CMIP5 models have been shown to exhibit rapid cooling events in their projections of the North Atlantic subpolar gyre. Here, we analyze the CMIP6 archive, searching for such rapid cooling events in the new generation of models. Four models out of 35 exhibit such instabilities. The climatic impacts of these events are large on decadal timescales, with a substantial effect on surface temperature over Europe, precipitation pattern in the tropics—most notably the Sahel and Amazon regions—and a possible impact on the mean atmospheric circulation. The mechanisms leading to these events are related to the collapse of deep convection in the subpolar gyre, modifying profoundly the oceanic circulation. Analysis of stratification in the subpolar gyre as compared with observations highlights that the biases of the models explain relatively well the spread in their projections of surface temperature trends: models showing the smallest stratification biases over the recent period also show the weakest warming trends. The models exhibiting abrupt cooling rank among the 11 best models for this stratification indicator, leading to a risk of encountering an abrupt cooling event of up to 36.4%, slightly lower than the 45.5% estimated in CMIP5 models.

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关于 CMIP6 模型中北大西洋副极地环流突变的风险

CMIP5 模型已被证明在其对北大西洋次极地环流的预测中表现出快速冷却事件。在这里，我们分析 CMIP6 档案，在新一代模型中搜索此类快速冷却事件。35 个模型中有四个模型表现出这种不稳定性。这些事件的气候影响在年代际尺度上很大，对欧洲的地表温度、热带地区的降水模式（尤其是萨赫勒和亚马逊地区）产生重大影响，并可能对平均大气环流产生影响。导致这些事件的机制与次极地环流中深层对流的崩溃有关，深刻地改变了海洋环流。与观测相比，对次极地环流分层的分析强调，模型的偏差相对较好地解释了它们对地表温度趋势预测的传播：显示最近一段时间最小分层偏差的模型也显示出最弱的变暖趋势。在该分层指标的11个最佳模型中表现出突然冷却的模型，导致遇到突然冷却事件的风险高达36.4%，略低于CMIP5模型中估计的45.5%。