Mean characteristics

The lower boundary of the thermal tropopause at 85-90° N, based on soundings from the NP program for 1954-91, has a minimum altitude of about 8.4 km in April and a maximum of 9.4 km in August according to Nagurny (1998). The mean thickness of the tropopause layer is only about 600 m over the Kara-Laptev seas in January,

Figure 4.7 The annual mean and summer (June-August) tropopause pressure (hPa) based on the lapse rate definition derived from ECMWF data for 1200 UTC, 1979-93 (adapted from Hoinka, 1998, by permission of AMS).

whereas in summer it exceeds 1.6 km thickness in a belt from northern Norway to northern Novaya Zemlya and eastward to Wrangel Island (Makhover, 1983).

There is a secondary minimum (maximum) in tropopause height in October (December-January). By contrast, the tropopause temperature shows a simple annual cycle with a minimum of -62 °C in January and a maximum of -49 °C in July, apparently following the annual cycle of temperature in the lower stratosphere as identified for the 100 hPa level by Wilson and Godson (1963). Highwood et al. (2000) also point out that tropopause temperatures in winter are lower over the Eurasian sector and higher over the Canadian-Atlantic sector.

The existence of a double maximum/minimum in tropopause height, first noted by Gaigerov (1967) and Makhover (1983), is attributed to several factors. The primary August maximum is attributed to the lag in the heating of the surrounding landmasses, while the secondary one in January is related to enhanced meridional heat transport by the planetary waves. The spring minimum is associated with the seasonal weakening of this transport. The autumn minimum relates to the cooling of the landmasses and a decrease in cyclone activity compared with late summer. Two patterns of the annual cycle in tropopause height are identified in different regions. Over North America and sub-Arctic Siberia there is a simple annual wave (winter/summer - tropopause pressure maximum/ minimum) while in the High Arctic, northern Europe and western Siberia there is double wave (spring/autumn - maxima, summer/winter - minima).

Figure 4.7 shows the spatial pattern of the mean thermally defined tropopause height (hPa) for 1979-93 for the annual mean and summer. The first order pattern is that the Arctic tropopause is encountered lower in the atmosphere (i.e., at a higher pressure) than in lower latitudes. This is consistent with the schematic shown in Figure 4.1. The annual mean height field is nevertheless strongly eccentric about the Pole with a center of 280 hPa located over Devon-Ellesmere islands. In summer, the minimum extends from Ellesmere Island to the North Pole, and its average center is about

290 hPa (Hoinka, 1998). Zangl and Hoinka (2001) extended this work to examine the tropopause based on dynamic and thermodynamic criteria. Their January map is similar to the annual mean. Highwood et al. (2000) also obtained similar results.

A key feature of the structure of the atmosphere in middle-high latitudes is the existence of tropopause folds. These are situated above strong baroclinic zones -associated with intense cyclones. In these regions, stratospheric air is extruded into the troposphere by intermittent folding of the tropopause (Danielsen, 1968). The associated mass removal of stratospheric air is maximized in late spring. The depression of the tropopause associated with the mean frontal zones is illustrated in Figure 4.1.

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