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Supplementary File #2
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Atmospheric events drive ocean processes on many time scales
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Arctic storms drive the generation of surface waves, storm surge and related upper ocean
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processes, particularly with larger open water expanses and decreases in summer sea ice
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extents. In coastal regions, waves and storm – driven ocean currents influence sea ice and
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coastal shallow water regions. Open areas create long fetches for the generation of more
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energetic storms and waves, potentially causing enhanced retreat and breakup of summer ice.
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Here, we consider Arctic storms over open water regions of the southern Beaufort and Chukchi
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Seas, propagating over the southern Beaufort region, and making landfall in the Mackenzie
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Delta region, creating storm-generated storm surge and waves as they impact this region.
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a) Storm surge
We first consider storm surge. On short time scales, those that relate to cyclone wind
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events, one of the most notable features of the southern Beaufort Sea are the interactions
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between the Mackenzie River plume and the near-shore and coastal dynamics of the
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neighboring region, particularly in the later summer and autumn, when plume waters from the
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River are notably warmer than coastal Arctic waters. These river-coastal interactions are
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strongly influenced by synoptic-scale winds impacting the Mackenzie Delta area. Mulligan et
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al. (2010) investigated the characteristics of the Mackenzie River flow as it passes the mouths
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of the Delta channels, forming a plume over the Beaufort Shelf. The time period of their study
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was 12 days in August 2007 during which a data set of in situ field, ship-based and satellite
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observations were collected, providing a composite picture of the plume. In situ field data
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recorded currents, temperatures, salinity and related properties in depths 2~6 m, out to about 30
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km from shore with complementary ship-observations extending to ~ 100km offshore, and
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satellite MODIS temperature data, several 100s km. They showed that the flows in this shallow
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near-shore – coastal region of the Delta are dominated by wind-driven motions of surface –
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trapped plume water and underlying shelf water. Inside the 2 m depth contour (about 20km
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from shore) the water is mostly vertically mixed, fresh river water, with strong vertical
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gradients in temperature, salinity and currents developing in 2-3 m water depths. Wind forcing
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can move temperature fronts by 40km / day.
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Hydrodynamic model simulations suggest stratification is in balance with surface wind
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stress, and that synoptic-scale winds are responsible for driving plume motions that can extend
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several 100s km into the Beaufort, as shown in the satellite imagery of Figure 1. Winds are the
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key in causing rapid responses and horizontal extensions of the plume to changes in winds,
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with strong vertical gradients in this coastal region. Mulligan et al. (2010) examined a period of
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offshore transport and mean water level set-down, which if followed by changing winds could
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result in set-up, storm surge, and coastal flooding. Similar studies of river plumes for other
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coastal regions were conducted by Fong and Geyer (2001), and Lentz (2004). More reliable
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simulations of the Mackenzie Delta would use a 3D ocean model coupled to a detailed
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hydrological model of the Delta, allowing reliable simulations of flow rates and water
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properties from each channel from the Delta.
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b) Waves
Second, we consider storm-generated surface waves, following Xu et al. (2011). There
are two notable features about wave studies involving the Mackenzie Delta waters. One is the
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scarcity of observations, and the other is unique very shallow topography relevant to waves
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associated with landfalling storms, which cause potential coastal erosion.
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Studies of ocean waves involve long-fetches, else large waves don’t tend to grow unless
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extreme hurricane-type intensity storms generate and drive them. Extended open water
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expanses that are present in the Chukchi and Beaufort Seas with decreased ice cover provide
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basins for wave growth, in deep, intermediate and shallow coastal waters. In studies of
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shallow water dissipation effects of the coastal waters off the Delta, we use data collected
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during a field experiment conducted by S. Solomon in the summer of 2008 to calibrate wave
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dissipation parameterizations used in a modern shallow water wave model that is then used in
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test studies in simulation and verification studies of an earlier storm in August 1991, where
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observations were also available. Thus our methodology is to select observation data that
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highlight the dominant shallow water wave physics through analyzing in-situ wind and wave
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developing processes, determine the dominant shallow water wave physics parameterizations
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through numerical wave model simulations, focusing on bottom friction and depth-induced
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breaking in waters off the Delta.
