | | |

13 14 15 16 17 18 19 20

0 20 40 60 80 100 120 140 160 180 20022.5

23.0

23.5

24.0

24.5

25.0

25.5

26.0

26.5

27.0

27.5| | | | | | | || | | | | | |

13 14 15 16 17 18 19 200

50

100

150

200

Dep

th (m

)

0 20 40 60 80 100 120 140 160 180 200

Distance (km)

8

9

10

11

12

13

14

15

16| | | | | | | || | | | | | |13 14 15 16 17 18 19 20

FluorescenceSalinity DensityTemperature

FluorescenceSalinity DensityTemperatureSum

mer

Win

ter

seasonal changesA major component of the along-shelf flow and a critical regulator of cross-shelf transport is the shelfbreak front, visible in both seasons near the 100 m isobath. On the inner shelf temperature stratification changes dramatically from summer to winter. Surface heating in early summer results in the formation of a seasonal pycnocline over the entire shelf and caps the cold, fresh, well-mixed winter water on the mid-shelf. Fresh surface water on the inner shelf originates in the Gulf of Maine. The subsurface maximum in chlorophyll fluorescence is elevated in summer and coincident with the seasonal pycnocline.

velocity and transportThe along- and cross-shelf fluxes of salt, heat, and nutrients in this region are highly variable and poorly understood. One objective of this study is to investigate the utility of sustained glider measurements for estimating these fluxes. Glider-based transport calculations are complicated by strong tidal velocities and intense, episodic meteorological forcing, both of which are inadequately resolved by the slow-moving gliders.

Previous studies have shown that the shelf/slope flow in this region is predominantly baroclinic. Over the shelf the flow is more barotropic and hence the reference velocity inferred from the glider is more important. We have calculated the cross-shelf baroclinic velocity structure for a section occupied in October 2007. The geostrophic velocity field contains weak surface intensified flow very near shore, a broad region of eastward flow over the middle shelf, and a surface intensified westward jet near the shelf edge. The westward transport across the section is roughly 0.25 Sv with the majority centered at the shelfbreak. This is reasonable given that the glider sampled only a portion of the current.

A more accurate estimate of the velocity structure and resultant transport will be possible if we can accurately predict and remove the tidal component of the depth-averaged reference velocity. A method that uses historical current meter observations to remove the tides from vessel mounted ADCP records has been used successfully in this region. We hope to adapt this method to support our analysis of the glider data.

Above: In order to minimize the small scale cross-shelf density gradients that result from tides and local wind forcing, the density field (a) is filtered with a 10-km low-pass second-order Butterworth filter. Geostrophic velocity is calculated from the smoothed density section and referenced to zero at the bottom (b). Depth-averaged horizontal velocity (c) is estimated from the glider navigation data. A 50-km wide boxcar filter (roughly 60-hr low-pass) is then applied in order to minimize tidal fluctuations. Finally, the absolute geostrophic velocity (d, e) is computed by referencing the vertically averaged geostrophic velocity to the glider-measured slab velocity.

Left: Transport as a function of distance offshore (f) and depth (g) computed from the velocity section in (e) above.

Johns Hopkins University Applied Physics Laboratory

Gulf of Maine

Gulf Stream

71˚W 70˚W 69˚W 68˚W 67˚W 66˚W

40˚N

41˚N

42˚N

43˚N

44˚N

Georges Bank

Gulf of Maine

AtlanticOcean

Boston

Portland

Line W Moored Array

Martha’s Vineyard Coastal Observatory

44008

summaryThe stratification of the continental shelf is critical to a variety of physical and biological processes and is highly sensitive to both advective and atmospheric forcing. A thorough understanding of the mechanisms responsible for the evolution of the shelf stratification on storm-to-seasonal timescales requires persistent high-resolution measurements over a large spatial footprint.

Previous work, both moored and ship-based, has indicated the need for both highly-resolved spatial measurements (to describe features on scales of 1-10 km) and a sustained at-sea presence (to capture episodic events and facilitate robust statistical inferences). Ship-based measurements on the continental shelf are generally expensive and weather dependent. Moored observations generally lack spatial resolution.

Autonomous vehicles, when used appropriately relative to their inherent capabilities, can provide sustained, low-cost, measurements of key physical and biological variables. However, There are logistical impediments to using slow, relatively fragile autonomous platforms in a demanding operational setting such as the New England shelf. These include strong tidal flows, frequent and intense meteorological events, and vigorous fishing activity. The goal of this program is to ask “How can these novel assets be most effectively employed in a coastal observing system?”

Since October 2006 we have used several gliders to occupy a 200 km transect spanning the New England shelf from the nearshore Martha’s Vineyard Coastal Observatory (MVCO) to the Line W moored array on the continental slope. We present here an initial synthesis of nearly 20 independent realizations of this section, consider seasonal variations in stratification, and evaluate our ability to compute along-shelf baroclinic transport.

Left: Seasonal changes in stratification are evident in sections from (top) July 2007 and (bottom) January 2007.

Below: A pronounced annual cycle is evident in air and sea temperature from NDBC buoy 44008 located near the midpoint of the glider section.

Jan06 Apr06 Jul06 Oct06 Jan07 Apr07 Jul07 Oct07 Jan08−10

−5

0

5

10

15

20

25

30

Sea TemperatureAir Temperature

Tem

pera

ture

Air/Sea Temperature at NDBC 44008

20 40 60 80 100 120 140 160 180-20

-15

-10

-5

0

5

10

15

200

50

100

150

200

Dep

th (m

)

0 20 40 60 80 100 120 140 160 180 200

Distance (km)

22.5

23.0

23.5

24.0

24.5

25.0

25.5

26.0

26.5

27.0

27.5

20 cm/s

20 cm/s

20 40 60 80 100 120 140 160 180-20

-15

-10

-5

0

5

10

15

20

20 40 60 80 100 120 140 160 18031.5

32.0

32.5

33.0

33.5

34.0

34.5

35.0

35.5

a dcb e

sigma-0geostrophicvelocity ref tobottom (cm/s)

geostrophicvelocity (cm/s)

geostrophicvelocity (cm/s)and salinity

East

West

East

West

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

Tran

spor

t per

uni

t wid

th (S

v)

0 100 200

Distance (km)

West

East

f

0

100

200

Dep

th (m

)

-0.01 0.00 0.01

Transport per unit depth (Sv)

West Eastg

Exploring the feasibility of glider-based transport, stratification, and ecology measurements on the New England shelf

John M. Lund, Paula Sue Fratantoni, Benjamin A. Hodges, and David M. Fratantoni

Autonomous Systems Laboratory, Physical Oceanography DepartmentWoods Hole Oceanographic Institution, Woods Hole, MA 02543

dfratantoni@whoi.edu

Acknowledgements: This work was supported by the Woods Hole Oceanographic Institution’s Coastal Ocean Institute and WHOI’s Access-to-the-Sea Program. We thank the captains of the research vessels Mytilus and Tioga for assistance with deployment and recovery operations. Vehicles used in this program were acquired with support from the Office of Naval Research. aSLaSLAutonomous Systems Laboratory

Woods Hole Oceanographic Institution