Final Report to the Northeast Consortium
Drifter Study of a Front in the Maine Coastal Current System off Penobscot Bay, Maine
July-August 2005
Lead PI: Lewis S. Incze, Aquatic Systems Group, University of Southern Maine, 350 Commercial Street, Portland, ME 04101 ([email protected])
Co-PIs: Proctor Wells, Phippsburg, Maine 04562 ([email protected])
James Manning, NEFSC, NMFS, Woods Hole, MA 02543 ([email protected])
D. Brooks, Texas A & M University, College Station, TX 77843 ([email protected])
Final Report Submitted June 2008
Abstract
Inexpensive satellite-tracked GPS drifters were used during summer 2005 to examine the residual near-surface circulation of water inshore of the 100 m isobath on the coastal shelf between the Pemaquid Peninsula and Matinicus Island, Maine. This area is near the confluence of the Eastern and Western Maine Coastal Currents (EMCC and WMCC, respectively), but inshore of GoMOOS Buoy E, which is the nearest to our study area. Details of circulation inshore of the GoMOOS buoys are of interest for understanding the transport of plankton, including larval fish and invertebrates and harmful algal species, closer to the coastal environment. We deployed drifters at up to four sites between July 28 and August 2, 2005, and recovered the last drifters on August 5. Drift was generally southwestward and consistent within each cluster (=date of deployment), ranging from 209-216°T over the entire data set. There were 14 successful drifter deployments. Net speeds (calculated from deployment to recovery locations) ranged from 1.7-16.1 cm/s and varied from day to day, with an over-all mean of 7.4 cm/s and SD of 4.1 cm/s. Drifter speeds were similar to speeds at Buoy E on some days, but the average velocity of the drifters (average drogue depth = 3 m) was about half (56%) of that recorded at 2 m depth at Buoy E. Average transit time through the study area was about one week. Retrospective modeling shows that summer 2005 was a “flow-through” year, with a generally strong connection between eastern and western portions of the coastal current system. We compare the present results with 20 other drifters that passed through our study area in other years, mostly in 2004.
Background
The Maine Coastal Current system is comprised of two recognized segments commonly referred to as the Eastern and the Western Maine Coastal Current (EMCC and WMCC, respectively; Pettigrew et al. 2005). The former is characterized by its perennially cold surface temperatures, which remain relatively cold during summer months due to strong tidal mixing off eastern Maine and rapid westward transport as a plume (residual velocity of ~20 cm s-1 during summer measured at GoMOOS Buoy I, Fig. 1; Pettigrew et al. 1998, 2005). In contrast, the WMCC is stratified and has a warm surface layer during summer months. At GoMOOS Buoy E during summer, the WMCC has speeds less than half that measured at Buoy I in the EMCC (both shown in Fig. 1; also see www.gomoos.org). The reduction in velocity may be due in part to divergence of some of the EMCC water away from the coast in the vicinity of Penobscot Bay (Pettigrew et al. 1998, 2005). The specific locations of the buoys may also affect these results, since they measure currents at only one place on the shelf. Recent drifters and modeling results indicate that much of the EMCC flow that starts to move offshore near the mouth of Penobscot Bay follows the coastal shelf edge (approximately along the100 m isobath) around Jeffreys Bank (Fig. 1B) and continues to flow southwestward along the shelf edge. Such topographic steering is not unexpected. What happens inside of this shelf-edge flow is not well known, however.
Off Penobscot Bay, where the EMCC and WMCC meet, there is a steep horizontal temperature gradient in the surface layer that often extends most of the way across the coastal shelf (as in Fig. 1). There are occasional flow-through “events” that can be seen in satellite SST data as cold water pushing westward through this region. It is now known that there can be substantial interannual differences in the flow regime. Xue et al. (2008) describe July and August of 2002, 2003 and 2004 as years where the continuity of flow from east to west was low, high and intermediate, respectively. Such differences have been seen in other years as well (Pettigrew et al. 2005).
