Science Source

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

States that in 2015, the sea ice extent (SIE) over the Sea of Okhotsk (Okhotsk SIE) in the Arctic hit a record low since 1979 during February–March, the period when the sea ice extent generally reaches its annual maximum
To quantify the role of anthropogenic influences on the changes observed in Okhotsk SIE, this study employs a fraction of attributable risk (FAR) analysis to compare the probability of occurrence of extreme Okhotsk SIE events and long-term SIE trends using phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel simulations performed with and without anthropogenic forcing
Finds that because of anthropogenic influence, both 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 4 times (FAR = 0.76 to 1)
Finds that a strong negative phase of the North Pacific Oscillation (NPO) during midwinter (January–February) 2015 also contributed to the 2015 extreme SIE event
Conducts an analysis based on multiple linear regression to quantify relative contributions of the external forcing (anthropogenic plus natural) and the NPO (internal variability) to the observed SIE changes
Finds about 56.0% and 24.7% of the 2015 SIE anomaly was estimated to be attributable to the external forcing and the strong negative NPO influence, respectively
Finds the external forcing explains about 86.1% of the observed long-term SIE trend