We tend to focus on the end of the melting season, also known as the minimum, because that's when the ice covers the least amount of water. That's all fine and dandy, but of course, it's just an arbitrary measuring point. For instance, this year's minimum for the JAXA sea ice extent data set was 6th lowest on record, but flash forward to the first week of October and this year's trend line is third lowest on record and could soon be second lowest on record:
This, as always, had to do with the weather, which has been pretty crazy the past two weeks. First, fierce winds pushed back against the ice edge in most of the Arctic Ocean, delaying the ice pack's expansion, while at the same time continuing to melt that last, stubborn sliver of ice extending towards the East Siberian Sea. The result has been a continuing decline of sea ice extent in the Arctic Basin, and to a lesser extent in the East Siberian Sea, where, normally, the trend line should have been going up already (image courtesy of Wipneus, ASIG Regional graphs):
Another factor that has come into play these past few days, is heat. It's mostly coming from the Pacific, where sea surface temperatures are running quite hot again, as can be seen on these DMI sea surface temperature anomaly maps, comparing this year's start of October with those of 2016 and 2017:
When winds blow over the zone of red colours, as is happening right now, it is bound to have an effect on surface air temperatures. In fact, it has already led to an excursion into record territory on the DMI SAT map for the region north of 80°, as can be seen Zack Labe's version of the DMI graph:
We have become more or less accustomed to these extreme spikes, but usually they're caused by Arctic waters releasing their heat so that they can start to freeze over. Given that they're freezing over relatively slowly right now, it's clear that the heat comes from elsewhere. This may be about to end, and when it does, the waters will start to release that heat, causing the spike to keep going for a while, setting new daily records north of 80° along the way.
It has led NOAA PMEL to project freeze onset on the Chukchi Sea continental shelf northwest of Icy Cape at a record late date, 47 days later than the long-term mean (1981-2016):
Their rationale corroborates the data on the DMI SST anomaly maps:
Data from autonomous floats and AXBTs indicate water column temperatures in the Chukchi Sea in 2018 are warmer than 2016 (+5 °C) and 2017 (+3 °C) Bottom-layer temperatures in 2018 are 6.5 °C warmer than 2016 and about 1.5 °C warmer than 2017. AXBT deployments 12-14 September show these conditions are broadly representative between Bering Strait and Point Barrow.
If freeze onset is later in the Chukchi, it will most likely be later in the Bering Sea as well, and we have seen how extreme things got there for all of last year's winter, breaking every record imaginable.
Whether this Arctic winter is going to get as crazy as it has been for the past couple of winters, remains to be seen, but this start doesn't inspire a lot of confidence.