Geoscience Reference
In-Depth Information
Figure 10.15 Regions in North
America east of the Rocky Mountains
dominated by the various airmass types
in July for more than 50 per cent and
75 per cent of the time. The 50 per
cent frequency lines correspond to
mean frontal positions.
Source : After Bryson (1966).
55°N, 165°W, splits into two, with one centre in the Gulf
of Alaska and the other over northern Manchuria.
In late June, there is a rapid northward displacement
of the Bermuda and North Pacific subtropical high-
pressure cells. In North America, this also pushes
the depression tracks northward with the result that
precipitation decreases from June to July over the
northern Great Plains, part of Idaho and eastern Oregon.
Conversely, the southwesterly anticyclonic flow that
affects Arizona in June is replaced by air from the Gulf
of California, and this causes the onset of the summer
rains (see B.3, this chapter). Bryson and Lahey suggest
that these circulation changes at the end of June may be
connected with the disappearance of snow cover from
the Arctic tundra. This leads to a sudden decrease of
surface albedo from about 75 to 15 per cent, with
consequent changes in the heat budget components and
hence in the atmospheric circulation.
Frontal wave activity makes the first half of
September a rainy period in the northern Midwest states
of Iowa, Minnesota and Wisconsin, but after about
the 20 September, anticyclonic conditions return with
warm airflow from the dry southwest, giving fine
weather - the so-called Indian summer. Significantly,
the hemispheric zonal index value rises in late
September. This anticyclonic weather type has a second
phase in the latter half of October, but at this time there
are polar outbreaks. The weather is generally cold and
dry, although if precipitation does occur there is a high
probability of snowfall.
2 The temperate west coast and Cordillera
The oceanic circulation of the North Pacific closely
resembles that of the North Atlantic. The drift from the
Kuroshio current off Japan is propelled by the westerlies
towards the west coast of North America and it acts as
a warm current between 40° and 60°N. Sea-surface
temperatures are several degrees lower than in com-
parable latitudes off western Europe, however, due to
Search WWH ::




Custom Search