7B.5 Impacts of Air-Sea Fluxes on the Evolution of an Arctic "Bomb"

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7B.5 Impacts of Air-Sea Fluxes on the Evolution of an Arctic “Bomb”
Lujun Zhang*
Nanjing University, Nanjing, China; Dalhousie University, Dartmouth NS, Canada
William Perrie and Zhenxia Long
Fisheries & Oceans Canada, Bedford Institute of Oceanography, Dartmouth NS, Canada
1. INTRODUCTION
The Arctic is important because of its unique
dynamical-thermo teleconnections and its potential
role in global climate change. Intense Arctic storms
are examples of "extreme" weather which can
impact coastal oceanographic processes in the
southern Beaufort Sea and the west Canadian
Arctic. This area is important because the coastal
marine environment is an integral part of the life
style of Canadian Northerners, and because of
hydrocarbon exploration and potential development
in the near future. Factors such as open water and
ice, and the oceanic surface fluxes can modulate
storm development and winds. Climate change may
endanger coastal settlements and marine
environments.
It is well known that hurricane intensity is
influenced by factors such as the storm's initial
intensity, the spatial extent of the storm, the
thermodynamic state of the atmosphere through
which the storm moves, the storm propagation
speed, and sea surface fluxes along the storm track.
Although several of these factors are also known to
modulate the strength of low- and mid-latitude
cyclone systems, little is known about the impact of
atmosphere-ocean-ice interactions on Arctic storms.
The primary focus of this study is to model the
oceanic responses to an Arctic “bomb” in
September 1999 which made landfall as an
unusually intense storm along the southern coast
of the Beaufort Sea. We investigate the ability of
* Corresponding author address: Lujun Zhang,
Nanjing University, 22 Hankou Road, Nanjing,
China, 210093; e-mail: ljzhang@nju.edu.cn.
surface heat fluxes to influence the storm’s life
cycle and air-ocean-ice dynamics.
2. BACKGROUND
The Arctic storm from September 1999 is
mesoscale in size. It developed over the NE Pacific
and western Bering Sea at 1800 UTC on 21
September 1999. It intensified explosively in the
Gulf of Alaska, developing into a meteorological
bomb at 1800 UTC 22 September 1999. The storm
made landfall with surface winds > 25 m s-1 at Cape
Newenham, Alaska, at 1200 UTC 23 September
and rapidly moved north northeastward. Thereafter,
it crossed the Rocky Mountains to the Yukon and
Northwest Territories and re-intensified over the
coastal waters of the southern Beaufort Sea, over a
zone of high sea surface temperature gradients,
causing extensive damage to coastal communities.
After half a day, the system moved northeastward
along the coast of the Beaufort Sea and continued
to fill. Finally, it dissipated over the northern
Canadian Archipelago, just after 1800 UTC on 26
September.
Initially, the storm lay 200 km off the Alaskan
coastline at 0000 UTC on 22 September with a
central sea level pressure (SLP) of 980 mb as
analyzed by the NARR, NCEP and CMC (Canadian
Meteorological Centre) datasets. The subsequent
18 h saw it develop as a superbomb tracking
northward and re-intensify to 953 mb near the
southern shore of Alaska. During with its mature
stage, satellite images reveal a mesoscale size and
spiral cloud bands of unusual symmetry, that
suggest the presence of a strong midlevel trough
interacting with the system over this period,
contributing to a rapid spinup of the lower level

vortex via baroclinic processes. The track of the low
pressure center passed over Anchorage, Alaska
where observed time series show a pronounced
maximum in equivalent potential temperature at the
storm’s core. Storm tracks and central SLP are
given in Fig. 1 from reanalysis data.
Figure 1: CMC, NCEP and NARR analysis data
showing (a) storm track and (b) central SLP
(hPa), from 00 UTC 22 Sep to 18 UTC 26 Sep.
3. MODEL DESCRIPTIONS
All simulations are performed by using
Mesoscale Compressible Community (MC2)
atmospheric model coupled to the Coupled
Ice-Ocean Model (CIOM, Wang et al. 2002), in
which the SSTs, ice concentration and
thickness from CIOM are passed to MC2,
while the surface air temperature, wind, sea
level pressure, short-wave radiation, clouds,
precipitation and specific humidity (i.e. the
momentum, heat, and moisture fluxes) from
MC2 are passed back to CIOM.
The MC2 model is a state-of-the-art fully
elastic nonhydrostatic model, using a semi-
Lagrangian advection and a semi-implicit time-
differencing dynamic scheme (Tanguay et al.,
1990). As a modeling tool, MC2 is very
versatile and has been successfully used in
simulations of extratropical cyclones (Benoit et
al., 1997; McTaggart-Cowan et al., 2001, 2003;
Ren et al., 2004; Fogarty et al., 2006). The
MC2 model domain covers the entire Arctic
Ocean, its coastal areas as shown in Fig. 1.
The number of horizontal grid points is 235 x
279, with horizontal resolution of 30km and 30
vertical layers. The central grid point is located
at (76ºN, 170ºW). We use north-polar
projection, and the integration time step is
600s. Initial conditions and boundary
conditions are determined from the CMC 6-
hourly analysis fields (Chouinard et al., 1994).
POM is used to simulate the oceanic
component of our coupled model system
(Blumberg and Mellor 1987; Mellor, 1998). To
accurately represent the cyclone-related
mixed layer dynamics, 23 vertical layers are
used, with higher resolution in the upper
ocean mixed layer (8 levels within the upper
80 m). Ocean topography is determined from
the Earth Topography and Ocean Bathymetry
Database (ETOPO2V2) at 2-min resolution,
interpolated to POM’s model grid (U.S.
National Geophysical Data Center,
http://www.ngdc.noaa.gov/).
The sea ice component of the coupled
model is a thermodynamic model based on
multiple categories of ice thickness distribution
function (Throndike et al., 1975; Hibler, 1980)
and a dynamic model based on a viscous-
plastic sea ice rheology (Hibler, 1979). For
Ice-Ocean coupling, heat and salt fluxes at the
b
a

