Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 107

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Figure 4.1 Winter
meridional circulation in the
Northern Hemisphere. (After
Palm´n and Newton 1969 )

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Upper-air westerlies
Carl Gustoff Rossby et al.( 1939 ) provided a physical or process-based explana-
tion for the wave-like features (now called Rossby waves) that characterize the
upper-level westerlies. The main reason for the existence of a general atmo-
spheric circulation is the need for redistribution of energy over the globe.
Rossby, and many subsequent researchers, demonstrated that the need for
equator-pole energy transfer and the strength of the wind flow aloft combine
to produce waves in the zonal westerlies. The physics of air movement through
ridges and troughs of Rossby waves results in upper-level convergence and
divergence in selected portions of the waves.
If the positions of longwave troughs and ridges during a given season are
predisposed toward specific locations and orientations, then surface pressure
systems or the centers of action can be related to the Rossby wave pattern and
position. The surface thermal properties of the Earth, specifically the tempera-
ture differences between continental and oceanic areas, play a significant role in
anchoring the position of certain Rossby wave features. In winter, for example,
air movements from over relatively warmer ocean waters toward an intensely
cold landmass result in a tendency for anticyclonic curves in the flow (a ridge);
the Western North American Ridge is a dominant feature of the winter climate of
North America. In contrast, air flowing off a cold landmass and over a warm
ocean current will be given a cyclonic torque; major troughs are climatologically
preferred winter season features for the east coasts of both North America and
Asia. A seasonal analysis of the frequency distribution of trough and ridge axes at
458 N (Harman 1991 ) is shown in Figure 4.2 . The most clearly defined modes are
seen in the January distribution where well-defined and persistent ridges occur at
908 E (continental Asia), 208W (North Atlantic), and 120-1408W (Eastern
Pacific-Western North American Ridge). Airflow away from a ridge and toward
the next downstream trough produces subsident motion and surface high pres-
sure. When this sinking motion occurs above a cold and snow-covered landmass,
major ''centers of action'' are produced. The North American (Canadian) and
Asian (Siberian) highs are positioned just downstream from major ridges.