3.2 Recent changes in extremes
We focus first on the nodes with significant frequency changes during winter (#4 and #12) and examine the corresponding impacts on extreme temperatures and precipitation (Fig. 4 ). Increasing frequency of node #4 implies that warm winter extremes occurred more often across much of the domain, but especially in northcentral North America, while cold extremes were more prevalent over much of Alaska and the Gulf of Alaska. The decreasing frequency of node #12 suggests a reduction in anomalously warm temperatures across the mid-latitudes of the domain, along with less frequent intense cold spells in northern Canada and near Alaska. The changes in frequency of occurrence of these two patterns, therefore, result in opposing changes in the frequency of cold events in the Alaskan region. In terms of precipitation extremes (Fig. 4c ), the increase in node #4 is associated with more frequent heavy events along the Pacific Northwest and northcentral Canada, while a decrease in node #12 would tend to offset these tendencies along the west coast.
During summer (Fig. 5 ), the increased frequency of node #1 suggests more (fewer) anomalously warm (cold) days along the west coast, Alaska, and high latitudes along with more frequent cold spells in the central and eastern parts of the continent, consistent with the declining trend in extreme summer heat over the Upper Midwest (Mueller et al., 2017). Unusually wet days are more likely in southern California, the southern Mississippi Valley, and the Canadian Maritimes. The increased occurrence of node #3 suggests more warm days in the Hudson Bay/Canadian archipelago region, more cool extremes in western U.S. states, and wet conditions along the west coast of Canada and southeast Alaska.