Studies of the Early-1990s Climate Shift in the Pacific and Its Linkage to the Atlantic Multi-decadal Oscillation

太平洋气候和上覆大气的变化具有世界性影响，而这种变化的原因和后果是气候动力学中的重要课题。其中一个变化是1990年代初的太平洋气候变化，其中1)厄尔尼诺/南方涛动(ENSO)事件的位置从赤道东太平洋转移到赤道中部太平洋；2)北半球季风的年际变化急剧减少；3)热带西太平洋海平面开始以更快的速度上升；4)北赤道流和逆流向南迁移。这个项目测试了这样一个假设，即1990年代的气候变化与副热带太平洋高压的加强有关，副热带太平洋高压是一个准静止的高海平面压力区域，从加利福尼亚州海岸延伸到日期变更线之外，覆盖了副热带北太平洋的大部分地区。当副热带太平洋高压加强时，其周围的地表风速加快，且风速越大，对该地区海气耦合的作用越强。该奖项下的工作审查了海洋-大气耦合的加剧可能导致上述四种气候变化影响的程度。该项目进一步检验了这一假设，即太平洋高压的加强是对同时发生的大西洋多年代际涛动(AMO)逆转的反应。AMO是一种缓慢变化的海表面温度(SST)变化模式，其中赤道以北的大西洋要么比平均温度高，要么比平均温度低，在1990年左右有一个从冷到暖的转变。这项研究是使用一套气候模型的模拟来进行的，该模型以不同的方式配置以检验假设。在一种配置中，在全球范围内规定SST，并在大西洋施加AMO模式，以便评估太平洋高压对AMO的响应。其他配置使用部分或完全交互的海洋模式，允许太平洋高压的变化影响SST和次表层海洋。这些构型可以检验海-气耦合的变化及其假设的后果。除了科学上的兴趣外，20世纪90年代太平洋气候变化由于其世界性影响而具有社会重要性。季风系统的变化对水资源是重要的，而从东太平洋ENSO事件向中太平洋ENSO事件的转移对美国天气和气候的影响也有影响。此外，该项目还为一名博士后助理和两名研究生提供支持和培训，从而为这一领域的下一代研究人员提供支持。

Changes in the climate of the Pacific Ocean and overlying atmosphere have worldwide consequences, and the causes and consequences of such changes are important topics in climate dynamics. One such change is the Pacific climate shift of the early 1990s, in which 1) the location of El Nino/Southern Oscillation (ENSO) events shifted from the eastern equatorial Pacific to the central equatorial Pacific; 2) the year-to-year variability of the Northern Hemisphere monsoons decreased dramatically; 3) sea level in the western tropical Pacific began rising at a faster rate; and 4) the north equatorial current and countercurrent migrated southward. This project tests the hypothesis that the 1990s climate shift is related to an intensification of the subtropical Pacific high, a quasi-stationary region of high sea level pressure which extends past the dateline from the California coast and covers much of the subtropical North Pacific. When the subtropical Pacific high intensifies the surface winds circulating around it speed up, and the higher speed has the effect of strengthening the coupling of the ocean and atmosphere in the region. Work under this award examines the extent to which the intensification of ocean-atmosphere coupling can lead to the four climate shift impacts listed above. The project further tests the hypothesis that the intensification of the Pacific high occurred in response to the reversal of the Atlantic Multi-decadal Oscillation (AMO) which occurred at the same time. The AMO is a slowly varying mode of sea surface temperature (SST) variation in which the Atlantic north of the equator is either warmer or colder than average, with a transition from colder to warmer SSTs around 1990. The research is conducted using a suite of simulations from a climate model configured in various ways to test the hypotheses. In one configuration SSTs are prescribed globally, with the AMO pattern imposed in the Atlantic so that the response of the Pacific high to the AMO can be assessed. Other configurations use a partially or fully interactive ocean model, allowing the changes in the Pacific high to affect SSTs and the subsurface ocean. These configurations allow the changes in ocean-atmosphere coupling and their hypothesized consequences to be examined.Aside from its scientific interest, the 1990s Pacific climate shift has societal importance due to its worldwide impacts. The variability of monsoon systems is consequential for water resources, while the shift from eastern to central Pacific ENSO events has implications for the impacts of these events on the weather and climate of the US. In addition, the project provides support and training for a postdoctoral associate and two graduate students, thereby providing for the next generation of researchers in this field.