Attributing Causes of 2015 Record Minimum Sea-Ice Extent in the Sea of Okhotsk

Abstract

In 2015, the sea-ice extent over the Sea of Okhotsk (Okhotsk SIE) hit a record lowest since 1979 during February-March, in which the sea-ice extent generally reaches annual maximum. To quantify the role of anthropogenic influences on the changes observed in Okhotsk SIE, this study employed a Fraction of Attributable Risk (FAR) analysis to compare the probability of occurrence of extreme Okhotsk SIE events and long-term SIE trends using Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model simulations performed with and without anthropogenic forcing. It was found that due to anthropogenic influence, the probability of extreme low Okhotsk SIEs that exceed the 2015 event, and the observed long-term trends during 1979？2015, have increased by more than four times (FAR = 0.76 to 1). In addition, it is suggested that a strong negative phase of the North Pacific Oscillation (NPO) during mid-winter (January-February) 2015 also contributed to the 2015 extreme SIE event. An analysis based on multiple linear regression was conducted to quantify relative contributions of the external forcing (anthropogenic plus natural) and the NPO (internal variability) to the observed SIE changes. About 56.0% and 24.7% of the 2015 SIE anomaly were estimated to be attributable to the external forcing and the strong negative NPO influence, respectively. The external forcing was also found to explain about 86.1% of the observed long-term SIE trend. Further, projections from the CMIP5 models indicate that a sea ice free condition may occur in the Sea of Okhotsk by the late 21st century in some models.