158
M.L. Van Woert
4.2 Model Sensitivity
It is commonly argued that fluctuations in the intense offshore katabatic winds at
Terra Nova Bay are responsible for fluctuations in polynya extent. Implicit in this
statement is the assumption that it is the sensible heat fluxes that are primarily
responsible for fluctuations in polynya extent. However, as noted by Pease [10], for
a polynya that is driven solely by sensible and latent heat fluxes, polynya width
does not dépend on the wind speed, but dépends rather on the reciprocal of the
air-sea température différence. This is easily seen by using Eq. (3) as an approximation for polynya extent and taking QNet = -Qs = raCpChï 5Uï 5(J0 - Ti 5), where
ru is the density of air, Cp is the spécifie heat of air, Chl 5 is the sensible heat transfer coefficient at 1.5-m height, Ul 5 is the wind speed at 1.5-m height, and T15 and
To are respectively the températures at 1.5-m height and the océan surface. The
maximum polynya extent, Lm, is then given by:
0.03 x Lf x p, x Hj
where AT = To - Tl 5 and a =---------- ------------ , which dépends on constants that
pa x Cp x Chl 5
hâve ail been previously defined. Figure 4a indicates that the Terra Nova Bay
polynya is not well described by this temperature-dependent model. Including
the latent heat flux in the définition of P(t) increases the complexity of the température dependence in Eq. (5), but the resuit remains independent of the wind
speed and no additional variance is explained by the inclusion of the term.
A significant finding of this study is that changes in the longwave heat flux are
equally as important as the sensible heat flux in forcing changes in polynya width.
To assess the impact of the longwave heat flux on fluctuations in polynya extent,
QNel is taken to include both the sensible heat flux and net longwave heat flux.
Substitution of these terms into Eq. (3) and (4) and retaining the fïrst two terms
in a Taylor expansion yields:
T = —M
b(Q"-Q^
m
AT
ATCJ15
(6)
where Qu- Qd is the net longwave flux and b = ————— , which are constants that
Pa ^p
hâve ail been previously defined. It is clearly seen that the wind contributes to
fluctuations in polynya extent by amplifying the net longwave flux. However, for
strong winds and cold air températures, Eq. (6) asymptotically approaches Eq. (5)
and the maximum polynya width becomes independent of the wind speed.
The sensitivity of polynya extent to changes in wind speed and air température can be quantified by noting that:
AT
d
A T 1
d
A TT
~ —— AT +-AU.
dT
ÔU
(7)
M.L. Van Woert
4.2 Model Sensitivity
It is commonly argued that fluctuations in the intense offshore katabatic winds at
Terra Nova Bay are responsible for fluctuations in polynya extent. Implicit in this
statement is the assumption that it is the sensible heat fluxes that are primarily
responsible for fluctuations in polynya extent. However, as noted by Pease [10], for
a polynya that is driven solely by sensible and latent heat fluxes, polynya width
does not dépend on the wind speed, but dépends rather on the reciprocal of the
air-sea température différence. This is easily seen by using Eq. (3) as an approximation for polynya extent and taking QNet = -Qs = raCpChï 5Uï 5(J0 - Ti 5), where
ru is the density of air, Cp is the spécifie heat of air, Chl 5 is the sensible heat transfer coefficient at 1.5-m height, Ul 5 is the wind speed at 1.5-m height, and T15 and
To are respectively the températures at 1.5-m height and the océan surface. The
maximum polynya extent, Lm, is then given by:
0.03 x Lf x p, x Hj
where AT = To - Tl 5 and a =---------- ------------ , which dépends on constants that
pa x Cp x Chl 5
hâve ail been previously defined. Figure 4a indicates that the Terra Nova Bay
polynya is not well described by this temperature-dependent model. Including
the latent heat flux in the définition of P(t) increases the complexity of the température dependence in Eq. (5), but the resuit remains independent of the wind
speed and no additional variance is explained by the inclusion of the term.
A significant finding of this study is that changes in the longwave heat flux are
equally as important as the sensible heat flux in forcing changes in polynya width.
To assess the impact of the longwave heat flux on fluctuations in polynya extent,
QNel is taken to include both the sensible heat flux and net longwave heat flux.
Substitution of these terms into Eq. (3) and (4) and retaining the fïrst two terms
in a Taylor expansion yields:
T = —M
b(Q"-Q^
m
AT
ATCJ15
(6)
where Qu- Qd is the net longwave flux and b = ————— , which are constants that
Pa ^p
hâve ail been previously defined. It is clearly seen that the wind contributes to
fluctuations in polynya extent by amplifying the net longwave flux. However, for
strong winds and cold air températures, Eq. (6) asymptotically approaches Eq. (5)
and the maximum polynya width becomes independent of the wind speed.
The sensitivity of polynya extent to changes in wind speed and air température can be quantified by noting that:
AT
d
A T 1
d
A TT
~ —— AT +-AU.
dT
ÔU
(7)