Geoscience Reference
In-Depth Information
was the subject of a focused study (Serreze Barrett,
2008
). They find that the pat-
tern is associated with the influx of lows generated over the Eurasian continent as
well as cyclogenesis over the Arctic Ocean itself; very few enter from the North
American continent. Cyclones migrating into the cyclone maximum region, along
with those generated over the region itself, tend to track around the vortex and even-
tually occlude and dissipate within the region. The seasonal onset of the summer
cyclone maximum is linked to an eastward shift in the Urals trough, development of
a separate region of high-latitude baroclinicity, and migration of the 500-hPa vortex
core near the pole. The latter two features are consistent with differential heating
between the Arctic Ocean and snow-free land, an issue that is discussed shortly.
However, the strength of the summer cyclone pattern is highly variable. When well-
developed, the 500-hPa vortex is especially strong and symmetric about the pole.
When poorly developed, the opposite pattern holds.
There have been a number of case studies of cyclone development processes over
the Arctic Ocean. The series of papers by E. LeDrew (
1984
;
1988
;
1989
) is recom-
mended. These studies made use of forms of the omega equation to diagnose the
contributions of different mechanisms to the vertical motions in cyclones. As with
mid-latitude systems, the effects of differential vorticity advection and temperature
advection (see Holton,
1992
) tend to dominate. Of particular interest, however, is
the sometimes significant role of local heat sources in the Arctic basin (LeDrew,
1984
).
4.5.3
Frontal Activity
Early studies of frontal activity in the Arctic, such as those of Reed and Kunkel
(
1960
) and Barry (
1967
), were based on manual analysis. While extremely time
consuming, manually depicted fronts always contain an element of subjectivity.
With the advent of fast computers, thinking turned to the application of automated
methods. T. Hewson (
1998
) provides a comprehensive review. Of the various meth-
ods that can be found in the literature, one that seems to work fairly well is a thermal
front parameter (TFP). The TFP is defined as:
TFP = −∇|∇
τ
|•(∇
τ
/|∇
τ
|)
(4.1)
where
τ
is a thermodynamic variable. The TFP magnitude will be largest where
there is a rapid change in the thermal gradient (the first term on the right) with a
large component parallel to the direction of the (unitized) thermal gradient (the sec-
ond term on the right). The minus sign places the frontal boundary on the warm-air
side of the concentrated baroclinic zone (corresponding to a ridge line in the field of
TFP). The simplest application is to define fronts based on ridge lines of TFP, with
the requirement that the TFP exceed a selected threshold value. Further details and
application of more robust approaches are described by Hewson (
1998
).
Serreze et al. (
2001
) used the TFP approach to assess frontal frequencies over
a twenty-year period (1979-1998) for the region north of 30°N. The intent was to
reexamine the concept introduced by Dzerdzeevskii (
1945
) and Reed and Kunkel
