1. In the vicinity of mooring D, the bottom slope is α ≈ 0.005. We show that for the stratification shown in Figure 1 (let N 0 =2f ) the frequency ω of bottom-trapped topographic Rossby waves satisfies the simple relationship:. The period of these waves would be about 50 days, consistent with the observed wave. Above the bottom layer, the waves decay with height (Figure 4) with decay scale. Rossby waves at the basin boundary could play a role in the generation of bottom-trapped topographic Rossby waves in the deep interior Canada Basin. Moored observations of bottom-intensified motions in the deep Canada Basin, Arctic Ocean Mary-Louise Timmermans 1 *, Luc Rainville 2, Leif Thomas 3, Andrey Proshutinsky 4 1. Yale University 2. Applied Physics Laboratory 3. Stanford University 4. Woods Hole Oceanographic Institution *Mary-Louise.Timmermans@Yale.edu Abstract In the deep Canada Basin, below the sill depth (about 2400 m) of the Alpha-Mendeleyev ridge complex, potential temperature and salinity first increase with depth, then remain uniform from about 2700 m to the bottom (approximately 3500 m). Year-long moored measurements of temperature, salinity and pressure in these deep and homogeneous bottom waters reveal significant vertical excursions at sub-inertial frequencies (periods of about 50 days). The observed isopycnal displacements have amplitudes up to 100m at the top boundary of the bottom layer; moored profiler measurements in the intermediate water column indicate that displacements decay exponentially above the homogeneous bottom layer. We show how the sub-inertial excursions are consistent with a bottom-trapped topographic Rossby wave. Given the magnitude of the bottom slope in the vicinity of the mooring, the observed vertical velocities correspond to only relatively weak (about 1 cm/s) cross-slope horizontal velocities. Hence, while the generation mechanism for the observed waves remains an open question, it seems only a moderate energy source is required. Ongoing mooring deployments will enable us to further understand the generation, energetics and propagation of these waves in the deepest waters of the Canada Basin. Observations: The Beaufort Gyre Observing System Canada Basin Deep Water The well-mixed bottom layer in the Canada Basin implies that convective mixing is occurring as a consequence of geothermal heating. A staircase structure, suggestive of double-diffusive convection, is observed between the temperature minimum and the top of the homogeneous bottom layer (Timmermans et al., 2003). The staircase and bottom homogeneous layers extend laterally across the entire Canada Basin. Mooring D – Deep Measurements Sub-inertial Motions Figure 3: Depth-time section of potential temperature from the deep instruments on mooring D. An inferred vertical isopycnal displacement η (offset by 2600 m) is shown by the black line (the moving average over the M 2 tidal cycle). η is derived by matching the CTD profile of potential temperature (Figure 1) to the deep time series measurements. At each sample time, η is found by minimizing the variance between the temperature data and vertically displaced CTD profile. Depth-time series from pressure sensors are shown by the gray lines; note that mooring motion is negligible compared to vertical displacements. Figure 4: Left: Depth-time section of potential temperature from the McLane Moored Profiler (MMP) sampling between 50 and 2050 m on mooring D. Middle: Isopycnals and the inferred displacement η (offset by 2600 m) from the deep instruments. Right: Vertical profile of rms vertical displacement calculated from the isopycnals shown in the middle panel. The dotted line shows the exponential fit to the vertical profile. Acknowledgements Further information: www.whoi.edu/beaufortgyre Funding was provided by the National Science Foundation Office of Polar Programs Arctic Sciences Section under awards ARC-0806115, ARC-0632201 and ARC-0806306. We appreciate the support of the captain and crew of the Canadian Coast Guard icebreaker Louis S. St-Laurent, and also acknowledge support from Fisheries and Oceans Canada. Figure 1: Left: The Canada Basin in the Arctic Ocean. Beaufort Gyre Observing System (BGOS) mooring locations are marked by diamonds. The location of a 2002 pilot study (Timmermans et al., 2007) is marked by the blue dot. Right: schematic of instrument configuration for mooring D. The BGOS was initiated in 2003, with instruments deployed on the deep section of mooring D since 2007. We analyze the dynamics of the deep water using these measurements at site D between Aug. 2007 - Aug. 2008. Figure 2: Potential temperature and salinity profiles taken at site D on 8/23/2007 prior to deployment of the mooring. Data were collected from the CCGS Louis S. St- Laurent. Depths of 16 instruments (measuring pressure, temperature and salinity), placed to span the deep staircase structure and homogeneous bottom layer, are shown by the red bars. The dashed sloped line in the right panel indicates stratification corresponding to N ≈ 2 f, where f is the inertial frequency at 74 0 N ( f = 1.4x10 -4 rad s -1 ). References Figure 5: Top: Daily-averaged time series of vertical displacement η. Bottom: Mortlet wavelet transform of daily-averaged η. The dashed cone-of-influence indicates the maximum period of useful information at that particular time. Periods longer than this are subject to edge effects. The peak wave period is about 50 days. Bottom-trapped Topographic Rossby Waves  Rhines, P., 1970: Edge-, bottom-, and Rossby waves in a rotating stratified fluid. Geophys. Fluid Dyn., 1, 273–302.  Timmermans, M.-L., C. Garrett and E. Carmack, 2003: The thermohaline structure and evolution of the deep waters in the Canada Basin, Arctic Ocean. Deep-Sea Res., 50, 1305-1321.  Timmermans, M.-L., H. Melling and L. Rainville, 2007: Dynamics in the deep Canada Basin, Arctic Ocean, inferred by thermistor-chain time series. J. Phys. Oceanog., 37, 1066-1076. Mooring D – Intermediate Depths