Peter C. Smith, Robert W. Houghton, Richard G. Fairbanks, David G. Mountian

1. Interannual variability of boundary fluxes and water mass properties in the Gulf of Maine and on Georges Bank Peter C. Smith, Robert W. Houghton, Richard G. Fairbanks, David G. Mountian

2. 1. Introduction—Earlier Research Petrie and Drinkwater, 1993 Bigelow, 1927 fresh SSW, deep inflow of shelf water NECE; Local river runoff; heat fluxes The annual mean volumetric transport of SSW near Cape Sable in winter was nearly double the mean but in summer was near the zero (Smiths, 1983) the interannual variability of SSW was found influenced by atmospheric forcing and warm core rings offshore rather than St. Lawrence River runoff (Smiths, 1989b). The peak deep inflow of slope water through Northeast Channel (NEC) occurred in late summer (Ramp et al, 1985), while in the deep basin in GOM, the downward mixing of cold intermediate water caused a maxima in May-June followed by gradually increasing T,S through December due to the advection of NCE inflows.

3. 1 Introduction--Objective • to describe the interannual variability of boundary fluxes and water mass properties in GOM/GB on shorter terms of 1~3yr with reference to former observations, and to investigate the sources of this variability.

4. 2 Data and Methods • Long-term (October 1993~September 1996) moored measurements of current (surface, mid-depth, near bottom), temperature and salinity were made at Cape Sable (C2), Northeast Channel East (NECE) and Northeast Channel (NECW) sites. • The current measurements were resolved into along-isobath and cross-isobath components and were filtered to removed tides and other high-frequency signals. All variables were averaged into months and fit with a mean plus annual cycle. The local depth-integrated monthly mean transport functions for volume (q), fresh water (b) and temperature (f) are estimated using the monthly mean values of current (U), salinity (S) and temperature (T) in appendix. Missing data were interpolated from adjacent instruments with which correlations were typically high. Finally the transport were interpolated into shallow (<75m) and deep (>75m) parts to reveal differences between the layers.

5. 2 Data and Methods • Hydrographic data includes GLOBEC broadscale survey cruises (Feb~Jul.1995, Jan~June 1996 and 1997) and from surveys conducted by Northeast fisheries Sciences Center (NEFSC). For each survey, the average surface layer temperature and salinity were calculated in each of four regions representing the three major basins (Wilkinson, Jordan and Geoges) and Northeast quarter of Georges Bank. • Coincident isotope and salinity samples were collected at standard depths on all GLOBEC broadscale surveys • Freshwater data were collected from BIO environmental data, NOAA data archive and the REDIMS distributed data archive for the Gulf of Maine. Local origin data included Boston precipitation (NOAA), Maine River discharge (REDMIS), St. John River discharge (BIO) , RIVSUM (BIO), Gulf of St. Lawrence Ice cover (BIO), Labrador Shelf ice cover south of 55°N (BIO), and Station 27(47°33’N,52°35’N) salinities at 0 and 50m.

6. 2. Data and Methods—GLOBEC long term moorings

7. 2. Data and Methods—NESFC stations/Oxygen Isotope

8. 3 Results—Annal cycle/NECEAxial an cross-channel currents surface opposed to bottom. the different governing processes of shallow and deep flows  There is significant annual cycle in NECE cross-channel current Early Spring Surface cross Surface Axial Late Summer, similar Bottom Axial Bottom Cross

9. 3 Results—Ｃ2 and NECE volumetric and freshwater transports C2 surface peak inflow peaks in early winter (not shown) Early Winter May~June , different C2 volume NECE volume April, even earlier Early Winter NECE freshwater C2 freshwater

10. 3 Results-- NECE surface/bottom volumetric and freshwater transports Surface volume consistent with V transport surface freshwater transport reflect the entire freshwater transport Apr~May Jul, consistent NECE surface volume NECE bottom volume NECE bottom freshwater NECE surface freshwater

11. 3 Results-- NECE deep layer(100m,150m,190m) inflow current, temperature, salinity • coherent • Obscured winter • Three events • a period of enhanced warm, salty inflow at the beginning (October 1993- September 1994) •  followed by a sharp reduction of the inflow and transition to cooler, fresher than normal conditions during the winter of 1994/1995 •  and a second period of greatly reduced inflow and cold, fresh conditions during the first nine months of 1996. Inflow current temperature salinity

12. 3 Results-- NECE deep layer(100m,150m,190m) T-S diagram 100m the water is a mixture of mid-depth SSW and Warm Slope Water (WSW) 150m/190m the mixtures include some Labrador Slope water (LSW) and Gulf Stream (GS). 100m (diamonds), 150m(squares) and 190m (triangles)

