MASIE and NSIDC are two different sea ice indexes. The difference between the two methodologies is here.

“MASIE relies on data from the Interactive Multisensor Snow and Ice Mapping System (IMS) that runs at the National Ice Center (NIC). The IMS product uses several satellite data sources including passive microwave, but it is also based on visual analysis and other data sources and undergoes a form of manual data fusion.”

“The Sea Ice Index (SII) relies on satellite passive microwave data as its only data source. These data are automatically processed using an algorithm and have known biases and limitations;”

Ron Clutz made a comment here that had me thinking what was the difference between the two methodologies over time? The comparison starts in 2006 because that is the oldest MASIE data available.The value being graphed is the NSIDC SII value minus the MASIE value.

As you can see … once upon a time the NSIDC SII was generally higher than MASIE. Now MASIE is generally higher.

What does it mean? I don’t know.