Fields of estimated monthly precipitation

Mean annual precipitation is shown in Figure 2.25. Maps for the four mid-season months based on the same data sources appear in Figure 6.3. A number of different

Figure 6.3 Mean precipitation north of 60° N for the four mid-season months, based on data from land stations and the Arctic Ocean with bias adjustments, the NCEP/NCAR reanalysis and satellite retrievals (over open ocean). Contours are at every 10 mm up to 80 mm and at every 50 mm (dotted) for amounts of 100 mm and higher (by the authors).

Figure 6.3 Mean precipitation north of 60° N for the four mid-season months, based on data from land stations and the Arctic Ocean with bias adjustments, the NCEP/NCAR reanalysis and satellite retrievals (over open ocean). Contours are at every 10 mm up to 80 mm and at every 50 mm (dotted) for amounts of 100 mm and higher (by the authors).

monthly station data sets (e.g., Groisman et al., 1991; Mekis and Hogg, 1999; Yang, 1999) were employed for land areas and for the Arctic Ocean. These records, which are of highly variable length, were gridded and then adjusted for gauge undercatch using the same coefficients employed by Legates and Willmott (1990). The fields were then blended with precipitation forecasts from the NCEP/NCAR reanalysis for which systematic biases were removed using statistical distributions from the bias-adjusted gridded station records. The techniques represent an outgrowth of those described by Serreze et al. (2003b). Over open (ice-free) ocean, use was made of updated and improved satellite-derived records from the Global Precipitation Climatology Project (Huffman et al., 1997). Precipitation values over central Greenland rely on interpolation from coastal sites and should be viewed with due caution - results for this region based on snow accumulation and modeling will be examined separately.

For a large part of the Arctic, winter is quite dry. January precipitation totals over much of eastern Eurasia, northern Alaska, northern Canada and the central Arctic Ocean are between 10 and 20 mm. These are regions where cyclone activity is relatively infrequent and which are far removed from Atlantic and Pacific moisture sources. Winter month precipitation totals are much higher in the Atlantic sector. This arises from frequent cyclone activity along the North Atlantic cyclone track, especially in the vicinity of the Icelandic Low, and associated moisture flux convergence. Along the Scandinavian and East Greenland coasts, precipitation is enhanced by orographic uplift of moist airmasses. The decrease in precipitation poleward toward the Kara Sea manifests the poleward decay of the primary cyclone track. The other area of high precipitation is along the Pacific side, and is associated with the northern end of the East Asian storm track and orographic uplift.

By April, the primary cyclone tracks have weakened, seen as a decline in precipitation in the Atlantic sector and on the Pacific side as compared with January. Precipitation over northern Canada, Alaska, eastern Eurasia and the central Arctic Ocean is still quite low. As seen in the July field, the precipitation pattern for summer is quite different. While the primary storm tracks are weakest at this time, the highest totals are still generally found in the Atlantic and Pacific sectors. However, precipitation is at its annual maximum over most land areas. The onset of the summer pattern features a strong transition between May and June. Several processes are responsible for the terrestrial summer maximum. Cyclone activity over land areas is more frequent than in winter (see Chapter 4). Also, following melt of the snow cover, strong heating of the land surface promotes evaporation. Convective precipitation becomes fairly common, at least in the Low Arctic. Summer convective precipitation over land is not unknown even at quite high latitudes. The first author observed convective precipitation during the summer of 1982 over the Hazen Plateau of northern Ellesmere Island (82° N). His fascination turned to dismay when it was discovered that the short aluminum tower on which the meteorological instruments were mounted acted as a lightning rod!

A regional contributor to the summer precipitation maximum over land areas and the central Arctic Ocean is the summer Arctic frontal zone (Serreze et al., 2001). As outlined in Chapter 4, summer sees development of a band of high frontal frequencies along Eurasia from about 60-70° N, best expressed over the eastern half of the continent. A similar band is found over Alaska. Although best developed in summer, the Alaskan feature is present year-round. The high frequency of frontal activity appears to be linked with heating contrasts between the Arctic Ocean and snow-free land, with the coastal baroclinicity sharpened by coastal orography. Cyclogenesis is common in preferred areas over eastern Eurasia, Alaska and Canada where the frontal zone is best expressed.

The hydrologic impact of the frontal zone is evident when plotting the longitudinal distribution of summer frontal frequencies (fronts per day) and the fraction of annual precipitation that falls in summer, averaged from 62.5° N to 67.5° N (Figure 6.4). At 140° E, where the Eurasian frontal zone is well expressed, nearly 60% of annual precipitation falls during summer. Where the Alaskan frontal zone is well expressed,

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Figure 6.4 Longitudinal distribution of summer frontal frequency (fronts per day) and the fraction of annual precipitation falling in summer, averaged from 62.5° to 67.5° N. The precipitation data include bias adjustments (from Serreze etal, 2001, by permission of AMS).

over 50% of annual precipitation falls during summer. The late summer and early autumn precipitation maximum over the Arctic Ocean occurs as cyclones from the weakened, but still present north Atlantic cyclone track and systems generated over Eurasia (many along the frontal zone) migrate into the region. From Yang's (1999) analysis of the North Pole records (Figure 6.1), mean precipitation over the central Arctic Ocean peaks in September at about 30 mm, roughly three times the total for April, the driest month. Note that without bias adjustments, peak precipitation would occur in July and August.

The October field in Figure 6.3 illustrates the transition back toward the winter pattern. For land areas except west-central Eurasia, precipitation has declined from its summer maximum. This is also true for most of the Arctic Ocean (again see Figure 6.1). However, it picks up sharply in the Atlantic and Pacific sectors as the primary storm tracks gain strength.

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