The spring NPO accounts for 29% of the interannual variability of the following September sea ice cover in the Beaufort Sea. It leads to the occurrence of earlier sea ice retreat in the Beaufort Sea by as much as one week. During positive NPO events, due to the intensified and northeastward extended Aleutian Low and the strong Beaufort High, the strong easterly winds in the Beaufort Sea enhance ice advection and reduce ice thickness. On the synoptic scale, there are more occurrences of the pack ice becoming detached from the coast due to the strong easterly winds in early spring. Moreover, due to the ice loss in the Beaufort Sea, the ocean gains more shortwave radiation and the melting of ice is accelerated. Meanwhile, the increased warm air advection over the northwestern America results in increased downward longwave radiation along the Beaufort Sea coast. Our results also show that thinner ice in spring fosters a stronger summer ice-albedo feedback in the following summer. Therefore, the springtime NPO can act as a potential predictor for sea ice melting in the Beaufort Sea. Although the anomalous warm air associated with spring NPO accelerates snow melting in the Mackenzie River Basin, the resulting increased discharge of the Mackenzie River occurs in early spring, when the river water is still close to zero. Therefore, the heat content associated the increased river discharge has a minor impact on the melting of ice in the Beaufort Sea.
Although AO is the dominant climate mode in the Arctic, there is a weak correlation between spring-time AO and September sea ice in the Beaufort Sea (figure not shown). The cyclone activity associated with the AO is mainly located in the eastern Arctic (see previous studies by Proshutinsky et al., 2015), and its impacts in the Western Arctic Ocean are relatively weak. In contrast, NPO captures the variations of atmospheric forcing over the Beaufort Sea, suggesting that the local atmospheric forcing plays an essential role in the interannual variability of sea ice melt in the Beaufort Sea.
Finally, the NPO is an atmospheric dynamic mode that can maintain itself through baroclinic conversion of available potential energy from the climatological mean flow (Tanaka et al. 2016). However, the strengths and positions of the troughs and ridges related to the planetary waves can be modified by the thermal state of the surface boundary layer, including the land snow depths and sea surface temperatures needed to trigger the NPO pattern (Horel and Wallace 1981; Frankignoul et al. 2011; Hurwitz et al. 2012). For example, previous studies have pointed out a linkage between the NPO pattern and ENSO (e.g., Horel and Wallace 1981; Trenberth et al. 1998). Frankignoul et al. (2011) and Hurwitz et al. (2012) have argued that SST anomalies in the western North Pacific are also likely to trigger the NPO pattern as well. We find that the spring NPO shows a significant correlation with the preceding winter “El Nino Modoki” with a coefficient of 0.39, implying the possibility to investigate the linkage or predictability of summer Beaufort Sea ice from conditions related to the tropical SST.
Proshutinsky et al., (2015) Arctic circulation regimes. Phil. Trans. R. Soc. A, 373(2052), 20140160.
Tanaka, S. et al. (2016) Vertical Structure and Energetics of the Western Pacific Teleconnection Pattern. J. Clim., 29(18), 6597-6616.
Horel, J. D., and J. M. Wallace, (1981) Planetary-scale atmospheric phenomena associated with the Southern Oscillation. Mon. Wea. Rev., 109, 813–829.
Frankignoul, C., et al., (2011) Influence of the meridional shifts of the Kuroshio and the Oyashio Extensions on the atmospheric circulation. J. Clim., 24, 762–777,
Hurwitz, M. M. et al. (2012) On the influence of North Pacific sea surface temperature on the Arctic winter climate. J. Geophys. Res., 117, D19110, doi:https://doi.org/10.1029/2012JD017819.
Trenberth, K. E.et al. (1998). Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures. Journal of Geophysical Research: Oceans, 103(C7), 14291-14324.