13. 3 Results-- The 5-month filtered transport C2/NECE shallow/Deep 6 • clearer three events near the bottom at NECE site •  an out-of-phase between deep NECE and C2 inflow transports • the NECE surface transport transits into reduced flow earlier in 1994, the 1996 reductions are weaker, and 1995 features are enhanced C2 Transport NECE shallow NECE bottom

14. 3 Results-- The 5-month filtered freshwater volumetric transport C2/NECE shallow/Deep • NECE In phase with volumetric in surface • Out of phase in bottom • transport salinates the Gulf in the deep water • ||Net transport trend are similar to surface transport/Fresh inflow C2 surface freshwater NECE surface freshwater NECE bottom freshwater

15. 3 Results– Water properties in GOM/NESFC Surface salinity anomalies • Positive only in 1994 for inner basin freshwater reduced • persist on NWGB until early 1995 • overall period the water is fresher during the 1990s

16. 3 Results– Water properties in GOM/NESFC Deep-layer salinities • Similar events are detected in the bottom water layers of the Gulf • most years decrease •  return-to-normal 1997 • Also found in BOF

17. 3 Results– Water properties/T-S diagram in GOM (NESFC) • fluctuation of water properties during the 1990s is mainly due to the variable mixing ratio between SSW and WSW rather than the effect from LSW as in 1960s’ cold events

18. 3 Results– Water properties/GLOBEC in NWGB • Similar salinity decrease from GLOBEC

19. 3.Results--Oxygen Isotope data • The freshwater invaded into Georges Bank is derived largely from Scotian Shelf Water • is devoid Maine Coastal Water (unclear) • Suggesting strong variations in mixing ratio of SSW and MCW

20. 4 Discussions—Warm Core effects(Oct.1993~Sep.1994) • Evidence1: Warm core observed from satellites Not a reliable indicator of enhanced inflow of slope water

21. 4 Discussions—Warm Core effects(Oct.1993~Sep.1994) • Evidence 2: • Contemporaneous hydrographic T/S sections • Warm core stream: 15°C，35.5psu • transport throughout the deep inflow and mixed vertically into surface

22. 4 Discussions- Box model • Using a simple box model driven by observed boundary fluxes •  in 1995~1996, the volumetric flow rate increased by 17% of total transport •  increased freshwater inflow at C2 and NECE induced a decrease of 0.73psu in salinity of the outflow.

23. 4 Discussion—source for 1995~1997 freshwater inflows? • Local runoff not exceptional • Nor is St. Lawrence River runoff • Ice-cover suggests northern origin • Newfoundland the most likely origin

24. 4 Discussions—Sources/ Salinity anomalies • Good correlation between  • Positive C2/NECE/GB/BOF Negative SJ/Nfd/Scotian Shelf/NECE/C2 • 8~9 month lag

25. 4 Discussions—Impact from climate aspect? • GSA in 1980s and 1990s are associated with high NAO index. • Icelandic low deepens and intensifies, producing strong, cold wind events •  along with possible contribution of freshwater through Canadian Archipelago • Lead to cold, fresh surface waters transporting around • Effect of cold 1960s Quasi-decadal fluctuation

26. 5 Conclusions • The annual cycles in NECE are found different for surface (with peak in spring) and deep inflow (with peak in later summer), which suggest different dynamic control factors for shallow and deep layers. In later winter there is a maximum near-surface cross-channel flow toward Georges Bank from NECE, which suggests a climatological “cross-over” tendency during that season. • There is no clear annual cycle in the salinity variation over most of the water column. • (3) The inflow rates from C2 and deep NECE are generally out of phase, with increase C2 inflow and associated reduced inflow at deep NECE (C2), while both sites show cooler and fresher conditions. Freshwater inflow into GOM is largely negative over the first half of observation period with maximum transport near the surface. The hydrographic data in GOM and GB shows consistency with timing of the freshwater inflows, with peak salinity in 1994 and declined salinity in 1995 and 1996.

27. Oxygen data suggest that in 1997 freshwater on central cap of GB is from Scotian Shelf in 1997 as opposed to 1994 and 1995, when Maine river water contributed 38% and 26%. (4) box model driven by observed boundary fluxes, the author finds in 1995~1996, the volumetric flow rate increased by 17% of total transport, and that the increased freshwater inflow at C2 and NECE induced a decrease of 0.73psu in salinity of the outflow. (5) The origin of 1996~1997 freshwater is proven in the northern Labrador Sea/Baffin Bay and results from exceptionally cold winters in the early 1990s. Analysis of similar event in the early 1980s suggests that the occurrence is part of quasi-decadal climate signal that follows NAO.

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