Prolonged Periodicity and Eastward Shift of the January North Pacific Oscillation Since the Mid-1990s and Its Linkage With Sea Ice Anomalies in the Barents Sea

The present study mainly investigates changes in the periodicity and location of the January North Pacific Oscillation (NPO) and the underlying mechanisms during 1980-2018. The periodicity of the January NPO prolongs from 2-3 years to 5-8 years around the mid-1990s. Corresponding to the prolonged periodicity, both the southern and northern lobes of the January NPO shift eastward, and their meridional movement is relatively weak. As a result, the impacts of the January NPO on surface air temperature in North America are enhanced. The changes in the periodicity and location of the January NPO differ from those of the NPO of other winter months. Further study shows that the changes in the January NPO are closely related to the sea ice anomalies in the Barents Sea in the preceding November-December and January. The sea ice anomalies also display a periodicity of 5-8 years after the mid-1990s, and their impacts on the January NPO are enhanced. The January NPO is significantly and negatively correlated with the sea ice anomalies after the mid-1990s. The sea ice anomalies contribute to the prolonged periodicity and eastward shift of the January NPO by exciting a quasi-stationary Rossby wave propagating eastward along the coast of northern Eurasia and resultant atmospheric dynamic processes. The physical mechanism is reasonably reproduced by the numerical experiment with observed sea ice anomalies in the Barents Sea prescribed. In addition, possible reasons for the enhanced impacts of the sea ice anomalies on the January NPO are discussed.