One condition that could cause the steep horizontal temperature gradient is that the EMCC would turn offshore in the vicinity of Penobscot Bay (Fig. 2). In this case, water that is west of the front might have a comparatively slow westward movement, with some contribution from western Penobscot Bay. There also may be some subsidence of the EMCC below the WMCC in this area, but the evidence for this is circumstantial. Subsurface temperature and salinity values west of the front are similar to EMCC water and suggestive of this process (N. Pettigrew, Univ. Maine, unpubl. data, L. Incze, unpubl. data, and other sources), but there are no direct measurements of the subsurface flow to substantiate this. Convergence at the front could result in increased concentrations of buoyant or upward swimming organisms in the frontal region. Past observations have shown high concentrations of lobster (Homarus americanus) postlarvae west of the front, a pattern that could be explained by either of the above flow conditions, and part of the motivation for the present study was to understand the processes that could account for these distributions.
There are two important implications for the situation where flow is not continuous along the shore: (1) plankton and other material west of the front could have relatively long residence times in that region; and (2) much of the plankton and material carried in the EMCC would be carried around this region (along the outer edge of the shelf) and transported southwestward. Farther to the west, in the vicinity of Casco Bay, the 100 m isobath comes closer to the shore. Drifters (Fig 3) and model results (Xue et al. 2008) suggest it is more likely that across-shelf forcing could bring material into the coast in this region, rather than the mid-coast.
The detail of what happens at the EMCC/WMCC front is essential for understanding along-shelf, east-to-west, connectivity of plankton, nutrients and contaminants. There are two large programs now collaborating in this region (MERHAB and Lobster SYNTHESIS) that are asking many of the same questions with respect to transport through this region, albeit for different organisms (see, for example, Keafer et al. 2005, Incze et al. 2006). Drifters used in both programs have concentrated on the outer part of the shelf for numerous practical reasons. Most prominent among these has been the need to understand and satisfactorily model the large-scale coastal circulation first. The coastal models do not currently resolve the frontal dynamics well, and the best data for model validation come from the GoMOOS buoys located on the outer coastal shelf in about 80+ m of water. A high-resolution model of Penobscot Bay by Huijie Xue (University of Maine; Xue et al. 2000) shows the front and the offshore transport associated with it, but the model does not cover a large area at this resolution (Fig. 2) and is a research model, not an operational (regularly available) product. A second factor that has steered the majority of early studies to the outer portion of the shelf is that drifters deployed closer to shore often get caught in obstacles such as ledges, islands, headlands and lobster gear. A large vessel cannot tend to these occurrences. A final reason for this study is that previous studies have taken place mostly in late May-early June. The timing has been dictated in part by ship schedules and the timing of algal blooms (the subject of many studies), but also by the need to avoid fixed lobster gear which is very abundant from mid June through September. For lobster larvae and postlarvae, July through early September is the time that has to be studied. We know from work we conducted from a UNOLS ship in August 2002 (R/V Weatherbird II) that it can be difficult to maneuver through the area at that time of year.
Objectives and Methods
Our objective was to use relatively inexpensive drifter technology and a small vessel to deploy, monitor and recover drifters in the region of the front during summer, concentrating on the area inshore of Monhegan Island (Figs. 4 and 5). We used Poctor Wells’ fishing vessel Tenacious to conduct hydrographic surveys, deploy drifters, monitor their progress for 2-3 days (using cell phone contact with a land station receiving the GPS data), and recover and redeploy drifters as needed. The Gulf of Maine Lobster Foundation helped to notify lobstermen about the project. Some of the drifters malfunctioned, and lobstermen contacted us when they picked them up.
We used a modified Davis-type drifter (Davis 1985) that had been strengthened to survive more frequent handling from small boats (see design changes and discussions at http://www.nefsc.noaa.gov/epd/ocean/MainPage/lob/driftdesign.html). The drifter consists of a vertical PVC cylinder a little over 1 meter long that housed electronics for a GPS satellite transmitter, antenna and strobe light (Fig. 6). The cylinder was equipped with a band of floats, and buoyancy was adjusted by placing weights on the bottom hoop of the drogues. The GPS units were inexpensive devices used to monitor delivery trucks, and they were selected due to the low cost of the units and the reporting service. This allowed us to build a number of drifters at a reasonable cost, but performance was compromised (see below). In the traditional Davis drifter, there are four arms protruding outward from the cylinder that hold flotation and support four vanes made of durable but flexible (usually vinyl) material extending 1 m deep. Instead of this shallow drogue, we suspended a 3-m long “holey sock”–type drogue on a bridle 1 m below the PVC housing, giving a center of effort approximately 3 m below the surface (1/2 the PVC pipe + 1 m bridle + ½ the 3 m drogue). The sock was made of a durable synthetic screen cloth held open by 1 m diameter aluminum hoops The drifters were assembled by students at Southern Maine Technical (now Community) College in South Portland in a collaboration between Professor Tom Long of SMCC and Jim Manning.
