Precipitation in Polar regions: Arctic and adjoining regions

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General

The two Polar Regions are usually identified as key regions for monitoring global climate change, because the surface air temperature and precipitation is expected to increase especially rapid in these two regions along with the ongoing increase of atmospheric CO2. You will find an outline of the main reasons for the alleged polar temperature amplification by clicking here. As air temperature rises, also precipitation is expected to increase according to climate models, because warm air can contain more water vapour than cold air.

Distribution of the average annual 20th century precipitation north of 50oN, in mm w.e. (water equivalent). The observation network is thin especially within the Arctic Ocean, and details of individual contour lines may well represent artifacts of interpolation, only. The general aridity (low precipitation) of this area, however, is correct. In general, the annual precipitation is high over oceans and over the windward part of adjoining continents. The importance of high ground (e.g. Rocky Mountains, southern Greenland, Scotland and Norway) for generating orographic precipitation is also clear from this map.

 

The distribution of precipitation in the Arctic represents a complex problem, subject of long-standing debate and compounded by the paucity of meteorological stations. While air temperatures today are registered at Arctic meteorological stations with relative small technical difficulties, except for sites with icing conditions, precipitation is considerably more complicated to measure correctly, especially when in solid form. Many Arctic meteorological stations have simply avoided measuring precipitation due to severe problems by doing so. In addition, little is known about the local and regional effect of altitude and topography on precipitation. Also the local and regional importance of redistribution of snow by wind is usually virtually unknown (Humlum 1987; Humlum 2002; Humlum et al. 2003; Nordli and Kohler 2003). Finally, much of the information that does exist on precipitation within the Arctic tends to be widely scattered in the scientific literature and is often viewed only in the context of a particular local problem, with little emphasis on the regional amount of precipitation (Humlum 2002). In addition, high-latitude trends in measured precipitation are influenced by gauge under catch. At a meteorological station exposed to warming, the fraction of annual precipitation falling as snow diminishes, and vice versa. As the gauge under catch is substantially larger for solid than for liquid precipitation, this implies that a fraction of any observed positive precipitation trend is fictitious, caused by reduced under catch in the precipitation gauges (Førland and Hanssen-Bauer 2000).

Notwithstanding all these limitations, the existing meteorological records of precipitation still provide the mean to test the association between air temperature and precipitation back in time. At the bottom of this page a series of diagrams show how precipitation and air temperatures have varied during the 20th century in regions north of 50oN. First, however, it is useful to see how the average 20th century precipitation has been distributed according to the existing meteorological records. This is illustrated by the three diagrams below. Click here for information on data sources, etc.

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Distribution of the average May-October 20th century precipitation north of 50oN, in mm w.e. (water equivalent). The observation network is thin especially within the Arctic Ocean , and details of individual contour lines may well represent artifacts of interpolation, only. As May-October represents the warm season north of 50oN, most of the precipitation will fall as rain during this period, except over the highest ground (e.g. central Greenland ). The east-west precipitation gradient across continents is less pronounced than shown above for the annual precipitation. This is partly because of importance of local precipitation events over continental interiors, e.g. in connection with thunderstorms.

 

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Distribution of the average November-April 20th century precipitation north of 50oN, in mm w.e. (water equivalent). The observation network is thin especially within the Arctic Ocean , and details of individual contour lines may well represent artifacts of interpolation, only. As November-April represents the cold season north of 50oN, most of the precipitation will fall as snow during this period. The east-west precipitation gradient across continents is pronounced compared to the warm season (see diagram above), as most precipitation falls in connection with cyclonic activity. Extensive Arctic regions in Canada, Alaska and Siberia are very arid (dry) during the cold season, and only a shallow snow cover will accumulate during the winter. By this, the ground surface has little protection towards low air temperatures and will cool rapidly. The most extensive permafrost regions in northern hemisphere are found within these dry regions. Conversely, snow and glaciers will more easily accumulate in the mountains of Norway, compared to the mountains in Siberia.

 

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Arctic seasonal snow cover

The general problem of reliable records on Arctic precipitation has implications for our knowledge on the duration and thickness of the seasonal snow cover, significant for the ground thermal regime (Ballantyne 1978; Humlum et al. 2003). Snow plays a key role in protecting plants and animals from cold dry winter conditions. It is also important for the seasonal water cycle. Variations in the snow cover may therefore have profound impact on biological activity and landforming geomorphic activity in the Arctic. In addition, the snow cover also has a direct effect on the distribution of permafrost on both local and regional scale (Humlum et al. 2003). In arid parts of the Arctic land regions the average winter snow cover is thin and the ground surface cools rapidly during the winter. Conversely, in more maritime areas the snow cover usually is thicker and reduces heat loss from the ground surface during winter (see diagram below). Interannual variations in timing of the establishment of the snow cover are also important. A dry and cold autumn enables enhanced cooling of the active layer and topmost permafrost, while high snowfall during late winter and late onset of snow melt protect the ground against thawing in early summer. The combination of these two meteorological phenomena is likely to be beneficial for conservation and growth of permafrost. Variations in the timing and duration of the seasonal snow cover presumably also have an influence on active layer thickness, but the effect is still not known in detail (Humlum et al. 2003).  