ice-ocean interface are governed by the
boundary processes as discussed by Mellor
and Kantha (1989) and Kantha and Mellor
(1989).
The coupled ice-ocean model grid size is
about 27.5 km with 23 sigma layers. The
number of horizontal grid points is 143 x 191.
The domain includes the central Arctic Ocean
(Canada and Eurasian Basins), the Beaufort
Sea coastal areas, Canadian Archipelago and
the northern GIN Seas. In this study, eight ice
categories are used. Because Bering Strait
and the southern boundary are open, the
radiation condition is applied at the lateral
open boundary of the CIOM model, with
specified depth-averaged transport taken from
an extension of the CIOM model. The initial
conditions and boundary conditions for
temperature and salinity are given by monthly
averaged profiles from the polar Science
Center Hydrographic Climatology (PHC).
4. ANALYSIS OF SIMULATION RESULTS
To investigate the role of air-sea-ice
interactions for the superbomb storm the
simulated went from 1800 UTC 22 Sep to
1800 UTC 26 Sep 1999. Control runs are
simulations in which MC2 is used alone,
uncoupled to the ice-ocean model, using CMC
analysis data to specify fixed SSTs during the
integration period. Coupled runs use the
MC2–CIOM model system.
1) STORM TRACK AND SST
Comparisons between the simulation
(coupled and uncoupled) tracks and the CMC
storm track are shown in Fig. 2. The
uncoupled simulation track differs from the
coupled simulation after the storm reaches the
Beaufort Sea coast and the Archipelago. The
coupled simulation is closer to the CMC storm
track.
The storm induces a cool wake in the
upper ocean. The SST cool wake is weak
during superbomb’s early stages, as the storm
lingers in the Gulf of Alaska. But within 24
hours, the storm moves to the Beaufort Sea
and influences the ocean surface by strong
winds (> 20 m·s-1), so that a cool wake
becomes widely distributed around the
Beaufort coastal areas as shown in Figure 3b.
Figure 2: Comparisons between the control storm
tracks simulation (uncoupled), with the coupled
simulation and the CMC analysis storm track.
During the second day of the simulation
(to 48 h), the storm’s propagation slows (> 15
m·s-1) and it still lingers along the Beaufort Sea
southern coast. Thus, the cool wake
strengthens in the coastal waters. During the
third day of the simulation (to 72 h), the cool
wake central area moves eastward, as the
storm moves over the Canadian Archipelago.
The maximum SST cooling is almost 2°C and
occurs in the coastal waters off the Mackenzie
Delta.
Because of extensive ice cover in the
Arctic Ocean in September 1999, the SST
cooling and associated oceanic mixed layer
currents, produced by the storm’s cyclonic,
asymmetric wind fields (Fig. 3a) mainly occur
in open waters of the southern Beaufort Sea.

2) UPPER-OCEAN RESPONSES
Figure 3: Coupled model results: (a) he 10
Uwinds
(1
ms
−
⋅) at 24h for Superbomb starting at 1800
UTC 22 Sep and (b) the SST at 24h, minus the
initial SST, and surface current ( 1
ms
−
⋅).
The storm-induced cool wake is not
simply an ocean surface feature. In
September, there is typically an oceanic
autumn sea temperature profile whereby a
shallow mixed layer overlays colder water, and
a sharp temperature gradient occurs in the
upper thermocline. The warmest sea
temperatures occur in the southern Chukchi
Sea, because of warm currents from the NE
Pacific via Bering Strait.
Figure 4: Horizontal–depth section from the coupled
simulation along line A–B (Fig. 3a) giving
temperature difference (0.2°C contours) after 96-
h simulation minus the initial state.
Figure 4 shows the storm-induced
impacts on the upper-ocean temperature from
the coupled simulation. The largest change in
sea temperature occurs to along the south
Beaufort Sea coast. At this time, the storm is
moving northward over the coast. A cooling
zone extends over the Chukchi Sea, Barrow,
and Beaufort Seas to the Archipelago, to the
10-m depth, with warming in deeper (20 to 50
m) waters, by as much as 1°C (Fig.4). The
latter is consistent with Price (1981) and Ren
(2004) results whereby entrainment causes
cooling in the mixed layer and warming at
depths below the initial mixed layer. Storm-
induced currents are a dominant mechanism
in forming a given storm’s SST depression.
Because of the asymmetry of the storm’s wind
fields (Fig. 3a), the storm track and the
distribution of ice cover in Arctic, the strongest
surface currents are produced in coastal
waters. This is shown in the coupled
simulation in Fig. 3b, with current speeds up to
1.5 m·s-1, which is similar to currents
generated by tropical and extratropical
24 h
a
b
24 h
A B
Depth
(m)