Our general expectations were that: (1) drifters deployed between Matinicus and Metinic Islands (drop site A in Fig. 5) would tend to move offshore and pass outside (south) of Monhegan Island; (2) drifters deployed northeast of Monhegan Island (drop sites B and C) would tend to drift toward the island or toward the west; (3) drifters deployed between Monhegan Island and the Pemaquid Peninsula (site D) would drift more slowly than the others and could move in any direction, although a slow westward net movement would be the most likely; and (4) net westward flow inside of Monhegan (Sites B, C and D) would be substantially less than at GoMOOS Buoy E.
We attached VEMCO minilogger temperature recorders at the bottom of each drifter housing (1 m depth) and on the bottom hoop (5 m depth). Temperatures were recorded every 30 minutes. Our objective was to use water temperature to help us understand the movements of drifters with respect to local ocean currents, given the strong horizontal thermal gradients in the region. By comparison, satellite data provide coarser resolution (ca. 4 km), are available only a few times a day, and are not available in cloudy or foggy weather. Thermistors were connected to the GPS transmitters in the initial design. This set-up was discontinued on most units due to very small leaks that nonetheless proved fatal to the GPS circuit boards.
Results and Discussion
We selected four deployment sites, A-D, to evaluate differences in trajectories from what we considered would be two different sides of the EMCC/WMCC front. We had foggy weather during the initial deployments and could not be guided by satellite imagery, and we had limited capacity to deploy and service drifters that were much farther apart than this. We had difficulties with drifters in the first set of deployments leaking in the relatively rough weather (drifter housings were submerged by the crest of most of the passing waves). On inspection, we concluded that the weak point in the electronics housing was the bulkhead connectors, which allowed very small amounts of water (usually just drops) to enter the housing, but these were enough to cause malfunction if they landed in the wrong place inside. We replaced the PVC pipe with straight units (no bulkhead pass-throughs) and had much better results. A few of the drifters were nonetheless lost due to failing GPS units. We recycled the working drifters on 1-4 d rotations (Table 1) depending on weather, their transit toward the boundaries of the study area, and other factors. We were not able to get GPS fixes very often, and this added to the time required to locate and collect them, especially in foggy weather.
We were able to complete four clusters of drifter deployments (on 28 and 30 July, 1 and 3 August: see Table 1 and Fig. 5). The direction of drift was generally southwestward and very consistent within each cluster, ranging from 209-216°T over the entire data set (n=14 successful drifter deployments). Net speeds (calculated from deployment to recovery locations) ranged from 1.7-16.1 cm/s and varied from day to day. Because we could not de-tide the data (positions were not reported often enough and the data set is small), the longer deployments may be better averages (the actual times are given in Table 1, from which the tidal bias can be estimated).
There was no regular spatial pattern of higher and lower speeds, so we averaged over all drifters to derive a mean speed of 7.4 cm/s (SD = 4.1 cm/s, n=14). Cluster 1 showed speeds from 5.3-8.8 cm/s, substantially slower than the mean daily flows of 11.2-18.1 cm/s at GoMOOS Buoy E during the same time. In contrast, speeds > 10 cm/s occurred at one or two of the deployment sites in clusters 2 and 3, and the higher drifter velocities were similar to the higher daily average speeds at Buoy E during those time periods. Buoy E daily values are given in Table 2, from which it can be seen that the daily averages can vary by a factor of two, but were generally > 10 cm/s during this study (mean +/- 1 SD = 13.3+/-2.8 cm/s). Over-all, the average velocity of our inshore drifters (average drogue depth = 3 m) was about half (56%) of that recorded at 2 m depth at Buoy E. Average drifter directions were slightly offshore compared with Buoy E (209-216 °T vs. 233-272 °T, respectively). This is probably due to topographic steering of the currents at Buoy E (see Fig. 5). Buoy E is on the southern side of Jeffreys Bank where the isobaths curve back toward shore.
