Arctic Year End: Okhotsk Surprise.

Arctic ice extent grows slowly this time of year since many regions are already covered completely.  In recent days 2017 reached 84% of the maximum last March, but dropped below the 10 year average.  As we can see, most of the action was a dramatic seesaw in Okhotsk Sea, a Siberian basin in the North Pacific.

Mid-Decenber Okhotsk Sea on the left began growing ice steadily to reach half of 2017 March max, then inexplicably lost over 300k km2 of ice in just four days.  Meanwhile, Chukchi (upper right) froze completely, then retreated somewhat.  Bering Sea on the right has been advancing steadily but more slow than average.

The overall effect in December is shown in the graph below:
Note that 2007 matches the 10 year average, while 2012 is well above.  Lagging behind are 2016, 2017 and SII 2017.  This year’s deficit to average appears in the last 11 days, mostly due to Bering and the Okhotsk year end surprise.
Most of the month combined Bering and Okhotsk (B&O) extents were average, until the inexplicable drop starting day 361. Except for those two seas, the month was unremarkable.
At year end, 2017 NH ice, excluding B&O, is down about 200k km2, or 2% of average.  Below is the analysis of regions on day 365.

Region 2017365 Day 365
Average
2017-Ave. 2007364 2017-2007
 (0) Northern_Hemisphere 12628187 13148939 -520752 13049737 -421550
 (1) Beaufort_Sea 1070445 1070225 220 1069711 734
 (2) Chukchi_Sea 943883 966001 -22118 965971 -22089
 (3) East_Siberian_Sea 1087120 1087134 -14 1087120 0
 (4) Laptev_Sea 897845 897841 4 897845 0
 (5) Kara_Sea 892689 888842 3848 871851 20839
 (6) Barents_Sea 331819 448769 -116950 334577 -2758
 (7) Greenland_Sea 555757 584649 -28892 666135 -110378
 (8) Baffin_Bay_Gulf_of_St._Lawrence 978074 1025197 -47123 1074827 -96753
 (9) Canadian_Archipelago 853109 853054 55 852556 553
 (10) Hudson_Bay 1260838 1227587 33251 1260856 -19
 (11) Central_Arctic 3191526 3207659 -16133 3199726 -8200
 (12) Bering_Sea 194350 446066 -251716 373942 -179592
 (13) Baltic_Sea 13345 34329 -20984 9972 3374
 (14) Sea_of_Okhotsk 336595 387522 -50928 371241 -34646

Most of 2017 deficit is in Bering and Barents, less so in Okhotsk with yesterday’s recovery.  Perhaps the Polar Vortex sent frigid air into the US and Canada and replaced it with warmer southern air, and ice receded as a result.  I have reported on Arctic incursions in a post Arctic Inversions and Intrusions but the speed of the effect on ice (if that is the phenomenon) is still surprising.  We shall see if it persists or not.

Background:  Winter Forecast by Dr. Judah Cohen, Dec. 25

On Christmas Day Dr. Judah Cohen published his updated Arctic Oscillation and Polar Vortex Analysis and Forecast  Excerpts below

As I have been discussing the past two blog posts I continue to believe that this is the most critical period of the winter and will ultimately determine the character of the winter. In my opinion, we have approached a fork in the road and the atmosphere can take two possible paths one is a path where the rest of the winter is relatively mild across the mid-latitudes of the NH and the second is a colder path or solution. The tropospheric polar vortex has been relatively weak for much of the month of December as illustrated by the relatively warm polar cap geopotential heights (PCHs) for the first half of December that then boomeranged off the mid-stratosphere and after a short respite with colder tropospheric PCHs, warmer PCHs in the troposphere are predicted through the end of the month and into early January.

Based on the easterly phase of the Quasi-Biennial Oscillation (QBO) I have been favoring a significant PV disruption most likely in January for much of the fall.  A significant stratospheric PV disruption would then be followed by an extended period of severe winter weather across the mid-latitudes of the NH.  However, I have to admit as of yet there are no signs of this scenario.  There is still time but not much.  However, there is another scenario that could still yield a relatively cold winter and that is one analogous to what happened in the winter of 2013/14 where no significant disruptions of the stratospheric occurred but rather repeated minor disruptions that yielded a cold winter for central and eastern North America. Though at first, I acknowledged this possibility I was hesitant to favor this scenario.  However, the longer the delay in a major disruption of the stratospheric PV the more I favor this scenario.

The Arctic Oscillation (AO) is currently slightly positive and is predicted to trend slowly negative through the end of the week towards neutral. The forecast is for the AO to remain close to neutral next week, likely a sign of uncertainty.

Figure 7. Forecasted snowfall anomalies (mm/day; shading) from 31 December 2017 – 4 January 2018. The forecasts are from the 00Z 18 December 2017 GFS ensemble. Note the new projection to provide finer resolution.

The current positive AO is reflective of mixed pressure/geopotential height anomalies across the Arctic and mostly positive pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is also currently slightly positive with weak pressure/geopotential height anomalies across Greenland and Iceland and positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic.

A return to Ural ridging/blocking at the turn of the calendar year favors active energy transfer from the troposphere to the stratosphere. Another pulse is predicted to commence this week and peak the first few days of January. The pulse itself is likely related to relaxation of the pattern and mild temperatures across North America. However, the predicted resultant perturbation to the stratospheric polar vortex (PV) especially the building of heights over northwestern North America in my opinion could once again favor the return of cold temperatures across eastern North America later in January.

Figure 9. Forecasted snowfall anomalies (mm/day; shading) from 5 – 9 January 2018. The forecasts are from the 00Z 25 December 2017 GFS ensemble. Troughing and/or cold temperatures will support the potential of additional snowfall across Eastern Europe, Siberia, West and East Asia, Alaska, Canada and the Northwestern US. Despite below normal temperatures, temperatures warm sufficiently to support snowmelt across the Eastern US (Figure 9).

Ice did not grow this past week on the North Pacific side of the Arctic basin, still I expect the negative ice anomalies in the region to shrink.  Sea ice is below normal in the Barents-Kara Seas as well and with time the greatest negative sea ice anomalies will likely reside in this region.  As I have been writing for much of the fall I do believe that the record low sea ice anomalies in the Chukchi and Bering Seas has focused ridging/blocking in this region.  Though model forecasts do not predict a return of the blocking in the foreseeable future.

Recent research has shown that regional anomalies are important and the sea ice region most highly correlated with the winter AO is the Barents-Kara Seas region where low Arctic sea ice favors a negative winter AO and a cold Eurasia.  Below normal sea ice in this region may be contributing to more active Wave Activity Flux/poleward heat transport predicted in the models that eventually could result in a negative AO.

As we have seen, the Polar Vortex did indeed return to North America, freezing Niagara Falls in the process.

Whichever fork the ice takes, the Polar Bears had a Merry Christmas

 

 

via Science Matters

http://ift.tt/2C4rtZ9

January 1, 2018 at 03:24PM

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