The Basic Atmospheric and Ocean Energy Budgets - The Arctic Climate System - page 82

3.3.3

Understanding the Annual Mean

As shown in Table 3.1 , in the annual mean, the change in atmosphere energy storage
from ERA-40 is zero - the atmospheric energy that is gained through spring and sum-
mer is lost through autumn and winter. By contrast, the annual average net surface flux
for the polar cap is positive, which together with the horizontal convergence of atmo-
spheric energy essentially balances the net radiation budget at the top of the atmosphere.
F sfc over the terrestrial parts of the polar cap domain, for which lateral transfers of heat
are small, should be close to zero in the annual mean; negative values through spring and
summer (a heat gain by the subsurface column based on the sign convention adopted
here) will be balanced by positive values through autumn and winter. The positive mean
net surface flux shown in Table 3.1 must therefore manifest oceanic processes that
provide a lateral heat source that allows the upward net surface flux to be maintained.
Looking back to Equation 3.6 and examined in more detail shortly, this is represented
by the combination the horizontal convergence of the oceanic sensible heat flux and the
horizontal divergence of the latent heat flux as sea ice.
The insightful student will point out that the Arctic climate system is changing,
which should be manifested in the energy budget terms. For example, there is no
reason to expect that in the annual mean, there will be zero change atmospheric heat
storage or a zero net surface heat flux over land (regarding the latter, recall from
Chapter 2 that permafrost is warming over large parts of the Arctic). In response,
one must remember that we are looking at fairly short records (ERA-40 ends in
2001) which are prone to error from a variety of sources.
With respect to linking the energy and freshwater budgets of the Arctic, recall
from Chapter 2 that in the annual mean, the Arctic Ocean and its contributing terres-
trial drainage are characterized by an excess of precipitation over evaporation - that
is, P-E is positive. This is the same thing, but in different units, as saying that there is
an annual mean convergence in the atmospheric transport of latent heat. The annual
mean P-E averaged over the polar cap domain based on ERA-40 is 193 mm.

3.4

Energy Budget of the Arctic Ocean Domain

3.4.1

Understanding the Annual Cycle

Attention now turns to the irregularly shaped Arctic Ocean domain ( Figure 3.6 ),
for which we can explicitly examine the energy budget of the ocean and its link to
the atmospheric budget. Recall from Equation 3.7 that change in ocean heat storage
∂O E /∂t is determined by the sum of the vertical net surface flux F sfc , the divergence
of latent heat in the form of snow and sea ice ∇• F i , and the oceanic convergence
of sensible heat −∇• F o . The change in ocean heat storage can in turn be portioned
into two major components: changes in sensible heat storage in the water (S o ), and
the latent heat storage in snow and sea ice (Li). i ). Other terms, such as the change in
the sensible heat of the sea ice and snow and the heat input by rivers draining into
the Arctic Ocean, are small and can be ignored. Serreze et al. ( 2007 ) were able