We compared our drifter results with drifters from other studies that drifted through our study area (Figs. 7 and 8 and Table 3). Note that these had deeper drogues, centered at 7.5 m. Most of these drifters were from 2004, and most were earlier in the season. Drifters labeled with trajectories A-D in Figure 7 all had east-west passages inside Monhegan Island and are therefore most comparable with our study in terms of location, but not timing. They had much higher average speeds (18-27 cm/s), but were in the month of May. Drifters passing through the outer part of our study area in July or August of 2004 had net speeds similar to the ones we observed or slower, due to complicated trajectories that resulted in small net movements (see, for example, O, M and P in Fig. 8).
Retrospective modeling results (GoMOOS circulation model) now show that summer 2005 can be characterized as a “flow-through” year (H. Xue, pers. Comm.., similar to results shown by Xue et al. 2008). The few drifters that entered our study area in summer 2004 showed similar or slower speeds than we observed, in a year that the GoMOOS ocean circulation model characterized as “intermediate” flow-through. Thus, there is agreement between the drifter and model results with respect to the relative flow through this region in these two years.
The temperature and salinity data from the along-shelf CTD transects on 28 July and 2 August show a modest gradient from east to west which is a little stronger on the latter date. Over all, they indicate a zone of gradual transition from eastern to western coastal characteristics similar to the larger scale AVHRR image shown in Figure 9. This is consistent with the similarity of drifter trajectories from the various deployment sites. Other CTD data from our cruises (Fig. 10) are included in our data delivery but do not add much to our analysis of drifter movements and are not presented or discussed in this report.
Temperature data from the thermistors placed on the drifters are shown in Figure 11. Temperatures at 5 m showed much higher frequency variation and generally a greater amplitude (up to 2 °C) than at 1 m. A likely source of the temperature variations at 5 m is the passage of internal waves, but we stress that such waves are not resolved by the 30- minute recording interval. Variations in drifter temperatures at 1 m depth show a generally positive correspondence with the 1 m temperature record at Buoy E (Fig. 12). The temperatures do not appear to be primarily wind-driven, but may have been partially driven by air temperature
Summary
There was less difference than we expected in the drifter directions at A vs. B and C: two of the drifters from “A” headed offshore as we expected, but two were headed toward a path inside of Monhegan Island. Inside of Monhegan (drifters deployed at “D” or drifters that passed that way), net westward velocities were similar to contemporaneous deployments at the other sites, contrary to our expectations. Over-all, our data indicate a general continuity of flow from east to west over the inner shelf. Indeed, we now know through retrospective modelling that summer 2005 was a “flow-through” period, unlike the conditions in 2001 that stimulated our observational design. Drifter velocities on the inner shelf in our study were appreciably slower than at Buoy E (56%), conforming to our expectations of slower residual transport inshore of the 100 m isobath. Average transit times through the area were on the order of one week from Matinicus Island to the Pemaquid Peninsula, less than the time it takes lobster larvae to develop. The few drifters that entered our study area in summer 2004 showed similar or slower speeds, in a year that the GoMOOS ocean circulation model characterized as “intermediate” flow- through, so there is generally good agreement between the drifter and model results with respect to the relative flow through this region in these two years. While we did not measure spatial variations in flow in a “blocked” year as we had originally hoped, the documentation of flow speeds over the coastal shelf is a valuable addition to our understanding and modelling of plankton transport along the coast.
Acknowledgements
We thank Nicholas Wolff for data processing, Tom Long and his students at Southern Maine Community College for helping us design and build the drifters, and the Northeast Consortium for funding support.
Literature Cited
Davis, R. 1985. Drifter Observations of Coastal Surface Currents During CODE: The Method and Descriptive View. J.Geophys. Res. 90: 4741-4755.
Incze, L. S., R. A. Wahle, N. Wolff, C. Wilson, R. Steneck, E. Annis, P. Lawton, H. Xue and Y. Chen. 2006. Early life history of lobster (Homarus americanus) populations in the Gulf of Maine. J. Crustacean Biol. 26: 555-564.
Keafer, B.A., J.H. Churchill, D.J. McGillicuddy, D.M. Anderson. 2005. Bloom development and transport of toxic Alexandrium fundyense populations within a coastal plume in the Gulf of Maine. Deep-Sea Res. II 52: 2674-2697.
