Springtime North Pacific Oscillation and Summer Sea Ice in the Beaufort Sea

Our study investigates the interannual variability of springtime NPO (North Pacific Oscillation) and its impacts on the following summer sea ice in the Beaufort Sea. The NPO is defined as the third rotated EOF mode of the spring (April-June) SLP (sea level pressure) poleward of 20^oN. It accounts for 10.5% of the total SLP variance. A positive NPO exhibits the intensified and northeastward extended Aleutian Low and North Pacific High. It also leads to a strong Beaufort high because of the decreased cyclone activity over the western Arctic Ocean. In positive NPO years, the East Asia trough is deep, and the polar vortex tends to be weak. Therefore, more cyclones in Siberia move to the sub-polar Pacific region and fewer into the Chukchi-Beaufort seas. Furthermore, weak baroclinicity in the Beaufort-Chukchi Seas does not favor the growth of cyclones. In addition, the intensified East Asia trough leads to an increased warm air advection from the Pacific Ocean toward North America and the coastal areas in the Arctic.

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.

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