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During the August 2008 field experiment, wave observations were collected at 1 shallow
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water site (Aquadopp) to test and also validate bottom friction dissipation parameterizations,
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with additional wave observations collected at two offshore deep water locations (Sites 1 and
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22) and wind observations were recorded at a meteorological station at Pelly Island (see Figure
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2). Results indicate the formulation for a wave bottom friction parameterization suitable for
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shallow waters off the Delta. These data are shown in Figure 3.
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The storm during August 4~6 1991 was chosen to test and validate a depth-induced
breaking parameterization formulation. In this study, depth-induced breaking was studied by
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modelling wave dissipation using observed winds at Tuktoyatuk, and reanalysis wind data over
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the entire simulation domain. Observed waves at a location in shallow water (MEDS291 in
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Figure 1) were used to validate the wave simulation results. Results determined the wave
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breaking parameter to give simulated results that compare well with observed data, confirming
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previous depth-induced breaking studies in areas at other locations with extremely mild slope.
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Therefore through IPY, bottom friction and depth-induced breaking dissipation processes
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are studied and validated in waters off the Mackenzie Delta, for the first time. Sparse data is a
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problem. Further validation and studies are needed, with more observational data. However, we
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were able to suggest convincing criteria for wave model simulations, and that wave forecasting
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can provide accurate predictions. Other factors, such as storms characteristics, structure and
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propagation speed, sea ice and the complex topography need further investigation in order to
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improve wave forecasts.
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References
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Fong, D., Geyer, W., 2001. Response of a river plume during an upwelling favorable wind
event. Journal of Geophysical Research 106 (C1), 1067e1084.
Lentz, S., 2004. The response of buoyant coastal plumes to upwelling-favorable winds. Journal
of Physical Oceanography 34, 2458e2469.
Mulligan, R. P., Perrie, W., Solomon, S., 2010. Dynamics of the Mackenzie River plume on
the inner Beaufort shelf during an open water period in summer. Estuarine, Coastal and
Shelf Science 89, 214-220.
Xu, F., W. Perrie, S. Solomon, 2011. Shallow water dissipation processes for wind-waves off
the Mackenzie Delta. Under review by Atmosphere-Ocean.
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(a)
(b)
Figure 1. Sea surface temperatures from MODIS satellite observations in August 2007 show
the extent of the Mackenzie River plume. Easterly winds in a) with mixing and upwelling
bring about lower temperatures; light winds in b) with less mixing and entrainment of
underlying shelf water allow warmer water to spread as a thin plume. Julian day is
indicated as “YD”.
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x 10
4
100
18
200
100
30
Site1 30
50
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90
30
100
South-North [ m ]
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Site11
12
10
Aqudrop
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2
2
2
2
5
70
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60
2
Tuktoyaktuk
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2
5
50
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30
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4
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2
2
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108
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2
6
10
Meds291
Pellly Island
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80
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2
10
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0.5
1
West-East [ m ]
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x 10
0
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Figure 2. Mackenzie Delta, showing locations of wind and wave observation stations.
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0
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1.5
Aquadopp location
Site1 station
Site11 station
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48
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96
Hours after 1600 UTC 15 Aug.,2008
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Significant wave height
Peak wave period
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Aquadopp location
Site1 station
Site11 station
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0.5
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0
24
48
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96
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Hours after 1600 UTC 15 Aug.,2008
（a）
(b)
Figure 3 Time series from data collected during 2008 summer experiment at locations in
Figure 2, showing (a) significant wave height and (b) wave period comparisons at the
Aquadopp, and Sites 1 and 11 from 1600UTC August 15, to 1600UTC August 20, 2008