Pettigrew, N. R, D. W. Townsend, H. Xue, J. P. Wallinga, P. J. Brickley, and R. D. Hetland. 1998. Observations of the Eastern Maine Coastal Current and its Offshore Extensions. J. Geophys. Res. 103, 30623-30640.
Pettigrew, N. R., J. H. Churchill, C. D. Janzen, L. Mangum, R. P. Signell, A. Thomas, D. W. Townsend, J. P. Wallinga, and H. Xue. 2005. The kinematic and hydrographic structure of the Gulf of Maine Coastal Current. Deep Sea Res. II. 52: 2369-2391.
Xue, H., F. Chai, and N. R. Pettigrew. 2000. A model study of seasonal circulation in the Gulf of Maine. J. Phys. Oceanogr., 30, 1111-1135.
Xue. H., L. Shi, S. Cousins, and N. R. Pettigrew. 2005. The GoMOOS nowcast/forecast system. Cont. Shelf Res., 25, 2122-2146.
Xue, H., L.S. Incze, D. Xu, N. Wolff and N. Pettigrew. 2008. Connectivity of lobster populations in the coastal Gulf of Maine. Part I: Circulation and larval transport potential. Ecol. Model. 210: 193-211.
Table 1. Drifter deployment and recovery schedule from this study, working drifters only, with calculated net speed and direction of drift. The three columns at right provide observations from GoMOOS Buoy E at 2 m depth for dates corresponding to the drifter measurements.
Daily Net Release Time of Net Speed Daily Release Time Pickup Last Fix Duration Duration Net Speed Direction Buoy E (2m) (cm/s; Mean Dir Drop Site Drifter ID Date (Local) Date (Local) (Days) (Hours) (cm/s) (T) Dates range) T (range)
Cluster 1 A 573923 28-Jul 8:05 1-Aug 4:00 3.83 91.92 5.3 210 B 573931 28-Jul 9:58 31-Jul 14:01 3.17 76.03 8.8 210 July 28-31 11.2-18.1 233-269 C 573932 28-Jul 9:24 31-Jul 13:20 3.16 75.92 5.2 210 D 573922 28-Jul 8:54 29-Jul 1:38 0.70 16.73 8.3 209 Cluster 2 A 573933 30-Jul 7:16 3-Aug 9:50 4.11 98.55 3.8 213 B 573924 30-Jul 8:18 2-Aug 11:11 3.12 74.88 8.5 212 July 30-Aug 2 7.4-11.2 233-272 C 573934 30-Jul 8:43 2-Aug 12:00 3.14 75.28 10.1 212 D 573921 30-Jul 9:11 31-Jul 13:11 1.17 28.00 13.5 211 Cluster 3 A 583932 1-Aug 7:09 2-Aug 12:40 1.23 29.52 16.1 213 B 583921 1-Aug 8:38 3-Aug 9:58 2.06 49.33 3.3 214 Aug 1-2 7.4-10.4 233-272 C 583922 1-Aug 8:08 2-Aug 12:15 1.17 28.12 5.7 213 Cluster 4 A 583934 3-Aug 10:24 5-Aug 6:31 1.84 44.12 9.9 216 C 583924 3-Aug 8:43 5-Aug 9:11 2.02 48.47 1.7 216 Aug 3-4 12.0-14.2 256-267 Extra B 583942 2-Aug 15:44 5-Aug 8:49 2.71 65.07 3.2 215 Aug 3-4 12.0-14.2 256-267
Table 2. Daily average speed and direction of currents at GoMOOS Buoy E during this study, 28 July-8 August 2005. Data were not de-tided.