 

Average 20th century insulation factor (N-factor) November-April. The N-factor expresses how well the snow cover is able to protect the ground from the penetration of low air temperatures. The maximum value of 1 indicate that there is no protection at all, while the minimum value of zero indicate maximum insulation. As an example, all other things being equal, the map shows that permafrost will more easily form in the dry parts of Siberia and Alaska, compared to the mountains in Iceland and Norway, where cold season precipitation is high.

 

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Arctic precipitation and temperature change during the 20th century

In the table below the decadal situation as to precipitation is compared to the average conditions for the whole 20th century, as shown in the diagrams above. Click on one of the small maps, and a new window with a larger map will open, showing areas north of 50oN in polar projection. In the precipitation diagrams (column 2-4) yellow-red colours indicate less precipitation than the 20th century average, while blue colours indicate higher precipitation than the average. It should be realised, that especially for the cold season, when most precipitation falls in the form of snow, precipitation records may underestimate the real amount of precipitation due to undercatch by precipitation gauges. Measuring solid precipitation correctly is still a major problem, and it is very likely that the real amount of winter precipitation is higher than shown in the diagrams below.

To analyse the association between changing air temperatures and precipitation maps showing the decadal temperature changes are also available (column 5-7), again compared to the average 20th century conditions. Warm colours indicates areas with higher temperature during the decade considered than the 20th century average, while blue colours indicate lower than average temperatures

The two rightmost columns show the calculated large-scale effects on ground temperatures and net accumulation of snow (~ glacier mass balance). These diagrams, however, ignore all small-scale effects derived from, e.g., topographic lee and shading, and should not be over-interpreted.

In the ground temperature diagram MAGST is short for mean annual ground surface temperature . Blue colours indicate areas where MAGST are below the 20th century average in the decade considered, while warm colours show areas where MAGST is estimated to have been higher than the average. In permafrost regions this would imply decreasing and increasing temperatures in the topmost part of the permafrost. The ground temperature deviation is calculated by considering the change in winter snow cover from changes in precipitation, and using this information to calculate the N-factor (Smith and Riseborough 2002). Together with the air temperature change, this information makes it possible to estimate the corresponding change in ground surface temperature. Lower than average MAGST will indicate better than average 20th century conditions for permafrost, and perhaps formation of new permafrost, while higher than average MAGST will have the opposite effect.

In the snow mass balance diagram warm colours indicate areas where seasonal snow will tend to disappear early in the year during the decade considered, compared to average 20th century conditions. Blue colours indicate areas where the seasonal snow cover will disappear later in the year than average 20th century conditions. For areas with glaciers, these two criterions correspond to below-average and above-average glacier mass balance compared to average 20th century conditions, respectively. The snow mass balance change is calculated by considering the corresponding changes in cold season precipitation and warm season air temperatures. Higher cold season precipitation (snow) will tend to improve the overall annual mass balance, while higher warm season air temperatures will lead to increased melting of snow and ice, and thereby have the opposite effect. A degree-day factor of 3.8 mm w.e. has been used in the calculations of summer ablation (melt and evaporation). As mentioned above, neither this nor the neighbouring ground temperature diagram should be over interpreted with regard to their details.

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Diagram table showing Arctic change during the 20th century

Period Precipitation annually Precipitation warm season Precipitation cold season

Temperature annually

Temperature warm season Temperature cold season Ground temperature Snow mass balance
1901-1910
1911-1920
1921-1930
1931-1940
1941-1950
1951-1960
1961-1970
1971-1980
1981-1990
1991-2000

Spatial distribution of decadal annual precipitation, surface air temperature, estimated ground temperature, and estimated snow mass balance in areas north of 50oN, in relation to the average for the 20th century. The significance of the colours used is described in the text above, and is also indicated in the individual diagrams by a scale. The time range is indicated by a number: 1 = January, 2 = February, etc. Click on the individual small diagrams to open full-size diagrams. Similar temperature diagrams showing both polar regions since 2005 can be seen by clicking here. Data source: NASA Goddard Institute for Space Studies (GISS).

