Ocean-atmosphere heat fluxes at the Ronne Polynya, Antarctica
The Ronne Polynya is a coastal polynya, a
region of thin ice or open water in sea ice,
caused by the offshore transport of the ice
by strong winds from the land (Figure 1). As
soon as the ice is transported offshore, new
ice forms on the exposed ocean surface and
is also advected offshore in a continual process,
earning this type of polynya the nickname
‘ice factory’. These polynyas have an
important impact on the regional meteorology
and oceanography of the high latitudes
as well as on the global ocean circulation.
The exposed ocean surface is relatively
warm compared to the cold polar atmosphere,
and the large temperature and
humidity differences result in large sensible
and latent heat fluxes from the ocean to the
atmosphere. This leads to a warming and
moistening of the atmospheric boundary
layer above and downwind of the polynya
and, through vigorous convective mixing,
the formation of a CIBL (convective internal
boundary layer) (Figure 1). A decrease in the
ocean-atmosphere temperature and humidity
gradients is caused by this warming and
moistening, which results in a decrease in
the surface heat fluxes with fetch from the
shore, or ice shelf front (Renfrew and King,
2000). The depth of the CIBL increases with
fetch due to the warming and also the
entrainment of warm air from above the
CIBL (Garratt, 1992).
As well as this turbulent heat transfer,
polynyas can also influence the balance of
radiative heat transfer through the generation
of ice fog (Smith et al., 1990) and
convective clouds or plumes (Pinto and
Curry, 1995). Polynyas, therefore, have the
potential to modify and induce mesoscale
atmospheric motion, impacting on regional
climate (Pinto et al., 1995).
High rates of ice production due to the
large ocean-atmosphere heat fluxes and
the continual removal of the newly formed
ice by the wind result in extensive brine
rejection, whereby sea water rejects salt
on freezing, leaving the sea ice relatively
fresh and the modified water column relatively
salty and therefore dense. This dense
water sinks, as shown in Figure 1, accumulating
on the continental shelf and forming
a water mass, which eventually contributes
to the temperature- and salinity-driven global
ocean circulation, known as the thermohaline
circulation (THC). Therefore the
ice-formation mechanism within polynyas
is important for the ventilation of deep
and bottom water in both the Southern
and Arctic Oceans (Morales Maqueda et al.,
2004). It follows that, in order to accurately
model the response of both the high latitudes
and global THC to a changing climate,
processes occurring within polynyas must
be investigated.
I was lucky enough to be given the opportunity
to participate in a British Antarctic
Survey (BAS) fieldwork campaign at the
end of the Antarctic summer field season
in February 2007. Using an instrumented
Twin Otter aircraft belonging to the BAS,
three flights were conducted over the
Ronne Polynya between 25 and 28 February
2007 to investigate ocean-atmosphere heat
fluxes. Quantification of these heat fluxes is
a step towards quantifying the surface heat
budget, which, together with the surface
salinity budget, will aid understanding of
the key processes governing deep-water
formation within polynyas.
Aircraft observations of the