Precipitation
The MMD simulations show a general increase in precipitation over the Arctic at the end of the 21st century (Table 11.1; Supplementary Material Figure S11.28). The precipitation increase is robust among the models (Table 11.1; Supplementary Material Figure S11.19) and qualitatively well understood, attributed to the projected warming and related increased moisture convergence (Section 10.3). The very strong correlation between the temperature and precipitation changes (approximately a 5% precipitation increase per degree celsius warming) across the model ensemble is worth noting (Figure 11.20). Thus, both the sign and the magnitude (per degree warming) of the percentage precipitation change are robust among the models.
The spatial pattern of the projected change (Supplementary Material Figure S11.28) shows the greatest percentage increase over the Arctic Ocean (30 to 40%) and smallest (and even slight decrease) over the northern North Atlantic (<5%). By the end of the 21st century, the projected change in the annual mean arctic precipitation varies from 10 to 28%, with an MMD-A1B ensemble median of 18% (Table 11.1). Larger (smaller) mean precipitation increases are found for the A2 (B1) scenario with 22% (13%). The percentage precipitation increase is largest in winter and smallest in summer, consistent with the projected warming (Figure 11.19; Table 11.1). The across-model scatter of the precipitation projections is substantial (Figure 11.19; Table 11.1). The Tebaldi et al. (2004a) 5th to 95th percentile confidence interval of percentage precipitation change in winter is 13 to 36% and in summer 5 to 19% (Supplementary Material Table S11.2).
Differences between the projections for different scenarios are small in the first half of the 21st century but increase later. Differences among the models increase rapidly as the spatial domain becomes smaller (ACIA, 2005). The geographical variation of precipitation changes is determined largely by changes in the synoptic circulation patterns. During winter, the MMD models project a decreased (increased) frequency of strong Arctic high (Icelandic low) pressure patterns that favour precipitation increases along the Canadian west coast, southeast Alaska and North Atlantic extending into Scandinavia (Cassano et al., 2006). Projections with RCMs support the broad-scale messages while adding expected local and regional detail (ACIA, 2005).
By the end of the 21st century, the MMD-A1B ensemble-projected precipitation increase is significant (Table 11.1), particularly the annual and cold season (winter/autumn) precipitation. However, local precipitation changes in some regions and seasons (particularly in the Atlantic sector and generally in summer) remain difficult to discern from natural variability (ACIA, 2005).