Descriptions and mechanisms for 1998/99 regime shift over the North Pacific

对1998/99年前后北太平洋的气候变化进行了一些研究，也进行了一些气候研究，这些研究可能是了解这种快速气候变化的基础。这里描述了一些主要的结果。这项研究的首席研究员先前提出，北太平洋的气候变化似乎是由20年振荡和50-70年振荡之间的同步引起的。为了了解这种共振的一些基本机理，研究了具有非线性反馈周期调制和具有周期强迫的简单延迟振荡模型。阐明了与阈值概念相关的非线性共振的基本机理。研究发现，1998/99年前后，北太平洋周边边缘海域的海冰发生了实质性变化。在白令海，东部白令海春季海冰在1999年前后减少。这种变化很可能是由低气压异常引起的。在鄂霍次克海，虽然海冰减少的长期趋势持续到20世纪90年代中期，但在20世纪90年代末和21世纪初，海冰面积比平时大。研究发现，对于海冰的这种长期波动，以前的研究重点是秋季的大气条件，而不是冬季的大气条件。对于大尺度的海洋锋面区，我们使用新网格化的高分辨率海温数据（该数据集可通过互联网获得）来研究海温的变化，包括1998/99年的变化。你可以通过谷歌找到“前田美子1dg SST”)。1998/99年副热带锋变化较大，次极锋变化不大。这与20世纪70年代的气候变化形成对比，当时的信号在次极锋中更为突出。另外，与气候变化有关的阿留申低压冬季强度在去掉线性趋势后呈现双峰型。这表明非线性反馈在状态转移的气候反馈中是重要的。少

A number of studies are conducted for a climate change over the North Pacific around 1998/99, as well as climate studies which may be a basis of understanding of such a rapid climate change. Here, some major results are described.The principal investigator of this research proposed previously that climatic regime shifts over the North Pacific appeared to be caused as synchronization between a 20-year oscillation and 50-70-year oscillation. In order to understand some basic mechanism of such resonance, a simple delayed oscillator model with periodic modulation of nonlinear feedback and that with periodic forcing are investigated. The basic mechanism of the nonlinear resonance associated with a concept of threshold has been clarified.It was found that substantial changes of sea-ice occurred around 1998/99 in marginal seas around the North Pacific. In the Bering Sea, spring sea-ice reduced around 1999 in the eastern Bering Sea. This change was likely to be caused by a low-pressure anomaly … More over Alaska. In the Okhotsk Sea, although long-term trend of sea-ice reduction continued until the mid-1990s, in the late 1990s and the early 2000s, sea-ice areas were larger than usual. It was found that for this long-term fluctuation of sea-ice, atmospheric conditions in autumn, not in winter previous studies focused, are important.For large-scale oceanic frontal zones, SST changes including those associated with 1998/99 change were investigated using a newly gridded high-resolution SST data (The dataset is available through the internet. You can find via google with "Minobe Maeda 1dg SST"). It was found that the subtropical front exhibited a large change in 1998/99, but not in the subpolar front. This is in contrast to the 1970s climatic regime shift, when a signal is more prominent in the subpolar front.Also, it is found that the winter strength of the Aleutian Low, which is a major atmospheric circulation changes associated with the regime shifts, exhibits bimodality after removal of a linear trend. This suggests that the nonlinear feedback is important in the climate feedback underlying the regime shifts. Less