Blog Cohen: impact mogelijke SSW vnl in N-A en N-Azië.

http://www.aer.com/science-research/climate-weather/arctic-oscillation/

Maandag verscheen de nieuwe blog van Judah Cohen. 

Een aantal punten:

-minor SSW is waarschijnlijk

-SSW half december is mogelijk

-Ontwikkelingen stratosfeer leiden tot lagedruk N-van Siberië en rug bij Alaska

-Strato AO negatief, troposfeer positief

-Als de SSW er komt, dan waarschijnlijk koud in het oosten van N-A en in het noorden van Azië.

-Als het blijft bij een minor SSW en een snel herstel, dan grote kans op zacht weer tot en met januari.

-Hij bekijkt SSWs in december, waarbij hij overeenkomsten ziet met de winter van 2002 (december 2001 dus).  

Impacts

In my opinion it is crunch time for the Northern Hemisphere (NH) winter. A minor sudden stratospheric warming (SSW where a warming of at least 25°C occurs in the polar stratosphere) is likely and a major mid-winter warming (MMW where the zonal mean zonal wind at 10hPa and 60N reverses from positive to negative) is possible in mid-December. I include in Figure i the temperature animation of the stratosphere and impressive warming is being predicted by the GFS, enough to at least qualify for a minor warming.  Based on the GFS forecast, some regions of the polar stratosphere could see a 70°C (126°F) jump in temperature in a matter of days!  I saw that some members of the GFS ensemble showed an MMW as early as early December, but I think this is likely too soon.

Figure i.  Forecasted average temperatures (°C/°K; countours) and  anomalies (°C/°K; shading) across the Northern Hemisphere for 18 November – 4 December 2019. The forecasts are from the 18 November 2019 GFS.

I believe regardless of the timing and magnitude of the event it will have impacts on the NH weather.  I would argue that some of the predicted features in the tropospheric circulation are related to the anticipated PV disruption.  The models are predicting a mid-troposphere low pressure over Northern Siberia starting next week.  This is very close to the predicted location of the stratospheric PV starting this week.  The other predicted main feature in the polar stratosphere is ridging/high pressure centered near Alaska.  This will likely be associated with a tropospheric feature/reflection as well.   Something similar occurred last December with ridging in the interior of North America and very mild temperatures across the continent.  A repeat is possible but my sense of the trends this fall is that the ridge will likely setup further west, forcing a colder solution but admittedly it’s a tough call.

In last week’s blog, I argued that the increase in the vertical energy transfer and the PV disruption is looking more like an “absorptive” event and less like a “reflective” event and that seems to be even more true this week.  Leading up to an “absorptive” event while the stratospheric AO trends negative the tropospheric AO trends positive with milder temperatures across the mid-latitudes and colder temperatures in the Arctic.  Though many of the trends are not particularly strong, based on today’s forecast plots included in today’s blog all those trends are apparent.  The forecast for Europe is consistent with these expectations with an increasing westerly flow and milder temperatures. Milder trends are also predicted for eastern North but those trends might run into more resistance due to record low sea ice in the North Pacific sector of the Arctic and the well above normal sea surface temperatures (SSTs) in the eastern North Pacific especially in the Gulf of Alaska.  Those features could help to promote ridging near Alaska/Gulf of Alaska with downstream troughing in North America with colder temperatures bucking the trends from the vertical energy transfer.

Regardless of the amplitude, I expect some cold weather from the SSW most likely in eastern North America and Northern Asia.  However, if the predicted SSW is relatively minor with a quick recovery in the stratospheric AO, even possibly becoming strongly positive, would favor a positive tropospheric AO.  Then an extended mild to very mild period across the NH mid-latitudes could ensue from late December through much of January.  I would expect at some point another PV disruption that would reverse the weather to colder but by then an overall mild winter would be almost a certainty.

Last December (before the AO blog was archived), I did analyze some MMW that were of the displacement variety.  First, event to event variability is large but there are two PV displacements that occurred relatively recently in mid-December – December 15, 1998 and December 16, 2000.  These are calendar dates close to when our polar vortex model predicts as the time most likely for an MMW this upcoming December.  If the SSW occurs mid-December then it takes about two weeks for the related circulation to propagate down from the stratosphere to the troposphere with a colder weather regime to follow, starting around the holidays.

The winter of 1998/99 and 2000/01 are two very different winters with winter 1998/99 being relatively mild and winter 2000/01 being relatively cold in eastern North America and Northern Asia.  There was also an MMW in late December 2001/early January 2002 followed by a mild winter.  However for now, I am leaning towards a colder solution more similar to 2000/01 more so than 1998/99 given the low sea ice in the North Pacific sector of the Arctic (though based on the weather forecasts, I expect a lot of ice to form over the next two weeks) and the very warm SSTs in the eastern North Pacific.  Also, in general the Arctic is warm.  I wrote many blogs last winter on the surprisingly cold Central Arctic that may have interfered with the downward propagation of a negative AO.  So far this fall, the central Arctic has not been cold.   Also the QBO (quasi biennial oscillation) is easterly or at least trending east.  An easterly QBO favors a more negative AO than a westerly QBO  (see Labe et al. 2019).

The following is some of the relevant text and figures copied from last December: “I have included in Figure ii the stratospheric PV a week or longer prior to when an MMW was observed and then in Figure iii the observed temperature anomalies across the NH the month when an MMW was observed and the following one or two months.  Winters included are 1998/99, 2000/01, 2001/02, 2003/04, 2005/06 (in the Cohen and Jones paper this is listed as PV split - the cold Europe and blockbuster February snowstorm along the mid-Atlantic are consistent with this - but at least the beginning resembles a PV displacement), 2006/07 and 2007/08.  Prior to 1998, the most recent stratospheric PV displacement was in 1987. My personal preference is not to use analogs prior to the era of amplified Arctic warming (pre-1990) for current winters.

Figure ii. Observed geopotential heights (contouring) and anomalies (shading) for a) December 1-17,
1998, b) December 1-10, 2000, c) December 15-28, 2001, d) December 21-31, 2003, e) January 1-14, 2006, f)
February 7-17, 2007 and g)  February 5-15, 2008.

In all of the stratospheric PV displacements since 1998 the stratospheric PV is displaced towards northern Eurasia with the exception of February 2007 when the stratospheric PV was displaced towards Greenland.  Also, the flow around the stratospheric PV in all winters was directed from Siberia towards eastern North America (5 winters) or to Europe (2 winters).  The predicted stratospheric PV displacement (see Figure 13 below) is consistent with all the previous stratospheric PV displacements of the past two decades.

Figure iii. Observed surface temperature anomalies (shading) for a) December 1 - February 28, 1998 b) December 1 - February 28, 2001, c) January 1 - February 28, 2002, d) January 1 - February 28, 2004, e) January 1 - February 28, 2006, f) February 1 - March 31, 2007 and g) February 1 - March 31, 2008.

Looking at the surface temperature anomalies during and following all seven stratospheric PV displacements shows more variability.  Temperatures across eastern North America are below normal four of seven two or three winter months during and following the stratospheric PV displacement.  Temperatures across much of Europe are below normal really only once for the two or three winter months during and following the stratospheric PV displacement.  Temperatures across Asia are below normal three of seven two or three winter months during and following the stratospheric PV displacement.”