 

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Comments to the diagram table above

The complexity of climatic change during the 20th century is apparent from the diagrams above. Some regions may become wetter, while other nearby simultaneously become more arid (dry). Some may become warmer, while other nearby simultaneously become cooler. Clearly the configuration of jet streams and local wind conditions by orographic effects often exercise the main control on the amount of recorded precipitation changes at individual sites (column 2-4), while the influence of air temperature is smaller. At the same time, there is a overall tendency to relatively low precipitation at many sites until the end of the Little Ice Age (around 1920), perhaps associated with low air temperatures. On the other hand, the cooling period 1941-1980 appears to have been characterized by generally increasing precipitation in many regions north of 50oN, which is in contrast to what would be expected from the air temperature change. Following the general warming after 1981, precipitation apparently has continued the overall, slow increase. It remains to be investigated how much of the apparent 20th precipitation increase (both during warming and cooling periods) are the result of gradually improved design of meteorological gauging stations.

The temperature diagrams (columns 5-7) are interesting to compare with known historical developments. At the end of the 1921-1930 Arctic warming, a number of very mild winters were experienced in West Greenland , most notably in 1929, where no landfast sea ice formed at all and hunters were able to use kayaks throughout the whole winter (Humlum 1999). Today, landfast sea ice usually forms in this region of Greenland . At the same time new fish species appeared in the central West Greenland region, causing a significant change in mans hunting habits towards open sea fishing (Humlum 1999). The most important new species was the Greenland cod, locally known as the big-headed cod, which presumably is identical to the Icelandic cod. Most likely, larvae of the Icelandic cod spread to SE Greenland waters in the early 1920’ies, driven by an enhanced Irminger Current, and for some decades were able to reproduce and survive in SW Greenland waters (Vilhjálmsson and Jakobsson 1998). This species represented a very important resource in Greenlandic economy for about 15-20 years, after which a decline set in, ending with a number of disastrous fishing seasons in the 1960s as climate and sea water around Greenland again was colong down. The retreating Arctic sea ice after 1920 also made the first navigation of the NE Passage without wintering possible in 1932 for the Russian trawler Alexander Sibiryakow. A few years later, in August 1942, the reduced sea ice along the coasts of the Arctic Ocean prompted the German naval high command to order the pocket battleship Admiral Scheer into the Kara Sea, east of Novaya Zemlya . This action was taken to intercept suspected allied convoys from US and Canada with supplies to the Red Army, supposedly taking advantage of the extraordinary open water conditions along the Russian and Siberian coasts at that time (Huan 1958). During this operation “Wunderland”, Admiral Scheer, operating without ice protection for its exposed propellers as far east as 100oE, met and promptly sank famous Alexander Sibiryakow. The background for the German naval concern was the fact that a German former freighter converted into an armed raider, the Komet, passed through the Northern Sea Route en route from the Atlantic to the Pacific Ocean in early summer 1940 (Barr 1975; Flaherty 2004). Passing north of Siberia , the Komet then ducked down through the Bering Strait to begin its raiding career in the Pacific. The Allies, however, did not have information on the reduced sea ice cover along the coasts of Russia and Siberia and therefore never attempted to make use of the NE Passage during the war.

The ground thermal diagrams (column 8) confirm the general tendency of increasing temperatures in the topmost part of permafrost, as recently documented from several Arctic regions. It is however also clearly seen, that the now 5-15 year long dataseries of permafrost temperatures represent very short periods, even when considered within the short instrumental period (the 20th century). As an example, the ground temperature diagrams give reason to believe that presently rising permafrost temperatures in Alaska and parts of Siberia may well have been preceded by 20-30 years of decreasing ground temperatures, and that there were periods of both ground warming and cooling before that.

The snow mass balance diagrams (column 9) reveal several geographical features documented by observed glacier variations and glaciological mass balance investigations. As one example, many glaciers in the Alps advanced in the years up to 1923, where a 20th century maximum frontal position was reached, today documented by well-defined moraine systems. Another example is the widespread positive mass balance for glaciers in the Alps between 1960 and 1980, a tendency also indicated by the snow mass balance diagrams. Other examples are advancing glaciers many places in Greenland between 1980 and 1990, and positive mass balance for glaciers western Norway 1990-2000 due to high winter precipitation.

 

 

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Current Surface Mass Budget of the Greenland Ice Sheet 

Date: 28 March 2017. Left: Map of the surface mass balance today (in mm water equivalent per day). Right: The average surface mass balance for today’s calendar date over the period 1990-2013. Click here for picture source and latest update. Courtesy of Danish Meteorological Institute (DMI).

 

 

 

Date: 28 March 2017. Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt) and the season 2011-12 (red) which had very high summer melt in Greenland. For comparison, the mean curve from the period 1990-2013 is shown (dark grey). The same calendar day in each of the 22 years (in the period 1990-2013) will have its own value. These differences from year to year are illustrated by the light grey band. For each calendar day, however, the lowest and highest values of the 22 years have been left out. Click here for picture source and latest update. Courtesy of Danish Meteorological Institute (DMI).

 

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Click here to se recent variations of air temperature in the northern hemisphere polar region.

Click here to read more about meteorological conditions in the Arctic. 

Click here for an update on present global, Arctic or Antarctic meteorological conditions.