Speed Direction Month Day cm/s T 72818.1 247 72913.2 238 73011.2 263 73114.9 269 81 7.4 233 8210.4 272 8312.0 256 8414.2 267 85 5.3 262 8617.4 259 87 8.7 225 88 6.8 231 Table 3. Statistics for Drifters from other studies that entered our study area (Trajectories are shown in Figs. 7 and 8). Transit times are given only for drifters completing an east- west transit of the area. *
Net Net Trajec- Time in Transit Speed Direction tory Drifter ID Project Month Year Area (d) Time (d) (cm/s) T A 45380 merhab 5 2004 1.97 1.97 18.37 254 B 453813 merhab 5 2004 1.29 1.29 27.96 265 C 66381 whcohh 6 2006 1.40 1.40 25.87 265 D 45382 merhab 5 2004 1.30 1.30 27.56 263 E 453814 merhab 5 2004 1.63 1.63 22.64 270 F 14475 ecohab 5 2001 3.45 3.45 10.12 265 G 453819 merhab 6 2004 1.83 1.83 19.73 264 H 55381 merhab 5 2005 0.97 34.02 248 I 47382 emolt 7 2004 1.71 11.88 225 J 45383 merhab 5 2004 2.22 12.13 255 K 45386 merhab 5 2004 2.16 10.30 261 L 453812 merhab 5 2004 0.42 13.04 263 M 48382 emolt 8 2004 3.18 2.43 001 N 45381 merhab 5 2004 0.42 21.76 270 O 66382 whcohh 6 2006 2.73 4.01 238 P 49382 emolt 9 2004 3.74 1.00 260 Q 45387 merhab 7 2004 1.38 12.55 279 R 453815 merhab 5 2004 1.70 19.23 279 S 48472 emolt 8 2004 0.58 3.52 199 T 453816 merhab 5 2004 0.62 22.48 277
* Project data are from several cruises and sources: N. Pettigrew, D. McGillicuddy, J. Churchill, R. Signell, J. Manning and L. Incze Figure 1A. Satellite sea surface temperature for August 1, 2001 with GoMOOS buoy locations and outer coastal shelf flows in the vicinity of the study area.
45 20 44.8 Buoy E Penobscot Bay 44.6 Buoy I 18
44.4 Buoy J 16 44.2
44 14
43.8 12 43.6
43.4 10 43.2
43 8 -70 -69.5 -69 -68.5 -68 -67.5 -67 -66.5 -66
Figure 1B. Detail view of the study area (blue ellipse), showing bathymetric features and place names.
Pemaquid Georges Islands Vinalhaven Peninsula Metinic Is. Matinicus Is.
m 0 Buoy E Monhegan 15 Is. 150 m Jeffreys Bank N Figure 2. July 2001 temperature and velocity at 5m from a high resolution (400 m) model of Penobscot Bay and the surrounding area (Dr. Huijie Xue, University of Maine). Matinicus Island is south of Vinalhaven and just outside the image; Metinic and Monhegan islands are also outside and to the west and southwest, respectively (cf. Fig. 1B).
25.0 cm/ s
Position of EMCC / WMCC Front Figure 3. Drifter Trajectories, drogued at 15 m, from Isle au Haut transect beginning 1 June, 2003. (Oceanus Cruise 391, EcOHAB, D. McGillicuddy, J. Manning and L. Incze). Note the much shorter trajectories and near-shore endpoints of the drifters launched inside the 100 m isobath (three drifters in the white shaded area) off Isle au Haut. The straight section at the end of drifter #89 (green line terminating in Friendship, Muscongus Bay, inside the study area) was aboard a lobster boat. The blue ellipse shows the area covered by the present study (cf. Fig. 1). Fig. 4. Satellite Sea Surface Temperature (SST) on 20 June 2005, prior to this study. The 50 m isobath is shown. Some of the offshore islands are named for reference to other figures; the gray areas surrounding these sites and other locations indicate unprocessed SST data. Metinic Island is in the warmer surface water west of the EMCC/WMCC front in this image.
Monhegan Matinicus Metinic Fig. 5. Drifter trajectories at four different times (clusters) in this study. Release sites (Table 1A) are designated by letters. The dashed line shows the CTD transect (station locations given in Table 1B). Drifter data are given in Table 2.
Pemaquid Georges Islands Metinic Is. Peninsula B A
D C
Buoy E Fig. 6. Various views of the drifters used in this study, on display with chief architect and production manager Jim Manning. Fig. 7. Drifters from other studies and time periods that entered the area from the east and exited through one of the boundaries . Drifters A-E passed north of Monhegan Island (similar to our drop sites), while drifters F and G passed to the south. These 7 drifters transited the region from East to West with a mean transit time of 1.8 ± 0.76 d. Drifters C, D, and H through K entered from the east but exited through the southern boundary. Data are given in Table 3.
Drifters Entering from the East
Metinic Is. Georges Islands Pemaquid Pen. A
B C D H I Monhegan Is. E J F
G K Fig. 8. Other historical drifters not shown in Fig. 7. Labels are placed near the beginning of the drifter paths. Data are given in Table 3.
