Variability of the Northern Annular Mode’s signature in winter sea ice concentration

Krahmann & Visbeck 2003: Polar Research 22(1), 51–57 The Northern Annular Mode (NAM) is, like the closely related North Atlantic Oscillation (NAO) (Visbeck et al. 2001; Visbeck et al. 2003), a large-scale pattern of atmospheric variability in the northern hemisphere (Thompson & Wallace 1998). It represents substantial variations of the northern hemisphere midand high latitude atmos pheric circulation on a broad range of time scales from weeks to multiple decades. As the winds change they exhibit infl uence on many parts of the climate system such as ocean cur rents, surface heat fl uxes and precipitation. A further component of the system that is strongly infl uenced by the NAM/NAO is the sea ice in the Arctic. Between high and low states of the NAM/NAO, thickness, concentration, extent and advec tion of the Arctic sea ice vary substantially (e.g. Deser et al. 2000). Observations revealed that the atmospheric circulation pattern over the Arctic changed substantially over the past decades (Walsh et al. 1996). This change is tightly linked to a shift in the preferred state of the NAM/NAO over the same period (Thompson & Wallace 1998) and has been named as the cause for the observed long term decrease in summer sea ice extent in the Arctic (Chapman & Walsh 1993; Parkinson et al. 1999; Deser et al. 2000). In contrast to the summer ice extent, the winter sea ice extent has not changed signifi cantly (Deser et al. 2000). It exhibits, however, signifi cant year-to year-variability, which is the subject of our study. Studies of Arctic sea ice have been able to establish the large-scale pattern of its variations and their evolution over the past decades (e.g. Mysak & Manak 1989; Deser et al. 2000; Vinje 2001). In particular, the strongest signal in winter sea ice extent (identifi ed as its fi rst empirical orthogonal function) has been found to be highly correlated with the NAM/NAO (Deser et al. 2000; Vinje 2001). It is characterized by a seesaw of concentration anomalies between the Labrador Sea and the Greenland, Iceland and Barents seas. Variability of the Northern Annular Mode’s signature in winter sea ice concentration