Energy Exchanges at the Surface - The Arctic Climate System - page 162

are some notable departures. There are a number of reasons for this, including dif-
ferences in ice and atmospheric conditions as well as measurement techniques. The
average fluxes in Figure 5.10 mask considerable day-to-day variability associated
with synoptic weather systems.

5.9

Partitioning of net Radiation

5.9.1

Characteristics over Sea Ice

Figure 5.12 , also from Persson et al. ( 2002 ), illustrates annual cycles of the tur-
bulent energy fluxes and conduction at the SHEBA site in comparison with other
estimates. The SHEBA results are representative of reasonably thick ice. They
show the sensible heat flux as directed toward the surface in winter and variously
toward or away from the surface in the other months. The salient point, however,
is that the fluxes are small, peaking in February at about 8 W m −2 . The latent heat
flux is also quite small, ranging from essentially zero in the winter months to
about −7 W m −2 upward (i.e., evaporation and sublimation is occurring) in June.
Again, there are some considerable differences with respect to other estimates.
As with the radiative fluxes, the turbulent terms show large day-to-day variability.
Typically, observed peaks correspond to synoptic events associated with increased
wind speed.
During winter, the sea ice and its overlying snow cover separate the cold atmo-
sphere from the relatively warm Arctic Ocean at its freezing point (−1.8°C for typ-
ical ocean salinities). Hence there is a temperature gradient in the snow and ice
cover and an upward heat flux to the surface, which in our framework is negative,
consistent with what is seen in Figure 5.11 . This conductive flux includes the effects
of latent heat release at the ice-ocean interface associated with ice growth. The
amount of ice growth (or ablation) at the bottom of the ice is represented by the sum
of the ocean heat flux, F w , and the conductive heat flux through the snow and ice,
K i ∂T i /∂z:

F W + K i ∂T i /∂z < 0 (growth) (5.7)

F W + K i ∂T i /∂z > 0 (ablation) (5.8)

F w has a mean annual value of about 2 W m −2 that is determined almost entirely by
shortwave input through leads and areas of thin ice. Compared to previous studies,
the winter conductive flux at the SHEBA site is rather small, peaking at about −5
W m −2 . Annually, the conductive heat flux to the surface of multiyear ice estimated
from different studies ranges from 2-8 W m −2 , with the SHEBA year falling in the
low end. According to Maykut ( 1986 ), the annual upward conductive flux is up to
40 W m −2 in the marginal ice zone of the northern North Atlantic Ocean. During
summer, as air temperatures increase, the conductive flux becomes small or nonex-
istent. For July, the SHEBA turbulent fluxes and conduction taken together are quite
small, meaning that the bulk of the net radiation surplus is going into melt.