220.127.116.11 Key Processes
Arctic climate is characterised by a distinctive complexity due to numerous nonlinear interactions between and within the atmosphere, cryosphere, ocean, land and ecosystems. Sea ice plays a crucial role in the arctic climate, particularly through its albedo. Reduction of ice extent leads to warming due to increased absorption of solar radiation at the surface. Substantial low-frequency variability is evident in various atmosphere and ice parameters (Polyakov et al., 2003a,b), complicating the detection and attribution of arctic changes. Natural multi-decadal variability has been suggested as partly responsible for the large warming in the 1920s to 1940s (Bengtsson et al., 2004; Johannessen et al., 2004) followed by cooling until the 1960s. In both models and observations, the interannual variability of monthly temperatures is at a maximum at high latitudes (Räisänen, 2002). Natural atmospheric patterns of variability on annual and decadal time scales play an important role in the arctic climate. Such patterns include the NAM, the NAO, the Pacific-North American (PNA) pattern and the Pacific Decadal Oscillation (PDO), which are associated with prominent arctic regional precipitation and temperature anomalies (see Box 3.4 and Section 3.6). For instance, the positive NAM/NAO phase is associated with warmer, wetter winters in Siberia and colder, drier winters in western Greenland and north-eastern Canada. The NAM/NAO showed a trend towards its positive phase over the last three to four decades, although it returned to near its long-term mean state in the last five years (see Section 3.6). In the future, global models project a positive trend in the NAO/NAM during the 21st century (see Section 10.3). There was substantial decadal-to-inter-decadal atmospheric variability in the North Pacific over the 20th century, associated with fluctuations in the strength of the winter Aleutian Low that co-vary with North Pacific SST in the PDO (see Section 3.6). A deeper and eastward-shifted Aleutian Low advects warmer and moister air into Alaska. While some studies have suggested that the Brooks Range effectively isolates arctic Alaska from much of the variability associated with North Pacific teleconnection patterns (e.g., L’Heureux et al., 2004), other studies find relationships between the Alaskan and Beaufort-Chukchi region’s climate and North Pacific variability (Stone, 1997; Curtis et al., 1998; Lynch et al., 2004). Patterns of variability in the North Pacific, and their implications for climate change, are especially difficult to sort out due to the presence of several patterns (NAM, PDO, PNA) with potentially different underlying mechanisms (see Chapter 3).