Surface and nearsurface circulation 451 Centers of action

Fields of mean sea level pressure (SLP) for the four mid-season months are provided in Figure 4.9. The mean January circulation at sea level, representative of winter, is dominated by the three well-known sub-polar "centers of action": (1) the Siberian High over east-central Asia; (2) the Icelandic Low off the southeast coast of Greenland; (3) the Aleutian Low in the North Pacific basin. The central Arctic Ocean appears as a saddle of relatively high pressures between the eastern Eurasian landmass and northwestern Canada.

The three centers of action are much weaker during April. The Aleutian Low exhibits a pronounced shift to the northeast, while the locus of the Siberian High has shifted slightly west. While the winter centers of action weaken, a closed high pressure cell develops over the Beaufort Sea. By July, the Aleutian Low has disappeared as a mean feature. The Icelandic Low is quite weak, and low pressures extend over the eastern Canadian Arctic. The Siberian High is replaced by a broad area of mean low pressure. An area of weak mean low pressure is also found near the pole in July. The mean October field illustrates the transition back to winter conditions.

The winter Siberian High is a cold, shallow anticyclone, driven largely by radiative cooling to space. The cold air is constrained by topography. There has been controversy over the reality of some of the extreme high pressures recorded in the region. This stems from difficulties in reducing station pressures recorded in upland valleys (where there are persistent cold air "lakes") to sea level (Walker, 1967). But the existence of the anticyclone is not in dispute. Low 1000 to 500 hPa thicknesses during winter are consistent with a cold anticyclone structure (Sahsamanoglou et al., 1991) and cold highs regularly move eastward from the mean locus of the winter Siberian High (around 45-50° N, 90-110° E). The system periodically intensifies and gives rise to cold air outbreaks or surges over East Asia (Ding, 1990). Table 4.1 summarizes mean and extreme pressures under the Siberian High for November, January and March. On average, the Siberian High is strongest in January and February.

The winter Icelandic and Aleutian lows are complex features. In part, they manifest low-level thermal effects of the comparatively warm ocean bordering the ice margin and cold land. Surface pressures will tend to be comparatively low over a warm surface as the overlying air will be warmer, and hence less dense than over a cold surface. One of the observations that supports this idea is that isobars are roughly parallel to and more tightly crowded along the coastlines of Greenland and eastern Asia-Alaska, respectively (Wallace, 1983). Furthermore, both lows are located downstream of the major mid-tropospheric stationary troughs (Figure 4.8) where cyclogenesis is favored by upper-level divergence (see Barry and Carleton, 2001). They are hence part of the primary North Atlantic and North Pacific cyclone tracks, respectively. Finally, there are strong regional cyclone development processes associated with enhanced baroclinicity (strong horizontal temperature gradients along the sea ice margin) and the orographic effects of the southern Greenland Ice Sheet (discussed further below). The winter Icelandic Low is part of a broad area of low-pressure extending into the Barents and Kara seas. This feature reflects the thermal influences mentioned above and the penetration of cyclones northeastward into the Arctic Ocean.

Table 4.1 Mean and extreme monthly values (hPa) of the Siberian Anticyclone central pressure for November, January and March (1873-1988)

November

January

March

Maximum

1042.3

1048.5

1038.8

Mean

1032.7

1037.5

1031.1

Minimum

1027.2

1029.8

1024.9

Source: Based on Sahsamanoglou et al., 1991

Source: Based on Sahsamanoglou et al., 1991

As the latitudinal distribution of solar heating becomes more even in spring and summer, the general circulation weakens. This is manifested in a weakening of the primary storm tracks and the low-pressure centers of action. Springtime weakening of the Siberian High represents a general response to the increasing solar heating and loss of the snow cover. Development of a closed anticyclone off the Canadian Arctic Archipelago in spring appears to reflect the infrequency of transient cyclones and the shift from eastern Siberia of the locus of lowest tropospheric temperatures.

The mean annual cycle of SLP varies across the Arctic. Based on a harmonic analysis using NCEP/NCAR data, Cullather and Lynch (2003) show that within the Atlantic sector, pressures tend to be highest in July and lowest in January. The area of the Siberian high is characterized by a February maximum and August minimum. By comparison, the Canada Basin-Laptev Sea region is dominated by a March maximum and September minimum. In an earlier effort, Walsh (1978) found that SLP averaged from 70° to 90° N exhibits a semi-annual cycle, with maxima in April and November, and minima in July and February. Cullather and Lynch (2003) examined this semi-annual cycle more closely and find it to be best expressed along the periphery of northern Greenland and extending to the Pole. Their harmonic analysis depicts pressure maxima in May and November. Dynamically, this semi-annual cycle can be related to seasonal variations in mass convergence as mass moves from Eurasia and into the Canadian Arctic Archipelago in spring and the reverse condition in autumn. The semi-annual cycle exhibits pronounced interannual variability, associated with mass exchanges with the primary storm tracks in the north Atlantic and Pacific.

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