Changes in the frequency of cold and warm synoptic weather types within eastern North America indicate the occurrence of a cool season climate shift in the late 1990s. This aligns with the timing of shifts toward earlier spring ice out on freshwater lakes within the region and less frequent cold air mass passage over the North American Great Lakes. While a preliminary shift occurred in the early 1980s, a pronounced shift toward more frequent warm weather types during November through April occurred after 1997, when prominent warming of the lower atmosphere over the region began. The late 1990s’ climate shift appears linked to middle atmospheric circulation change associated with phase changes for two influential low-frequency climate teleconnections originating within the Atlantic and Pacific Oceans, the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. The climate shift, approximately 20 years after warming within the global climate system commenced, further illustrates the importance of climate system variability within anthropogenically driven climate change.

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1990 年代后期北美东部凉爽的季节气候变化

北美东部冷暖天气类型频率的变化表明 1990 年代后期发生了凉爽季节气候变化。这与该地区淡水湖上较早的春季结冰以及北美五大湖上空冷空气团较少频繁通过的时间一致。虽然在 1980 年代初期发生了初步转变，但在 1997 年之后发生在 11 月至 4 月期间明显转向更频繁的温暖天气类型，当时该地区低层大气开始显着变暖。1990 年代后期的气候变化似乎与中间大气环流变化有关，这些变化与起源于大西洋和太平洋的两个有影响的低频气候遥相关的相变有关，大西洋年代际涛动和年代际太平洋涛动。在全球气候系统开始变暖大约 20 年后，气候变化进一步说明了气候系统变异在人为驱动的气候变化中的重要性。