Other Drifters
Metinic Is. Georges Islands Pemaquid Pen. L N
O M Monhegan Is.
P S R Q T Fig. 9. Temperature and salinity sections for 28 July and 2 August along the hydrographic transect 1 shown in Fig. 5. Station locations are given in Table 1B.
July 28, 2005 August 2, 2005 0 0 14 14
-20 12 -20 12
10 10 -40 -40 Depth (m) Depth Depth (m) Depth 8 8
-60 6 -60 6 Temperature °C -69.4 -69.2 -69 -69.4 -69.2 -69 JulyLongitude 28, 2005 AugustLongitude 2, 2005
0 32 0 32
31.5 31.5 -20 -20 31 31 -40 -40
Depth (m) Depth 30.5 (m) Depth 30.5 Salinity (psu)
-60 30 -60 30 -69.4 -69.2 -69 -69.4 -69.2 -69 Longitude Longitude Fig. 10. Locations of CTD transects (numbered) and casts from this study. Positions and times are given in Table 3. Sea surface temperature data are from AVHRR, 7/30/05 at 21:47 GMT, courtesy of A. Thomas, University of Maine.
. n e P
id 5 u q a m 3 4 e 2 P 7 1
6 Matinicus Is. Monhegan Is. Fig. 11 A-C. Temperature records from VEMCO Miniloggers attached to drifters at 1m and 5m below the surface. Data were recorded every 30 minutes. Figures are grouped by Cluster release and sorted by Drop Site. Drifter ID is also shown (see Table 1 for details, and Fig. 5 for trajectories). The bottom panel in each figure shows hourly wind speeds from Buoy E. X-axes show local time with dates labeled at the beginning of the day. Fig. 11A
Drifter 573933, Cluster 2, Drop Site A 18
C) 1m o 16 5m 14
12
Temperature ( Temperature 10 7/31 8/1 8/2 8/3 Drifter 573934, Cluster 2, Drop Site C 18
C) 1m o 16 5m 14
12
Temperature ( Temperature 10 7/31 8/1 8/2 8/3
Drifter 573921, Cluster 2, Drop Site D 18
C) 1m o 16 5m 14
12
Temperature ( Temperature 10 7/31 8/1 8/2 8/3 Hourly Wind Speed, Buoy E
6
4
2
Wind Speed (m/s) 0 7/31 8/1 8/2 8/3 2005 Fig. 11B
Drifter 583921, Cluster 3, Drop Site B 18 C) o 1m 5m 16 14 12
Temperature ( Temperature 10 8/1 noon 8/2 noon 8/3 noon
Hourly Wind Speed, Buoy E
6
4
2
Wind Speed (m/s) 0 8/1 noon 8/2 noon 8/3 noon 2005 Fig. 11C
Drifter 583934, Cluster 4, Drop Site A 18 C) o 1m 16 5m 14 12
Temperature ( Temperature 10 8/3 noon 8/4 noon 8/5 noon
Drifter 583924, Cluster 4, Drop Site C 18 C) o 16 14 12
Temperature ( Temperature 10 8/3 noon 8/4 noon 8/5 noon
Hourly Wind Speed, Buoy E
6
4
2
Wind Speed (m/s) 0 8/3 noon 8/4 noon 8/5 noon 2005 Fig. 12. Hourly wind speed, air temperature and surface water temperature (2 m) at Buoy E are shown in the first three panels; drifter surface temperatures (1m; sampled every 30 minutes) are in the bottom panel. Fig. 12 Buoy E Wind Speed
6
4
2
Wind Speed (m/s) 0 7/31 8/1 8/2 8/3 8/4 8/5 Buoy E Air Temperature 24 C) o 22 20 18 16
Temperature ( Temperature 14 7/31 8/1 8/2 8/3 8/4 8/5
Buoy E Water Temperature (1m) 20 C) o 18
16
14
Temperature ( Temperature 12 7/31 8/1 8/2 8/3 8/4 8/5 Drifter Water Temperature (1m) 20 C) o 18
16
14
Temperature ( Temperature 12 7/31 8/1 8/2 8/3 8/4 8/5 2005 c:2,d:A c:2,d:C c:2,d:D c:3,d:B c:4,d:A c:4,d:C