中文摘要：

太平洋年代际振荡（PDO）和北太平洋涡旋振荡（NPGO）是北太平洋（20°～60°N，120°E～120°W）海温（SST）的EOF前两个模态，本文通过比较1990年前后北太平洋冬季SST EOF前两个模态，揭示了PDO和NPGO在1990年前后特征，并从关键区海温变化、北太平洋涛动（NPO）、赤道太平洋中部变暖（CPW）和北极涛动（AO）的影响，揭示了北太平洋主模态在1990年之后调整的成因。我们发现，1990年之前，北太平洋SST场的EOF前两个模态与PDO和NPGO的空间结构类似，但是在1990年之后，SST的EOF第一模态的最大荷载中心向日界线移动，40°N以北的太平洋被正的SST异常控制，表现出与NPGO模态的负位相相似的空间分布特征，而 EOF 第二模态由偶极子演变成了三极子结构。北太平洋中部（28°～36°N，152°～178°W）和北太平洋北部（44°～49°N，151°～177°W）海温距平在1990年之后呈显著的负相关变化，是导致在1990年之后冬季NPGO成为主模态的内部原因，而NPO在1990年之后的显著增强则是重要的外部原因。分析显示，NPO在1980年开始表现出增强趋势，通过风生流机制，NPO可以增强北太平洋45°N附近的气压梯度和西风异常幅度，从而导致了1990年之后NPGO海温模态的加强。虽然CPW和AO对NPO的南支（夏威夷）和北支（阿拉斯加）的海平面气压异常中心加强有贡献，但是上述两个因子与 NPGO 之间的关系在1990年之前并不明显。因此，CPW 和AO与NPGO之间并不存在稳定的物理联系。

英文摘要：

The Pacific Decadal Oscillation （PDO） and North Pacific Gyre Oscillation （NPGO） are generally referred to as the first two EOF modes of Sea Surface Temperature （SST） in the North Pacific （20°–60°N, 120°E–120°W）. In the present study, the authors compared the first two EOF modes of winter SST anomalies in North Pacific for the period before and after 1990. Based on the impacts of key regional SST changes, the North Pacific Oscillation （NPO）, Central Pacific Warming （CPW）, as well as Artic Oscillation （AO）, the authors discuss the possible causes of the first mode of the NPGO after 1990. Our results suggest that before 1990, the first two EOF modes of SST exhibit the PDO and NPGO pattern, respectively. However, after 1990 the center of the maximal load of the first EOF tilts to the dateline, and gives rises to positive SST anomalies north of 40°N, resulting in a negative phase of NPGO; meanwhile, the previous NPGO changes from a dipole pattern to the triple mode. Analyses suggest that the negative correlation between the changes of SST in the northern （44°–49°N, 151°–177°W） and central （28°–36°N, 152°–178°W） North Pacific could have resulted in the internal shift of the dominant SST modes after 1990. Evidence shows that the NPO played an important external forcing in enhancing the NPGO after 1990. Based on wind-driven oceanic current theory, the increased amplitude of NPO in Sea Level Pressure （SLP） may intensify the polar gradient of the SLP along 45°N, enhancing the zonal sea surface winds, and causing the NPGO mode to prevail after 1990. The increase in CPW and AO may have enhanced the NPO by intensifying the southern and northern branch of the NPO in Hawaii and Alaska after 1990, but there is no evidence to show that both these factors were closely related to the NPO before 1990.