AMSR-E, November How do changes in river input and sea AMSR-E, April OS11D - 1164 ice affect the Hudson Strait outflow? AGU Fall Meeting, 2008 Dave Sutherland1 Fiamma Straneo1, Steve Lentz1, Stephen Déry2, Ken Drinkwater3 1 Woods Hole Oceanographic Institution ([email protected]), 2 University of Northern British Columbia, 3 Institute of Marine Research, Norway

1 Introduction 2 Location and Data The waters that flow out through Hudson Strait, a 100 km long, 400 km wide coastal system that connects Hudson Bay with A set of moorings was deployed in the outflow region of Hudson Strait from 2004-2006 and represents the first successful yearlong mooring records from the strait (Fig. 1). Each mooring the Labrador Sea, constitute the third largest freshwater contribution to the northern North Atlantic, behind only Fram was equipped with an upward looking ADCP, as well as upper and lower MicroCATs measuring temperature and salinity. In addition, the middle mooring (A in Fig. 1) had a McLane Moored Strait and Davis Strait. The majority of this freshwater comes from Hudson Bay, through an annual 940 km3/yr of river input Profiler (MMP) that collected temperature, salinity, CDOM, and optical backscatter data over the depth range ~40-170 meters. Meteorological data over the strait and in eastern Hudson Bay and a comparable amount of sea ice melt, with an additional portion coming from Davis Strait (Straneo and Saucier, 2008). were obtained from the six-hourly NCEP reanalysis fields. Over the last several decades, however, large interannual changes have been documented in Hudson Bay, including a de-

o crease in riverine input and a decrease in the length of seasonal sea ice coverage (Déry et al., 2005). 70 o N 63 N 20 30 Ba 0 30 f Foxe 0 fin How these interannual changes are reflected in the fresh Hudson Strait outflow is unknown. Recent observations have Baffin Is 0 Is. Figure 1. (a) Map illustrating how the cyclonic circulation in Hudson Bay 65 o Basin begun to shed light on the outflow’s structure on seasonal timescales, showing it to be a baroclinic buoyant coastal current, N . 30’ 100 connects through Hudson Strait to the Labrador Sea. In eastern Hudson Bay, Mansel Island (MI) and the Belcher Islands (BI) act as obstacles to the cy- with the highest velocities and lowest salinities in late fall and winter (Straneo and Saucier, 2008). Within this seasonal enve- o D 200 60 o 62 N A N MI Labrador C clonic boundary current. The star marks the location of the wind data used. lope, though, synoptic scale variability (on the order of 4-6 days) dominates the observed salinity and velocity records. Se 10 30 Latitude 20 0 0 30 a 30’ 0 (b) Zoom-in on Hudson Strait showing the mooring locations (black plus- 55 o Ungava Bay Thus, before examining how interannual trends in forcing affect the outflow, we must first understand what mechanisms 0 40 N 400 BI ses) and names (C, A, D) for the 2005-2006 deployment. The mooring array Quebec 0 cause the freshwater export to be concentrated to a series of discrete pulses. Since the freshwater outflow modulates how o 61 N spans about 30 km from the coast and encompasses the peak velocity of 50 o James Quebec high-stratification and high-nutrient water enters the northern North Atlantic, the fact that the outflow is confined to co- N Bay 100 the outflow current. Previous mooring deployments are also indicated, 94 o o herent eddy-like structures that preserve their properties for longer periods of time is important. The generation of these W 89 o W 30’ W o o 54 o o though they both lasted less than a full year (1986-87: orange, 1982: blue). 84 W o o o o W 75 o o o o o o low-salinity pulses appears to be related to the passage of storms across Hudson Bay that drives variability in the cyclonic 79 W 74 W 69 W 64 W 59 W 74 W 73 W 72 W 71 W 70 W 69 W 68 W (a) Longitude (b) Longitude boundary current circulation (Prisenberg, 1987).

3 Research objectives 5 Seasonal to interannual variations Utilizing a new and unprecedented set of observations, this research aims to 1) describe the series of discrete freshwater pulses that constitute the majority of the freshwater export leaving the Hudson Bay system through Hudson Strait, 2) examine the different mechanisms that could explain the observed variability, 320 320 310 3) and understand how these coherent, low-salinity eddy-like structures might respond to changes in forcing on interannual timescales. 310 300 300 Legend 290 290 Sept-Nov events 280 280 Dec. events 40

