The relative warmth in Barrow led me to wonder when the warming effect of reduced sea ice typically shows up on the calendar in Barrow. As we all know, Arctic sea ice has been much below normal in recent years and is well below-normal again this year (see chart below), despite earlier model forecasts of increased ice (as we've discussed before). The lack of autumn sea ice in the Arctic waters near Barrow has caused pronounced autumn warming in Barrow in recent years. The second chart below shows that September and November temperatures in Barrow have been higher than the long-term mean in just about every year since 1997, and the October mean temperatures have been tremendously elevated since 2002. In previous posts Rick has pointed out the collapse of the inter-annual variance of the October temperatures at Barrow, and of course this is directly attributable to the huge loss of sea ice; the open water in October simply prevents any seriously cold air from settling in over Barrow.
A more precise look at the seasonal timing of the recent warming can be found by looking at the mean daily temperature anomalies at Barrow since 2002; the dark blue line in the chart below shows that warming at Barrow relative to the 1981-2010 normal is maximized in the second half of October and starts to diminish at the beginning of November. Note that I applied a +/- 15 day smoothing to the 2002-2013 mean anomalies to remove some of the sampling variability; the unsmoothed daily mean anomaly from 2002-2013 actually peaks at +11.1 °F on October 28. The chart also shows the same data for several other coastal Arctic stations, from Kotzebue around to Deadhorse; the other stations show very similar patterns, with pronounced recent warming in October, although the timing of the peak is a little earlier at Kivalina and Kotzebue in the northwest.
The following two charts show that, interestingly, a similar October peak in recent warming is found at both Fairbanks-area stations and a collection of various other Alaska stations north of 64 °N. I am a little surprised by this, because I would not necessarily expect the pronounced warming effect from the sea ice reduction to extend so far to the south. However, the very similar timing of the peak warming at all the stations seems quite suggestive of being caused by the same phenomenon throughout, and there is very high confidence that the sea ice reduction is indeed the cause of the October warming on the North Slope. The only other possibility I can see is that the weather pattern has coincidentally (and strongly) favored southerly flow and warmer conditions in mid-late October on average in the past 11 years. A little more investigation will be required to rule out this possibility.
The winds (level?) in late Fall may have favored warmer conditions. The ocean temps along the southern and western perimeter of Alaska may have seasonally increased (?) due to changes in density caused by winds and differential water temperature and salinity along the coast.ReplyDelete
Are we sure it's the collapse of the ice that has led to the warming? Perhaps most of the warming is due to changing weather patterns. If we have a huge pressure block in Siberia causing southerly flow in all of Alaska then we would see everyone get warmer. The winds would force any ice to stay away from the coast and move more north of Russia. This lack of ice would increase temps by a few degrees (which is why Barrow and Deadhorse are a little higher than Kotsebue) and the variability has collapsed. Eventually any system regime would fall apart by the approaching winter and the ice would build. As an example, think about the weather patterns of this last winter.ReplyDelete
Thanks for the comments, Gary and Eric. It's good to think outside the box - I agree that wind pattern shifts could be involved in the seasonal temperature anomalies, and it remains to be seen if we can show what this contribution might be.ReplyDelete
However, I would argue that the collapse in October variability in Barrow is very likely a direct result of the sea ice loss. Not only does this make perfect physical sense, but in my experience atmospheric circulation anomalies (blocking patterns, etc) do not show this kind of year-to-year persistence even when there is a pronounced regime change. I'll look at this in more detail soon.
That pesky and experienced Rick has an answer for Barrow's Fall temps:Delete
Gary, thanks. I read that at the time when it was published, but had forgotten how well-written it is. A very nice piece, indeed.ReplyDelete
More opinions regarding the possible effect of ocean currents on warming in the Arctic:ReplyDelete
A link to recent papers regarding the thermal-haline regime in the North Pacific-Bering Sea that may be influencing Alaskan climate: