The Annua Cycle

Variations of cloud amount in the annual march in different parts of the Arctic are very similar and rather straightforward. Analysing them allows us to distinguish three states: winter, summer, and transitional (spring and autumn) (see Figure 5.1). In cold half-year (from November to April) the mean total cloud amounts are clearly at their iowesl and oscillate between 40% and

60%. In May an abrupt increase in cloudiness is observed, especially outside the Canadian Arctic. The highest cloudiness in the Arctic occurs from June to October (about 80%-90%). The autumn decrease of cloudiness is even faster than the spring increase. As can be seen from Figure 5.2, the spring transition and high summer cloudiness are entirely accounted for by low clouds in all regions. Middle cloudiness does not show great changes in the annual cycle, while high cloudiness is clearly at a minimum in the summer. Interesting maxima of middle and high cloudiness occur in October, which according to Huschke (1969) corresponds very well to the high degree of cyclonic activity over the Arctic during that month. Middle cloudiness also shows a small maximum in spring.

Figure 5.1. Mean monthly total cloud amounts (after Huschke 1969). I - West Eurasian Arctic, 2 - East Eurasian Arctic, 3 - Canadian Arctic, 4 - Central Arctic.

The mean annual marches of cloudiness presented here after Huschke (1969) do not indicate the existence of the three types distinguished by Vowinckel (1962). However, when we take into account data from individual stations rather than areally-averaged data, we find at least two of Vowinckcl's types. The third type (East Siberian) is characteristic of eastern Siberia, which lies outside the area defined as the Arctic in the present work. The present author would propose a change in VowinckeFs terminology (Norwegian-Sea and Polar-Ocean) to oceanic and continental, respectively. The occanic type (Figure 5.3) occurs mainly in the Arctic with the lowest degree of climate continentality (mainly the southern and central parts of the Atlantic region and the southern part of the Baffin Bay region). The continental type is most common in the Arctic and probably therefore Huschke's (1969) areally-aver-

age annua! courses from different regions, described earlier, can be classified as this type. The oceanic type is characterised by high cloudiness throughout the year, with a slight maximum in summer (Figure 5.3). It is worth noting that even in this area, which is so clearly governed by moving cycloncs from the Southwest, maximum cloudiness is observed in summer when the cyclonc frequency is lower than in autumn and winter.

Figure 5.2. Mean monthly high-cloud (a), middle-cloud (b) and low-cloud (c) amounts in selected regions of the Arctic (after Huschke 1969). Key as in Figure 5.1.

Bjornoya -Jan Mayen

Figure 5.3. Mean monthly total cloud amounts in the Atlantic region of the Arctic, 1951-1998.

Bjornoya -Jan Mayen

Figure 5.3. Mean monthly total cloud amounts in the Atlantic region of the Arctic, 1951-1998.

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