CDOM (ppb) 270 270 4 Low-salinity events 33 Jan-Mar events 60 260 260 Low-salinity waters exit Hudson Strait in a series of events that occur on a time scale of ~4-6 days during (a) (b) 32.6 250 250 the fall/winter season (Fig. 2). These pulses are surface-trapped, but can reach depths up to 120 meters. 80 30.6 31 31.4 31.8 32.2 32.6 33 −2 −1 0 1 2 3 Temperature (oC) Figure 3 illustrates the characteristics of a typical event, showing the coherent signals in salinity, CDOM, and 32.2 Salinity across-strait velocity, as a pulse propagates by the array. Three mechanisms could explain this observed 100 Figure 6. The occurrence of low-salinity events is limited to the fall/winter season due to the timing of the river input into Hudson Bay, but variability: 31.8 the sea ice cycle also imprints a seasonal signal. The entire Hudson Bay system undergoes a complete freeze/melt cycle every year, which can depth (m) 120 be seen in plots of CDOM vs. salinity (a) and temperature (b). High CDOM is associated with a high river water %, but once sea ice formation 1) movement of the front back and forth across the mooring array, 31.4 and the associated brine rejection begins, the S vs. CDOM mixing line changes (a), and the temperature goes to freezing (b). 2) individual river plumes exiting Hudson Bay, 140 3) or remotely-forced pulses of low-salinity water advected by the array. 31 160 MMP Salinity 30 850 Since the across-strait velocity signal is onshore then offshore, a simple salt balance (Fig. 4) shows that these 30.6 Figure 7. Since the river input into Hudson Bay and the annual sea ice events cannot be explained by the front moving across the moorings. Instead, these events are associated Sep Oct Nov Dec Jan Feb Mar Apr cycle modulate the low-salinity events seasonally, any interannual trends 2005-2006 in these forcings might also imprint interannual variations in the structure with synoptic-scale storms that pass over Hudson Bay and accelerate the waters there, causing a pulse of 26 775 /yr) freshwater to exit past Mansel Island and reach the strait ~10-14 days later (Fig. 5). Rivers certainly play a Figure 2. Salinity record from the MMP at mooring A for the 3 and timing of the pulses. From 1964-2003 the input of 42 rivers into fall/winter season. Low-salinity events occur on a 4-6 day times- role in supplying the freshwater for these events, but mixing and the long residence times in Hudson Bay Hudson Bay (red) decreased by 10% (Déry et al., 2005). In addition, satellite cale from late September until early April (triangles). suggest that these events are not individual river plumes. 22 700 measurements show that the number of weeks of 100% sea ice coverage over Hudson Bay (blue) has decreased as well, with the sea ice season # of weeks both starting later and ending earlier (Armstrong and Brodzik, 2005). 20 33 300 0.2 18 625 MMP salinity MMP CDOM Figure 3. Salinity, CDOM, and 32.8 (km discharge river The low-salinity events are common features in all previous years’ obser- 40 0.15 32 32 32 31.5 31.5 32 290 32 31.5 across-strait velocity from vations (1982, 1986-87, 2004-06). We are currently investigating how the 32.6 32 60 0.1 mooring A of a typical low- 14 550 32.4 5 changes in river input and sea ice have impacted their structure. 2.5 2.5 2. 1960 1964 1969 1974 1979 1984 1989 1994 1999 2004 3 3 3 280 0.05 salinity event. The along- Year 80 32.2 5 5 5 strait velocity (not shown) 32. 32 32. 32. 0 100 270 accelerates with the passage

depth (m) 31.8 −0.05 120 of these pulses as well, but 31.6 260 −0.1 the timing does not show a 31.4 Moor. A ADCP, U 140 across consistent pattern. 31.2 250 −0.15 6 Summary 25 26 27 28 29 30 31 25 26 27 28 29 30 31 25 26 27 28 29 30 31 Oct. 2005 Oct. 2005 Oct. 2005 • The series of discrete, low-salinity pulses observed in the Hudson Strait outflow are surface-trapped, anticyclonic eddies with vertically and horizontally coherent Mean characteristics (std. devs.) Fall/Winter 2005-06 1.5 15 salinity, CDOM, and velocity signals. 38 events, one per 4.4 days St = -vSy - uSx • These eddies dominate the salinity record during the fall/winter season, and make 1 10 horizontal scale ~ 25 km (12 km), 3-4 times Rossby radius up 40% of the volume transport and 50% of the freshwater flux of the outflow. 2 2 5 g’ ~ 0.011 m/s (0.003 m/s ) 0.5 • Generation of these eddies is related to the passage of storms over Hudson Bay y advective speed ~ 0.18 m/s (seasonal max/min) 0 and the forcing of low-salinity waters out of Hudson Bay near Mansel Island. 0 vertical extent ~ 75 m (10 m) −5 • Changes in river input and sea ice over interannual timescales have been psu / da wind speed (m/s) −0.5 −10 documented, but their effects on the low-salinity events is unknown.

−1 −15 09/10 09/20 09/30 10/10 10/20 10/30 11/09 11/19 11/29 12/09 12/19 12/29 01/08 Date, 2005-2006 −1.5 Acknowledgments and references 09/10 09/20 09/30 10/10 10/20 10/30 11/09 11/19 11/29 12/09 12/19 12/29 01/08 Figure 5. North-south winds in E. Hudson Bay (star, Fig. 1) from NCEP reanalysis. Strong Date Thanks to NSF OCE-0751554 for funding this project and to Jim Ryder, the Canadian Coast Guard ships, and crew who helped deploy and recover the moorings in Hudson Strait. DS also Figure 4. Two terms from the salt balance equation calculated from the moorings at a depth northward winds accelerate the flow out of the bay and correspond to the passage of thanks Pierre St. Laurent and Rocky Geyer for useful comments. the low-salinity events observed in Hudson Strait (red dots), at a lag calculated with the Déry, S.J., et al. (2005), Characteristics and trends of river discharge into Hudson, James, and Ungava bays, 1964-2000. J. Climate. of 45 meters (time rate of change in blue and the cross-strait advective term in red). The Prisenberg, S.J. (1987), Seasonal current variations observed in western Hudson Bay. J. Geophys. Res., 92, 10,756–10,766. along-strait advective term could not be estimated directly, but would be the residual. observed propagation speed of each pulse and the distance traveled (~310 km). Armstrong, R. L., and M. J. Brodzik (2005), Northern Hemisphere EASE-Grid weekly snow cover and sea ice extent version 3. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media. Straneo, F. and F.J. Saucier (2008), The outflow from Hudson Strait and its contribution to the Labrador Current. Deep-Sea Res. I.