Precursor role of winter sea-ice in the Labrador Sea for following-spring precipitation over southeastern North America and western Europe

Abstract

The role of winter sea-ice in the Labrador Sea as a precursor for precipitation anomalies over southeastern North America and Western Europe in the following spring is investigated. In general terms, as the sea ice increases, the precipitation also increases. In more detail, however, analyses indicate that both the winter sea-ice and the sea surface temperature (SST) anomalies related to increases in winter sea-ice in the Labrador Sea can persist into the following spring. These features play a forcing role in the spring atmosphere, which may be the physical mechanism behind the observational relationship between the winter sea-ice and spring precipitation anomalies. The oceanic forcings in spring include Arctic sea-ice anomalies and SST anomalies in the tropical Pacific and high-latitude North Atlantic. Multi-model Coupled Model Intercomparison Project Phase 5 and Atmospheric Model Intercomparison Project simulation results show that the atmospheric circulation response to the combination of sea-ice and SST is similar to that observed, which suggests that the oceanic forcings are indeed the physical reason for the enhanced spring precipitation. Sensitivity experiments conducted using an atmospheric general circulation model indicate that the increases in precipitation over southeastern North America are mainly attributable to the effect of the SST anomalies, while the increases overWestern Europe are mainly due to the sea-ice anomalies. Although model simulations reveal that the SST anomalies play the primary role in the precipitation anomalies over southeastern North America, the observational statistical analyses indicate that the area of sea-ice in the Labrador Sea seems to be the precursor that best predicts the spring precipitation anomaly.

摘要

当冬季拉布拉多海海冰增加时, 北美东南部和欧洲西部的春季降水会增加. 因此, 冬季拉布拉多海海冰可以作为春季降水的一个预测因子. 进一步的分析表明, 海冰异常和与之同时出现的海温异常都能够持续至春季, 并对春季大气环流产生影响, 从而导致降水异常. 这是冬季海冰和春季降水产生联系的主要原因, CMIP5的AMIP试验也证明了这一点. 在这一联系中, 海冰和海温各自起到何种作用呢? 大气环流模式敏感性试验表明, 北美东南部的降水主要是由海温异常造成; 欧洲西部的降水主要由海冰异常造成. 值得注意的是, 尽管模式试验表明海温异常主要造成北美东南部的降水异常, 但是冬季的拉布拉多海海冰异常是最好的统计预测因子, 因为我们无法从观测中分离出上述海温异常信号.