cyclones (Bender and Ginis 2000; Ren et al.,
2004).
3) SEA ICE RESPONSES
Arctic storm activity plays important roles
at various time and space scales, ranging from
the local scale, causing severe erosion along
coastal margins of the Beaufort and Alaska
and other Arctic regions, to the continental
scale, where storm corridor position and
strength strongly affect the moisture and heat
exchanges between the Arctic and lower
latitudes. Sea-ice, specifically the location of
the ice edge, plays an important role in the
location of storm tracks as well. Its presence
can impede storm progression into the Arctic
by creating a cold friction zone over which
storms lose energy. The ice edge often
defines a strong baroclinic zone which can
enhance storm activity by both strengthening
storms and by acting to preferential guide their
trajectory.
A recent NASA study shows that the
rising frequency and intensity of Arctic storms
over the last half century can be attributed to
progressively warmer waters, directly resulting
in enhanced acceleration in the rate of Arctic
sea ice drift.
For the summer Beaufort, Chukchi, and
East Siberian Seas we investigated the
response of the ice edge and interior ice to the
superbomb storm using the air-ocean-ice
coupled model. Specifically, the
peak 10
Uwinds are about 18 m s−1 (Fig. 5b).
Figure 5a shows the associated impacts on
sea ice drift from the coupled simulation during
the storm.
The ice edge currents imply that the
storm fractures the large floes into small floes,
some of which are advected into the adjacent
warm water. The ice interior thickness
suggests that the storm caused an increase in
the open water amount and a shift in the floe
size distribution toward smaller floes.
Figure 5: Distribution of the ice current (a) and
10
Uwinds ( 1
ms
−
⋅
) at 03h for the coupled
simulation starting at 1800 UTC 22 Sep., 1999.
4) SEA SURFACE FLUXES CHANGE
Comparisons of the associated sensible
and latent heat fluxes from the coupled
simulations are shown in Figure 6a and 6b. As
expected, latent heat flux constitutes a
dominant factor in the coupling of atmosphere
and ocean. Moreover, SSTs and the storm’s
propagation speed are important factors
affecting the ocean’s impact on latent heat
fluxes. While sensible and latent heat fluxes
have similar distributions, the Arctic storm
a
b

propagates rapidly and after 24 h it has moved
over cold water and both latent and sensible
heat fluxes are negative near the storm-center.
Although extensive positive sensible heat
fluxes remain, they are confined to the rear of
the storm and do not strongly affect further
storm development. The reaction of the lower
atmosphere to SST cooling is strong. Smaller
sensible and latent heat fluxes from the ocean
surface tend to cool and dry the atmospheric
boundary layer in the coupled simulation.
Concomitantly, the 10
Uwinds decrease and
the cyclone tends to weaken in the coupled
simulation. However, because most of the
significant interactive processes occur during
the peak intensification period, after the initial
96 h in our simulation of the storm, the ocean
impact on intensity is not large.
4. CONCLUSIONS
This study is concerned with the
implications of using a coupled atmosphere–
ocean-ice model to simulate intense arctic
storms and upper-ocean responses. To
illustrate the impacts of ocean surface
processes, we consider an Arctic superbomb
that developed over the NE Pacific and
western Bering Sea at 1800 UTC on 21
September 1999. The coupled model can
realistically simulate the atmosphere-ocean-
ice interactions in the storm. Model results
were shown to compare well with CMC
analysis data. We have shown the role of sea
surface fluxes on the storm's explosive re-
intensification over the Beaufort coastal waters.
We compared these processes to the other
factors that modify the storm's development as
it passes across the Rockies, to its final decay
region in the Arctic.
Figure 6: Distribution of sensible heat flux (a) and
latent heat flux (b) at 96h for the coupled
simulation starting at 1800 UTC 22 Sep., 1999.
(Units: 2
Wm
−
⋅)
5. ACKNOWLEDGEMENTS
Support for this research comes from the
Federal IPY (International Polar Year) Office
and the Panel on Energy Research and
Development (PERD) of Canada.
b
a

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